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40 Ar;39 Ar Age Spectra and Total-Fusion Ages 

of Tektites from Cretaceous-Tertiary Boundary 

Sedimentary Rocks in the Beloc Formation, Haiti 

By G. Brent Dalrymple, Glen A. Izett, Lawrence W. Snee, and John D. Obradovich 

U.S. GEOLOGICAL SURVEY BULLETIN 2065 

The age of tektites from the Cretaceous-Tertiary ( K-T) boundary 

layer within the marine Beloc Formation, Haiti, as determined 

by 40 Art39 Ar incremental heating and total fusion methods, 

records the time of a bolide impact at 64.4±0.1 Ma 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1993 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Robert M. Hirsch, Acting Director 

For sale by Book and Open-File Report Sales 

U.S. Geological Survey 

Federal Center, Box 25286 

Denver, CO 80225 

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does 

not imply endorsement by the U.S. Government. 

Library of Congress Cataloging-in-Publication Data 

40 Art39 Ar age spectra and total-fusion ages of tektites from Cretaceous-Tertiary boundary sedimentary rocks in 

the Beloc Formation, Haiti I by G. Brent Dalrymple ... [et al.]. 

p. em. - (U.S. Geological Survey bulletin ; 2065) 

Includes bibliographical references. 

1. Tektite-Haiti. 2. Argon-argon dating. 3. Cretaceous/Tertiary boundary-Haiti. I. Dalrymple, G. 

Brent. II. Title: 40 Art39 Ar age spectra and total-fusion ages of tektites from Cretaceous-Tertiary boundary 

sedimentary rocks in the Beloc Formation, Haiti. III. Series. 

QE75.B9 no. 2065 

[QE399] 

557.3 s-dc20 

[549'.112] 

93-31108 

CIP 
CONTENTS 

Abstract .................................................................................................................................... 1 

Introduction ............................................................................................................................... 1 

Description of the Tektites ....................................................................................................... 2 

Analytical Methods ................................................................................................................... 3 

Continuous Laser Systems (Menlo Park) ..................................................................... 5 

Internal Resistance Furnace System (Denver) .............................................................. 6 

Monitor Minerals ........................................................................................................... 6 

Results ....................................................................................................................................... 7 

Laser Total Fusion Analyses ......................................................................................... 7 

Laser Incremental HeatingAnalyses ............................................................................ 12 

Vacuum Furnace Incremental Heating Analyses ........................................................ 14 

Discussion ............................................................................................................................... 14 

References Cited ..................................................................................................................... 18 

FIGURES 

1. Photograph of polished slab of the "K-T marker bed," Beloc Formation, southern Haiti ................................................ 3 

2. Photographs showing clay tektite pseudomorph with relic glass core and a representative selection 

of unaltered glass tektite cores, like those that were dated .............................................................................................. .4 

3. ]-curves for the six irradiations that included Haitian tektites (HT) and ZCB sanidine samples .................................... 5 

4. Cross section of sample chamber, laser beam, and IR microscope used for 40 Art39 Ar dating of the 

single tektites with the Menlo Park continuous laser system ............................................................................................ 6 

5. Histogram of 40 ArP9 Ar laser system total fusion ages on 53 single Haitian tektites and on 28 samples 

of the Z-coal bentonite (ZCB) sanidine ............................................................................................................................. 8 

6. K/Ca ratios for the 53 laser system total fusion measurements on individual tektites, as calculated from 

the 39 Ar/37 Ar ratios ............................................................................................................................................................ 8 

7. 40 ArP9 Ar age spectra for four single Haitian tektites measured with the laser system .................................................. 15 

8. K/Ca diagrams for the four tektites measured with the laser system ............................................................................. 15 

9. 40 ArP6 Ar vs 39 ArP6 Ar isochron diagrams for (a) one of the laser incremental heating measurements 

on single tektites, and (b) one of the resistance furnace incremental heating experiments on bulk 

tektite samples .................................................................................................................................................................. 16 

10. 40ArP9 Ar age spectra for tektite samples analyzed with the resistance furnace system ............................................... 16 

