Thennoluminescence 

Dating of Hawaiian Basalt

By ROOD J, MAY

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1095

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1979
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

May, Rodd J.

Thermoluminescence dating of Hawaiian basalt.

(Geological Survey Professional Paper 1095)

Bibliography: p. 41-43.

1. Thermoluminescence dating. 2. Basalt-Hawaii. I. Title. II. Series: United States. Geological Survey.

Professional Paper 1095. 

QE508.M36 551.7*01 78-26826

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402

Stock Number 024-001-03142-6
CONTENTS

Page

Abstract _______________________________ 1

Introduction ____________________________ 1

Previous work ________________________ 2

Purpose and goals _______________________ 4

Basic physical properties of thermoluminescence _______ 4

lonization and electron trapping ______________ 4

Trap depth __________________________ 5

Stability of trapped electrons _______________ 6

Nonthermal drainage _____________________ 6

Thermoluminescent emission _______________ 6

Radiation damage in dosimeter crystals _________ 9

Experimental equipment and technique ____________ 10

Sample collection ______________________ 10

Sample preparation ______________________ 12

Major-elementplagioclase compositions __________ 13

TL measuring and recording equipment __________ 14

Procedure ___________________________ 16

Analysis of the plagioclase emission spectrum _________ 18

Procedure and results ____________________ 18

Discussion ___________________________ 18

Summary ____________________________ 20

Linearity of TL acquisition, saturation effects, and

sensitization _______________________ 20

Linearity of TL acquisition and saturation ________ 20

Sensitization effects _____________________ 22

Fading of artificially induced TL ________________ 23

Page

Radioactive element contents of the alkalic and tholeiitic basalts and dose-rate calculations ______________ 25

Radioactive element contents of the basalts ________ 25

Calculation of radioactive decay energies _________ 27

Dose rate calculations ____________________ 28

Discussion ___________________________ 28

Summary ____________________________ 29

Natural TL ____________________________ 29

Alkalic basalt natural TL __________________ 30

Tholeiitic basalt natural TL _________________ 31

Artificial TL __________________________ 33

Alkalic basalt artificial TL _________________ 33

Tholeiitic basalt artificial TL _______________ 34

TL dating and analysis of errors ________________ 36

TL dating of the alkalic basalts _______________ 36

TL saturation and the age limits of the method _____ 38

TL dating of the tholeiitic basalts _____________ 39

Summary of the TL-dating procedures and predictions for 

future applications _____________________ 40

References cited __________________________ 41

Supplemental information ____________________ 44

Sample location and age data _______________ 44

Alkalic basalts ______________________ 44

Tholeiitic basalts ____________________ 45

Derivation of equations for the error in the TL ratios and

TLages _______________________ 46

Error equations for the TL age ratio _________ 46

Equation for the error in the TL ages _________ 47

ILLUSTRATIONS

FIGURE 1. Generalized TL glow curve ____________________________________________________ 2

2. Schematic representation of the distribution of energy levels in an insulator mineral __________________ 7

3. Simplified geologic index map of the Island of Hawaii showing sample localities _____________________ 11

4. Ternary diagram of plagioclase composition of alkalic and tholeiitic basalt ________________________ 13

5. Schematic diagram of the thermoluminescence reader ____________________________________ 14

6. Spectral response curves for the PM tube and filter CS5-60 _________________________________ 15

7. An approximate segregation of an artificial TL glow curve into its four constituent peaks _______________ 17

8. Transmission curves for the interference filters used for spectrum analysis ________________________ 18

9. Spectral composition of the 300° C artificial TL emission from selected alkalic and tholeiitic basalt plagioclase separates 19

10. Rate of TL accumulation with increasing radiation exposure in both the alkalic and tholeiitic basalts ________ 21

11. Artificial TL decay during a 40-day storage of irradiated samples _____________________________ 24

12. Curves of average percentage of 300°C peak TL remaining in alkalic and tholeiitic basalt samples after each decay

	interval _____________________________________________________________ 24

13. Natural and artificial TL glow curves for selected alkalic and tholeiitic basalts ______________________ 30

14. Alkalic basalt natural TL relative to sample age ________________________________________ 31

15. Tholeiitic basalt natural TL relative to sample age ______________________________________ 32

16. Alkalic basalt 300°C artificial TL relative to sample age ___________________________________ 34

17. Tholeiitic basalt 300° C artificial TL relative to sample age _________________________________ 35

18. 300°C artificial TL peak-height averages for historic tholeiitic basalts ___________________________ 36

19. Alkalic basalt TL ratios relative to sample age ________________________________________ 37

20. Tholeiitic basalt natural TL peak-height averages for the carbon-14-dated prehistoric basalts relative to sample age 39

21. Tholeiitic basalt NEa/AEn ratios plotted relative to sample age ______________________________ 40

	III
iv CONTENTS

TABLES

Page

TABLE 1. Major-element chemical compositions of selected plagioclase separates ___________________________ 13

2. Percentage of total TL in the 4,000-A to 5,000-A bandwidth for the 300° C alkalic basalt plagioclase artificial TL peak _ 20

3. Percentage of TL remaining in the 300° C peak of both the alkalic and tholeiitic basalts at the end of each decay interval 23

4. Radioactive element contents of the alkalic basalts ______________________________________ 26

5. Radioactive element contents of the tholeiitic basalts _____________________________________ 27

6. Abbreviated radioactive decay series and decay energies for the radioactive isotopes of uranium, thorium, and potassium 27

7. Natural abundances and half-lives of the isotopes _______________________________________ 28

8. Energy constants in rads per parts per million per year adjusted for natural isotopic abundances ___________ 28

9. Annual dose rates for the alkalic and tholeiitic basalts ____________________________________ 28

10. Alkalic basalt natural TL peak-height averages for crushing stages 1 and 2 ________________________ 31

11. Individual and cumulative errors in the energy-normalized peak-height average for the alkalic basalts ________ 31

12. Tholeiitic basalt natural TL peak-height averages for crushing stages 1 and 2 _______________________ 32

13. Individual and cumulative errors in the energy-normalized peak-height average for the tholeiitic basalts _______ 33

14. Alkalic basalt 300° C artificial TL peak-height averages for crushing stages 1 and 2 measured with and without filter

	CS5-60 _________________________________________________________ 34

15. Tholeiitic basalt 300° C artificial TL peak-height averages for crushing stages 1 and 2 measured with and without filter

	CS5-60 ________________________________________________________ 35

16. Alkalic basalt TL ratios, TL ages, and calculated errors ____________________________________ 37
THERMOLUMINESCENCE DATING 

OF HAWAIIAN BASALT

By RODD J. MAY

ABSTRACT

The thermoluminescence (TL) properties of plagioclase separates from 11 independently dated alkalic basalts 4,500 years to 3.3 

million years old and 17 tholeiitic basalts 16 years to 450,000 years 

old from the Hawaiian Islands were investigated for the purpose of 

developing a TL dating method for young volcanic rocks. Ratios of 

natural to artificial TL intensity, when normalized for natural radiation dose rates, were used to quantify the thermoluminescence 

response of individual samples for age-determination purposes. 

The TL ratios for the alkalic basalt plagioclase were found to increase with age at a predictable exponential rate that permits the 

use of the equation for the best-fit line through a plot of the TL 

ratios relative to known age as a TL age equation. The equation is 

applicable to rocks ranging in composition from basaltic andesite 

to trachyte over the age range from about 2,000 to at least 250,000 

years before present (B.P.). The TL ages for samples older than 

50,000 years have a calculated precision of less than ± 10 percent 

and a potential estimated accuracy relative to potassium-argon 

ages of approximately ± 10 percent. An attempt to develop a similar dating curve for the tholeiitic basalts was not as successful, primarily because the dose rates are on the average lower than those 

for the alkalic basalts by a factor of 6, resulting in lower TL intensities in the tholeiitic basalts for samples of equivalent age, and also 

because the age distribution of dated material is inadequate. The 

basic TL properties of the plagioclase from the two rock types are 

similar, however, and TL dating of tholeiitic basalts should eventually be feasible over the age range 10,000 to at least 200,000 years 

B.P.

The average composition of the plagioclase separates from the 

alkalic basalts ranges from oligoclase to andesine; compositional 

variations within this range have no apparent effect on the TL ratios. The average composition of the plagioclase from the tholeiitic 

basalts is labradorite.

The natural radiogenic dose rates for the alkalic basalts calculated on the basis of assumed secular equilibrium range from 0.228 

to 0.462 rad per year and average 0.335 rad per year exclusive of the 

cosmic-ray energy dose and with the alpha-particle component 

equal to one-tenth of the total alpha decay energy. The TL measurements were made using material of a 37- to 44-micrometer size 

range; the crushing required during sample preparation was found 

to have a negligible effect on natural TL.

Both natural and artificial TL were filtered to the bandwidth 

3,500 A to 5,000 A to restrict the light detected to that from the 

plagioclase emission peak centered at about 4,500 A and associated 

with structural defects. Within this bandwidth, the natural TL 

from both the alkalic and tholeiitic basalt plagioclase consists of a 

single peak with a maximum amplitude at about 350° C; the artificial TL glow curves produced by an exposure of the drained samples to a standard dose of X-radiation consist of four broad,

variably overlapping peaks with maxima at about 110°C, 150°C, 

225°C, and 300°C. The maximum amplitude of the 350°C natural 

and 300°C artificial TL peaks, both produced by the same general 

activation energy distribution of trapping centers, were used for 

TL dating. The high-temperature artificial TL peak occurs at a 

lower temperature than the corresponding natural TL peak owing 

to the presence of a large number of electrons retained in traps 

near the lower end of the trap-depth energy range in samples 

whose TL is measured a short time after intense artificial irradiation. These traps remain essentially empty in the natural environment owing to spontaneous decay and do not produce measurable 

low-temperature natural TL peaks. With prolonged storage after 

irradiation, the 300° C artificial TL peak migrates to higher temperatures and decreases in amplitude.

INTRODUCTION

Thermoluminescence (TL) is the transient emission 

of light in or near the visible spectrum that accompanies the heating of certain semiconductor and insulator minerals to temperatures below that of 

incandescence. The phenomenon is incompletely understood but originates in the ionization of lattice 

atoms of the dosimeter crystals by decay particles 

from radioacitve elements incorporated in the rock or 

mineral. If enough energy is transferred to the excited 

electrons during ionization, they will be elevated to 

the conduction band, where a proportion will become 

trapped in electrically charged defect structures in the 

crystal lattice. These trapped electrons lose energy in 

the form of light (TL) as they return to the valence 

band during heating.

TL is distinguished from the other two principal 

types of luminescence, fluorescence and phosphorescence, by the existence of a significant delay time between excitation and the emission of stored energy 

and by the temperature-dependent nature of the 

emission process. In general, all types of luminescence 

are distinguished from incandescent glow by their occurrence in the spectral region where the mineral or 

other crystal is nonabsorbing (optically transparent), 

whereas incandescent emissivity by Kirchoff 's Law is 

strong only in the spectral region where the crystal is 

strongly absorbing.
THERMOLUMINESCENCE DATING OF HAWAIIAN BASALT

The phenomenon of TL was first reported more 

than 300 years ago by Boyle (1663) and has been studied in various contexts since that time. Only in the 

past two decades, however, following a discussion of 

the potential of TL for a variety of applications, including dating (Daniels and others, 1953), has a concentrated effort been made to understand the TL 

phenomenon and to employ it in solving such diverse 

problems as radiation dosimetry, stratigraphic correlation, and the dating of various archeologic and geologic materials.

The use of TL for dating is based on the cumulative 

retention of a representative proportion of the electrons excited to the conduction band by radioactive 

decay and their detectable release during laboratory 

heating. With increasing temperature, thermal stimulation provides sufficient kinetic energy to free electrons from progressively deeper (higher activation 

energy) traps. The untrapped electrons then return to 

the valence energy band at defect or impurity "recombination centers," emitting the energy differential between the excited and ground states as light. The low 

levels of light emitted are typically measured with a 

photomultiplier tube, and concurrently the voltage 

produced is plotted in relation to temperature using 

an X-Y plotter to obtain what are referred to as glow 

curves.

A generalized glow curve, figure 1, shows a curve 

representing TL emission and one representing the 

combined electronic noise and blackbody (incandescent) radiation background, which is measured during 

a second heating of the sample. The magnitude of the 

natural or artificial (laboratory induced) TL emission 

can be quantified either as the area between the two 

curves within specified temperature boundaries or in 

terms of peak heights by the vertical distance between 

the two curves at the point of maximum amplitude of 

the particular TL peak being studied.

I

TEMPERATURE

FIGURE 1. Generalized TL glow curve, g, TL emission; bb, blackbody radiation (incandescence) background.

Absolute TL ages can be calculated directly from 

the natural TL data if the natural radiation dose rate, 

the sensitivity (trap density) of the samples, and the 

ionization efficiency of the different components of 

natural radiation are known. Ideally, dividing the 

magnitude of the laboratory (artificial) radiation dose 

required to reinstate an artificial TL peak equivalent 

to the natural peak by the natural annual dose rate 

should yield the age of the sample. Unfortunately, this 

direct approach, in which quartz is used as the TL dosimeter, has not been generally successful for samples 

older than about 30,000 years, mainly because of 

spontaneous natural TL decay. The derivation of an 

empirical TL-dating method of more widespread applicability is the principal goal of this investigation.

PREVIOUS WORK

After what was probably the first recorded description of the TL phenomenon by Boyle in 1663, relatively little research was done until the 1950's. Significant early papers include the classic study of both 

phosphorescence and thermoluminescence by Randall and Wilkins (1945) and the general review of TL 

research by Daniels and others (1953), which includes 

one of the earliest proposals for using TL in dating.

More recently TL has been a subject of increasing 

interest and investigation, with applications in a number of fields. One of the more important of these outside the general field of dating archeologic and 

geologic materials has been the use of TL in radiation 

dosimetry and personnel monitoring. In this application, synthetic dosimeter crystals, of which lithium 

fluoride is the most widely used, are employed to monitor a wide range of radiation energies and total doses 

in industrial and laboratory situations.

Numerous attempts have been made in recent years 

to develop TL- dating procedures for a variety of materials and a wide range of geologic ages, but most 

have encountered difficulty. The greatest degree of 

success has been achieved with the dating of young archeologic artifacts using several different materials 

and techniques. These methods exemplify the "absolute" dating approach in that sample ages are calculated directly from measurements of TL, trap density 

(sensitivity), and natural radiation dose rate. They are 

distinct from "relative" dating, which requires a comparison of individual TL values with an age curve 

based on independently dated samples of similar composition for an assignment of geologic age.

Aitken, Zimmerman, and Fleming (1968) have developed a TL-dating method using crushed and sized, 

but mineralogically unseparated, potsherds, for which 

they have obtained ages from 640 to 3,500 years B.P.
INTRODUCTION

(before present) with a precision of ± 10 percent relative to carbon-14 ages. The same basic experimental 

procedure has been used in authenticity testing of 

more recent artifacts for which apparent ages are 

known but for which questions of origin (that is, authenticity) exist (Fleming, 1970; Fleming and others, 

1970). Refinement of this method has led to the development of a "fine-grain" technique that promises a 

routine precision of less than ± 10 percent (Aitken, 

1970; Zimmerman, 1971). This technique, which now 

largely supersedes the earlier technique, involves the 

separation of a 2- to 8-/mi(micrometer) "whole-sherd" 

(mineralogically unseparated) fraction from carefully 

crushed sherds. The grain-size constraint is imposed 

by the very short range in a crystalline matrix of the 

decay particles that are critical in both defect formation and ionization. At the same laboratory at Oxford 

University, a "quartz-inclusion" method has also been 

developed for application to the same types of artifacts to which the "fine-grain" method is applied. This 

method, which involves the preparation, from gently 

crushed sherds, of a pure quartz separate composed of 

grains 95 to 105 n in diameter, has been shown to yield 

ages near 1,500 to 2,000 years B.P. with an average deviation of the TL ages from the archeologic ages of 

only 5.4 percent (Fleming, 1970). The theoretical considerations and analytical techniques for the two 

methods are essentially identical.

Another TL method, based on the use of thin slices 

from pyrolyzed chert fragments recovered from archeologic sites as the TL dosimeter, yields ages in the 

range 12,000 to 50,000 years B.P. with an accuracy of 

better than ± 15 percent relative to cultural ages as 

reported by Goksu and others (1974). These workers 

predict the eventual extension of their method, which 

is similar to that developed for pottery, to possibly 

200,000 years.

Relatively little work has been done to develop a 

TL-dating method for geologic materials, and most 

attempts have been unsuccessful, partly because 

much work has been attempted with samples older 

than the probable saturation limit of TL accumulation and partly because of the limitations of the materials and procedures used. Although the development 

of a reliable dating method has proved difficult, 

most investigations have identified a general tendency 

for natural TL to increase with age in a variety of 

materials.

The potential of using calcium carbonate (principally limestone) for dating has been investigated by 

several workers, among them Zeller and others (1957), 

Johnson (1963), and Zeller (1968). Hutchinson (1968) 

attempted to date tectonic and magmatic events using 

limestone, with generally poor results. An investigation of the TL of foraminifers collected from independently dated pelagic sediment cores by Bothner and 

Johnson (1969) identified a systematic increase in the 

natural TL of the calcium carbonate tests to an age of 

approximately 105 to 106 years. This predictable increase could conceivably provide the basis for a reliable dating procedure. In general, however, the TL 

properties of calcium carbonate are complex and are 

frequently subject to interference from spurious TL 

emission. The mineral is at the present time of uncertain value for age determinations, although research is 

continuing.

Studies of the age of deposition of fluorite using 

that mineral as the TL dosimeter by Blanchard (1966) 

.and Kaufhold and Herr (1968) have been generally 

unsuccessful. The alkali halide minerals do not appear 

at this time to provide a reliable basis for age determination. Attempts to determine the age and thermal 

history of meteorites (Houtermans and Liener, 1966) 

and tektites by TL (Durrani and Christodoulides, 

1969; Durrani and others, 1970) have yielded similarly 

discouraging results. A moderate degree of success 

was reported by Vaz and Senftle (1971) in the correlation of radiometric and TL ages obtained for a collection of zircon crystals from Ceylon (now Sri Lanka) 

that range in age from about 1X109 to 3X109 years. 

Their method, however, is more an evaluation of the 

extent of radiation damage in the crystals of high radioactive element content than of simple TL accumulation.

Slightly more attention has been devoted to the 

minerals of intrusive igneous rocks, particularly 

quartz and feldspar, two of the most sensitive and 

widely occurring natural TL dosimeters. One attempt 

to date vein quartz of approximate Precambrian age 

(Hwang, 1972) failed, apparently because saturation 

effects produced lower than expected TL values. 

Panov and others (1970) identified progressively increasing TL intensities for both quartz and feldspar 

from four intrusive complexes in Transbaykalia that 

range in age from about 83 to 227 million years old; 

however, they did not attempt to use the data to calculate TL ages, and the suitability of the data for such 

calculations was not evaluated.

Mineral separates and whole-rock fractions from 

young volcanic rocks used in several TL studies appear to offer the most promise for the successful derivation of a TL age method. Sabels (1963) performed 

one of the earliest and most definitive TL studies using crushed whole-rock fractions from a number of 

dated basalt flows as old as 15 m.y. He was not successful in assigning reliable TL ages to most of the 

rocks but did establish the potential viability of TL 

dating for rocks younger than 1 m.y. and identified
THERMOLUMINESCENCE DATING OF HAWAIIAN BASALT

many of the problems inherent in TL dating. Several 

subsequent dating attempts have employed mineral 

separates from volcanic flows as the basis for age determination. Hwang (1970), in dating orthoclase separated from an 1,800-year-old flow from Mount 

Vesuvius, reported a good correlation of the TL and 

archeologic ages. Two younger flows studied as part of 

the same investigation did not possess any measurable 

natural TL. Aitken, Fleming, Doell, and Tanguy 

(1968) used feldspar separates to study the TL from 

flows of known age from several volcanoes but were 

unable to derive realistic TL ages from their data. A 

more recent attempt by Aitken and Fleming (1971) to 

date volcanic products including lava, tephra, and 

pumice by the fine-grain technique gave an approximately correct age for the lava but unsatisfactory ages 

for the tephra and pumice owing to suspected spurious TL interference from an unidentified mineral. 

Wintle (1973) attempted to date several flows ranging 

in age from 5,000 to 50,000 years B.P. by using the TL 

from feldspar separates but was unable to obtain accurate dates owing to lower-than-expected natural TL 

values, which were attributed to "anomalous fading" nonthermal drainage of trapped electrons during geologic time.

A preliminary investigation by Berry (1973) of the 

TL properties of plagioclase separates from a suite of 

six samples of tholeiitic and alkalic basalt from the island of Hawaii ranging in age from approximately 225 

to 17,000 years led to the identification of a progressive increase in the TL of the samples with increasing 

age. No attempt was made on the basis of the preliminary results to develop a dating method for these 

rocks, but the data display the same basic TL relative 

to age relation identified by other workers for prehistoric pottery.

PURPOSE AND GOALS

The investigation by Berry (1973) prompted May to 

initiate a more detailed and comprehensive study of 

the potential of using TL to date Hawaiian basalts. 

The principal impetus for the research was the possibility of using TL dating to bridge the significant age 

gap that exists between the limits of the two principal 

methods for dating young rocks, radiocarbon and potassium-argon. The age range of specific interest was 

that from about 35,000 years, the approximate upper 

limit of the carbon-14 method, to about 100,000 years, 

the approximate lower limit for the potassium-argon 

method for most kinds of rocks.

The rocks selected for use in this investigation included samples of both alkalic and tholeiitic basalt 

from flows of independently determined young age

from the Hawaiian Islands. These rocks were chosen 

because (1) a reasonably large selection of well-dated 

material was available; (2) some of the flows had already been studied on a preliminary basis; (3) the Hawaiian rocks are of considerable geologic interest; and 

(4) except for those few flows that have associated carbon, young Hawaiian lavas have proved difficult to 

date by other methods.

The experimental procedure will begin with the investigation of critical aspects of the TL process followed by a presentation and discussion of the TL 

measurements and an analysis of the feasibility of using the data in the development of a TL-dating technique.

BASIC PHYSICAL PROPERTIES 

OF THERMOLUMINESCENCE

Thermoluminescence, as defined briefly in the introduction, is the energy in excess of thermal radiation emitted as light during the heating of an 

activated dosimeter mineral. The phenomenon can be 

depicted spatially by a three-dimensional array of 

orthogonal axes representing light intensity, activating temperature, and spectral composition.

Details of the TL process are still only partially understood, even though a number of empirical and 

theoretical investigations have been made over the 

past 30 years. Nonetheless, understanding of the phenomenon is sufficient to permit a qualitative discussion of specific aspects of the TL process.

IONIZATION AND ELECTRON TRAPPING

Thermoluminescent emission is observed only (1) 

in semiconductor and insulator minerals having a forbidden energy band-gap of at least several, but probably not more than 10, electronvolts between the 

valence and conduction band and (2) to varying degrees in different minerals exhibiting this basic property. Igneous minerals that possess a significant 

innate thermoluminescent capability include quartz, 

feldspar, apatite, and zircon.

During the process of spontaneous decay of the isotopes 238U, 235U, 232Th, and 40K, which occur in varying 

amounts in rocks and minerals, high-energy alpha, 

beta, and gamma radiations are emitted. This internal 

radiation is supplemented in rocks near the Earth's 

surface by the more penetrating component of cosmic 

radiation. In the event these high-energy radiations 

intercept dosimeter minerals, such as quartz or feldspar in a rock or pottery sherd, they can produce ionization of constituent atoms of the crystals. If only a 

small amount of energy is transferred to an electron of
BASIC PHYSICAL PROPERTIES OF THERMOLUMINESCENCE

a struck atom during the coulomb interaction of a decay particle with the electronic field of the atom, the 

ionization event will result in "pair" formation. In this 

case, the electron and positively charged "hole" in the 

valence band of the atom remain spatially associated 

by mutual charge attraction and recombine with the 

transfer of excess energy to the lattice as heat. If, however, sufficient energy is imparted to an electron during ionization, it will be excited to the conduction 

band of the crystal. Dislocation of atoms from normal 

lattice sites can also occur, as discussed in a later 

section.

Once in the conduction band, an electron wanders 

through the lattice until it either becomes trapped or 

encounters a recombination center, which can serve to 

degrade it back to the valence band with the emission 

of the energy differential as a photon of light.

The properties of traps and their influence on TL 

are complex. In general, a trap is a localized condition 

of inhomogeneity in the electronic field of the crystal. 

It may be associated with any of a number of physical 

defects including vacancies, interstitial ions, impurity 

ions, or dislocation defects and may have either a net 

positive or negative charge. Both electrons and positively charged holes can become trapped in these locations of unbalanced charge for periods of time that are 

both temperature and time (probability) dependent.

The probable lifetime (p) of an electron in a trap of 

depth (E) has been described by Randall and Wilkins 

(1945) as

where s is the "attempt to escape" frequency, T the 

absolute temperature, and k the Boltzmann constant. 

From this relation, it is apparent that electrons in 

deep traps (E large) would be more stable than those 

in shallow traps (E small). For an isothermal situation, the decay rate for an initial population (n) of 

electrons in traps of a single activation energy (depth) 

according to the Randall-Wilkins equations is

dn

dt

This model assumes first-order (monomolecular) kinetics and does not provide for nonthermal drainage 

by quantum mechanical "tunneling" or other possible 

decay processes that do not require that trapped electrons return to the conduction band before combining 

with an impurity or defect center or for retrapping of 

thermally released charge carriers. An investigation of 

the effect of retrapping by Garlick and Gibson (1948) 

led to the conclusion that it was negligible. More recently, Durrani and others (1973) demonstrated that 

there was no detectable retrapping of electrons released from deep traps by shallower (lower temperature peak) traps.

The Randall-Wilkins model has since been found to 

be incomplete but still represents the best available 

approximation to the TL process. The basic equation 

has served as the basis for a number of more recent 

derivations, including those by Christodoulides and 

Ettinger (1971). Braunlich (1968), and Land (1969b). 

More complex models based on second-order kinetics 

have been derived (see for example, Curie, 1963; Medlin, 1968) but do not provide substantially better explanations of the TL process in compositionally 

complex natural minerals such as feldspar.

TRAP DEPTH

Electron trap "depths" are measured in units of 

electronvolts whose calculated values represent the 

activation energy required to release an electron from 

a trap of specified depth. Trap depths have been determined both from theoretical considerations and by 

measurement of actual TL properties (for a comprehensive compilation of trap depth determinative 

methods, see Braunlich, 1968). Physical methods include "isothermal annealing" experiments in which 

the rate of decay of artificially induced TL is measured over a specified time interval for samples stored 

at constant temperature and "initial rise" techniques 

that derive pertinent information directly from the 

morphology of the rising portion of the TL glow curve 

and from, in at least one instance (Land, 1969a), inflection points and peak maxima of the glow curves. 

The two basic physical methods have been used essentially interchangeably by many workers in terms of 

the accuracy and reliability of the data obtained. Recently, however, they have been shown by Wintle 

(1975) to yield significantly different results when 

carefully applied to the same material, in the particular example cited, quartz.

The trap depth calculations typically are made by 

using artificial radiation to fill traps in samples from 

which the natural TL has been drained; the results are 

then extrapolated to the natural TL situation.

Trap depths calculated for two of the prominent 

peaks of quartz by the isothermal annealing technique 

by Hwang and Goksu (1971) gave values of 0.97 ±0.1 

eV (electronvolts) for the 110°C peak and 1.1 ±0.1 eV 

for the 370°C peak. Using the same basic isothermal 

annealing approach, Wintle (1975) obtained a value of 

1.7 ±0.1 eV for the 325°C peak of quartz. The difference in trap depth and peak maximum temperature 

obtained during these two experiments for what is 

probably the same high-temperature peak (trap type 

and depth) is probably a result of differences in equip-
THERMOLUMINESCENCE DATING OF HAWAIIAN BASALT

ment and in sample handling and irradiation techniques.

STABILITY OF TRAPPED ELECTRONS

The probable lifetime of electrons in traps of a single depth or narrow range of depths is critical because 

the TL peak selected for dating use must be sufficiently stable that the traps that produce it retain a 

representative proportion of the charge trapped over 

the time period of dating interest. Peak stability is 

usually defined in terms of half-lives (£i/2 ) expressed 

with the equation

In2 ,E/kT

based on the Randall-Wilkins model for electron retention, where, as before, k represents the Boltzmann 

constant. Differences in the assigned values for the 

trap depth (E), storage temperature (T), and frequency factor (s), for which estimated values in the literature range from 107 to 1012 sec"1 , commonly produce 

differences in the half-lives calculated for the same 

TL peak by different workers. For example, the halflife of a hypothetical peak produced by an electron 

population stored in traps with an arbitrary depth of 

1.1 eV and a frequency factor of 5X108 sec" 1 , as calculated with the equation listed above, is 3.7 X 102 years. 

Increasing the trap depth by 0.1 eV, the limit of precision reported by many workers for trap depth calculations, to 1.2 eV increases the calculated half-life by a 

factor of approximately 51 to 1.9 X 104 years. This 

large difference in calculated half-lives produced by a 

difference in trap depth, equal in many cases to the 

limit of resolution of the determinative methods, severely limits the usefulness of such calculations for detailed peak stability evaluation. The reliability of the 

calculations is further reduced by the fact that the TL 

peaks are probably produced by a narrow continuum 

of trap depths rather than a single discrete depth and 

that nonthermal drainage of electrons from traps may 

be significant. Neither of these factors is adequately 

accounted for by existing models. Moreover, as there 

are large differences in the dose rate between natural 

and artificial radiation, the use of artificially induced 

TL to model detailed properties of natural TL is probably not strictly valid.

There is at this time no definitive method for accurately determining natural TL peak half-lives that incorporates an assessment of all the variables 

mentioned above. In general, natural TL peaks that 

occur at 300° C or more are probably sufficiently stable to be of use in TL-dating experiments.

NONTHERMAL DRAINAGE

The spontaneous decay of artificially induced TL in 

samples stored in the dark after irradiation in excess 

of that caused by thermally related processes as predicted by existing theory has been termed anomalous 

fading by Wintle (1973). This anomalous fading is 

probably in fact nonthermal drainage, the spontaneous escape of electrons from traps by tunneling or other processes that do not require that the trapped 

electrons reenter the conduction band before being 

degraded to the valence band. The existence of this 

phenomenon has been studied recently in connection 

with the TL properties of both terrestrial and lunar 

plagioclase, with apparently conflicting results. 

Anomalous fading of TL in artificially irradiated samples of terrestrial feldspar from several volcanic regions, and significantly lower than expected natural 

TL values in the same samples, were attributed by 

Wintle (1973) to nonthermal drainage. A more recent 

investigation has shown the process to be essentially 

nonluminescent within limits of detection (Wintle, 

written commun., 1975). Garlick and Robinson (1972) 

reported significant nonthermal drainage over a wide 

range of trap depths in both terrestrial and lunar plagioclase, as manifest by similar isothermal TL decay 

rates for artificial TL peaks with a wide range of activation temperatures. Blair and others (1972) stored 

artificially irradiated samples at -196°C and noted 

some TL decay, which they attributed to nonthermal 

leakage. Durrani and others (1973) restudied the 

problem, and although they could not detect significant nonthermal drainage, they could not rule out its 

possible existence or significance. The preponderance 

of the evidence seems to support the existence of 

nonthermal drainage; the general significance of the 

phenomenon for dating applications of TL has been 

discussed by Garlick (1973). This is an example of the 

type of complexity that makes the calculation of reliable values for such parameters as peak half-lives extremely difficult.

THERMOLUMINESCENT EMISSION

Luminescent emission during thermal stimulation 

is a complex phenomenon that has been extensively 

studied, but like many aspects of TL, is only partially 

understood. The process has been qualitatively modeled and can perhaps be most easily visualized in 

terms of a band-theory representation of the distribution of energy levels in a hypothetical crystal, as 

shown in figure 2.

Electrons elevated to the conduction band by the 

ionization of any of the atoms of a dosimeter crystal
BASIC PHYSICAL PROPERTIES OF THERMOLUMINESCENCE

Conduction band 

C

Trapping levels

Impurity activator 

levels

_ A_ Hole

Valence band

FIGURE 2. Schematic representation of the distribution of energy 

levels in an insulator mineral. A, activation of a trapped electron; C, mobility of a free electron in the conduction band; T, 

recombination of a conduction band electron with an impurity 

or defect center, with thermoluminescent emission characteristic of the center; I, ionization of an impurity by a hole in the 

valence band; N, nonthermal untrapping of an electron and 

recombination with an impurity center.

may return to the valence band only through recombination centers consisting of certain impurity atoms 

and some types of structural defects. According to 

Mott and Gurney (1964), activator impurities are effective as recombination centers because they contribute a valence level above the upper energy limit for the 

valence band of the pure crystal, as shown schematically (fig. 2). When a hole produced by an ionization 

event moves through the valence band and encounters 

a suitable activator impurity, an electron from the impurity atom theoretically combines with the hole 

(process I in fig. 2), eliminating the state of positive 

charge. The result is exactly the same as if the impurity had been ionized initially; owing to energy differentials, this vacant level is not mobile.

During heating, electrons released from traps to the 

conduction band (A in fig. 2) move about until they 

encounter an ionized impurity or suitable defect, then 

combine with it, emitting radiation characteristic of 

the particular recombination center (T in fig. 2). Once 

combination has occurred, the center is capable of repeating the process anew.

In natural minerals, the number of impurity recombination centers is essentially a constant fixed at the 

time of crystallization, whereas the defect center population can increase with prolonged exposure to defect-producing natural radiation.

The emission spectrum that results from the recombination process consists of broad peaks on a wavelength relative to intensity plot whose maxima occur 

within restricted wavelength bands. The spectral position of the peaks are characteristic both of the host 

mineral and the activating defect or impurity. In plagioclase the luminescence spectrum measured at or 

near room temperature consists of three broad peaks 

in the blue, green, and red-near infrared portions of 

the spectrum. The peaks have maxima at about 4,500 

A, 5,600 A, and between 7,000 and 7,800 A, respectively (Geake and others, 1973). This spectral distribution 

has been found to remain essentially unchanged with 

respect to peak position at temperatures to 400° C, although there may be some change in relative intensities. The 4,500-A peak remains prominent in the 

plagioclase spectrum to at least 400° C (Durrani and 

Hwang, 1974).

The transition metals, particularly iron and manganese, have been shown to be the principal impurity activators in natural plagioclase, although in some 

synthetically grown feldspars, relatively high concentrations of several rare-earth elements have been 

shown to be capable of stimulating characteristic luminescent emission.

Variations in the overall emission spectrum of plagioclase have been shown by Smith and Stenstrom 

(1965) and Sippel and Spencer (1970) to have no systematic correlation with plagioclase mineralogic type. 

This is perhaps to be expected because emission is determined by the content of minor- and trace-element 

impurities rather than the major-element constituents. The position of the emission peaks has been 

shown by luminescence studies by Leverenz (1968), 

Garlick and others (1971), and Geake and others 

(1972) to be independent of the type of artificial exciting radiation, whether X-rays, electrons, or protons.

The plagioclase emission spectrum is not unique in 

wavelength distribution properties. It has been shown 

in general that the peak positions and associated activators are common to all feldspars and to other natural silicate minerals, including quartz and sodalite. 

Some spectral components have been observed in synthetic silicate minerals. The emission peak at approximately 4,500 A has been found to occur widely in 

natural silicate minerals, including, in addition to 

feldspar, quartz (Smith and Stenstrom, 1965; Sippel 

and Spencer, 1970), sodalite (Van Doom and Schipper, 1971), and forsterite and iron-free enstatite 

(Geake and Walker, 1966). It has been identified in 

synthetic SiO2 (Leverenz, 1968) and in some nonsilicate minerals, including pure ZnS (Mott and Gurney, 

1964). The origin of the 4,500-A emission peak in feldspar has been attributed by Geake and others (1972) 

to a specific but as yet unidentified type of lattice defect on the basis of both deductive reasoning and theo-
8 THERMOLUMINESCENCE DATING OF HAWAIIAN BASALT

retical calculations using crystal field theory. The 

peak was postulated by Leverenz (1968) to originate 

in the anion radicals (for example, SiO2 tetrahedra) of 

framework silicate minerals such as quartz and feldspar.

Structural damage produced by impact shock has 

been reported by Sippel and Spencer (1970) to cause 

broadening and shifts in the relative proportions of 

the emission peaks of both terrestrial and lunar feldspar and to result in enhanced emission in the blue 

band for some shocked plagioclase crystals.

The rare-earth elements cerium and europium have 

been found to stimulate emission peaks in the ultraviolet and blue portions of the spectrum when incorporated in synthetically grown CaAl2Si2O8 (anorthite) 

in quantities to 1 percent (Laud and others, 1971; Issacs, 1971). The correlation of blue emission with europium content in plagioclase was noted by Drake 

(1972).

The effect of these elements on the luminescence 

spectra of natural minerals in the blue band is probably small relative to structural defects. Cerium is 

present in basaltic plagioclase in quantities that average only about 2 to 5 ppm (parts per million) and is 

almost always less than 10 ppm. Europium, although 

it has a high partition coefficient for plagioclase relative to the melt and most other mineral phases, occurs 

only in basaltic plagioclase in amounts that average 

0.3 to 0.5 ppm and is almost always less than 1 ppm 

(Smith, 1974).

Geake and his coworkers (1971,1973) have recently 

shown by theoretical calculations and doping experiments with both natural and synthetically grown plagioclase that the broad plagioclase emission peak at 

approximately 5,600 A is caused by manganese, probably substituting in calcium lattice sites. Doping of a 

natural plagioclase with manganese by heating the 

sample with MnSQ, (Geake and others, 1971) resulted 

in a 16-fold increase in the amplitude of the green 

peak accompanied by a halving of the amplitude of 

the blue and infrared emission peaks. An attempt by 

these workers to develop a detailed correlation between the magnitude of the green peak and manganese content failed, probably because analytical 

measurements of total manganese content are not 

necessarily equivalent to the number of Mn2+ ions in 

Ca2+ sites, the presumed critical relation for emission. 

The substitutional manganese is inferred to be in the 

Mn2+ oxidation state to preserve electrical neutrality 

in the crystals.

The 5,600-A peak has been observed by a number of 

workers (Nash and Greer, 1970; Sippel and Spencer, 

1970; Geake and others, 1971,1973; Durrani and others, 1973) to occur in nearly all terrestrial and lunar

plagioclase examined. The common occurrence of the 

peak can probably be attributed to the relative abundance of manganese in igneous rocks. It occurs in basaltic plagioclase in quantities that typically range 

from 50 to 100 ppm (Smith, 1974). The peak and its 

manganese origin have been observed in other silicate 

minerals, including sodalite (Van Doom and Schipper, 1971) and ZnSiO4 :Mn (Leverenz, 1968).

The position of the peak maximum has been found 

by Geake and others (1971) to occur consistently in 

the interval between 5,550 A and 5,650 A independent 

of feldspar composition. Sippel and Spencer (1970) 

found the amplitude of the peak to be prominent in 

samples for which the anorthite molecule exceeds 11 

mole percent and to be low or missing in the spectra of 

alkali feldspar, including albite, presumably owing to 

low concentrations of manganese.

The third prominent peak in the plagioclase emission spectrum is that occurring in the red-near infared 

between 7,000 A and 7,800 A. Theoretical calculations 

and doping experiments similar to those used to identify the origin of the 5,600-A peak have demonstrated 

that the red peak is produced by Fe3+, probably substituting in tetrahedral AP+ sites (Geake and others, 

1973, Telfer and Walker, 1975), although it could possibly be substituting in Ca2+ sites (Geake and others, 

1972b, 1973).

The red peak has been shown by Sippel and Spencer (1970) to have a mean position at 7,300 A and a 

range of 400 A over which peak position correlates 

roughly with mole percent albite. In general, they 

found the red peak of alkali feldspar to be less intense 

than that of plagioclase. The position of the red peak 

was found by Geake and others (1973) to vary over 

about a 500-A interval and to correlate with composition, emission from albites tending to be at longer 

wavelength than that from anorthites.

Comparisons of the emission spectra from lunar and 

terrestrial plagioclase have shown that the red peak is 

typically prominent in terrestrial feldspar but small or 

absent in lunar plagioclase. This difference has been 

attributed by Geake and others (1972) to a low oxidation potential in the lunar environment during the period of formation of the rocks that would have the 

effect of inhibiting the oxidation of iron, resulting in a 

high Fe2+:Fe3+ ratio in lunar plagioclase. In terrestrial 

feldspars, the amplitude of the red peak is almost always greater than that of the blue peak and for most is 

equal or greater than the green peak. The prominence 

of the red peak probably derives from the fact that 

iron is the second most abundant minor element in 

natural feldspar. In basaltic plagioclase, it usually occurs in amounts that range from 0.2 to 0.5 weight percent, the higher concentrations being in samples with
BASIC PHYSICAL PROPERTIES OF THERMOLUMINESCENCE

a higher anorthite content (Smith, 1974). The average 

value for plagioclase from basaltic rocks is about 0.5 

weight percent iron.

Other transition metals such as titanium and vanadium (Geake and others, 1972) and chromium (Geake 

and others, 1973) are probably capable of producing 

luminescence in the red band but, owing to their proportionally minor abundance in natural feldspar, have 

an effect on luminescence that is subordinate to that 

of iron and is probably not detectable in most samples. The rare-earth element samarium has been identified as causing luminescent emission in the red band 

(Drake, 1972) but is present in basaltic plagioclase in 

amounts of 1 ppm or less (Smith, 1974) and should not 

have a noticeable effect on the observed spectra.

Certain elements, among them iron, cobalt, and 

nickel, have been proposed by some workers to act as 

inhibitors to suppress TL emission. These elements 

are apparently effective in promoting radiationless 

transitions from the conduction to the valence band at 

the expense of radiative transitions (Leverenz, 1968). 

There is some specific evidence to indicate that Fe2+ 

acts as a TL suppressant, whereas Fe3+ is an activator 

(Geake and others, 1971), as described.

RADIATION DAMAGE 

IN DOSIMETER CRYSTALS

Radioactive decay particles of sufficient energy, in 

addition to producing ionization, are capable of displacing atoms from normal lattice positions. Few 

studies have been made of the extent of naturally produced radiation damage in TL dosimeters and the effects of such damage on luminescence spectra and TL 

efficiency. Most studies have used high-intensity 

doses of artificial radiation to create defects; the production rates and effects are then extrapolated to the 

natural situation. For example, bombardment of a lunar fines sample with ~ 1010 fission fragments from a 

252Cf source was found to produce no significant alteration in the TL response of the sample to a standard 

dose of beta radiation (Hoyt and others, 1972). A calculation of the number of new traps theoretically produced in a mineral by natural radioactive decay over a 

period of 104 years by Tite (1968) showed the effect to 

be negligible. As a consequence, in most archeologic 

dating schemes, the trap population is assumed constant over the geologically short time span for which 

these methods are used (Aitken, 1970). There is considerable evidence, however, to indicate that the defect density of minerals is increased by prolonged 

exposure to natural radiation and that this increase 

has an augmentive effect on TL response.

Alpha particles and recoil nuclei from both uranium 

and thorium atoms are particularly efficient in terms

of the number of defects produced per unit of radiation dose. They are relatively inefficient in producing 

ionization, as the ionization efficiency of alpha particles has been found to be about an order of magnitude 

less than that of an equivalent dose of beta or gamma 

radiation (Aitken, Zimmerman, and Fleming, 1968).

Alpha particles from uranium and thorium decays 

range in energy from about 4 to 7 MeV (million electronvolts) and, for particles to 5 MeV, produce an 

average 30 to 100 stable dislocations per decay event 

in a material of Z < 30 (Levy, 1968). Recoil nuclei 

from alpha decays produce many more dislocations 

along a very short (~ 500 A), highly disturbed path 

through the crystal lattice that can be revealed for 

study by etching; such paths are referred to as 

"tracks" (Walker, 1973). Only about 2 percent of the 

total alpha decay energy is partitioned to the recoil 

nucleus.

The effectiveness of alpha particles in producing 

defects is severely limited by the fact that they have 

an average range of only about 25 nm in a crystalline 

matrix (Fleming, 1972). Since most of the uranium 

and thorium in rocks and pottery is located in the 

groundmass and clay matrix, respectively, of these 

two important materials, this restricted range limits 

both the number of alpha particles that are likely to 

reach TL dosimeter grains and the effect in large 

grains of the few that do. This constraint was the principal reason for developing the fine-grain dating technique for pottery, where the fraction used has a 

dimension range of 2 to 8 /*m (Aitken, Zimmerman, 

and Fleming, 1968). For grains much larger than 10 

/um, alpha particles probably penetrate only the periphery. This limit can theoretically result in a discrepancy between the anticipated TL intensity based on 

the calculated natural dose rate and the actual measured TL.

Beta particles and gamma rays are capable of displacing atoms from lattice positions if an appropriate? 

collision occurs before the particles have been degraded below the necessary energy threshold by ionization 

events, the principal form of energy dissipation. In the 

case of gamma rays, displacement events are rare and 

require as an intermediate step the formation of a 

Compton electron, which is then indistinguishable 

from other high-energy electrons (beta particles).

Travel distances for beta particles and gamma rays 

in a crystalline matrix vary widely in accordance with 

the broad range of particle energies but in general 

average a few millimeters for beta particles and a few 

centimeters for gamma rays. Even parent nuclides located in the matrix can emit radiation that has an essentially unattenuated effect on enclosed dosimeter 

grains. In plagioclase, the beta radiation from 40K,
10 THERMOLUMINESCENCE DATING OF HAWAIIAN BASALT

much of which is located in the crystals, supplies 

about 75 percent of the total beta and gamma radiation energy released during the decay of all the nuclides in a rock or other material and about 50 percent 

of the total dose rate adjusted for diminished alphaparticle efficiency.

According to Hwang and Goksu (1971), the minimum amount of energy that must be transferred to 

the struck atom in a displacement event is about 30 eV 

(electronvolts). Levy (1968) estimated the critical displacement energy to be about 25 eV and further stated 

that the minimum beta-particle energies required to 

displace oxygen and silicon atoms are 1.6X105 eV and 

2.6X105 eV, respectively. These threshold energies indicate that a significant proportion of the beta and 

gamma radiation emitted during radioactive decay 

has sufficient energy to cause displacements, even 

though the probabilities of such events are rather low, 

being only 10~2 to 10"1 per beta decay event for particles from 0-5 MeV in energy and 10~3 per gamma-ray 

emission for gamma rays of 0-3 MeV (Levy, 1968). Although the potential of these radiations for creating 

displacements is small, in view of their penetrating 

power relative to alpha particles, they could produce 

enough defects to add significantly to the total defect 

population.

EXPERIMENTAL EQUIPMENT !AND TECHNIQUE

The experimental dating technique developed at 

the U.S. Geological Survey is technically similar to 

methods used in other laboratories but differs significantly in the philosophy of the approach. In an attempt to overcome the principal problem encountered 

with existing "absolute" dating methods the apparent limitation of the range of the methods to probably 

the past few tens of thousands of years, principally because of anomalous fading and other factors this investigation was undertaken with the specific goal of 

developing a "relative" dating procedure based on 

rocks of known age against which other samples of 

similar composition but unknown age could be compared. The concept embodied by this approach requires that natural TL accumulate in a predictable 

and monotonic manner but does not require that the 

storage of trapped charge be 100 percent cumulative 

with time. It also requires that the measurements of 

TL parameters be reproducible and representative 

but minimizes the need for the determination of absolute magnitudes.

'Any use of trade names and trademarks in this publication is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey.

SAMPLE COLLECTION

The alkalic and tholeiitic basalts analyzed during 

this investigation, with the exception of three samples 

(5X340, Klpa lOa, Mol le), were collected from the Island of Hawaii at locations shown on the index map 

(fig. 3). A detailed description of sample locations is 

given in the list of sample locations and age data at the 

end of this report.

Most of the samples were collected in or near roadcuts mainly for convenience and the availability of 

fresh material, but also because that is where the 

original samples of flows dated by the potassium-argon method were obtained. Sample-collection criteria 

stressed collection of the freshest material available. 

Thin sections prepared during later laboratory processing revealed no alteration of the plagioclase laths 

in any of the samples collected. The samples were obtained as far beneath the exposed surface as possible 

to minimize the heating effect of isolation, which can 

be significant in black basalt at 20° N, lat. In roadcut 

sampling, an effort was made to avoid collecting near 

fractures produced by blasting operations, since the 

possible effects of shock on TL had not been evaluated. Where contacts were visible, samples were collected at least 1 m below the tops of flows to avoid any 

possibility of thermal TL drainage by overlying younger flows.

The alkalic basalt samples are from the Hawi Volcanic Series of the Kohala Volcano, Hawaii, with four 

exceptions: The youngest sample (023) is from Mauna 

Kea Volcano, Hawaii; the three oldest are from the islands of Molokai (Klpa lOa and Mol le) and Oahu 

(5X340).

The Hawi flows presented critical sampling problems, as all are prehistoric and are not identified on 

existing maps as discrete dated units. A few samples 

obtained from Donald Swanson (U.S. Geological Survey), including H71-3 and H71-10, represent extra 

material from earlier potassium-argon dating studies 

(McDougall and Swanson, 1972). The others (033, 

035, 036, 041, and 042) were collected in the field on 

the basis of published distance measurements from 

recognizable landmarks. The obvious potential for error in sample collection under these conditions was 

minimized by comparing whole-rock K2O contents for 

the samples collected with those published with the 

potassium-argon data for the samples they were supposed to duplicate. Concordance to within less than 

± 2 percent, or an average of about 0.025 weight percent K2O, was stipulated as the acceptable maximum 

limit of variation outside which samples were discarded as unreliable. This test should be reliable because most adjacent alkalic basalt flows from the 

Hawi Volcanic Series differ in whole-rock K2O content
EXPERIMENTAL EQUIPMENT AND TECHNIQUE 11

156 C 155 C 

—I—

20°

19 C

EXPLANATION

i * * a 0 T3'"!r »*!'** "l^ _ ^ I * I* °_ * 1° * ^

Historic flows (since 1750) 

Puna Volcanic Series (Kilauea Volcano) 

Kau Volcanic Series (Mauna Loa Volcano) 

Laupahoehoe Volcanic Series (Mauna KeaVolcano] 

Hamakua Volcanic Series (Mauna Kea Volcano) 

Hualalai Volcanic Series (Hualalai Volcano) 

Hawi Volcanic Series (Kohala Mountains) 

Pololu Volcanic Series (Kohala Mountains)

Sample location data

Alkalic basalts 

Map No. Sample No. 

023 

H71-10

035 

H71-3

036

10 KILOMETE-RS -H

5 MILES

041

042

033

Map No.

9

10

11

12

13

14

15

16

Tholeiitic basalts 

Sample No. Map No. 

012 

015 

026 

030

050

051

032

024

17

18

19

20

21

Sample No. 

070 

018 

037

P71-3,4,5 

P71-13

FIGURE 3. Simplified geologic map of the Island of Hawaii showing sample localities. Samples are listed in order of increasing age for 

each rock type, as given under sample locations and age data. Geology generalized from G. A. Macdonald, Atlas of Hawaii, 1973 (R. 

W. Armstrong, ed.).

by at least 10 percent, commonly by as much as 30 

percent. The test requires, though, k that both K2 0 

analyses used in the comparison, if made by different 

laboratories, be highly accurate.

Sample 023 was collected from a young flow on the 

flank of Mauna Kea Volcano using location data supplied by Stephen C. Porter (Univ. of Washington). 

Samples Klpa lOa and Mol le from Molokai were supplied by M. H. Beeson (U.S.G.S.). Sample 5X340, 

from the Waianae Volcanic Series on the island of 

Oahu, was obtained from short drill cores (Doell and

Dalrymple, 1973).

The historic thpleiitic basalts were collected from 

roadcuts and flow surfaces on the basis of flow locations and eruption dates shown on U.S. Geological 

Survey topographic series 1W quadrangle maps and 

on the geologic map of Hawaii by Stearns and 

Macdonald (1946). Samples dated by the carbon-14 

method were recollected on the basis of careful sample 

location data compiled by G. B. Dalrymple (U.S.G.S.) 

during his initial collection of these flows for TL dating in 1966 and 1970.
12 THERMOLUMINESCENCE DATING OF HAWAIIAN BASALT

Samples from the older tholeiitic flows from the Pololu Volcanic Series of Kohala Volcano whose identification numbers are prefaced by the letter "P" 

represent, like alkalic basalts H71-3 and H71-10, extra material from samples dated by I. McDougall 

(Australian National Univ.). They were provided for 

TL study by D. A. Swanson (U.S.G.S.). Sample 037 

was collected in a roadcut identified by road mileage 

measurements. Its validity is suspect because its K2 0 

content differs by 0.04 weight percent from the sample it was supposed to duplicate. It was tentatively accepted as valid.

SAMPLE PREPARATION

Measurements of thermoluminescent intensity 

were made on both alkalic and tholeiitic basalts using 

pure plagioclase separates of a carefully restricted size 

range as the TL dosimeter. The sample-preparation 

procedure involved crushing, separation of a precisely 

sized whole-rock fraction by sieving, and the final segregration of plagioclase with heavy liquids.

Crushing during sample preparation has been 

found by several workers to alter the natural TL of 

several materials and minerals. The effect involves either enhancement of the natural TL signal by a stressrelated process involving the ionization of constituent 

atoms (tribothermoluminescence), or by TL drainage. 

Tribothermoluminescence produced during crushing 

was found to contribute significantly to the natural 

TL signal of calcite (Zeller and others, 1955) and tektites (Durrani and others, 1970) but to produce less 

than a 2-percent change in the measured TL from 

crushed meteorite samples (Houtermans and Liner, 

1966). No detectable effects were found in crushing 

experiments with fossil shells unless the <88-^m fraction was used (Johnson and Blanchard, 1967), nor 

were such effects as the filling of existing empty traps 

or the draining of filled traps found during crushing 

experiments with pottery (Tite, 1966). Negligible tribothermoluminescence effects were found by Hoyt 

and others (1970) in an experiment in which they vigorously shook samples of lunar fines for periods of half 

an hour. Drainage of natural TL during crushing has 

been observed and was found by Aitken, Zimmerman, 

and Fleming (1968) to result in an average of 30 percent more natural TL in pottery fragments crushed 

gently in the jaws of a vise than in the same size range 

of material from the same shards crushed with a mortar and pestle. A preliminary crushing experiment 

performed as part of this investigation involved the 

crushing of a piece of tholeiite basalt (018) in a press 

by several applications of moderate stress of approximately 2 seconds duration each. Nearly all measureable natural TL was found to be drained by the 

crushing process.

The wide disparity in apparent crushing effects is 

probably a function of the wide variety of materials 

studied, some being more susceptible to modification 

by crushing than others. The duration and (or) intensity of the crushing process and the size range of the 

material used for TL measurement are critical parameters. In order to investigate the effects of crushing on 

plagioclase in more detail, a careful crushing experiment was conducted as part of this investigation. The 

results of the crushing experiment are discussed in the 

section entitled "Natural TL."

The crushing experiment consisted of two successive crushings of each sample at different applied 

stress intensities. All sample handling from this stage 

on was under indirect or subdued light conditions to 

avoid any possible modification of natural TL properties by either artificial or natural ambient light.

The first, or stage one, crushing consisted of putting 

fist-size and smaller pieces of basalt, obtained by reducing larger blocks with a minimum number of blows 

from a 1-kg rock hammer, through a chipmunk (jaw) 

crusher whose minimum discharge opening was about 

1.5 cm. Two to 3 kg of basalt was usually required to 

be certain of an adequate plagioclase yield during subsequent separation procedures. The fraction greater 

than 74 jim from the stage-one crushing was separated 

by sieving with metal screens and a Tyler sieve shaker. 

A preliminary separation of the«37-to 44-^m (<325, 

>400 mesh) fraction used in the TL-dating studies 

was conducted simultaneously at a lower position in 

the sieve stack. The coarse fraction (>74 ^m) was 

subsequently run through a roller crusher with the 

rollers set 0.16 cm apart. This constituted crushing 

stage two, the fines from which were sized to 37 to 44 

jim by a preliminary sieving identical to that described for stage one.

Final sizing consisted of a precision separation of 

the 37- to 44- jim whole-rock fraction using an AlienBradley sonic sifter. This instrument consists of a sonically driven vibrating air column incorporating 

screens of desired size. Cycles of 10 minutes' duration 

produced separates that were both dimensionally uniform and free of all adhering finer material.

A restricted range in grain size is a critical requirement in TL work because variations in grain size have 

been shown to produce significant differences in TL 

response from the same material. The effect is probably produced by differences in sample surface area 

per unit weight involving a lesser probability of self 

absorption for TL photons emitted from within 

smaller grains. Sabels (1963) showed that TL emission 

from the <37-jim fraction was nearly 100 percent
EXPERIMENTAL EQUIPMENT AND TECHNIQUE 13

greater than emission for the 105- to 177-/mi fraction 

in crushed whole-rock basalt.

A size range of 37 to 44 /^m was selected for use in 

this research because it generally yielded monomineralic plagioclase separates with a minimum of binary 

grain contamination.

Plagioclase was separated from the whole-rock material with a centrifuge using bromoform adjusted to 

the desired densities with water-free ethanol. The 

procedure used for all samples was successive separations at carefully controlled densities of 2.80 and 2.65. 

For most samples, the separates obtained had a purity 

of better than 90 percent. The few percent of contaminants consisted mainly of binary grain attachments 

of nonthermoluminescent silicate minerals (pyroxene 

and olivine) and oxides (principally magnetite).

MAJOR-ELEMENT PLAGIOCLASE COMPOSITIONS

In an attempt to determine the range in average 

feldspar composition represented by the plagioclase 

separates from the samples of each rock suite and to 

provide a basis for an evaluation of the effect that differences in major-element composition have on TL response, analyses of the plagioclase separates for the 

elements calcium, sodium, and potassium were obtained for those alkalic and tholeiitic basalt samples 

for which adequate material remained after completion of the TL measurements. The analyses, made in 

U.S. Geological Survey Analytical Laboratories 

Branch, are reported in terms of the oxides of the elements (table 1). CaO was determined by atomic absorption; Na-P and K20 by flame photometry. Minor 

and trace elements were not determined because adequate material for such detailed measurements was 

not available.

The CaO, Na2O, and K2O contents of each separate 

when recalculated to 100 percent and plotted on a 

CaO-Na2O-K20 ternary diagram (fig. 4) were used to 

estimate an average plagioclase composition for each 

sample. The assignment of approximate mineralogic 

type was made by association of the plotted points 

with the plagioclase compositional line calibrated on 

the basis of the stoichiometric proportions of CaO and 

NagO in each of the members of the plagioclase solidsolution series, as tablulated in Winchell and Winchell (1951).

The chemical composition of the plagioclase separates from the tholeiitic basalts all fall in a tight group 

within the labradorite field (fig. 4). The average composition for the group as estimated from the graph is 

about An^^s. This plagioclase composition is typical 

for tholeiitic basalts and compares favorably with a

TABLE 1. Major-element chemical compositions of selected plagioclase separates, in weight percent

[Elements are reported as weight percent of the oxides. Analysts: CaO, 

Fred Simon; Na20 and K20, Lois Schlocker]

Sample 

No. CaO Na!0 K20

Alkalic basalts

023

H71-10

035

H71-3

036

041

042

033 _

7.11

4.09

2.32

5.14

4.68

7.80

5.90

6.14

7.53

6.78

6.77

6.66

2.02

2.87

3.25

2.32

2.40

2.18

Tholeiitic basalts

032

024

070

074 _

018 _

019 _

P71-13

037 _

P71-4

P71-3

P71-5

11.0

11.5

12.8

11.9

11.0

11.1

10.5

11.5

3.84

3.66

3.61

3.61

Insufficient material

3.22

3.29

4.54

Insufficient material

Insufficient material

3.64

0.398

.364

.215

.322

.490

.136

.720

.182

ANORTHITE A CaO 

An,

BYTOWNITE 

An

P71-13 

P-71-5

LABRADORITE/032 

An

ANDESINE 

An

EXPLANATION 

a Alkalic basalt 

o Tholeiitic basalt

OLIGOCLASE

K 2 O

FIGURE 4. Ternary diagram of alkalic and tholeiitic basalt plagioclase compositions expressed in terms of CaO, Na20, and K20. 

Plotted points represent analyses for the three oxides in 

weight percent recalculated to 100 percent. The dashed lines 

represent the mineralogical increments into which the plagioclase solid-solution series is divided, expressed in terms of 

CaO and Na20.

petrographically determined compositional range of 

An59_70 .

The plagioclase separates from the alkalic basalts 

have a considerably wider range in major-element 

chemistry. The compositional averages vary from 

about An20 to An40, or from oligoclase to andesine, in a 

manner that reflects reasonably closely the bulk composition of the parent basalts in terms of position in
14 THERMOLUMINESCENCE DATING OF HAWAIIAN BASALT

the magma-differentiation series. The alkalic basalts 

range in composition from hawaiite (036) through mugearite (042 and H71-10) to trachyte (035). The effects of this range in feldspar compositions on the TL 

properties of the samples are discussed in later sections.

TL MEASURING AND RECORDING EQUIPMENT

The TL measurement apparatus used in this investigation, shown schematically in figure 5, is a modification of the instrument described by Dalrymple and 

Doell (1970a). It consists of a black anodized aluminum sample chamber whose main function is to exclude external light and provide a sealed environment 

in which the composition of the atmosphere can be adjusted to exclude oxygen. The chamber incorporates 

fused silica (quartz) windows at both top and bottom; 

the upper window also serves as a thermal barrier to 

minimize heat conduction to the photomultiplier 

(PM) tube. The construction of the chamber permits 

installation of one or two optical filters in the TL light 

path to remove undesired portions of the emitted 

spectrum. Immediately above the sample is a screen 

designed to limit the emission detected to that from 

the sample alone.

The sample planchets used in the TL measurements consist of circular discs, 4.7 cm in diameter and 

0.015 cm thick, pressed from fine (99.99+ percent 

pure) silver sheet. Silver was selected as the planchet 

material because it has one of the lowest emissivities 

of all the metals and one of the highest thermal conductivities, which minimizes thermal gradients during 

heating. The planchets have a central circular depression 1.2 cm in diameter and 0.04 cm deep for sample

window 

Sample 

planchet^ 

Nitrogen-1

Temperature controller

Quartz-iodide lamps (3) 

Magnin mirrors

FIGURE 5. Schematic diagram of the thermoluminescence reader.

placement. The backs of the planchets were coated 

with a thin layer of graphite (Aerodag G) to reduce the 

reflectivity of the silver and enhance absorption from 

the infrared heat source. The planchets were installed 

in the sample chamber in the position shown in figure 

5 and sealed with silicone rubber O-rings above and 

below the periphery of the disc to prevent light from 

the heat source from reaching the PM tube.

The heating system consists of three symmetrically 

arranged quartz-iodide lamps whose radiation is focused on the bottom of the planchets with mangin 

mirrors, as shown in the diagram. Voltage to the lamps 

is increased linearly with a variable autotransformer 

driven by a variable speed DC motor.

Temperature during heating was monitored with a 

chromel-alumel thermocouple (T.C. in fig. 5) welded 

to each planchet with a 0.0076-cm-thick disc of pure 

nickel as an intermediary to overcome problems arising from spot-welding directly together two materials 

of widely differing resistivities, thermocouple wire, 

and silver. The nickel disc and attached wires were 

welded to each planchet immediately adjacent to the 

sample depression to insure accurate temperature 

measurement. Connection of the wires from each 

planchet to the rest of the thermocouple system was 

made with a simple overlapping compression joint at 

the edge of the sample chamber. The thermocouple 

output was connected to the X-Y recorder abscissa 

through a thermocouple reference junction, which 

provided electronically a room-temperature-compensated 0°C reference voltage. Temperature measurement reproducibility was estimated to be about ±3 

percent.

The TL was detected with an EMI 6255S "Q" 

quartz-window photomultiplier tube with a Cs3Sb 

cathode and an extended range in the near ultraviolet, 

mounted in an Electro Optics Associates (type PM103) housing. The tube has a gain of about 3X107 and a 

spectral response from 1,700 A to about 6,500 A with a 

maximum sensitivity near 3,900 A (fig. 6). It was operated at an experimentally determined optimum potential of 1,100 volts. As the dark current at room 

temperature was found to be negligible, the tube was 

not cooled.

The pulsed signal from the PM tube was measured 

with an SSR model 1105/1120 photon counter/amplifier-discriminator, which has a deadtime of 25 nanoseconds and counts pulses of electrons at the anode of 

less than 10 nanoseconds duration as discrete events. 

All measurements were made with an instrumental 

statistical counting precision of ± 2 percent. The output of the photon-counting system was used to drive 

the ordinate of the X-Y recorder. The high-voltage 

power supply, photon counter, X-Y recorder, and
EXPERIMENTAL EQUIPMENT AND TECHNIQUE 15

^ 2000 4000 6000 u.

WAVELENGTH, IN ANGSTROMS

FIGURE 6. Spectral response curves for the photomultiplier (PM) 

tube and filter CS5-60. Curve 1, response for PM tube 6255S 

"Q" in percent quantum efficiency, reproduced from manufacturer's calibration curve; 2, transmission curve for Corning filter CS5-60 in percent transmittance, reproduced from 

manufacturer's data; 3, schematic representation of the relative intensity and spectral distribution of the TL measured by 

this combination of filter and PM tube.

heating system were all powered through an isolation 

transformer and a voltage regulator to remove irregularities in the line current.

In all natural and artificial TL measurements made 

during this investigation that were used for age calculation, a Corning long-wavelength-absorbing blue filter (CS5-60) was used to inhibit the transmission of 

blackbody radiation. The filter, inserted in the light 

path above the upper silica window of the sample 

chamber, effectively delayed the onset of blackbody 

radiation interference to about 350°C. The wavelength detection range of the PM tube-filter combination is limited to the bandwidth from 3,500 A to 

approximately 5,000 A, with a maximum sensitivity at 

about 3,900 A (fig. 6). This restricted bandwidth limits the TL detected to that from the 4,500-A plagioclase emission peak.

Calibration of the TL detection system was performed at frequent intervals with a carbon-14-activated standard light source of known luminous flux to 

determine the extent of equipment drift, which was 

found never to exceed 5 percent from day to day or 

over an 8-hour series of experiments. Unfortunately, 

the light source has a markedly greater luminous intensity than any of the TL emissions observed in this 

investigation; in order to calibrate the detection system at the same level of light intensity as the TL observed, it was necessary to filter the output of the light 

source. Since the extent of filter attenuation of the intensity of the light source is not precisely known, it 

was not possible to use the light source to calibrate the 

TL signal in standard light units.

Radiation sources available at the laboratory for the 

production of artificial TL consist of both an X-ray 

and a beta source. The beta source is a plane-surface 

45-millicurie 90Sr-90Y irradiator; the beta particles 

have an essentially bimodal energy distribution with 

maximum intensities from the two isotopes of about 

0.2 and 0.9 MeV, respectively. This beta source has a 

flux of about 100 rad min" 1 (air-equivalent). The Xray source consists of a CA8S tungsten tube with 

beryllium windows that is mounted horizontally in a 

lead-lined aluminum irradiation chamber. The tube is 

equipped with a solenoid shutter and powered by a 

General Electric XRD-5 generator. The tube is operated at a power setting of 50 kilovolts and 25 milliamperes; the unfiltered radiation has a measured 

effective average energy of 14 keV at a tube-to-sample 

distance of 29 cm. Radiation output is approximately 

525 rad min-1 (air-equivalent).

Samples were centered in the X-ray beam path by 

means of a fixed sample holder on a sliding tray, the 

guides and stops for which were carefully set to insure 

that the positioning was reproducible. An air-equivalent ionization chamber probe (Victoreen model 

555-100 ma) was installed immediately adjacent to 

the sample depression of each planchet but was located such that it did not shadow any part of the sample depression during irradiation. The probe was used 

with a Victoreen model 555 Radocon II integrating radiation detection system to measure total radiation 

dose and control exposures. Radiation exposures 

could be automatically terminated by a circuit in the 

integrating system that tripped the shutter on the 

X-ray tube at a preselected total dose.

Calibration of the X-ray source was performed periodically using premeasured 30-mg batches of lithium 

fluoride TLD-100 thermoluminescent dosimeter. 

Each 30-mg sample was used only once owing to serious annealing effects produced in lithium fluoride by 

the heating required during TL determinations. The 

energy calibration measurements were made in the 

same experimental fashion as those for geologic samples. Comparison of peak heights provided the means 

of checking for uniformity of energy output, which 

was estimated to be constant over the duration of the 

investigation to within ± 1 percent.

An experiment designed to determine the relative 

efficiency of the two radiation sources in producing 

TL showed that to within measurable limits the TL 

produced by comparable total exposures (in air-equivalent rads) of beta and X-radiation were identical. In 

all irradiations made during this investigation, the X-- 

ray source was used because of a higher radiation intensity, which reduced exposure times, and the convenience of the automatic exposure-termination system.
16 THERMOLUMINESCENCE DATING OF HAWAIIAN BASALT

PROCEDURE

Samples were isolated from direct laboratory light 

as much as possible during processing in order to 

minimize the detrimental effects that light has on TL. 

Ultraviolet light of sufficiently high intensity has a 

well-documented capability for draining TL from all 

trapping levels in most materials. It has been used to 

drain natural TL in studies where such drainage was 

desired, but an initial heating for that purpose was 

found^to alter the response of the sample to subsequent doses of artificial radiation. Ultraviolet illumination has been found to be capable of inducing TL in 

many of the same materials, preferentially in traps 

contributing to low temperature peaks. Ultraviolet 

light from fluorescent bulbs was found to fill low-temperature traps in separates from pottery while draining high-temperature traps (Tite, 1966) and similarly 

to induce TL in the 150°C peak of lunar fines while 

draining the TL in the 200°-250°C peak of the same 

samples (Hoyt and others, 1972). A detailed study by 

Durrani and others (1973), using lunar fines and a relatively intense ultraviolet source, found that ultraviolet radiation will induce TL in all trapping levels but 

will do so preferentially in shallower traps if the sample is drained of preexisting TL. It will drain some of 

the TL from all trapping levels in irradiated samples.

An experiment performed as part of this investigation showed that exposure of drained plagioclase samples to a 150-watt frosted incandescent bulb at a 

distance of 60 cm for a period of 18 hours produced 

only a moderate low-temperature TL peak and no 

measureable TL in the high-temperature peak that is 

used for TL-dating studies. A 96-hour exposure of the 

same samples to the same light source did not produce 

any additional TL in the low-temperature peak but 

did produce a level of TL in the high-temperature 

(300° C) peak range that had an amplitude of about 15 

to 20 percent of the previously measured natural TL. 

In general, these experiments indicate that to avoid 

possible modification of TL properties, sample handling should be under subdued light conditions, 

avoiding light sources with a large ultraviolet component such as fluorescent bulbs. If samples are stored in 

the dark between experiments and exposure to laboratory lighting is minimized during sample handling, 

incandescent lighting should not produce measurable 

adverse effects on the TL of plagioclase.

Preparation of samples for TL measurement consisted of spreading 4- to 7-mg aliquants of plagioclase 

as uniformly as possible over the sample depression of 

each planchet. The depressions were coated with a 

thin film of nonluminescent silicone fluid (Duxe Products) before sample placement to increase thermal 

contact and inhibit sample movement during subsequent handling. Seven aliquants (from each crushing 

stage) were measured for all samples for which adequate material was available, because this number of 

measurements was found to maximize the statistical 

benefits of multiple determinations within the limits 

of practicality.

The amount of plagioclase used in each TL measurement is critical. Differences in the intensity of light 

emitted by each of the several aliquants measured for 

a given sample are largely the result of variations in 

the amount of emitter present. In order to compensate 

for these variations in emission, the amount of material must be known. Unfortunately, although TL intensity increases with sample weight, above a certain 

optimum weight equivalent to a uniform layer one 

grain thick over the bottom of the sample depression, 

the increase in TL with further increase in sample 

weight is not linear. This is the result of the fact that 

the addition of material in excess of the optimum 

amount produces multiple grain layering, which reduces the emission effectiveness for those grains below the surface layer.

The most effective of the several empirical methods 

tried during this investigation for compensating for 

differences in TL response due to differences in sample weight is based on the observation that about 

4.5 mg of material is required to uniformly cover the 

bottom of the sample depression; more material produces multiple grain layering. The method devised for 

calculating an effective sample weight, and used in all 

subsequent TL calculations, is based on an equation 

that assigns an effectiveness of 0.5 to the portion of 

the sample weight in excess of 4.5 mg:

Wa = 4.5 mg +

Wt - 4.5 mg

where Wa is the adjusted sample weight and Wt is 

the total or measured weight.

After each planchet had been loaded with sample, it 

was placed in the sample chamber, the chamber 

sealed, and the PM tube placed in position directly 

over the sample depression. The chamber was then 

flushed with dry nitrogen containing less than 5 ppm 

oxygen at a rate of about 50 L hr*1 for 7 minutes prior 

to heating, and continuously during the heating process, to strip oxygen in the chamber and any adhering 

to the sample grains. This procedure is necessary because many materials give a spurious emission when 

heated in the presence of oxygen (Johnson and Blanchard, 1967; Aitken, 1968), possibly a phenomenon 

caused by oxidation reactions on the surfaces of the 

dosimeter grains (Johnson and Daniels, 1961) or ignition of organic material. Radiative oxidation processes in general have been referred to by several
EXPERIMENTAL EQUIPMENT AND TECHNIQUE 17

workers as chemiluminescence. The effect can obscure 

the natural TL in some low TL materials and may be 

partly responsible for the failure of several early TLdating attempts. The levels of oxygen that have been 

observed to cause spurious TL are small; nitrogen containing about 30 ppm oxygen was found to produce a 

substantially greater spurious component in the TL of 

pottery than nitrogen with less than 5 ppm oxygen 

(Aitken, Fleming, Reid, and Tite, 1968). The use of 

dry nitrogen was found by Johnson and Blanchard 

(1967) to substantially reduce, or eliminate entirely, 

the spurious emission observed in TL measurements 

on fossil shells.

For the present study, samples were heated at a rate 

of 20° ±0.5°C sec' 1 from room temperature to a maximum of 450° C. This heating rate was selected primarily because it provides an optimum compromise 

between slower rates, which maximize peak resolution 

at a cost of reduced amplitude and faster rates, which 

produce maximum TL amplitudes with somewhat 

poorer peak resolution. Heating was terminated at a 

maximum temperature of 450° C because, even with 

filter CS5-60, blackbody radiation caused saturation 

of the TL detection equipment in the range 400- 

450°C.

Following an inital heating during which the natural TL was measured and drained, the samples were 

allowed to cool to a maximum of 50° C, then heated 

again to obtain the blackbody radiation background 

(see fig. 1). Upon cooling, the samples were removed 

from the chamber and exposed to a standard dose of 

X-radiation. The purpose of the irradiation was to refill the traps that had originally stored the natural 

TL-producing electron population. Radiation doses 

were measured electronically in roentgens (air ionization) and converted to rads, the unit of absorbed dose 

typically used in dosimetry. The conversion coefficient for air from ionization (roentgens) to absorbed 

dose (rads) is approximately 0.87. This factor was 

used in this investigation to approximate the dose absorbed by the feldspar because a specific conversion 

coefficient for compositionally complex feldspar has 

not yet been determined.

For age-determination purposes, the alkalic basalts 

were exposed to an 870-rad dose of X-rays and the 

tholeiitic basalts, which had a markedly lower sensitivity, to 1,740 rads. Both of these dose levels are low 

relative to those used by many other workers for a 

similar purpose but were found to be adequate since 

in this investigation the only requirement of a specific 

artificial radiation dose is that it produce a measurable peak that constitutes a reliable relative measure 

of the trap density in individual samples. Samples 

were sequenced such that none was irradiated within

1 hour of the last heating.

Following irradiation, the samples were replaced 

immediately in the sample chamber and allowed to 

equilibrate in the dark for 7 minutes (with the nitrogen flowing) to permit spontaneous decay of the most 

unstably trapped electrons. The samples were then reheated in the same manner as that used during natural TL measurement to measure the artificial TL and 

blackbody radiation background. The artificial TL 

was used during age calculations to normalize differences in natural TL due to differences in trap density.

TL magnitudes were quantified in terms of peak 

heights measured vertically from the blackbody curve 

to the glow curve at the point of maximum amplitude 

on the TL curve of the particular peak being studied. 

Peak heights were used instead of peak areas because 

measurement of peak heights minimized the interference from overlapping tails of adjacent lower temperature peaks. Low-temperature peak interference 

was not a problem in measuring the high-temperature 

natural TL peak that occurs between 340° and 365° C, 

because spontaneous decay over geologic time eliminates the electrons in shallow traps. In the measurement of artificial TL, where as many as four peaks 

with maxima at about 110°, 150°, 225°, and 300° C 

were observed in most samples, as shown in figure 7, 

the high-temperature tail of the 225° C peak occasionally interfered with the measurement of the 300° C 

peak. The interference, when present, did not usually 

add more than a few percent to the amplitude of the 

300° C peak maximum.

The effectiveness of peak heights for quantitative 

TL measurement was tested by comparing peak 

height and area data using samples for which interference from the adjacent low-temperature peak was 

minimal. The peak heights were found to be as reliable a relative measure of intensity as the corresponding areas.

100 200 300 

TEMPERATURE, IN DEGREES CELSIUS

400

FIGURE 7. An approximate schematic segregation of a plagioclase 

artificial TL glow curve into the four peaks identified as the 

principal constituents of artificial TL emission.
18 THERMOLUMINESCENCE DATING OF HAWAIIAN BASALT

ANALYSIS OF THE PLAGIOCLASE 

EMISSION SPECTRUM

The TL spectra of plagioclase from several of the 

alkalic and tholeiitic basalts were analyzed for the 

purpose of identifying the extent to which variations 

in the emission spectra influence the peak heights recorded during TL measurement and the possible effect of such variations on calculated TL ages. The 

data would also provide an opportunity to evaluate 

possible correlations between basic TL properties and 

major feldspar composition.

PROCEDURE AND RESULTS

Interference filters (Spectracoat Varipass Filters 

manufactured by Optics Technology) were used to 

segregate the portion of the emission spectrum of the 

300°C artificial TL peak between 4,000 A and 5,000 A 

into 500-A bandwidths. The transmission characteristics of the filters, shown in figure 8, were designed for 

use with normally incident light. Their use to resolve 

uncollimated TL introduces a systematic error in the 

results because the effective cut-off limit of each filter 

migrates to progressively shorter wavelengths as the 

angle of incidence of the TL deviates increasingly 

from the normal. In view of this limitation, the data 

presented must be considered to be approximate.

The experiment was performed by exposing two aliquants of each sample to a standard dose of X-radiation followed by TL measurement. The natural TL 

emission spectrum of these samples was not analyzed, 

because material available was insufficient for the 

multiple determinations required.

Alternate irradiations of each aliquant were measured without an interference filter to provide a check 

on possible changes in TL sensitivity resulting from 

the repeated irradiation-heating cycles. In practice, 

the two unfiltered measurements preceding and following each filtered measurement were averaged to 

provide an accurate unfiltered peak height against 

which the filtered measurement could be compared. 

The percentage of TL in the 4,000-5,000-A and 4,500- 

5,000-A bandwidths as well as that in the bandwidths 

1,700-4,000 A and 5,000-6,000 A were obtained by calculating the difference between the amplitude of the 

300° C peak transmitted by each of the filters and the 

unfiltered average peak height described above.

The results of the spectrum analysis for both the alkalic and tholeiitic basalts are shown by a histogram 

(fig. 9) representing the average of the data for the two 

aliquants from the sample. The data have been corrected for both the approximate attenuation of the interference filters in the bandwidth transmitted and 

the bias imposed on the measurements by the spectral 

response of the PM tube.

The individual bandwidth values for the two aliquants from each sample were surprisingly similar, 

differing in most cases by less than 5 percent. With all 

sources of error taken into account, the individual 

spectral bandwidth values are probably accurate to 

within an estimated ± 20 percent.

DISCUSSION

The natural TL emission spectra at 300° C of these 

samples, though not studied, are expected to be simi100 r

LU 80 

o

60

40

20

4000-A filter

4000 5000 4000 5000 

WAVELENGTH, IN ANGSTROMS

4000 5000 6000

FIGURE 8. Transmission curves for the interference filters used for spectrum analysis. Curves reproduced from manufacturer's data.
ANALYSIS OF THE PLAGIOCLASE EMISSION SPECTRUM 19

EACH BANDWIDTH LIN

TOTAL) (PERCENT OF 

C w -Jto oo ui 

H 

0

_ S«imple: 023

o m o o 9 ^ in <o

V o m o ^ ^- m

H71-10 H71-3 041

100

75

50

25

100 r

50

SB

<=iu 25

. Q_

0 L

WAVELENGTHS, 

IN ANGSTROMS x10 2

042 5X340 024 070 018

100

75

50

25

J 0

FIGURE 9. Spectral composition of the 300° C artificial TL emission from selected alkalic and tholeiitic basalt plagioclase separates. 

The spectra are normalized for filter attenuation and PM-tube bias; the wavelength increments are the same for all analyses. 

Alkalic basalts: 023, H71-10, H71-3, 041, 042, and 5X340. Tholeiitic basalts: 024, 070, and 018.

lar to spectra defined for the artificial TL because the 

same traps and recombination centers are responsible 

for both types of emission. The probable similarity of 

the natural and artificial TL spectra is discussed in 

more detail under "Artificial TL."

There are no consistent correlations between spectral characteristics and measured chemical components for either the alkalic or tholeiitic basalts. One of 

the most significant aspects of the data is the prominent component of TL below 4,000 A that appears in 

the spectrum of all the alkalic basalts and two of the 

three tholeiitic basalts. Emission in this bandwidth 

has not heretofore been reported for plagioclase. The 

emission occurs between 1,700 A, the approximate 

lower limit of detection of the PM tube, and 4,000 A, 

the nominal cut-off limit of the shortest wavelength 

interference filter available. Since the peak has not 

been reported previously, the upper limit of significant emission must actually occur at less than 3,500 A, 

the lower limit of detectability for the systems used by 

most other workers (for example, Sippel and Spencer, 

1970; Geake and others, 1971,1972,1973). The magnitude of the short-wavelength component reported for 

each sample in figure 9 may be in error (too large) by 

as much as 10 percent owing to reflection at the lower 

glass-air interface of the interference filter. This reflection was not present in unfiltered measurements. 

The short-wavelength component does not appear to 

vary systematically with either increasing sample age 

or chemical composition among the alkalic basalt 

samples studied. It does appear to increase with age in 

the three tholeiitic basalts, but an explanation for this 

relation is not available at this time.

The overall spectral characteristics of the alkalic 

and tholeiitic basalts differ significantly. The only 

recognizable spectral trend common to both rock 

types is the poorly defined tendency for the proportional amplitude of the 4,500-A peak, which can be approximated by summing the percentage values in the 

4,000-4,500-A and 4,500-5,000-A bandwidths, to increase with age. The percentage values in this band for 

the alkalic basalts are listed in table 2. This trend, 

which could be a function of increase in the number of 

crystal defects with age, is shown more clearly for both 

the alkalic and tholeiitic basalts by the rate of increase 

in the amplitude of the 300° C artificial TL peak measured (with filter CS5-60) for these and the other samples studied. Within the alkalic basalt suite, the enhanced 4,500-A emission from samples H71-3 and 

041, both of which have relatively low whole-rock K2 0 

contents, provides the basis for a circumstantial correlation whose validity and significance are unknown.
20 THERMOLUMINESCENCE DATING OF HAWAIIAN BASALT

TABLE 2. Percentage of total TL in the 4,000-A to 

5,000-A bandwidth for the 300°C alkalic basalt plagioclase artificial TL peak

Percent TL in

Sample Sample the band 4,000 A Whole-rock K2O 

No. age (years B.P. 1 ) to 5,000 A (weight percent)

023

H71-10

H71-3

041

042

5X340

4,500

62,000

150,000

181,000

184,000

3.3X108

21.8

21.8

44.2

38.2

28.6

29.8

2.02

2.82

1.81

1.61

2.34

1.23

1 See sample location and age data at end of report.

One of the most significant differences in the spectra of the two rock types is the marked enhancement 

of the >5,000-A component in the tholeiitic basalts 

relative to the alkalic basalts. The emission correlates 

with the 5,600-A emission peak, and its prominence in 

the tholeiitic basalts may be the combined effect of a 

lower density of defects and higher manganese contents in the plagioclase from these rocks. This relation 

agrees in general with the findings of Sippel and Spencer (1970) that the green peak is relatively low in plagioclase near the albite end of the plagioclase series, 

presumably in response to low concentrations of manganese.

For most of the samples analyzed, the broad composite TL peak between 150° and 250°C (fig. 7) was 

found to consist of a greater percentage of > 5,000-A 

(green peak) emission than that observed for the 

300°C peak. For the alkalic basalts, the >5,000-A 

component accounts for between 15 and 50 percent of 

the middle temperature peak TL emission. The origin 

and significance of this difference in emission spectra 

between the different temperature TL peaks is not 

understood at the present time.

The significant differences in the TL emission spectra of plagioclase of differing compositions do not necessarily present a serious problem for use of the data 

in dating work provided the natural and artificial TL 

spectra are closely similar. Differences in spectra, and 

consequent differences in TL peak amplitudes, are 

largely cancelled by the natural-to-artificial TL peak 

height ratio used in the final age determination.

SUMMARY

The results of this experiment indicate that there 

are large differences in the emission spectra of the plagioclase of the tholeiitic and alkalic basalts and significant variations among samples of each of the two 

rock types. These variations show no consistent correlation with identified chemical properties but, in the 

case of the 4,500-A emission peak, do show a tentative 

correlation with sample age. Differences in emission 

spectra between samples do not influence age calculations if the natural and artificial spectra are closely 

similar. There is significant TL emission in the bandwidth 1,700 A-4,000 A that has not been reported before and whose origins were not identified during this 

study. The emission for the tholeiitic basalts in the 

bandwidth >5,000 A is significantly enhanced relative to that for the alkalic basalts. The difference 

probably originates in a lower concentration of defects 

and higher manganese contents in the plagioclase 

from the tholeiitic basalts. The emission spectra from 

different temperature TL peaks from a given sample 

differ, that from the composite 150°-250°C peak having a larger percentage of > 5,000-A emission than the 

300°C peak.

LINEARITY OF TL

ACQUISITION, SATURATION

EFFECTS, AND SENSITIZATION

A series of experiments was performed to evaluate: 

possible changes in the TL properties of plagioclase 

caused by progressively increasing exposures of X-radiation, the appearance of saturation effects from 

large radiation exposures, and changes in TL response 

resulting from repeated irradiation and heating. The 

experimental procedure used to study the first two effects consisted of exposing two aliquants of plagioclase from each of the alkalic and tholeiitic basalts 

selected for study to increasing doses of X-radiation, 

followed by measurement of the unfiltered TL; data 

for the third was obtained during the spectrum analysis experiment.

LINEARITY OF TL 

ACQUISITION AND SATURATION

During the linearity experiment, the 300° C peak 

was measured for all samples in which it was detectable. For the tholeiitic basalts, the 150° C peak was 

measured as well. The values reported are the averages for the two aliquants analyzed from each rock.

The radiation exposures used in this experiment 

ranged from 217 to 34,800 rads. This range of radiation exposures is relatively small compared to that 

used by other workers; the maximum exposure of approximately 35,000 rads was similar to the level used 

by many workers for normal dating studies. In previous studies of saturation effects, exposures exceeding one million rads have been reported (Hwang, 1972; 

Durrani and others, 1973).

The TL sensitivities of the plagioclase separates 

from the two rock types varied so widely that no one 

sample was exposed to the whole exposure range. The 

alkalic basalt response was studied with lower exposures, and that for the tholeiitic basalts was studied 

with exposures which started higher and progressed to 

the upper limit.
LINEARITY OF TL ACQUISITION, SATURATION EFFECTS, AND SENSITIZATION 21

The TL intensities produced in the plagioclase from 

both rock types by progressively increasing exposures 

of X-radiation increased at a slightly lower than linear 

rate, as shown in figure 10. The data are presented in 

terms of peak heights plotted against the activating 

X-ray exposures. For comparison and convenience, 

the data for the 150° C peak for alkalic basalt sample 

023 are plotted with that for the tholeiitic basalts.

The artificial radiation exposures used to study TL 

production in the alkalic basalt plagioclase ranged 

from 217 to 6,960 rads. The plagioclase had such a 

high TL efficiency, however, that for several samples, 

including H71-10 and Mol le, an exposure of only 

1,740 rads produced saturation of the photon counter. 

Values reflecting equipment saturation are not shown 

in the accumulation plots (fig. 10), since actual TL 

saturation of the 300° C peak was not observed in any 

of the samples studied.

The tholeiitic basalt samples had a lower TL sensitivity to radiation than the alkalic basalts. As a result, 

radiation exposures were started at 1,740 rads and increased to an arbitrary maximum of 34,800 rads. The 

lower sensitivity for the tholeiitic basalt is shown by 

the markedly lower X-intercept values obtained by 

extending the TL accumulation lines for both rock 

types to the abscissa. These values provide a relative 

assessment of the overall TL sensitivity of the samples, a complex quantity composed of chemical and 

crystal defect influences on TL storage and emission. 

The X-intercept values could be used to quantify TL 

response for age calculation purposes and might provide an increase in accuracy over estimates of trap 

density obtained at a single radiation exposure.

The rate of increase in TL from greater exposures of 

X-radiation can be approximated by an exponential 

equation of the form.

y = (i - e-X)

where y is peak height in units of centimeters per milligram and x a representation of the radiation dose in 

rads. The lines drawn through the data points in the 

plot are described by this equation, which has also 

been shown to define the artificial TL acquisition rate 

for fluorite and reportedly for other materials as well 

(Kaufhold and Herr, 1968).

The results of this study are not applicable to an 

assessment of either the accumulation rate or maximum storage capacity for natural TL, because the 

dose rates and exposure times for natural and artificial radiation differ radically and this difference, in 

particular the duration of exposure, produces large 

differences in the effects of thermal and nonthermal 

drainage on the stored-electron populations.

The parallel arrangement of the empirically derived

1 °4 - ALKALIC BASALTS 

- 300° C peak

103

21 I I I I I I I I I I I I I I I I I III

105

- THOLEIITIC BASALTS 

I 300° C peak

8 10«

103

101 102

Mil

103

10V THOLEIITIC BASALTS 

150° C peak

104

10' 102

ARTIFICIAL TL PEAK HEIGHTS,

IN CENTIMETERS PER MILLIGRAM

103

FIGURE 10. Rate of TL accumulation with increasing radiation 

exposure in both the alkalic and tholeiitic basalts.

best-fit lines through the data for each sample of both 

the alkalic and tholeiitic basalts is significant, as it indicates the existence of a uniform process governing 

the rate of TL acquisition with increasing radiation 

dose that is independent of mineral composition. In
22 THERMOLUMINESCENCE DATING OF HAWAIIAN BASALT

the case of the tholeiitic basalts, the similarity of the 

TL growth rates for both the 150° and 300° C peaks 

indicates that the trapping parameters and partitioning of untrapped electrons among the recombination 

centers during TL measurement remain constant with 

increasing radiation dose.

The progressive deflection of some of the plotted 

points for the 150° C peak of sample 018 to the right of 

the TL acquisition line for that sample reflects an increase in TL sensitivity produced by some aspect of 

the repeated irradiation and heating cycles. The extent of sensitization can be quantified, and compensated, with the aid of low-dose calibration irradiations 

interspersed through the irradiation series. Comparison of the calibration peak heights with the peak from 

an initial equivalent radiation exposure provides a 

means for calculating a sensitivity correction factor. 

An example of such a correction is shown for the 

150° C peak from 34, 800 rads for sample 018. The direction and magnitude of the correction is indicated 

by the arrow between the points.

With the exception of samples 037 and 074, for 

which both the 150° C and 300° C peaks were affected 

by equipment saturation at high exposures (values not 

shown in fig. 10), the only apparent sample saturation 

effects are those shown by the deflection of some of 

the 150° C peak values for sample 023 to the left of the 

TL acquisition line. The general absence of saturation 

effects is in agreement with the work of most investigators, who report that the short-term shortage capacity of many minerals is not exceeded for exposure 

levels of less than many tens of kilorads of artificial 

radiation.

SENSITIZATION EFFECTS

The heating of some materials during TL measurement and annealing experiments has been found to 

produce systematic changes in TL sensitivity (Fornaca-Rinaldi, 1968; Hwang, 1972), which may be either increased or decreased, with increases being more 

commonly observed. The effect is commonly variable 

among samples of the same material. In some materials, the tendency for TL to increase is exacerbated by 

exposure to a large radiation exposure prior to heating.

Data on sensitivity changes in feldspar are limited 

but generally indicate either no change or slight enhancement in TL response; no significant decrease in 

TL sensitivity with heating has been reported. Experimental studies include an annealing experiment 

by Hoyt and others (1970) in which samples of lunar 

fines that contain plagioclase were heated to 800°C. 

Lattice defects should be mobile at this temperature, 

but no change in TL response was detected. A similar

experiment by Garlick and others (1971) in which 

samples of lunar fines were annealed in an atmosphere 

of pure argon at 600°C also produced no detectable 

change in TL response. More recently, Durrani and 

others (1973) found that heating lunar fines to 550° C 

during TL measurement enhanced the response of the 

heated material to a subsequent 800-kilorad dose of 

gamma radiation by 40 percent in the 300°-500° C 

range but that after the initial heating there was no 

further change. In another experiment, Durrani and 

others (1973) exposed a sample of lunar fines to a 

gamma-ray dose of 1.4 megarads, then annealed the 

sample at 450° C for 1 hour. The sensitivity of the sample to a subsequent radiation exposure was increased 

by 50 percent. Geake and others (1971) heated feldspar to 1,050°C for 30 minutes and found no change in 

the position of the spectral emission peaks.

PROCEDURE AND RESULTS

Most of the data used in this study to evaluate 

changes in TL sensitivity were obtained during the 

spectrum analysis experiment. The data show that for 

the two heating cycles and one irradiation required for 

age determination, there is a negligible change in the 

TL sensitivity of plagioclase from the alkalic and tholeiitic basalts within the limits of experimental error. 

This result may be a function of the relatively short 

(<25 s) heating times and low radiation exposures 

used in this investigation; it is significant because it 

indicates that the artificial TL peaks provide an accurate relative measure of TL sensitivity.

The effect of the first heating (for natural TL measurement) on TL sensitivity was tested for the six alkalic basalt samples selected for spectrum analysis. The 

samples were exposed to 870 rads of X-radiation prior 

to the first heating. The artificial TL peak heights obtained were compared with those from equivalent 

samples that had been thermally drained. The peak 

amplitudes were identical to within less than the normal 5 percent experimental error. A comparable analysis of four tholeiitic basalts gave similar results.

The exposure of aliquants of both alkalic and tholeiitic basalt plagioclase to more than four to six irradiations and heatings produced a definite but 

somewhat erratic increase in sensitivity in most samples. The effect was more pronounced in the tholeiitic 

than in the alkalic basalt separates. In the 8 to 10 cycles required during analysis of the spectral composition of plagioclase from the alkalic basalts, increases 

in the sensitivity of the 300° C peak were found to 

range from 0 to 15 percent and to average about 4 percent. For plagioclase from the tholeiitic basalts, calculations of sensitivity were based on the 150° C peak
FADING OF ARTIFICIALLY INDUCED TL 23

because in unfiltered runs involving young samples 

the 300° C peak was not well enough defined for measurement. The percentage increases in sensitivity calculated for the 150°C peak are probably comparable 

to the sensitivity changes in the 300° C peak in most 

cases. Sensitivity increases for the plagioclase from 

the tholeiitic basalts averaged about 20 percent in 10 

cycles and increased to an average of about 25 percent 

in 15 cycles although behavior was very erratic. Sensitivity increased by as much as 50 percent or more in 15 

runs in a few cases. There was no discernible correlation of sensitivity changes with measured plagioclase 

composition for either rock type.

SUMMARY

The results of the linearity study show that the rate 

of TL accumulation with increasing radiation dose 

follows a precise pattern that is independent of both 

dose level (within limits) and mineral composition. 

The only difference in the data between samples is the 

X-intercept dependency of each TL accumulation 

line on trap density and TL emission properties. The 

uniformity of TL acquisition over a wide range of radiation exposures provides justification for the use of 

low exposures to determine relative differences in TL 

sensitivity between samples. No evidence of TL saturation was observed in the 300° C peaks of any of the 

samples studied; sensitization problems were significant only for the 150° C peak of sample 018.

The results of the sensitization analysis indicate 

that although plagioclase is sensitized by repeated 

heatings and irradiations, there is no perceptible 

change in response during the two cycles necessary for 

dating. For other experiments requiring repeated irradiation and heating, the calibration irradiations discussed for sample 018 can provide a means of 

compensating for the enhanced response.

FADING OF ARTIFICIALLY INDUCED TL

An experiment was performed to assess the extent 

of peak interference caused by the tail of the 225° C 

peak overlapping the peak maximum of the 300° C 

peak (fig. 7). The experiment involved storing irradiated samples in the dark at room temperature for 

varying lengths of time to produce a greater degree of 

decay of the 110°, 150°, and 225°C peaks than of the 

300° C peak, owing to the greater probability for the 

spontaneous untrapping of electrons in shallower 

traps.

The experiment provided an opportunity to study 

the rate and extent to which TL fade (drainage) occurs in plagioclase. Spontaneous fading has already

been shown to be a complex process involving both 

thermally dependent and nonthermal decay processes 

(see sections on "Stability of Trapped Electrons" and 

"Nonthermal Drainage"). It poses a potentially serious problem for "absolute" TL dating methods that 

require the cumulative storage of all electrons trapped 

over geologic time.

PROCEDURE AND RESULTS

The experimental procedure consisted of exposing 

at least 8, and in a few cases 12, of the aliquants from 

each of the rocks selected for study to 1,740 rads of Xradiation. The aliquants were those that had been 

used previously for TL-dating studies. The reirradiation of these aliquants permitted the use of previous 

measurements of the unfiltered 300° C peak artificial 

TL in the calculation of the amount of peak decay that 

had occurred during each storage interval. Two aliquants were measured 7 minutes after exposure, and 

the peak amplitudes were compared with those obtained previously for the same samples to provide a 

check on possible changes in sensitivity resulting from 

storage of the drained aliquants between the initial irradiation experiment and this study. These measured 

changes listed in table 3 show a tendency for the sensitivity of the samples to decrease slightly during storage. The changes were considered to be generally 

small enough not to require comprehensive sensitivity 

correction for all samples.

The remaining samples were stored in the dark at 

room temperature after irradiation. Three, and where 

the number available permitted four, aliquants from 

each rock were removed and run at intervals of 10, 20, 

and, for samples for which enough material was available, 40 days. The reported decay values given in table 

3 are the averages of the three to four aliquants measured at each decay interval.

TABLE 3.—Percentage of TL remaining in the 300°C peak of both 

the alkalic and tholeiitic basalts at the end of each decay interval

[The decay percentages were calculated by comparing the peak heights obtained at 

each decay interval with the initial peak from the same radiation dose measured 7 

minutes after irradiation, both without filters CS6-60. +, increase; —, decrease, in 

sensitivity relative to the initial radiation series; n.d., values not determined]

Sample 

No.

Sensitivity 

change (percent)

Percent TL left

10 days 20 days 40 days

023___

H71-10

035___

H71-3

036___

042_ 

033_ 

Molle _ 

Klpa lOa 

5X340 _ 

P71-13 _ 

P71-4 _ 

P71-3 _ 

P71-5 _

n.d.

+ 4.4

- 9.9

- 2.0

- 4.6

- 2.2

- .4

- .1

- 2.1

+ 8.8

+ 1.2

- 9.0

-14.6

- 4.2

+ 3.0

33.6

37.2

21.7

23.6

29.7

22.5

27.1

31.9

28.0

35.0

30.5

14.7

34.6

39.0

35.6

26.7

34.6

18.3

21.8

25.3

21.0

26.4

29.2

25.5

25.0

27.9

10.3

38.3

31.9

30.4

n.d. 

25.9 

n.d. 

21.4 

n.d. 

20.7 

26.3 

26.1 

22.1 

23.7 

24.0 

7.4 

20.6 

30.6 

22.0
24 THERMOLUMINESCENCE DATING OF HAWAIIAN BASALT

The extent of TL decay during each storage interval 

was determined by comparing the peak amplitudes 

measured for each aliquant after one of the three storage intervals with those produced by the same radiation exposure and measured 7 minutes after 

irradiation. This technique worked well for the tholeiitic basalts, for which the TL measured 7 minutes 

after irradiation did not produce equipment saturation. It was not satisfactory for the alkalic basalts, 

however, because for several of them a 1,740-rad dose 

did produce saturation of the TL equipment. This 

problem was circumvented by using the peak data for 

the 217- and 870-rad exposures and the TL-accumulation-rate-with-increasing-radiation-dose relation described in the section on "Linearity of TL Acquisition 

and Saturation" to calculate an approximate TL peak 

amplitude at the 1,740-rad dose level for those alkalic 

basalts that produced saturation. The projected values are accurate to within an estimated ±10 percent.

The results of the decay experiment were generally 

as expected in terms of the relative amounts of decay 

observed for the different temperature peaks. The delayed TL measurements provide a much improved 

resolution of the 300° C peak in cases of peak overlap. 

Reproductions of the TL glow curves for alkalic basalts numbers 033, H71-10, and Mol le (fig. 11) show 

the amount of decay of the various temperature peaks. 

The more rapid decay of the low-temperature peaks 

relative to the 300° C peak is indicative of a largely 

thermally controlled decay process similar to that described by the Randall-Wilkins equation. Unfortunately, the data do not facilitate determination of the 

extent to which nonthermal drainage contributes to 

the decay process.

16

W

cc

UJ

h 12

SAMPLE

-—— H71-10/ 

- Molle/

\ \

/ 

A1

\ \ I Measured 7 minutes

\ \. !• . ... after irradiation

Meesured 40 days 

after irradiation

100 200 300 400 

TEMPERATURE, IN DEGREES CELSIUS

FIGURE 11.—Artificial TL decay during 40-day storage of irradiated samples.

Averages for the percentage of 300°C peak TL remaining at the end of each decay interval for both alkalic and tholeiitic basalts, table 3 and figure 12, 

clearly show that the largest proportion of the total 

decay recorded over the 40-day duration of the experiment occurred during the first 10 days. All samples 

lost at least 60 percent of the trapped electron population responsible for the 300° C peak during this initial 

period, many nearly 70 percent. The decay rate decreased sharply following this initial period. Except 

for four samples, less than 10 percent additional decay 

occurred during storage up to the maximum 40-day 

limit.

The decay rate shown by the curves (fig. 12) appears 

to be exponential, but careful examination of similar 

data by Garlick (1973) has shown the decay process to 

be more accurately represented by a summation of a 

number of exponential decays. The individual exponential functions define untrapping rates for the 

range of trap depths that contribute to individual TL 

peaks.

DISCUSSION

The similar TL decay rates for plagioclase from the 

alkalic and tholeiitic basalts is significant because the 

rates appear to be approximately independent of both 

trap density and mineral composition. The relatively 

uniform reduction of the trapped electron population 

to approximately 20 percent of the intitial level in 40 

days shows that many more electrons are trapped during laboratory irradiation than can be stored stably 

and suggests that the number trapped initially is proportional to the number of stable traps available. This 

observation has not been reported previously. The 

phenomenon may be produced by a metastable clustering of a limited number of electrons around traps 

capable of the stable retention of only one electron, or

100

THOLEIITIC BASALTS

20 40 10 

DECAY TIME, DAYS

20 40

FIGURE 12. — Average percentage of 300° C peak TL remaining in 

the alkalic and tholeiitic basalt samples after each decay interval. Vertical bars indicate the range of values observed in each 

interval.
RADIOACTIVE ELEMENT CONTENTS OF THE ALKALIC AND THOLEIITIC BASALTS 25

by some other process that produces an equivalent result.

The variation in decay rates observed for the 300° C 

peak in individual samples, particularly within the alkalic basalt suite, shows a rough correlation with the 

proportional amplitude of the adjacent 225° C peak in 

the 10- and 20-day intervals of the decay experiment. 

The samples with the least amount of overlap interference from the tail of the 225°C peak tend to show 

the greatest amount of TL remaining during the two 

initial decay intervals. Those with more extensive interference, such as samples 035 and 041, tend to have 

the lowest percentage of TL remaining. An analysis of 

the effect based on measurements at the 10-day interval showed that overlap interference from the tail of 

the 225° C peak may contribute a maximum of about 

10 percent to the amplitude of the 300° C peak; the 

average is considerably less.

The TL decay patterns identified in this investigation are in qualitative agreement with observations of 

a similar nature made by Dalrymple and Doell (1970 

a, b) and Hwang (1973) on lunar fines. Wintle (1973) 

investigated TL fading in a number of minerals and 

found a wide variation in decay rates. Her study included several feldspar samples whose TL faded by 

less than 5 percent in 4 weeks. Causes for this variation in TL retention properties have not been proposed.

It is not certain how the data developed on the basis 

of high-intensity doses of artificial radiation in this investigation apply to comparable retention properties 

for natural TL except that it is clear that long-term 

natural TL storage capacity is only a fraction of the 

apparent capacity identified on the basis of shortterm storage of artificial TL. The extent of this difference is demonstrated by the fact that the natural TL 

peak amplitudes for the alkalic basalts vary from a low 

of 0.67 percent of the artificial TL amplitude from 870 

rads of X-radiation for sample 023 to a high of 6.43 

percent for sample 033. The natural TL peak heights 

were produced by adjusted natural radiation doses 

that were approximately equal to or greater than the 

artificial dose for sample 023 and more than 50 times 

greater for sample 033. Some of the difference is due 

to enhanced artificial TL peaks caused by the metastable short-term storage of laboratory-excited electrons, some probably due to spontaneous drainage of 

natural TL-producing electrons over geologic time.

Using decay-diminished peak amplitudes to provide a measure of trap density for use in TL age calculations was not investigated in this experiment, 

principally because the measurements were made 

with unfiltered TL. It is uncertain what effect differences in the extent of decay at, say, the 20-day interval

would have on using these peaks to quantify the relative magnitudes of trap densities. Possibly the extent 

of decay is indicative of the natural TL storage properties of individual samples, in which case the decay 

amplitudes would be valuable, but this hypothesis is 

untested.

SUMMARY

Spontaneous decay of artificial TL reduces the 

300°C peak amplitudes of both the alkalic and tholeiitic basalts to about 20 percent of their initial value in 

40 days. This rapid and extensive decay provides a 

measure of the metastability with which most of the 

electrons trapped during relatively high intensity laboratory irradiations are stored.

The artificial TL decay produced by storage of irradiated samples for 10 to 40 days is faster and more 

complete for the low-temperature peaks than the 

300° C peak and yields a much-improved resolution of 

the high-temperature peak in cases of peak overlap. 

Interference from the 225° C peak contributes a maximum of 10 percent to the measured amplitude of the 

300° C peak in the samples studied. The use of TL 

peaks measured after storage of irradiated samples to 

quantify TL response characteristics for dating applications was not investigated in this study but could 

conceivably provide reliable data. The decay rates 

suggest that the use of artificial TL measured a short 

time after irradiation to define natural TL storage capacity is invalid. The same data indicate that the 

short-term artificial TL peaks are suitable for defining relative differences in trap density.

RADIOACTIVE ELEMENT CONTENTS

OF THE ALKALIC AND THOLEIITIC

BASALTS AND DOSE-RATE CALCULATIONS

RADIOACTIVE ELEMENT CONTENTS OF THE BASALTS

The production of natural TL and probably secondary defects in basaltic plagioclase, as well as in 

other geologic materials, is dependent on the interaction of decay particles and recoil nuclei from radioactive elements and cosmic rays with the lattice atoms of 

the dosimeter crystals. The most important radioactive nuclides in igneous rocks are 238U, 235U, 232Th, and 

40K and the intermediate daughter products of the 

uranium and thorium decay series. For the purpose of 

energy calculations in this investigation, the decay 

chains were considered to be in secular equilibrium. 

For rocks less than one million years old, this assumption is only approximately correct owing to probable 

disequilibrium in the uranium and thorium decay 

chains, but in the absence of detailed isotopic analy-
26 THERMOLUMINESCENCE DATING OF HAWAIIAN BASALT

ses, it represents the most reasonable basis for doserate calculation. The actual extent of disequilibrium 

in the 238U series for several historic and prehistoric 

tholeiitic and alkalic Hawaiian basalts has been determined by Somayajulu and others (1966), who found 

that although 238U and 234U were in secular equilibrium, 230Th exceeded the amount necessary for equilibrium by 10 to 50 percent in all the samples studied; the 

excess averaged about 25 percent. 210Pb was found to 

be depleted in most samples, possibly as a consequence of radon loss, by an average of about 25 percent of the quantity required for equilibrium.

The effect that this disequilibrium has on dose rates 

calculated on the basis of assumed secular equilibrium 

can be determined if it is assumed that the depletion 

of 210Pb is indeed due to radon loss. Approximately 60 

percent of the decay energy in the 238U series originates 

from the portion of the chain including and beyond 

radon. If the average amount of radon loss and 230Th 

excess for the Hawaiian rocks is used as the basis for 

computation, the disequilibrium dose rates for the 238U 

series are about 80 percent of those calculated on the 

assumption of secular equilibrium. Variations in the 

extent of disequilibrium result in a range of approximately ±10 percent around the average value. The 

extent of disequilibrium in the decay chains of 235U 

and 232Th were not determined for the Hawaiian rocks, 

but if the extent of disequilibrium is comparable to 

that for the 238U series, then the average total dose rate 

for the Hawaiian basalts may be as much as 20 percent 

lower than that calculated on the basis of secular equilibrium. This apparent discrepancy between the probable actual average annual dose rates and those 

calculated on the basis of secular equilibrium would 

not adversely affect relative TL-dating studies if the 

difference was consistent between samples. The apparent range in actual dose rates caused by variations 

in the extent of disequilibrium between samples is potentially more critical, but since available data suggest 

that this does not exceed a maximum of about ± 10 

percent, and the average is probably less, it does not 

appear that this variation will be a significant source 

of error in the final TL ratio.

PROCEDURE AND RESULTS

Radioactive element contents were measured on 

whole-rock material because the plagioclase dosimeter crystals, in addition to being exposed to the decay 

products from radioactive elements incorporated in 

the crystals themselves, can intercept decay particles 

from nuclides located in the matrix. Radiation from 

the matrix is important because geochemical partitioning of atoms among the coexisting minerals in a

crystallizing basalt melt results in the incorporation of 

most of the uranium and thorium in minerals other 

than plagioclase or in the residual glassy groundmass.

Uranium and thorium were measured by delayed 

neutron activation (DNA); K2 0 was measured by 

flame photometry. The precision of the K20 measurements is uniformly equal to or less than 1 percent. The 

errors in the uranium and thorium measurements are 

considerably greater because the contents of these elements in the Hawaiian basalts are near or below the 

lower detection limit of the DNA method (1 ppm for 

uranium, 10 ppm for thorium) for reasonable precision and accuracy. The DNA method was used in this 

investigation to test the feasibility of this relatively 

accessible and rapid means of analysis. More sensitive, and expensive, methods such as isotope dilution 

exist and could be used to achieve better measurement precision and accuracy if necessary.

The uranium, thorium, and K2 0 contents of the alkalic basalts are given in table 4. The K20 values represent the total content of that element, consisting of 

all the naturally occurring isotopes; the 40K content 

can be calculated by multiplying the total K20 content by 0.8301 to convert to weight percent K and the 

potassium content by the natural abundance of 40K, 

1.9X10-4 mol/mol, to obtain the amount of that isotope. In practice, the proportionality constants are expressed so that the dose rate from 40 K can be 

calculated directly from weight percent natural K.

Each of the uranium and thorium measurements is 

accompanied by an estimate of measurement precision expressed as a coefficient of variation (cv). The 

magnitude of the errors for the alkalic basalts are generally acceptable relative to the other errors in the TL 

measurements, as they constitute less than about 25 

percent of the total error in the TL ratio. The data 

show that for samples with radioactive element contents equivalent to, or greater than, the alkalic basalts, delayed neutron activation is an acceptable 

method of analysis.

TABLE 4.—Radioactive element contents of the alkalic basalts

[Uranium and thorium measured by Hugh Millard; K2O measured by Lois Schlocker. 

cv, coefficient of variation]

Sample 

No.

PpmU ±cu 

(percent)

PpmTh ± cv Weight percent 

(percent) K2O

023 ___ 

H71-10 _

035 ___ 

H71-3 _

036 ___

041 ___

042 ___ 

033 ___ 

Molle _ 

Klpa lOa 

5X340 _

1.07

2.57

1.62

1.39

1.27

1.00

1.79

1.38

1.06

.22

.67

5.5 

4.8 

4.8 

5.7 

6.7 

6.6 

5.7 

4.9 

6.6 

23.1 

8.7

3.65

7.07

5.27

3.96

7.67

2.79

4.69

2.98

2.23

1.46

2.85

13.6

13.5

11.7

15.7

10.0

19.3

17.4

17.7

24.6

31.1

17.7

2.016

2.82

2.92

1.807

2.137

1.613

2.34

2.178

1.293

.293

1.228
RADIOACTIVE ELEMENT CONTENTS OF THE ALKALIC AND THOLEIITIC BASALTS 27

The uranium, thorium, and K2 0 contents of the 

tholeiitic basalts are given with the errors in the measurements in table 5. Coefficients of variation greater 

than 30 percent mean that the accompanying parts 

per million values are not reliable. The poor precision 

of the measurements, particularly for thorium, indicates that the DNA method is not very well suited to 

rocks with radioactive element contents equivalent to 

or lower than those for the ocean island tholeiites. An 

attempt to measure the uranium concentrations in the 

tholeiitic basalts by fission-track spark counting 

(FTSC) gave values that were in good agreement with 

the DNA values for about half of the samples (Hugh 

Millard, written commun., 1975). The data are preliminary, but the reproducibility of the two measurements reported for each sample appears to be 

comparable to that for the DNA method. As estimates 

of analytical precision were not included with the 

analyses, it is difficult to tell whether the method provides a substantial increase in accuracy over the DNA 

method for low-uranium rocks. The FTSC values were 

not used for dose-rate calculations.

DISCUSSION

The tholeiitic basalts show an apparently greater 

range in uranium and thorium contents than the alkalic basalts, although some of this variation may be attributable to the larger experimental error in the 

measurements. The amounts of the elements, lower 

than those for the alkalic basalts by a factor of three to 

five or more, result in calculated annual dose rates for 

the two rock types that differ by approximately a factor of six. This large difference in dose rates, perhaps 

coupled with the significant difference in average 

feldspar composition, has a marked effect on TL 

properties. Differences in dose rate within each of the 

two suites are smaller and have a lesser effect, which

TABLE 5.—Radioactive element contents of the tholeiitic basalts

[Uranium and thorium measured by Hugh Millard; K2O measured by Lois Schlocker; 

CD'S (coefficient of variation) are uniformly equal to or less than 1.0 percent; n.d., not 

detectable]

Sample

No.

012

015

026

030

050

051

032

024

070

074

018

019

P71-13

037

P71-4

P71-3

P71-5

PpmU

0.38

.35

.20

.21

.27

.18

.13

.16

.17

.12

.15

.34

.08

.49

.11

.20

.11

±cv 

(percent)

10.7

12.3

21.6

20.7

15.0

21.9

26.6

20.3

19.7

28.4

23.8

16.7

73.7

8.8

44.6

25.2

51.2

Ppm Th

0.88

1.201

.35

.61

1.32

.98

.25

n.d.

.31

.25

.69

.92

.69

.94

.78

.62

.63

±cu \ 

(percent)

42.7

32.8

120.7

66.4

28.3

40.4

127.9

99.7

121.5

50.6

54.1

84.6

40.2

55.5

69.4

83.6

V eight percent 

K2O

0.587

.680

.379

.455

.609

.423

.410

.454

.504

.341

.344

.369

.091

.868

.120

.329

.143

in most cases can be normalized by dividing the peakheight averages by annual dose rates calculated from 

the radioactive element contents.

CALCULATION OF RADIOACTIVE DECAY ENERGIES

The most direct method of quantifying the TL-producing effectiveness of natural radiation consists of 

calculating the amount of energy evolved during the 

successive disintegrations in the decay chains of each 

of the primary nuclides. Decay energies represent the 

maximum possible effective energy because only part 

of the total energy emitted during a decay is likely to 

be absorbed in the plagioclase crystals and only a 

small part of that absorbed energy will result in ionization and stably trapped electrons. If, however, the 

distribution of radioactive elements in the host rock is 

reasonably homogeneous relative to the dosimeter 

grains (a reasonable assumption for most rocks), the 

plagioclase crystals should intercept a representative 

proportion of the total radiation emitted. Dose rates 

based on decay energies should then provide a reasonably accurate relative measure of the level of internal 

radiation to which the dosimeter grains have been exposed and a reliable measure of differences in dose 

rate between samples. This relative dose rate is adequate for use in age calculations in this investigation.

Natural radiation dose rates calculated from decay 

energies are expressed in rads, the standard unit of 

absorbed dose in radiation dosimetry. One rad is equal 

to 100 ergs of energy absorbed per gram of any absorber (10-2 J kg-1 ). The representation of energy emitted 

during radioactive decay in terms of absorption units 

is an approximation, especially in view of the implicit 

assumption that the absorption will produce mainly 

ionization, but is accepted practice.

The end members of the decay series, the number of 

each of the principal particles emitted during a complete decay, and the approximate energies evolved are 

given in table 6. Beta particles were assigned an effective energy equal to one-third the maximum beta energy (Mladsenovic, 1973) except where the energy of 

the dominant emission for a particular isotope was

TABLE 6.—Abbreviated radioactive decay series and decay energies for the radioactive isotopes of uranium, thorium, and potassium

[Weast (1974); Lederer and others (1967). EC, electron capture]

End members 

Isotope of the series

238U 238U 2MPb

23SU Z35U 207Pb

Th 232Th 208Pb <0K-40Ca (0.89)

«K "K-'-Ar (0.11)

Principal 

decay 

particles

8a, 6/J~> 7 

la, 4/J-- 7

'/J~ ' 

EC

Particle energies (MeV)

a

42.97 

41.40 

36.80

P

1.45 

1.02 

1.41 

.60

7

1.91 

1.17 

.689

1.46

Total

46.33 

43.59 

39.80

'.69

'Total average emission energy adjusted for the proportions of the two decay modes.
28 THERMOLUMINESCENCE DATING OF HAWAIIAN BASALT

specified in the literature. For 40K, the maximum beta 

energy is 0.60 MeV (Pertsov, 1964). Gamma-ray energies were calculated by weighing the dominant emissions by their relative proportions.

DOSE-RATE CALCULATIONS

PROCEDURE AND RESULTS

The calculation of dose rates from measured quantities of the radioactive elements requires conversion 

of particle energies into energy constants in terms of 

rad per parts per million per year. The energy constants are adjusted for natural abundances of the isotopes to facilitate calculation of dose rates directly 

from the reported amounts of the natural elements. 

With the isotopic abundances, half-lives, and decay 

constants for uranium, thorium, and potassium given 

in table 7, energy constants (Ec) can be calculated 

from decay energies with the equation

Ec = AQa^a,

where A, the specific activity of each isotope, is defined by the equation

A = — N\a, 

m A l

where A and m are the decay constant and gram atomic weight for each isotope, respectively, NA Avogadro's 

number, and al a constant, 3.1536X107 s/yr. In the 

equation for energy constants, Q is the decay energy 

for each particle in the complete disintegration of 

each primary isotope (1 MeV = 1.6021X1Q-6 erg); the 

values for the constants «2 and «3 are 10 ~2 rad/erg, and 

106 g/ppm.

Energy constants in rads per parts per million per 

year adjusted for the natural abundances of the isotopes are listed for alpha and beta plus gamma radiation in table 8.

Dose rates reflecting the estimated TL-producing 

effectiveness of natural radiation were obtained by 

adding one-tenth of the calculated alpha-particle dose 

to the sum of the beta and gamma doses. The alpharadiation coefficient reflects the reduced efficiency of

TABLE 7.—Natural abundances and half-lives (t) of the 

isotopes

[Data from Lederer and others (1967)]

Isotope

238U

2KU

232Th

"K

Natural 

abundance

percent

99.276

.7196

100

.0119

t'fl (years)

4.51X109

7.13X10"

1.41X1010

1.28X109

Decay constant(X) 

loge2

"~ t*

1.54X10-'° yr- 1

9.72X10-'° yr-'

5.42X10- 10 yr-'

TABLE 8.—Energy constants in rads per parts per million per year adjusted for natural isotope abundances

Isotope a energy (8 + 7 energy Total effective 

energy

35U _ _ _ _ 0.0278

.0075

0.0215

.0043

.108

'0.0493

1 .0118

2 .108

'Alpha-particle energies in the energy total are adjusted to 0.1 of the 

nominal calculated energy; see text for explanation. 

'Dose rate is in terms of rads per weight percent natural K-yr.

alpha particles in producing ionization. It was derived 

from experiments with lithium fluoride (Morehead 

and Daniels, 1957) and other materials (Aitken, 1968) 

in which the trap-filling efficiency of alpha particles 

per rad of radiation was compared with that of beta 

and gamma radiation. This coefficient has been used 

by other workers and found to be approximately correct. The annual dose rates and errors in the dose 

rates for both the alkalic and tholeiitic basalts are 

given in table 9.

DISCUSSION

The cumulative errors in the annual dose rates of 

table 9 were calculated with a standard propagationof-errors equation using only the analytical errors in 

the isotopic measurements. The cumulative errors do 

not include compensation for probable variations in 

the degree of disequilibrium in individual samples or

TABLE 9.-Annual dose rates for the alkalic and tholeiitic basalts

Sample 

No.

U1 

(rad/yr)

Th' 

(rad/yr)

K

(rad/yr)

Total

(rad/yr ±<r2 )

Alkalic basalts

023 _ _ 

H71-10 

035___ 

H71-3 _ 

036 __

042 _ _ _ . 

033____ 

Mol le _ . 

Klpa lOa 

5X340 .

0.053

.127

.080

.069

.062

.050

.088

.068

.052

.011

.033

0.043

.083

.062

.048

.091

.033

.055

.039

.026

.017

.034

0.181

.253

.262

.162

.192

.145

.210

.193

.116

.026

.110

0.277: 

.463: 

.404: 

.278: 

.345: 

.228: 

.353: 

.300: 

.194: 

.054: 

.177:

: 0.007 

: .013 

: .009 

: .009 

: .010 

: .007 

: .010 

: .008 

: .007 

.006 

: .007

Tholeiitic basalts

012

015

026

030

050

051

032

024

070

074

018

019

P71-13

037

P71-4

P71-3

P71-5

0.018

.017

.010

.010

.013

.009

.006

.008

.008

.006

.008

.017

.004

.024

.006

.010

.005

0.011

.016

.005

.008

.017

.013

.003

.004

.003

.009

.012

.009

.012

.010

.008

.008

0.052

.060

.034

.040

.054

.038

.036

.040

.045

.030

.031

.033

.008

.077

.011

.035

.013

0.081 ±0.005

.093 ±

.049 ±

.058 ±

.084 ±

.060 ±

.045 ±

.048 ± 3

.057 ±

.039 ±

.048 ±

.062 ±

.021 ±

.113±

.027 ±

.053 ±

.026 ±

.006

.006

.006

.005

.006

.002

.002

.004

.004

.005

.007

.008

.005

.006

.006

.007

'Alpha particles assigned an energy efficiency of 0.1.

^he cumulative error in the total energy dose was calculated with the equation

um =((7U2 + <7Th2 + <7K2)1/2 -

3Dose rate for sample 024 does not include radiation from thorium.
NATURAL TL 29

for small errors in the energy-conversion coefficients.

The magnitude of the analytical errors in the thorium measurements for the tholeiitic basalts indicates 

that the isotopic concentrations, and therefore the 

dose rates calculated from these concentrations, are 

not very reliable. This limits the usefulness of the 

dose-rate data, but since the dose rate from thorium 

constitutes less than 20 percent of total dose rate (the 

average is in the range of 10 to 15 percent), it does not 

completely abrogate the usefulness of the cumulative 

dose-rate figure for normalizing differences in TL 

caused by variations in the natural radiation environment.

The annual dose rates in table 9 do not include energy from cosmic radiation. The mean cosmic-ray 

dose at sea level has been calculated by Herbst (1964) 

to be about 0.033 rad/yr, by Spiers and others (1964) 

to be about 0.037 rad/yr, and by Schilling (1964) to 

range from about 0.035 rad/yr at the equator to about 

0.050 rad/yr at lat 50°. The increase in intensity of 

cosmic radiation with altitude has been estimated by 

Pertsov (1964) to be approximately a factor of three in 

3 km from sea level.

The annual cosmic-ray dose rate has been estimated by Aitken (1968) to be attenuated to about 0.015 

rad/yr at a depth of about 1.5 m below the surface in 

soil. An annual cosmic-ray dose rate of 0.014 to 0.016 

rad/yr is used routinely by several investigators in 

work with archeologic materials (Tite, 1968; Aitken, 

1968,1970; Fleming and others, 1970).

In this investigation, energy from cosmic radiation 

was not included in the dose rate, because the absolute 

magnitude of the dose for individual samples could 

not be accurately determined. The omission of the 

cosmic-ray dose infers that the dose rate is constant. 

The probable error introduced by this assumption can 

be evaluated by determining the proportional influence of the probable maximum cosmic-ray dose rate of 

0.015 rad/yr on the internal radiation dose rate for the 

alkalic and tholeiitic basalts. Since it represents only 

4.5 percent of the average radiogenic dose rate for the 

alkalic basalts from the Hawi Volcanic Series, the error added to the internal radiation dose rates by variations in the cosmic-ray component between samples 

should be negligible. The cosmic-ray dose of 0.015 rad/yr is equivalent to about 25 percent of the average 

annual radiogenic dose rate for the tholeiitic basalts, 

and it is likely that the assumption of uniformity of 

the cosmic-ray dose introduces an error of perhaps 

several percent in the calculated radiogenic dose rates 

for samples collected within less than about 1 meter of 

the surface.

The average annual dose rate for alkalic basalt samples 023 to 033 in table 9 is 0.335 rad/yr. This average

is approximately 20 percent lower than a typical dose 

rate for pottery of 0.410 rad/yr as recalculated from 

data tabulated by Aitken (1970), using an a-radiation 

efficiency of 0.1 and excluding energy from cosmic radiation. The average dose rate for the alkalic basalts is 

similar to an annual dose rate of approximately 0.336 

rad/yr recalculated from data for a lava sample from 

the volcano Thera by Aitken (Aitken and Fleming, 

1971), again excluding cosmic radiation and using an 

alpha-particle efficiency of 0.1.

The average annual dose rate for the historic and 

carbon-14-dated tholeiitic basalts (samples 012 

through 019 in table 9) is 0.055 rad/yr. This is significantly lower than the dose rates for most of the materials for which TL dating has been attempted.

The radioactive element contents of the ocean-island basalts studied in this investigation are in general lower than those for rocks of comparable bulk 

composition but continental origin. This should make 

continental rocks, particularly those of basalt composition, better suited for TL dating than their oceanisland equivalents.

SUMMARY

The uranium, thorium, and potassium contents of 

the basalts were measured on crushed whole-rock material, uranium and thorium by delayed neutron activation, and K2O by flame photometry. The amounts of 

the elements and the energy emitted during a complete decay of each of the primary nuclides were used 

to calculate an annual dose rate for each rock. The 

dose rates represent the maximum possible TL and 

defect-producing radiogenic energy; the actual effective dose rates are lower since energy is dissipated in 

modes other than TL production in plagioclase.

The average radiogenic dose rate for the alkalic basalts, 0.335 rad/yr, is higher than that for the tholeiitic 

basalts (0.055 rad/yr) by a factor of six. These dose 

rates do not include cosmic-radiation energy, which 

for samples buried more than a meter or so below the 

surface has a maximum intensity of 0.015 rad/yr (Aitken, 1968). The dose rates for the alkalic basalts are 

approximately 40 percent lower than the typical dose 

rate for pottery and lower than the dose rate for rocks 

of comparable bulk composition but continental in 

origin.

NATURAL TL

The amplitude of the high-temperature natural TL 

peak with a maximum intensity between 325° and 

350° C was used to quantify TL response in this investigation. Natural TL glow curves were reproduced 

from actual TL graphs for four alkalic basalts and four
30 THERMOLUMINESCENCE DATING OF HAWAIIAN BASALT

tholeiitic basalts, as shown in figure 13A and figure 

13B. Corresponding artificial TL glow curves are 

shown for comparison.

ALKALIC BASALT NATURAL TL

PROCEDURE AND RESULTS

Seven natural TL peak-height measurements, normalized for adjusted sample weight, were averaged for 

each crushing stage for individual samples to yield the 

values given in table 10 (in centimeters per milligram). The natural TL values for both crushing stages 

were then averaged and plotted against the independently determined ages of the samples, as shown in 

figure 14. The line through the data is a least-squares 

fit calculated by weighting the data to reflect the estimated degree of deviation from the mean trend pro1200

r 800

400

Natural TL

——— 033

——— H71-10

——— Mol 1e

100 200 300

TEMPERATURE, IN DEGREES CELSIUS

400

120

80

40

duced in individual samples by variations in spectral 

composition and trap density. Sample 023 has a disproportionately large influence on the best-fit line by 

virtue of its distal position relative to most of the 

other samples. Its TL value appears to be approximately correct for its age in comparison with values 

for both the older alkalic basalts and the tholeiitic basalts of young age; it was weighted more heavily in recognition of its apparent reliability.

DISCUSSION

The best-fit line on the plot of TL relative to age, 

figure 14, in effect defines the average rate of natural 

TL increase in basaltic plagioclase. The TL values 

show a moderate degree of scatter with respect to this 

best-fit line, but they have not been normalized for 

differences in trap density or spectral composition, 

both of which strongly influence measured natural TL 

amplitudes. The equation for the best-fit line in figure 

14 is of the form

y = aebx.

The reasonably close approximation of the data to a 

predictable curve is critical because it is the first demonstration that the increase in natural TL over exNatural TL

Artificial TL

SAMPLE 

036

100 200 300 

TEMPERATURE, IN DEGREES CELSIUS

400

Artificial TL

—— 033 /

———— H71-10

——-MoMe

070

100 200 300 400 

TEMPERATURE, IN DEGREES CELSIUS

100 200 300 

TEMPERATURE, IN DEGREES CELSIUS

B

400

FIGURE 13.—Natural (solid lines) and artificial (dashed lines) TL glow curves for selected alkalic basalts (A) and tholeiite basalts (B). 

Both natural and artificial TL were measured with filter CS5-60; curves were measured 7 minutes after irradiation. The artificial TL 

(A) is from a standard 870-rad exposure and (B) from a standard 1,740-rad exposure of X-radiation.
NATURAL TL 31

TABLE 10. — Alkalic basalt natural TL peak-height averages for 

crushing stages 1 and 2

Sample Sample age Type of age Natural TL Natural TL Average TL 

No. (years B.P.) determination cr. st. I 1 cr. st. 21 cr. st. 1 and 22 

(cm/mg) (cm/mg)

023 4,500 "C 0.15 + 0.03

H71-10 ___ 62,000 K-Ar 3.44+ .16 

035 140,000 do. 4.84+ .27

H71-3 ____ 150,000 do. 20.71 ± .60 2 

036 153,000 do. 10.46+ .85 1

041 181,000 do. 7.57+ .86

042 184,000 do. 22.23+ 2.04 1

033 196,000 do. 21.97 + 2.04 2

0.13 ±0.04 0.14 + 0.03 

3.70+ .21 3.57 ± .13 

4.48+ .15 4.66 ± .15 

0.91+ .67 20.81 ± .44 

D.69+ .49 10.57 ± .49 

9.28 ± .97 8.42 ± .65 

9.00 ±2.37 20.61 + 1.61 

D.42±1.08 21.19 + 1.14

Molle____ ^I.SSXIO6 Est. 14.45+ .84 14.52 ± .64 14.48 ± .52 

KlpalOa __ ^I.SXIO6 do. 4.55± .18 4.67±1.23 4.59+ .61 

5X340 ___ 3.3X106 K-Ar 25.14 + 1.18 22.36±1.01 23.75± .78

'Errors are 1 standard deviation. 

2The cumulative error in the peak-height average for cr 

calculated with the error equation CV = '/2((CU2 , + CU2

go, 00,

** «t

K 1 

UJ <.

> DC 

<" ^

I -1 10 : -T.H71-10 /^ 

2i : * ^

£oc : /^

•*- UJ . ^^ X a. /^

< g ' /^ 

UJ OC s^

°- UJ 1 ^ _i H- 1 E ^

H 5 if 

^ p 3023

cc -z 

D uj H- 0

^^ 01 ——— ' ——— ' ——— ' ——— ' ' '

ushing stages 1 and 2 was 

)V4).

T.H71-3 033/

* ZSl*

03S^£°*2 

-fc- ^041

,-5-. ^035

Sample Sample 

No. weighting

023 3 

H71-10 1 

035 2 

H71-3 1 

036 2 

041 2 

042 2 

033 2

i i i i

0 40 80 120 160 200 

SAMPLE AGE, IN YEARS x10 3

FIGURE 14.— Alkalic basalt natural TL, 350° C peak, relative to 

sample age. The plotted points are the peak-height averages for 

crushing stages one and two, normalized for both sample weight 

and individual natural radiation dose rates. The horizontal error 

bars represent the uncertainty in the independently measured 

ages, where large enough to be shown; the vertical error bars represent the cumulative error in the normalized peak-height averages 

(table 11).

tended periods of time is monotonic and because it 

suggests that the rate of increase is approximately exponential. In most previous studies covering shorter 

periods of time, which have used quartz as the principal TL dosimeter, the increase in TL with age has 

been assumed to be linear (see for example Aitlfen, 

1970). This conclusion may be correct for the quartz 

from pottery, which is likely to be geologically old and 

to have a relatively high defect density, but it does not 

appear from the data available in this study to be applicable to geologically young basaltic plagioclase. 

The errors in the TL measurements and the dose 

rates and the cumulative sum of these errors calculated for each sample with a standard error propogation equation, given in table 11, are for most samples 

less than 10 percent.

TABLE 11. — Individual and cumulative errors in the energy-normalized peak-height average for the alkalic basalts

[Cr. st., crushing stage; cv, coefficient of variation]

Sample Cr. st. 1 Cr. st. 2 cucum Annual Sample Cumulative 

No. + cv ± cv (percent) dose weight error 

(percent) (percent) rate + cv (percent1 ) 

cv (percent) 

(percent)

023 20.1 31.3 18.6 2.45 4.0 19.2

H71-10 4.7 5.6 3.7 2.81 4.0 6.1 

035 5.5 3.4 3.2 2.13 4.0 5.5 

H71-3 2.9 3.2 2.2 3.11 4.0 5.5 

036 8.1 4.5 4.6 2.95 4.0 5.5

041 11.4 10.4 7.7 3.21 4.0 9.3

042 9.2 12.5 7.8 2.72 4.0 9.2

033 9.3 5.3 5.4 2.63 4.0 7.2

Molle 5.8 4.4 3.6 3.79 4.0 6.6 

Klpa lOa 3.9 26.4 13.3 10.88 4.0 17.6 

5X340 __ 4.7 4.5 3.3 6.14 4.0 8.0

'The cumulative error in the energy-normalized natural TL peak-height averages was 

calculated with the error equation ci>cum = (CI>TL cum2 + [Cl)dose rate2 ! + Cllwt2 ) •

The TL data for samples Mol le, Klpa lOa, and 

5X340 are not shown on the plot in figure 14, because 

they are much older than the rocks from the Hawi 

Volcanic Series and because their natural TL peakheight values are lower than the age of the samples 

and the TL-accumulation rate defined in figure 14 for 

alkalic basalts would predict. The dose rates for these 

older samples are lower than those for the younger basalts, but it is not certain that the lower TL amplitudes are entirely a function of the differences in dose 

rates between the two sample groups. It is probable 

that the older samples are affected to some extent by 

TL saturation, which probably begins to diminish the 

rate of increase in stored natural TL at an age of not 

much more than 200,000-300,000 years. The TL averages for both crushing stages of each sample are essentially identical, differing in most cases by less than the 

statistical error in either set of measurements. This 

similarity demonstrates that the stage 2 crushing does 

not measurably alter the trapped-electron population 

of the samples relative to crushing stage 1. It does not 

prove that there is no modification of TL during the 

initial crushing, but the fact that the natural TL peak 

heights for both crushing stages of each sample differ 

in the same direction for many samples by approximately the same percentage amount as the corresponding artificial TL (table 14A) strongly suggests 

that any changes are small. The proportionality of the 

natural and artificial TL would require that any significant drainage of electrons during crushing to be 

accompanied by an equivalent destruction of traps, 

and this is unlikely.

THOLEIITIC BASALT NATURAL TL

PROCEDURE AND RESULTS

Natural TL peak heights for the tholeiitic basalts 

given in table 12 for those samples for which TL was
32 THERMOLUMINESCENCE DATING OF HAWAIIAN BASALT

TABLE 12.—Tholeiitic basalt natural TL peak-height averages for 

crushing stages (cr. st.) 1 and 2

[n.d., not detectable]

Sample Sample age 

No. (years B.P.)

012

015

026

030

050

051

032

024

070

074

018

019

P71-13 ___ 

037

P71-4 

P71-3 ___ 

P71-5 _ __

16 

21 

69 

114 

~225

=^500 

2,500

3,740 

10,140 

> 10,140 

17,360

> 17,360 

327,000 

400,000 

403,000 

414,000 

447,000

Type of Natural TL1 Natural TL1 TL average 

age cr. st. 1 cr. st. 2 cr. st. 1 and 2Z 

determination (cm/mg) (cm/mg)

Historic 

do. 

do. 

do. 

do. 

do. 14C

"C

UC 

n.d. 

"C

n.d. 

K-Ar 

do. 

do. 

do. 

do.

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

0.33 ±0.02 

.26 ± .20 

.26 ± .03 

.20 ± .04 

.20 ± .03 

.28 ± .09 

.46 ± .05 

.47 ± .02 

.69 ± .06 

.36 ± .09 

.80 ± .08 

.17 ± .03 

2.52 ± .12 

3.20 ± .11 

10.43 ± .71 

2.06 ±.04

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

n.d. 

0.12 ±0.09 

.07 ± .05 

.09 ± .04 

.21 ± .12 

.38 ± .05 

.32+ .02 

.31 ± .17 

.28 ± .09 

.64+ .02 

.17± .03 

2.90 ± .03 

2.06+ .07 

12.32 ±1.34 

2.53 + 0.11

0.19 ±0.07 

.13± .05 

.14 ± .03 

.24 ± .08 

.42 ± .04 

.39 ± .02 

.50 ± .14 

.32+ .06 

.72 ± .06 

.17+ .02 

2.71 ± .13 

2.63 ± .07 

11.37 ± .73 

2.29 ± .05

'Errors are 1 standard deviation.

2The cumulative error in the peak-height average for crushing stages 1 and 2 was 

calculated with the same error equation used for the alkalic basalt (table 10).

Note: Samples 074 and 019 were collected below samples 070 and 018, respectively.

measureable show that at least 2,500 years is required 

for TL to accumulate to a level that is reliably distinguishable from ambient background and that reasonable statistical precision is not attained for rocks 

much younger than 10,000 years of age. The low TL 

signal is principally a function of the much lower internal radiation dose rocks for these rocks. What produces the poor precision in the measurements is not 

clear. Some of the TL variation within crushing stages 

may originate in the proportionately large influence of 

normal experimental errors on the small natural TL 

signal, but the variation appears to be too large to be 

entirely attributable to experimental errors. The variation in the TL properties of the aliquants does not 

appear to be related to sample size.

The peak-height data for samples 032 through 019 

display a consistent pattern of lower TL amplitudes 

for crushing stage 2 than for crushing stage 1. This 

may be an indication of TL drainage produced by the 

second stage relative to the first; it could be responsible for some of the TL variability observed within 

crushing stages. Why TL drainage should occur in the 

tholeiitic basalts, if it does, but not noticeably affect 

the alkalic basalts is not clear.

The significant increase in TL amplitudes from the 

second crushing stage relative to the first for several of 

the older tholeiitic basalts from the Pololu Volcanic 

Series (P71-13 through P71-5) cannot be explained at 

this time. It is almost certainly not the result of enhancement of the inherent natural TL signal; it could 

result from the release of a larger proportion of grain 

boundary material from the enclosing matrix during

the second crushing. The boundaries of the plagioclase crystals probably have higher natural TL intensities in these older samples because of a greater 

production rate for both TL and trap density in the 

near-surface regions of the crystals by short-range alpha particles from uranium and thorium atoms located in the matrix.

Where two TL values are listed in table 12 for each 

crushing stage of a particular sample, they represent 

separate determinative series starting with fresh 

pieces of basalt. These duplicate analyses were performed to evaluate the reproducibility of the TL measurements from these generally low TL tholeiitic 

basalts. For young samples, the reproducibility is generally poor, although the averages for crushing stage 1, 

in particular, do not vary irj many cases by more than 

the standard deviation error of the individual sets of 

measurements.

Peak-height averages for crushing stages 1 and 2 

were plotted for the four prehistoric carbon-14-dated 

basalts in figure 15. The two points for each of the

1.0

HI HI

II 1,

0.01

032 018

070

024

8 12 

SAMPLE AGE IN YEARS x10 :

16 20

FIGURE 15.—Tholeiitic basalt natural TL, 350°C peak, relative 

to sample age. The TL data have been normalized for variations in 

dose rate and sample weight. Except for sample 032, the points are 

the peak-height averages for crushing stages 1 and 2. Only the 

stage 1 crushing data is shown for sample 032. The two points for 

three out of the four samples are for separate determinative series. 

Vertical error bars represent the cumulative error in the dose rate 

and TL measurements (table 13). Horizontal error bars represent 

the uncertainty in the carbon-14 ages.
ARTIFICIAL TL 33

three out of four samples shown represent the crushing stage averages for the two separate determinative 

series for each sample. The error in the energy-normalized natural TL data, listed in table 13, exceeds 20 

percent in many cases.

DISCUSSION

The limited data on tholeiitic basalt shown in figure 

15 suggests that the rate of increase in TL with age in 

the tholeiitic basalts is exponential and similar to that 

identified for the alkalic basalts in figure 14. The rate 

of increase is lower than that for the alkalic basalts, 

however, possibly owing to the difference in dose 

rates.

The natural TL values for samples 074 and 019 are 

not shown in figure 15 because the samples have not 

been dated by independent means. The data for the 

older tholeiitic basalts (samples P71-13 through P71- 

5 in table 12) are not shown because their natural TL 

values are significantly lower than would be expected 

from their ages, dose rates, and the TL-accumulation 

rate for the carbon-14-dated basalts. These older samples are probably affected to some extent by TL and 

trap-density saturation. The time of appearance of 

saturation effects cannot be ascertained from the data 

available.

SUMMARY

The natural TL peak heights for both crushing 

stages for the alkalic basalts are closely similar, indicating minimal drainage of TL by crushing during 

sample preparation. The TL for these samples inTABLE 13.—Individual and cumulative errors in the energy-normalized peak-height average for the tholeiitic basalts

[Cr. st., crushing stage; cv, coefficient of variation]

Sample

No.

012

015

026

030

050

051

032

024

070

074

018

019

P71-13 

037

P71-4 

P71-3 

P71-5

Cr. st. 1 

± ci; 

(percent)

5.2

76.6

9.7

20.9

12.8

31.3

9.9

3.5

8.9

24.4

10.0

20.2 

4.8

3.3 

6.8 

2.0

Cr. st. 2 

± ci; 

(percent)

71.1

67.5

40.8

56.3

14.0

7.5

55.2

32.4

14.4

14.8 

8.2

3.6 

10.9 

4.2

. c"cuin (percent)

35.9

35.3

21.4

32.2

8.6

4.1

27.9

20.3

8.8

12.5

4.7

2.4 

6.4 

2.3

Annual 

dose 

rate 

ci; 

(percent)

6.30

6.11

13.10

9.85

6.21

9.37

4.72

3.44

7.59

10.35

10.30

11.44

38.89 

4.71

22.82 

11.53 

27.55

Sample 

weight 

± cv 

(percent)

4.0

4.0

4.0

4.0

4.0

4.0

4.0

4.0

4.0

4.0

4.0

4,0

4.0 

4.0

4.0 

4.0 

4.0

Cumulative 

error 

(percent1 )

36.1

36.1

22.7

33.1

13.5

11.1

29.7

22.8

14.4

40.8 

6.6

22.9 

13.2 

27.6

'The cumulative error in the energy normalized natural TL peak-height averages was 

calculated with the same error equation listed for the alkalic basalts in table 11.

creases at an exponential rate to at least 200,000 years 

B.P. The data for the tholeiitic basalts show that at 

least 2,500 years is required for measurable natural 

TL to accumulate and that reasonable measurement 

precision is not attained for samples much younger 

than 10,000 years of age. The rate of increase in TL for 

the tholeiitic basalts appears to be exponential but is 

slower than that for the alkalic basalts possibly because of the difference in dose rates.

ARTIFICIAL TL

Four distinct but variably overlapping TL peaks 

with maxima in the temperature ranges 90°-110°C, 

150°-175°C, 200°-225°C, and 300°-325°C occur in 

plagioclase that has been exposed to laboratory radiation at room temperature and heated for TL measurement a short time after irradiation. The 300° C 

peak could be clearly distinguished in every sample 

studied, but not all of the three lower temperature 

peaks could be resolved in every sample, probably 

owing to masking by more intense adjacent peaks. 

Only the 300° C peak was used to quantify differences 

in trap density between samples in this investigation.

Artificial TL glow curves for plagioclase from four 

alkalic and four tholeiitic basalts are shown in figures 

13A and 13R The only feldspar TL peak observed by 

other workers but not detected in this investigation is 

the one at approximately 450°C (Hoyt and others, 

1970; Brito and others, 1973; Durrani and others, 

1973), which is beyond the blackbody-saturation limit 

of the detection system used in this study.

ALKALIC BASALT ARTIFICIAL TL

PROCEDURE AND RESULTS

The 300° C artificial TL peak heights were measured both with and without filter CS5-60 for both 

crushing stages with the results given in table 14. The 

unfiltered TL data were not used for age calculation 

but are included for comparison. Attenuation factors 

given represent the ratio of unfiltered to filtered TL. 

Attenuation factors for samples H71-3 and 041 are artificially low because the unfiltered TL peaks for these 

samples were diminished by saturation of the TL detection equipment.

The artificial TL peak-height averages for samples 

023 through 033 in table 14A, normalized only for 

sample weight, were plotted against the age of the 

samples in figure 16. The line through the data is a 

computer-derived least-squares fit based on the same 

sample-weighting factors used for the natural TL (fig. 

14). The relatively close approximation of the data to 

a predictable curve indicates an apparently exponen-
34 THERMOLUMINESCENCE DATING OF HAWAIIAN BASALT

TABLE 14. — Alkalic basalt 300°C artificial TL peak-height averages for crushing stages (cr. st.) 1 and 2 measured with and 

without filter CS5-60 

A. Averages with filter CS5-60

Sample 

No.

023

H71-10

035

H71-3

036

041

042

033

Molle

Klpa lOa 

5X340

Sample age 

(years)

4,500 

62,000 

140,000 

150,000 

153,000 

181,000 

184,000 

196,000 

1.35X106 1.5X106 

3.3X10"

Artificial TL 

cr. st. I 1 

(cm/mg)

22.7 ± 0.9 

129.3 ± 3.0 

89.0 ± 3.4 

515.0 ±35.1 

221.5 ±19.3 

473.4 ±34.1 

306.6 ±11.0 

350.0 ±16.4 

186.3 ± 7.1 

20.5 ± 1.5 

227.8 ±13.6

Artificial TL 

cr. st. 2' 

(cm/mg)

18.2 ± 2.6 

134.1 ± 8.3 

80.8 ±12.6 

632.2 ±17.1 

227.6 ±11.2 

507.9 ±19.3 

300.4 ± 14.7 

306.0 ±18.7 

197.8 ± 5.9

Average 

artificial TL 

cr. st. 1 and 22

20.4 ± 1.5 

131.7+ 4.3 

84.9+ 6.8 

623.6 ±17.5 

224.5 ±11.2 

490.6 ±20.1 

303.5+ 9.4

328.0+12.5 

192.0+ 4.6

'Errors are 1 standard deviation.

'The cumulative error in the peak-height average for crushing stages 1 and 2 was 

calculated with the error equation listed in table 10.

B. Averages without filter CS5-60

[n.d., not determined]

Sample 

No.

023

H71-10

035

H71-3 

036

041

042

033

Molle 

Klpa lOa 

5X340

Artificial TL 

cr. st I 1 

(cm/mg)

n.d.

n.d.

n.d.

1,273.6 ±39.5 

n.d.

1,193.0 ±69.2

1,006.5 ±43.4

n.d.

738.0 ±28.8 

96.1 ± 5.7 

816.0 ±71.0

Artificial TL 

cr. st. 2' 

(cm/mg)

82.3+ 3.6

632.3 + 26.6

369.6+17.4

1,212.8 + 25.4 

828.9 + 34.8

1,222.7 + 33.0

1,001.3 + 39.0

1,088.7 + 65.3

741.4 ±34.8 

87.3 ± 5.0 

813.9 ±34.2

Average 

artificial TL 

cr. st. 1 and 22

1,242.7 ±23.6

1,208.5 ±38.7

1,003.9 ±29.1

739.7 ±22.9 

91.7 ± 3.8 

814.9 ±39.1

Attenuation 

factors3

. 4.52

. 4.72

. 4.57

2.07 

. 3.64

2.52

3.28

. 3.56

3.96 

4.68 

3.53

'Errors are 1 standard deviation.

2The cumulative error in the peak-height average for crushing stages 1 and 2 was 

calculated with the error equation listed in table 10. 

'Attenuation factors are the ratio of the unfiltered to filtered artificial TL.

tial rate of increase in the storage capacity of progressively older samples for the electrons that produce TL 

in the 3,500-A to 5,000-A bandwith. A possible explanation for the increase in trap density is that although 

natural radiation produces defects at an approximately constant rate, as the number of defects increases, 

interlocking reduces the probability for spontaneous 

annihilation. The net result would be a faster-thanlinear increase in the aggregate defect population.

DISCUSSION

There is a close correlation between the proportional amplitudes of the natural and artificial TL peak 

heights (fig. 14 and 16) for all the alkalic basalts studied except one (sample 041). This relation suggests 

that in each case the level of natural TL storage was 

near the maximum storage capacity for a sample of 

that age; further significant increases in TL would require an increase in trap density. If this is correct, it 

could explain the observed exponential rate of increase in natural TL.

For sample 041, the artificial TL peak height is 

anomalously high relative to the natural TL peak

2 103

LU UJ

Q_ 0.

o

102

10

041

K-H

035

023

40 80 120 160 

SAMPLE AGE IN YEARS x103

200

FIGURE 16.—Alkalic basalt 300°C peak artificial TL relative to 

age. Plotted points are the peak-height averages for crushing 

stages 1 and 2, normalized only for sample weight. Horizontal error 

bars represent the uncertainty in the sample ages where large 

enough to be shown.

height for a sample of its age and dose rate. The reasons for this anomaly are not clear. It could be produced by a slight shift in the emission for the artificial 

TL relative to the natural TL; 041 has the largest percentage of 4,500-A artificial TL peak emission of all 

the samples analyzed.

The data for samples Mol le, Klpa lOa, and 5X340 

were excluded for the same reason they were omitted 

from the natural TL data (fig. 14); the TL values were 

lower than expected, in this case possibly because of 

trap-density saturation.

The attenuation factors in table 14B tend to decrease with increasing age among the samples studied 

because the proportional amplitude of the 4,500-A 

emission peak increases progressively with age. The 

trend confirms the change in the 4,500-A peak with 

age that was initially described during the spectrum 

analysis experiment.

THOLEIITIC BASALT ARTIFICIAL TL

PROCEDURE AND RESULTS

The 300° C artificial TL peak-height averages were 

measured both with and without filter CS5-60 with 

the results given in table 15. The precision of the artificial TL measurements is no better in most cases than 

that for the corresponding natural TL, even though 

the artificial TL for these samples is considerably 

more intense, which should reduce the influence of 

measurement errors. This suggests that much of the 

observed variability is produced by inhomogeneity in 

the TL properties of the plagioclase composing each 

aliquant. Attenuation factors for tholeiitic basalts (table 15B) are similar to those described for the alkalic 

basalts.
ARTIFICIAL TL 35

TABLE 15. Tholeiitic basalt 300° C artificial TL peak-height averages for crushing stages (cr. st.) 1 and 2 measured with and 

without filter CS5-60

A. Averages with filter CS5-60

Sample Sample age Artificial TL Artificial TL Average 

No. (years) cr. st. 1' cr. st. 2' artificial TL 

(cm/mg) (cm/mg) cr. st. 1 and 22

012 13 60.0 + 2.5 74.6 + 7.2 68.3 + 3.6

015 15 15.0 + 0.7 16.4 + 0.9 15.7 + 0.5

026 66 14.5 + 1.1 11.8 + 0.4 13.1+0.6

030 114 1.0+0.1 .8 + 0.2 .9 + 0.1

050 =225 4.8+0.2 2.9 + 0.3 3.8 + 0.2

051 =500 .5 + 0.1 .4 + 0.1 .4+0.0

032 2,500 .3 + 0.1 .5 + 0.0 .4 + 0.0

.3 + 0.1 .2+0.0 .2 + 0.0

024 3,740 .2 + 0.0 .2 + 0.0 .2 + 0.0

070 10,140 .9 + 0.1 .9 + 0.3 .9 + 0.2

074 > 10,140 9.3 + 1.2 6.8 + 0.3 8.0 + 0.5

10.3 + 0.7 10.3 + 0.6 10.3 + 0.4

018 17,360 2.7 + 0.8

3.5 + 0.3 2.4 + 0.3 2.9 + 0.2

019 > 17,360 13.7 + 2.0 14.3 + 1.4 14.0+1.2

P71-13 327,000 3.2 + 0.4 2.6 + 0.1 2.9 + 0.2

037 400,000 8.1 + 1.7 9.2 + 0.4 8.6 + 0.9

P71-4 403,000 5.6+0.3 3.9 + 0.1 4.7 + 0.1

P71-3 414,000 11.3+'0.8 12.8 + 1.6 12.0 + 0.8

P71-5 447,000 ^7.1+0.3 9.3 + 0.4 8.2 + 0.3

'Errors are 1 standard deviation. 

'The cumulative error in the peak-height averages calculated with the error equation 

listed in table 10.

B. Averages without filter CS5-60

[n.d., not detected]

Sample Artificial TL Artificial TL Average Attenuation 

No. cr. st. 1' cr. st. 2' artificial TL factors3 

(cm/mg) (cm/mg) cr. st. 1 and 22

012 137.2 + 6.7 155.0 + 12.7 146.1 + 7.0 2.14

015 72.6 + 5.2 76.2 + 5.3 74.4 + 3.6 4.74

026 67.3 + 5.6 50.4 + 2.5 58.8 + 2.8 4.48

030 n.d. n.d.

050 29.1 + 7.4 25.3(one

measurement)

30.0 + 3.9 28.9 + 4.2 29.4 + 2.6 7.61

051 n.d. n.d.

031 n.d. n.d.

024 n.d. n.d.

070 4.8 + 0.5 3.6 + 0.7 4.2+0.4 4.32

074 51.7 + 0.5

67.1 + 4.4 62.2 + 3.0 64.6 + 2.6 6.47

018 10.6 + 2.1 12.1 + 2.4 11.3 + 1.6

17.4 + 0.9 12.9 + 2.1 15.1 + 1.3 5.12

019 103.2+12.9 92.7 + 7.9 97.9 + 7.4 7.00

P71-13 ^ 13.8 ±0.8 13.5 ±0.8 13.6 + 0.6 4.77 

037 32.6 + 2.9 33.2 + 4.5 32.9 + 2.7 3.79

P71-4 25.5 ±1.6 17.5 ±2.1 21.5 ±1.5 4.51 

P71-3 28.7 ±1.3 35.8 + 3.2 33.2 ±1.6 2.68 

P71-5 ____ 28.8±2.4 37.0 + 2.1 32.9±1.7 3.99

'Errors are 1 standard deviation. 

'The cumulative error in the peak-height averages was calculated with the error equation listed in table 10. 

"Attenuation factors are the ratio of the unfiltered to filtered TL.

Artificial TL amplitudes were plotted relative to 

age (fig. 17) for the four prehistoric carbon- 14-dated 

tholeiitic samples whose natural TL peak heights were 

plotted in figure 15. Four samples is too limited a data 

base for broad conclusions, but the rate of increase in 

peak amplitude with age appears to be approximately 

exponential. 

The slope of a best-fit line through the artificial TL 

data in figure 17 (not shown) would be steeper than 

that through the natural TL data (fig. 15), possibly 

because of differences in the effect of short-term electron storage (artificial TL) relative to long-term stor-

^

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to age. 

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Thetw 

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Alt 

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- K^070

x^032 

- 032^fo24

1 1 1 1 1 1 1 1 1 1

1 4 8 12 16 20 

SAMPLE AGE IN YEARS x103

IRE 17.— Tholeiitic basalt 300°C peak artificial TL relative 

Plotted points are the peak-height averages for crushing 

1 and 2, normalized only for variations in sample weight. 

ro values for sample 032 represent data for the two separate 

linative series. Vertical error bars represent the error in the 

eight averages and the horizontal error bars the reported 

ainty in the carbon-14 ages.

atural TL) on TL emission. It is unlikely that it 

3 that trap density actually increases more raplan stored TL, since per decay event there is a 

>r probability for ionization than for defect prom. The relation is therefore likely an artifact of 

:perimental procedure, 

lough the historic tholeiitic basalts did not emit 

ireable natural TL, most of the samples did emit 

nal TL after excitation with X-radiation. The ar1 TL peak-height relations reveal significant inition concerning the structural reorganization 

ipparently occurs in plagioclase crystals during 

nst few hundred years following eruption. ArtifiL data for samples 012 through 051 (table 15A), 

18, show the peak heights to decrease progreswith age to a minimum somewhere in the neighod of 500 years after eruption. This decrease in 

enotes a gradual decrease by annealing of the 

number of primary defects incorporated in the 

>clase during crystallization from the melt. Exe to the low levels of internal natural radiation 

jrobaby causes a gradual increase in the defect 

ation from the minimum with increasing age 

7) until saturation is attained at an age of per-
36 THERMOLUMINESCENCE DATING OF HAWAIIAN BASALT

1U

oc

METERS PERMIL

O

JNCENTI

f2i

C3

UJ

I 

X 1

UJ 

Q.

I- 

-1

O

\L

oc 

n 1

0012 -I

-I 

-I

I 

-I 

0\015 

I 0 026

\

: \

: \

I \

\ °050

\

\

\

\

\ 

: oao o N x

"~ X,

°051

_

l l l i l i I i i i

12 24 36 48 

SAMPLE AGE IN YEARS x10'

60

FIGURE 18.—300°C artificial TL peak-height averages for the historic tholeiitic basalts.

haps 200,000 to 300,000 years. Note that the defect 

density in tholeiitic basalt plagioclase to 500,000 years 

old is only a fraction of that which exists in newly 

formed crystals.

The poorly defined increase in the attenuation factors with age for the historic samples (012 through 

050, table 15B) provides yet another indirect measure 

of the progressive decrease in defect density with age 

in the young samples. Since both the artificial TL 

peak heights and trap density decrease with age, it is 

likely that the increase in attenuation factors is largely produced by shifts in the spectral composition of 

the TL. The progressive decrease in the attenuation 

factors for the prehistoric samples (032 through P71- 

5) signifies a progressive increase in the trap density 

of the samples from natural radiation.

SUMMARY

The artificial TL peak heights for the alkalic basalts 

increase at an exponential rate with age, apparently 

indicating an exponential rate of increase in trap density with time. The artificial TL peaks are closely proportional in relative magnitude to the corresponding 

natural TL peaks, which suggests that natural TL for 

each sample is near the maximum long-term stable 

storage capacity for the particular trap density; further significant increases in TL are dependent on increases in trap density.

The artificial TL for the tholeiitic basalts also appears to increase exponentially but at a lower rate 

than for the alkalic basalts. The data for the historic 

samples reveals a rapid annealing of primary defects 

to a minimum achieved within approximately 500 

years after eruption. The defect density then increases slowly from the effects of natural radiation but 

even after 500,000 years does not equal the trap density that exists in newly formed crystals.

TL DATING AND ANALYSIS OF ERRORS

The formulation of a TL-dating method requires 

that differences in natural TL caused by variations in 

trap density, natural radiation dose rate, and TL spectral composition between samples be accurately compensated. The development of a normalizing 

procedure for natural TL will first be undertaken for 

the alkalic basalts because a more uniform distribution of material over an extended age range is available from these than for the tholeiitic basalts. 

Moreover, the lower measurement errors for the various quantities make the values for the alkalic basalts 

more reliable. The normalized natural TL will be used 

to define an equation that should be suitable for assigning ages to other samples of similar chemical composition but unknown age. The technique will then be 

applied to the tholeiitic basalts.

TL DATING OF ALKALIC BASALTS

The variables affecting measured natural TL amplitudes are age, dose rate, trap density, and TL spectral composition. In this experiment, sample ages are 

known, dose rates have been calculated, and trap density is represented on a relative basis by artificial TL. 

The only variable not directly measured for all samples is TL spectral composition. Compensation for 

this variable is automatically accomplished during 

calculation of the TL ratio, however, the spectra for 

both natural and artificial TL emission, being produced by the same traps and recombination centers, 

should be closely similar. If they were not, the close 

proportionality of the natural and artificial TL peak 

heights (figs. 14 and 16) would not exist.

Differences in the variables for the alkalic basalts 

were treated linearly because the differences were relatively small and no consistent departure from linear-
TL DATING AND ANALYSIS OF ERRORS 37

ity could be detected. This approach would probably 

not be applicable for a group of samples with a significantly wider range in chemical and TL properties such 

as would be encountered in the development of a single TL equation for both the alkalic and tholeiitic basalts.

PROCEDURE AND RESULTS

The normalized natural TL ratio (r) used in this investigation for TL dating is a dimensionless number 

defined by the equation

r = k (NEa/AEn)

where N is the average natural TL peak height (crushing stages 1 and 2, En the annual dose rate, and A, the 

average 300° C artificial TL peak height (crushing 

stages 1 and 2) normalized to a standard X-radiation 

exposure base (Ea). The constant k is 10~2 years. This 

numerical value 10~2 is required to shift the decimal 

point in the TL ratio so that the loge r is a positive 

number; the unit years is required to cancel the unit 

years in the TL ratio that results from En being in 

units of rads per year while Ea is in rads. The ratio 

represents natural TL normalized for all pertinent intrinsic variables.

The TL ratio is not original in general form to this 

investigation; the basic concepts embodied in an N/A 

ratio were probably first advanced during early attempts to apply TL dating to archeologic artifacts at 

Oxford University (Aitken and others, 1964). The ratio has been used widely (Aitken, 1970; Fleming, 1970; 

Hwang, 1972; Goksu and others, 1974), but as employed by these workers for "absolute" TL dating, the 

context in which the expression is used, and the meaning of the constituent parameters, differs significantly 

from that defined in this investigation.

The TL ratios calculated for alkalic basalt samples 

023 through 033 are given in table 16. The error listed 

for each ratio was calculated with the equation

where ar' is the fractional error in the TL ratio when 

the errors in the other quantities are expressed as 

fractional errors. The equation is derived in the section on derivation of equations at the end of this report.

In the plot of TL ratios relative to sample age, figure 

19, samples Mol le, Klpa lOa, and 5X340 were not included, because they are probably affected by saturation, as discussed. The uniform increase in the 

amplitudes of the ratios when plotted in terms of increasing age in figure 19 is a compelling indication of 

both the systematic nature of the TL process and the

inherent reliability of the TL ratio. The equation for 

the best-fit line through the data set, excluding sample 041, was calculated with the double-regression 

York II least-squares cubic fit analysis (York, 1969). 

The equation for the line is

r = debt

where r is the TL ratio, t sample age, b the slope, and a 

the y-intercept. The slope is 1.0056X10-5 ± 0.0422X 

10-5 , the y-intercept 2.146 ± 0.129. Both errors are 

one standard deviation. The final TL age equation is

r = 2.146l- 0056 *1(r5£

or, expressed in a more convenient form for age calculation.

t = 9,947 (loger 0.7637).

TABLE 16.—Alkalic basalt TL ratios, TL ages, and calculated 

errors

Sample 

No.

023

H71-10

035

H71-3

036

041

042

033

Sample age 

(years, B.P.)

4.500 ± 110

62,000-1-2,000

140,000 + 4,000

150,000-1-2,000

153,000 + 7,000

181,000 + 3,000

184,000 + 3,000

196,000 ±7,000

TL ratio (r1 )

2.41 + 0.60

5.77+1.88

13.45 + 2.07

11.92 + 2.04

13.37+1.86

7.46+1.66

18,97 + 3.83

21.53 ±4.21

TLage

(years2 )

-12,000 + 23,500

70,000 + 14,350

150,000 + 14,500

139,000+16,000

150,000+13,000

95,000 + 21,000

183,000 + 19,000

195,000 ±18,500

'Error calculated with the error equation derived in the section at end of report on 

derivation of equations. 

2 t = 99,447 (loggr - 0.7637), where t is sample age.

20

10

9

w 8

^041

023

40 80 120 160 

SAMPLE AGE IN YEARS x103

200

FIGURE 19.—Alkalic basalt TL ratios plotted in relation to sample 

age. The equation for the best-fit line through the data represents the TL age equation. Horizontal error bars represent the 

uncertainty in the independently determined sample ages 

where large enough to be shown; the vertical error bars represent the error in the TL ratios.
38 THERMOLUMINESCENCE DATING OF HAWAIIAN BASALT

The generalized form of the equation defines the basic 

properties of the TL ratio for alkalic basalt plagioclase. The specific form is applicable only to TL ratios 

determined in the same analytical fashion with the 

same equipment as that used for the measurements 

employed in the derivation of the equation. Changes 

in either equipment or experimental technique will almost certainly result in a modification of the slope and 

intercept values for the equation. The precision of TL 

ages calculated with the age equation can be estimated with the preliminary error equation.

where <rt' is the fractional error in the TL age. The error equation is derived in the section on derivation of 

equations at the end of this report. The equation does 

not include the statistical error in the best-fit line itself, because at this preliminary stage in the development of the method only the errors in the TL data are 

being considered. The error in the best-fit line itself, 

which includes the error in the potassium-argon ages 

as well as those in the TL ratios, is more properly in 

the realm of accuracy, as distinct from precision. A 

more comprehensive error analysis designed to include errors in the best-fit line will eventually have to 

be made, however, before routine assignment of TL 

ages to rocks can begin.

The TL ages calculated with the age equation for 

the individual samples used in its derivation differ by 

less than ± 10 percent from the respective potassiumargon ages, with the exception of samples 023 and 041. 

The TL ages, though not strictly independent because 

each of the ratios was used in the derivation of the 

equation, provide a preliminary indication of the 

probable degree of concordance of TL and potassiumargon ages. The statistical errors in the TL ages determined on the basis of the errors in the TL ratio can be 

seen from table 16 to average approximately ±10 percent for rocks older than about 50,000 years of age. Reduction of this error is feasible, but 5 percent is likely 

to be the minimum error attainable in routine TL age 

determinations; errors closer to ± 10 percent will 

probably prove more common.

DISCUSSION

The exponential rate of increase in the TL ratio (r) 

with age is a critical finding. The positive slope of the 

best-fit line indicates that the increase in trap density 

with age results in an even faster increase in natural 

TL storage capacity, possibly by an increased initial 

trapping probability for conduction band electrons. 

The uniform rate of increase in the TL ratios with age,

when compared with the scatter in both the natural 

and artificial TL data alone, demonstrates the effectiveness of the TL-normalizing procedure for canceling differences in natural TL due to differences in 

trap density between samples. It also indicates that 

small differences in plagioclase composition, at least 

within the range from sodic oligoclase to andesine (fig. 

4), do not have a detectable systematic effect on the 

ratio.

The close proportionality of the natural and artificial TL peak amplitudes for individual samples 

strengthens the hypothesis that natural TL storage is 

near maximum capacity. Since the artificial TL is significantly more intense than the corresponding natural TL, the process of short-term electron trapping 

and storage can at least for the sake of discussion be 

conceptualized as a metastable clustering of a number 

of electrons around trapping centers capable of the 

stable retention of probably only one electron.

Sample 041 was not included in the age-curve calculation, because the large difference between the natural and artificial TL amplitudes for the sample results 

in a TL ratio with a magnitude of about half the value 

appropriate for a sample of its age. The natural TL 

amplitude for sample 041 is approximately correct 

(fig. 14), whereas the artificial TL appears anomalously high (fig. 16). The reason for the difference is 

not clear; the possibility of its being due to a slight 

shift in emission spectra has been proposed here. 

Whatever the cause, the disparity of the two TL values and the anomalously low ratio were deemed adequate grounds for omitting the sample from the agecurve derivation. Comparable TL differences in samples of unknown age could be used as a basis for rejection of such samples as unsuitable for TL dating. The 

simple existence of a large difference in values is not 

sufficient grounds for rejection, however, since the 

difference in the natural and artificial TL values could 

possibly occur in a sample 100,000 years old, the approximate age obtained for sample 041 with the TL 

ratio. At this preliminary stage in the development of 

the method, some geologic control would be required 

for a reliable assessment of validity.

TL SATURATION AND THE 

AGE LIMITS OF THE METHOD

Samples Mol le and 5X340,1.5 and 3.3 m.y. old, respectively, have TL ratios that are significantly lower 

than expected; the ages obtained for these two samples with the TL age equation are 225,000 and 300,000 

years, respectively. Although both samples have annual dose rates that are lower than the average for the 

younger basalts, it is almost certain that the low dose 

rates are not entirely responsible for the erroneous
TL DATING AND ANALYSIS OF ERRORS 39

ages. The samples appear to be affected by both TL 

and trap-density saturation. Sample Klpa lOa was not 

included in the age analysis, because it is a transitional alkalic basalt with chemical and TL properties 

closer to the tholeiitic basalt group.

Saturation is probably initially manifest in a decrease in the rate of accumulation of defects. The specific cause of this approach to a limiting defect density 

has not been identified but is probably related to 

spontaneous annealing. The amount and rate of annealing defined by the artificial TL data for historic 

basalts (fig. 18) indicates a greater defect mobility 

than has been generally thought to occur in minerals 

at ambient surface temperatures. It suggests that 

spontaneous annealing probably occurs continuously, 

degrading a small percentage of the radiogenically 

produced defect population. Theoretically, at a high 

defect density, a small percentage annealing rate 

could produce a number of annihilations equal to the 

rate of defect production; the equivalence of the two 

rates would define saturation. Attenuation of the defect accumulation rate is expected to cause a proportionate decrease in the TL accumulation rate, and TL 

saturation should follow defect saturation fairly 

closely.

The defect density at which saturation occurs 

should be largely dependent on the natural dose rate. 

There is probably also a specific relation between dose 

rate and the length of time required for the saturation 

effects to appear, but whether an increase in dose rate 

is accompanied by an increase or a decrease in the 

elapsed time to saturation is not clear.

The age at which saturation effects begin to appear 

in the alkalic basalts cannot be determined from the 

data available. The TL ratios for samples Mol le and 

5X340 are clearly attenuated, but the ratio for the oldest of the young alkalic basalts, sample 033, is not. The 

TL ages for samples Mol le and 5X340 suggest that 

the upper age limit for the TL method may be as 

young as 250,000 to 275,000 for rocks of their approximate composition and dose rate.

Estimates of an upper limit for TL dating by other 

workers are few and inconclusive. They include an estimate by Sabels (1963) that TL would prove useful 

for dating lavas to several tens of thousands of years of 

age. Goksu and others (1974) anticipated the extension of their preliminary TL method for pyrolyzed 

chert to more than 100,000 years.

A lower limit for the TL method is specified by the 

age at which natural TL first becomes measurable 

with acceptable precision. For the ocean island alkalic 

basalts, this would probably be at an age of about 

2,000 years. For continental rocks with higher dose 

rates, the lower limit may be as young as 1,000 years.

TL DATING OF THE THOLEIITIC BASALTS

An attempt to use the limited data available for the 

tholeiitic basalts to establish a unique relation between TL and sample age similar to that developed for 

the alkalic basalts was not successful, as the age distribution of dated material available is limited and the 

statistical precision for both the TL measurements 

and the supporting data was generally poor. A systematic relation between natural TL and sample age exists, however, and future work involving samples of 

known age in the 10,000- to 200,000-year range should 

produce a reliable TL-dating method for these rocks.

PROCEDURE AND RESULTS

Natural TL peak-height averages for samples 

024,070, and 018, shown in figure 20, indicate a uniform rate of increase in natural TL with age that appears to be more rapid than that for the alkalic 

basalts. The natural TL data for samples 074 and 019 

are included in figure 20 at positions on the estimated 

best-fit line specified by their natural TL peak 

heights. Nether 074 or 019 has been independently 

dated, but both were collected from flows in known 

stratigraphic relation to dated flows. The apparent

10

5

cc 111

Q. -£ w

s tt

(E ui

si

(3 ui

< 0

as s

30

FIGURE 20.—Tholeiitic basalt natural TL peak-height averages for 

the carbon-14-dated prehistoric basalts. The data are the 

average of the peak-height values for the two separate TL series, where available, normalized for sample weight and annual dose rates. The estimated best-fit line is based on the data 

for samples 024, 070, and 018; the data for undated samples 

074 and 019 are included in the graph at positions specified by 

their natural TL peak-height averages.
40 THERMOLUMINESCENCE DATING OF HAWAIIAN BASALT

TL age of approximately 20,000 years for sample 074, 

collected from the lava flow stratigraphically below 

the flow from which sample 070 was obtained and separated from it by about 1 meter of volcanic ash from 

the widespread Pahala Ash (steams and Macdonald, 

1946) is geologically reasonable. Sample 074 occupies 

approximately the same stratigraphic position relative to the ash as sample 018. The two flows could be 

of similar age if the ash is coeval at the two sampling 

locations. The apparent TL age of approximately 

21,000 years for sample 019, collected either one or 

two flows (field relations were obscure) below the flow 

from which sample 018 was obtained, is also reasonable. The accuracy of the apparent TL ages for these 

two samples cannot be evaluated at this time; the plot 

of peak-height averages for tholeiitic prehistoric carbon-14-dated basalt (fig. 20) was prepared primarily 

to demonstrate that natural TL for the basalts appears to increase rather uniformly with age. Unnormalized natural TL is too dependent on variations in 

the other TL parameters to be of significant value 

alone for reliable age determination, although accuracy to within a factor of two, which the natural TL 

data may be capable of providing with some reliability, could be valuable for some applications.

To determine the trend of NEa/AEn ratios for the 

tholeiitic basalts with age, these ratios were plotted 

relative to sample age in figure 21. Samples 074 and 

019 are included in the figure at sample ages specified 

by their natural TL peak amplitudes (fig. 20). The ratios for the older tholeiitic basalts from the Pololu 

Volcanic Series, included in the figure even though 

they are probably affected by saturation, serve to illustrate in general the increasing trend that should be

Ir 10.0

CD 

O

= 1.0

UJ

P71-4

o024

-o070 

3018

D 074 

D 019

037

80 160 240 320 

SAMPLE AGE IN YEARS x103

400 480

FIGURE 21.—Tholeiitic basalt NEa/AEn ratios plotted relative to 

sample age. Samples 074 and 019 are included at the tentative 

sample ages indicated by their natural TL peak heights (fig. 

20).

followed by the ratios for younger rocks as they increase in age.

DISCUSSION

The progressive decrease in the NEa/AEn ratios for 

the tholeiitic basalts to an age of greater than 20,000 

years is an unexpected result. This trend is produced 

by an apparently more rapid rate of increase in artificial TL (trap density) than natural TL and as discussed earlier, may possibly be an artificial relation 

caused by the experimental procedure. A study of the 

TL properties of several of these same prehistoric carbon-14-dated flows using unfiltered TL by Berry 

(1973) led to the identification of a progressive increase in the TL ratios with age. In any case, the trend 

of decreasing ratios appears to reverse itself at around 

20,000 years. The ratios for samples older than approximately 20,000 years should increase with age, as 

suggested by the magnitude of the ratios for the older 

samples from the Pololu Volcanic Series.

The predictable increase in both the natural and artificial TL for the tholeiitic basalts suggests that a reliable TL-dating method for these rocks is probably 

feasible but would require considerable further research incorporating a wider age distribution of dated 

material. These rocks cannot be directly dated with 

the age equation for the alkalic basalts because the 

difference in dose rates and feldspar composition between the two rock types produces a significant nonlinear difference in the rate of TL and trap-density 

accumulation.

SUMMARY OF THE TL-DATING

PROCEDURES AND PREDICTIONS

FOR FUTURE APPLICATIONS

The preliminary TL-dating method defined in this 

investigation for Hawaiian alkalic basalts is capable of 

age resolution over the age range from 2,500 to perhaps 250,000 years B.P. with a precision for rocks 

older than about 50,000 years of approximately ± 10 

percent. The TL ages have a potential estimated accuracy of about ± 10 percent relative to potassium-argon ages.

The method was developed using plagioclase separates from rocks of independently determined age as 

the TL dosimeter. The separates range in average 

composition from oligioclase to andesine and have 

K2O contents that range from 2.3 to 3.3 weight percent. The whole-rock uranium contents of the samples 

range from 1 to 2.6 ppm, thorium from 2.8 to 7.6 ppm. 

Annual radioactive dose rates calculated on the basis 

of assumed secular equilibrium and exclusive of the 

cosmic-ray energy dose, and with an assumed TL-producing efficiency for alpha particles of 0.1 of the tabulated total alpha-particle energy, range from 0.240 to
REFERENCES CITED 41

0.500 rad/yr and average 0.350 rad/yr.

The TL measurements were made using 4- to 7-mg 

aliquants of plagioclase of a 37- to 44-^m size range 

sealed in a light-tight sample chamber in which the 

atmosphere was adjusted during TL measurement to 

exclude oxygen. Heating was at a rate of 20° C s- 1 by 

infrared radiation focused on the bottoms of the sample planchets. TL was detected with an EMI 6255S 

"Q" photomultiplier tube coupled to an SSR model 

1105/1120 photon-counting system. The TL was filtered with a Corning filter number CS5-60 to restrict 

the TL detected to that from the 4,500-A plagioclase 

emission peak. This spectral peak is apparently 

caused by a specific but as yet unidentified structural 

defect rather than an impurity element in a substitutional or intersitial lattice position.

Thermoluminescence is basically a three-dimensional phenomenon that can be depicted spatially by 

three orthogonal axes representing light intensity, activation temperature, and spectral composition. Plagioclase natural TL consists of a signal peak at about 

350°C; the artificial TL from laboratory irradiations 

constituted four overlapping peaks with maxima 

about 110°, 150°, 225°, and 300°C. The light from 

each of these TL peaks has a characteristic spectral 

composition of three broad peaks with maxima at 

about 4,500 A, 5,600 A, and 7,200 A. The relative intensities of the spectral peaks appear to vary between 

the different temperature peaks.

Both the 350°C natural TL peaks and 300°C artificial TL peaks appear to increase approximately exponentially in amplitude with increasing sample age. 

The increase in artificial TL is an indication of an increase in the aggregate number of crystal-defect-associated electron traps produced by internal radiation. 

The slightly faster rate of increase in natural TL is an 

indication of a rapid filling of these traps by excited 

electrons, and perhaps of an increased retention (retrapping?) probability for electrons in samples with 

progressively larger trap populations. The natural to 

artificial TL (N/A) ratios for the samples analyzed increase uniformly at an exponential rate with age; the 

equation for the best-fit line through the data represents a "relative" TL age equation that can be used to 

assign ages to other undated samples of similar chemical composition.

The TL method should be applicable to volcanic 

rocks similar in bulk composition to the alkalic basalts 

over the age range approximately 2,500 to at least 

250,000 years. Volcanic rocks of continental origin 

should be particularly well suited to TL dating because they typically have higher radioactive-element 

contents than Hawaiian rocks of equivalent composition.

The TL method for the alkalic rocks cannot at this 

time be used to directly date the ocean island tholeiitic basalts, because the average dose rate for the tholeiitic basalts is lower than that for the alkalic basalts 

by a factor of two and produces natural TL intensities 

in the tholeiitic basalts that are lower than those in 

the alkalic basalts for samples of equivalent age by a 

nonlinear factor. An attempt to develop an independent method using tholeiitic basalts of known age and 

the same general technique developed for the alkalic 

basalts was not successful, primarily because of an inadequate age distribution of dated material and poor 

precision in the TL and dose-rate measurements. 

Both natural and artificial TL appear to increase 

rather uniformly with age at an approximately exponential rate, however, and a reliable TL-dating method for these rocks should eventually be feasible over 

the age range of approximately 10,000 years to at least 

100,000 years.

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44 THERMOLUMINESCENCE DATING OF HAWAIIAN BASALT

SUPPLEMENTAL INFORMATION

SAMPLE LOCATION AND AGE DATA

ALKALIC BASALTS 

[Listed in order of increasing age]

Sample No.: 023

Source volcano: Mauna Kea Volcano; Hale Pohaku flow.

Rock type: Nominally an alkalic olivine basalt; precise petrologic 

type not determined.

Collected by: R. May, 1972; recollection of a previously dated flow.

Location: 19°43'54"N., 155°26'50"W.; collected from flow surface 

outcrop near the unimproved road from Hwy. 20 to the Hale Pohaku Ranger Station, about 0.6 km SE. of Puu Hookomo; Puu 00 

7 W quadrangle (1956); elevation: 2.25 km.

Age information: 14C age of 4,000 ± 110 years B.P. on carbonaceous 

ash from the top of the Humuula palesol, beneath the Puu Kole 

flow, which has been determined to be essentially coeval with the 

Hale Pohaku flow on the basis of field relation (Porter, 1971).

Sample No.: H71-10

Source volcano: Kohala Volcano; Hawi Volcanic Series.

Rock type: Mugearite.

Collected by: lan McDougall and Don Swanson, 1971.

Location: 20°07'10"N., 155°34'45"W.; collected from a roadcut

about 1 km west of Kukuihaele, Kukuihaele IVz' quadrangle

(1957); elevation: 300 m. 

Age information: Whole-rock K-Ar age of 62,000 ±2,000 years B.

P. (McDougall and Swanson, 1972).

Sample No.: 035

Source volcano: Kohala Volcano; Hawi Volcanic Series.

Rock type: Trachyte.

Collected by: R. May, 1972; recollection of a previously dated flow.

Location: 20°04'30" N., 155°45'50" W.; collected from a prominent 

roadcut about 11.5 km from the Kamauela Post Office on Hwy. 

25, and about 1.1 km WSW. of Puu Loa, Kawaihae 7 ¥2' quadrangle (1956); elevation: 1.04 km.

Age information: Duplicates sample GA 1378, for which a wholerock K-Ar age of 140,000 ± 4,000 years was obtained (McDougall, 

1969).

Sample No.: H71-3

Source volcano: Kohala Volcano, Hawi Volcanic Series.

Rock type: Mugearite.

Collected by: lan McDougall and Don Swanson, 1971.

Location: 20°12'20" N., 155°44'00" W., collected from a flow partly 

filling Pololu Valley, Honokane 1W quadrangle (1957); elevation: 37 m.

Age information: Whole-rock K-Ar age of 150,000 ±2,000 years 

B.P. (McDougall and Swanson, 1972).

Sample No.: 036

Source volcano: Kohala Volcano; Hawi Volcanic Series.

Rock type: Hawaiite.

Collected by: R. May, 1972; recollection of a previously dated flow.

Location: 20°04'45" N., 155°46/00// W.; collected from a roadcut

about 12.3 km from Kamauela Post Office on Hwy. 25, near

Keawewai Stream, Kawaihae 1W quadrangle (1956); elevation: 

1.06 km.

Age information: Duplicates sample GA 1377, for which a wholerock K-Ar age of 153,000 ±7,000 years B.P. was obtained 

(McDougall, 1969).

Sample No.: 041

Source volcano: Kohala Volcano; Hawi Volcanic Series.

Rock type: Mugearite.

Collected by: R. May, 1972; recollection of a previously dated flow.

Location: 20°06'30" N., 155°52'45" W.,; collected from a roadcut 

8.0 km south of Mahukona on the new section of Hwy. 26, 

Keawanui 1W quadrangle (1957); elevation approximately 61 

m. NOTE: Location is approximate because the new road does not 

appear on the current topographic map.

Age information: Supposed to duplicate sample HW-21, for which 

a whole-rock K-Ar age of 181,000 ± 3,000 years B.P. was obtained 

(McDougall and Swanson, 1972).

Sample No.: 042

Source volcano: Kohala Volcano; Hawi Volcanic Series.

Rock type: Mugearite.

Collected by: R. May, 1972; recollection of a previously dated flow.

Location: 20°04'30// N., 155°51/30" W.; collected from a roadcut 

about 1.45 km north of the North Kohala-South Kohala District 

Boundary, on the edge of Keawewai Stream, Kawaihae 7 Vz f 

quadrangle (1956); elevation: approximately 60 m. NOTE: The 

same location restrictions specified for sample 041 apply.

Age information: Supposed to duplicate sample HW-22, for which 

a whole-rock K-Ar age of 184,000 ± 3,000 years B.P. was obtained 

(McDougall and Swanson, 1972).

Sample No.: 033

Source volcano: Kohala Volcano; Hawi Volcanic Series.

Rock type: Hawaiite.

Collected by: R. May, 1972; recollection of a previously dated flow.

Location: 20°02'00" N., 155°42'25" W.; collected from a roadcut 

9.6 km from the Kamauela Post Office on Hwy. 25, Kamauela 

1W quadrangle (1956); elevation: 755 m.

Age information: Duplicates sample GA 1376, for which a wholerock K-Ar age of 196,000 ±7,000 years B.P. was obtained 

(McDougall, 1969).

Sample No.: Mol le

Source volcano: Unknown.

Rock type: Alkalic basalt.

Collected by: M. Beeson, 1969.

Location: 21°07'00" N., 156°44'30" W.; Halawa 7V&' quadrangle 

(1952).

Age information: Probable age of approximately 1.35 m.y. (M. 

Beeson, U.S.G.S., written commun.) based on K-Ar ages from 

associated basalts from East Molokai (McDougall, 1964).

Sample No.: Klpa Wa

Source volcano: Unknown. 

Rock type: Transitional alkalic basalt. 

Collected by: M. Beeson, 1971.

Location: 21°11'00" N., 157°00'05" W.; Kaunakakai 7'/2 ' quadrangle, island of Molokai (1952). 

Age information: Probable age of about 1.5 m.y. (M. Beeson,
SAMPLE LOCATION AND AGE DATA 45

U.S.G.S. written commun.) based on K-Ar ages ranging from 

1.47 to 1.49 m.y. on associated basalts (McDougall, 1964).

Sample No.: 5X340

Source volcano: Waianae Volcano; Waianae Volcanic Series, upper 

member.

Rock type: Not precisely determined; either an alkalic basalt or a 

Hawaiite.

Collected by: R. Doell and G. B. Dalrymple.

Location: 21°2014" N., 158°05'33" W.; collected near Hwy. 90 

near Kalealoa Road, Ewa 7 Vz' quadrangle, island of Oahu (1962).

Age information: Material consisted of extra sections of drill cores 

5X340 to 5X347 from site J, unit 56, collected originally for paleomagnetism and age-dating studies. The flow yielded a K-Ar 

age of 3.30 ±0.10 m.y. (Doell and Dalrymple, 1973).

THOLEIITIC BASALTS

[Listed in order of increasing age]

Sample No.: 012

Source volcano: East rift of Kilauea Volcano; Puna Volcanic Series.

Rock type: Tholeiitic basalt.

Collected by: R. May, 1972.

Location: 290 30'38" N., 154°50'40" W.; collected from a roadcut on 

Hwy. 132,0.6 km west of the intersection with Hwy. 137, Kapoho 

7 W quadrangle (1965); elevation 36.5 m.

Age information: Historic flow, 1960.

Sample No.: 015

Source volcano: Kilauea Volcano, Puna Volcanic Series.

Rock type: Tholeiitic basalt.

Collected by: R. May, 1972.

Location: 19°26'20" N., 154°56'50" W.; collected in a roadcut on 

Hwy. 13 at the intersection with the road to Opihikau, Pahoa 

South 7 V2' quadrangle (1966); elevation: 305 m.

Age information: Historic flow, 1955.

Sample No.: 026

Source volcano: Mauna Loa Volcano; Kau Volcanic Series.

Rock type: Thoeliitic basalt.

Collected by: R. May, 1972.

Location: 19°05'40" N., 155°47'30" W.; collected in a roadcut on

Hwy. 11, at the intersection with Ginger Blossom Lane, Pohue

Bay IVz' quadrangle (1962); elevation: 587 m. 

Age information: Historic flow, 1907.

Sample No.: 030

Source volcano: Mauna Loa; Kau Volcanic Series.

Rock type: Tholeiitic basalt.

Collected by: R. May, 1972.

Location: 19°49'45// N., 155°44'15" W.; collected in a roadcut on 

Hwy. 19,2.8 km east of Puuanahulu, Puu Anahulu 7 ¥2' quadrangle (1959); elevation: 678 m.

Age information: Historic flow, 1859.

Sample No.: 050

Source volcano: East rift of Kilauea Volcano; Puna Volcanic Series.

Rock type: Tholeiitic basalt.

Collected by: R. May, 1972.

Location: 19°22'30" N., 154°58'00" W.; collected from a road metal

pit adjacent to Hwy. 13 about 1.7 km north of the coast, Kalapana 7 W quadrangle (1966); elevation: 33 m. 

Age information: Historic flow, age approximately 1,750 A.D.

Sample No.: 051

Source volcano: Kilauea Volcano; Puna Volcanic Series.

Rock type: Tholeiitic basalt.

Collected by: R. May, 1972.

Location: 19°19'25" N., 155°17'30" W.; collected from an outcrop 

adjacent to the road from Park Headquarters to Hilina Pali, near 

BM 2829, Kau Desert 7¥2' quadrangle (1963); elevation: 869 m.

Age information: Historic flow, age approximately 1,420 A.D. 

(Howard Powers, written commun.).

Sample No.: 032

Source volcano: Kilauea Volcano, Puna Volcanic Series.

Rock type: Tholeiitic basalt.

Collected by: R. May, 1972; recollection of a previously dated flow.

Location: 19°26'00// N., 155°15/20" W.; collected from a roadcut on 

Hwy. 11 at the entrance to Kilauea Volcano National Park, Kilauea Crater 1W quadrangle (1963); elevation: 1.20 km.

Age information: 14C age of 2,500 ± 250 years B.P. on charcoal from 

a fern mold in pumice overlying the flow (sample W-201) (Rubin 

and Suess, 1956).

Sample No.: 024

Source volcano: Mauna Loa; Kau Volcanic Series.

Rock type: Tholeiitic basalt.

Collected by: R. May, 1972; recollection of a previously dated flow.

Location: 19°04'20" N., 155°36'50" W., collected from a roadcut 

adjacent to the churchyard in central Waiohinu from which the 

dated sample was obtained, Naalehu 7¥2' quadrangle (1962); elevation: 323 m.

Age information: 14C age of 3,740 ± 150 years B.P. on charcoal from 

beneath the flow exposed at the surface in the Waiohinu churchyard (sample W-856, Rubin and Corrine, 1960). A second age of 

3,620 ± 250 years B.P. was obtained for carbonaceous material 

from the same stratigraphic position collected from a different 

sampling site nearby (sample W2016, Sullivan and others, 1970).

Sample No.: 070

Source volcano: Mauna Loa; Kau Volcanic Series.

Rock type: Tholeiitic basalt.

Collected by: S. Gromme and Monte Marshall, 1972.

Location: 19°02'35" N., 155°33'30" W.; collected from the cliff section near the top of Maniania Pali about 1.2 km south-southwest 

of Kimo Point, near Naalehu 7 ¥2' quadrangle (1962); elevation: 

about 30 m.

Age information: 14C age of 10,140 ±300 years B.P. on carbonaceous ash baked by the flow sample (W-907, Rubin and Berthold, 1961).

Sample No.: 074

Source volcano: Mauna Loa; Kau Volcanic Series. 

Rock type: Tholeiitic basalt.

Collected by: S. Gromme and Monte Marshall, 1972. 

Location: 190 02'35" N., 155°33'30" W.; collected from the next basalt flow below the 10,140-year-old flow and separated from it by
46 THERMOLUMINESCENCE DATING OF HAWAIIAN BASALT

2.5 m of volcanic ash from the Pahala Ash.

Age information: Older than 10,140 on the basis of stratigraphic 

position.

Sample No.: 018

Source volcano: Kilauea Volcano; Puna Volcanic Series.

Rock type: Tholeiitic basalt.

Collected by: R. May, 1972; recollection of a previously dated flow.

Location: 19°16'20" N., 155° 1710" W.; collected near the top of 

the Kaone fault scarp, 200 m southwest of Puu Kaone, Kau Desert 7 Vi' quadrangle (1963); elevation: 213m (SE. of Hilina Pali).

Age information: 14C age of 17,360 ±650 years of B.P. on carbonaceous ash beneath and baked by the flow (sample W-905, Rubin 

and Berthold, 1961).

Sample No.: 019

Source volcano: Kilauea Volcano (?).

Rock type: Tholeiitic basalt.

Collected by: R. May, 1972.

Location: 19°16'20" N., 155° 1710" W.; collected stratigraphically 

below the dated flow from which sample 018 was obtained and 

separated from it by 3 meters of volcanic ash from the complex 

series of ash units referred to collectively as the Pahala Ash. Poor 

exposure precluded a definitive assessment of whether this particular flow immediately underlies the ash or if there is another 

thin flow between the bottom of the ash and the flow from which 

sample 019 was obtained, but at most this flow could not be more 

than two flow units below the dated flow.

Age information: Older than 17,360 years on the basis of stratigraphic position.

Sample No.: P71-13

Source volcano: Kohala Volcano, Pololu Volcanic Series.

Rock type: Tholeiitic basalt.

Collected by: lan McDougall and Dan Swanson, 1971.

Location: 20°0710// N., 155°35'20" W.; collected from Waipio Valley, Kukuihaele 1W quadrangle (1957); elevation: 170 m.

Age information: Whole-rock K-Ar age of 327,000 ± 45,000 year 

B.P. (McDougall and Swanson, 1972).

Sample No.: 037

Source volcano: Kohala Volcano; Pololu Volcanic Series. 

Rock type: Tholeiitic basalt.

Collected by: R. May, 1972; recollection of a previously dated flow. 

Location: 20° 1110" N., 155°5410" W.; collected from a roadcut in

Mahukona, Mahukona 7 1/2 / quadrangle (1957); elevation: 10 m. 

Age information: Supposed to duplicate sample HW-17, for which

a whole-rock K-Ar age of 400,000 ± 8,000 years B.P. was obtained

(McDougall and Swanson, 1972).

Sample No.: P71-4

Source volcano: Kohala Volcano; Pololu Volcanic Series.

Rock type: Tholeiitic basalt.

Collected by: lan McDougall and Don Swanson, 1971.

Location: 20°12'20" N., 155°44'00" W.; collected from Pololu Valley, about 110 m below the top of the exposed section of the Pololu Volcanic Series, Honokane 7 Vi' quadrangle (1957).

Age information: Whole-rock K-Ar age of 403,000 ± 141,000 years 

B.P. (McDougall and Swanson, 1972).

Sample No.: P71-3

Source volcano: Kohala Mountain; Pololu Volcanic Series.

Rock type: Tholeiitic basalt.

Collected by: I. McDougall and D. Swanson, 1971.

Location: 20°12'20" N., 155°44'00" W.; collected on the trail into 

Pololu Valley, about 40 m below the top of the exposed section of 

the Pololu Volcanic Series, Honokane 1W quadrangle (1957).

Age information: Whole-rock K-Ar age of 414,000 ± 18,000 years 

B.P. (McDougall and Swanson, 1972).

Sample No.: P71-5

Source volcano: Kohala Volcano; Pololu Volcanic Series.

Rock type: Tholeiitic basalt.

Collected by: I. McDougall and D. Swanson, 1971.

Location: 20°12'20" N., 155°44'00" W., collected on the trail into 

Pololu Valley about 90 m below the top of the exposed section of 

the Pololu Volcanic Series, Honokane 1W quadrangle (1957).

Age information: Whole-rock K-Ar age of 447,000 ±27,000 years 

B.P. (McDougall and Swanson, 1972).

DERIVATION OF EQUATIONS FOR THE ERROR IN THE TL 

RATIOS AND TL AGES

ERROR EQUATION FOR THE TL AGE RATIO

The TL age ratio r is expressed as

r = k(NEa/AEn)

where k is a constant, N the natural TL peak-height average, A/Ea 

the average artificial TL peak-height average for the same samples 

from a standardized exposure of artificial radiation (Ea) and En 

the natural annual radiation dose rate.

In the general case, if y - F (xj, X2-. • xn) and the changes Aaq in 

the jc's are small, then the differential dy approximates the change 

Ay in y,

—— 

dF 

-

OXl

——dF 

0X2

dF

and if the changes Ax in the x's are independent, then the variance in Ay may be approximated by

dF dF

+ ax9 2 +. . . +

dF 

dxn

with the partial derivatives evaluated at nominal values of *„ x2 , 

and xn (Crow and others, 1960, p. 69). 

Given r = NEa/AEn, and the relation established above,
DERIVATION OF EQUATIONS FOR THE ERROR IN THE TL RATIOS AND TL 47

Taking partial derivatives and combining terms,

dr =

if the changes AN, AA, A£n, and A£a are small, then AN2 may be 

replaced by aN2 , and so on, squaring each term in the previous 

equation and converting to variances

Converting to fractional errors in the form of 

err2 = ff/r2

where or'2 is the fractional error and combining terms and simplifying yields the final equation for the error in the TL ratio

Note that En, the natural radiation dose rate, is actually a composite function incorporating errors in the measurement of the uranium, thorium, and potassium concentrations. Using an error 

equation similar to the one employed for the eror in the TL ratio, 

the error in the quantity En is

EQUATION FOR THE ERROR IN THE TL AGES

The TL age equation is of the form

fl , ,1 t =\— In r — In a

LO J. 

where

t is the sample age,

r is the TL ratio,

b is the slope of the best-fit line,

a is the y-axis intercept.

Employing the same basic assumptions used in the preceding derivations regarding the size of the errors involved and permissible 

approximations in terms of substitutions of statistical quantities,

dt = L£_ 

Ldx

— (hi r - In a)\ Ay. b J

Taking partial derivatives, converting to fractional errors, and 

simplifying yields the equation for the fractional error in the TL

Note that the error in b, the slope of the age curve, is not included in the error equation. This is because the equation is designed to 

provide an estimate of the precision of the TL ages; errors in the 

slope of the line and in the y intercept are more properly in the 

realm of accuracy.

GPO 689-035