11. Summary of age measurements on deposits related to the K-T boundary impact marker bed in 

North America and Haiti ................................................................................................................................................ 19 

TABLES 

1. Parameters for the six irradiations that contained Haitian tektites .................................................................................... 5 

2. 40 ArP9 Ar laser total fusion age data on single Haitian tektites and Z-coal bentonite sanidine ........................................ 9 

3. Means and standard deviations of 40 ArP9 Ar laser total-fusion ages on Z-coal bentonite sanidine and 

single Haitian tektites (HT), in Ma ................................................................................................................................. 12 

4. Analytical data for 40 ArP9 Ar laser age spectra on single K-T tektites from Haiti ........................................................ 13 

5. Analytical data for 40 ArP9 Ar resistance furnace age spectra on K-T boundary tektites from Haiti ............................. 17 

6. Summary of 40ArP9 Ar age spectra and isochron results on K-T boundary tektites from Haiti ..................................... 18 

III 

40Ar/39Ar Age Specta and Total-Fusion Ages of Tektites 

from Cretaceous-Tertiary Boundary Sedimentary Rocks 

in the Beloc Formation, Haiti 

By G. Brent Dalrymple, Glen A. Izett, Larry W. Snee and John D. Obradovich 

ABSTRACT 

Fifty-two total-fusion 40 Art39 Ar ages, measured with a 

continuous laser system on single tektites from the Cretaceous-Tertiary (K-T) boundary layer within the Beloc Formation of southern Haiti, have a weighted mean of 

64.42±0.06( obest) Ma. 40 Art39 Ar age spectra on four single 

Haitian tektites obtained with the same laser system are 

flat and each has a plateau over more than 95 percent of 

the 39 Ar released. The weighted mean of the four plateau 

ages is 64.38±0.18 Ma( obest). Two age spectra obtained 

using a resistance furnace system on bulk samples of about 

50-70 of the tektites also have flat plateaus with a 

weighted mean age of 64.49±0.10 Ma. Our results indicate that the impact that formed the tektites occurred 

64.43±0.05 Ma. Sanidine from two bentonites that lie 50-

70 centimeters above the K-T boundary interval in continental sedimentary rocks in Montana was also dated with 

the laser system (28 measurements) and gives a weighted 

mean40Arf39Ar age of 64.77±0.07 Ma. 

The Haitian tektites occur at the paleontological K-T 

boundary along with an Ir abundance anomaly and 

shocked quartz. They were the product of an impact on 

Earth of an extraterrestrial bolide (asteroid or comet) at the 

end of the Cretaceous Period. All available data, from our 

study and from those previously published, suggest that 

this event occurred 64.6±0.1 Ma. 

INTRODUCTION 

Slightly more than a decade ago, Alvarez and others 

(1980) proposed that a large (10±4 km diameter) asteroid 

of chondritic composition struck the Earth about 65 Ma 

and that the debris injected into the atmosphere by the impact caused the mass faunal extinctions that mark the end 

Manuscript approved for publication June 28, 1993. 

of the Cretaceous Period. Their hypothesis was based on 

the presence of an unusually high content of Ir and other 

Pt-group metals, which are depleted in the Earth's crust 

relative to Solar System abundances, in a thin marine 

claystone bed at the Cretaceous-Tertiary (henceforth referred to as K-T for the sake of brevity) boundary in Italy, 

Denmark, and New Zealand. Since 1980, Ir abundance 

anomalies have been found in both marine and continental 

K-T boundary sedimentary rocks at nearly 100 sites worldwide (Alvarez and Asaro, 1990). 

Some investigators proposed that the Ir anomaly is 

due to extensive volcanism (for example, Officer and 

Drake, 1985; McLean, 1985; Officer and others, 1987, 

1992), possibly from the eruptions that emplaced the 

Deccan Traps of India. The general absence of significant 

concentrations of the Pt-group metals in terrestrial volcanic 

rocks, however, and the presence in the K-T boundary 

layer of shocked quartz, known to form only as the result 

of hypervelocity impact, makes the volcanism hypothesis, 

at least as an explanation of the K-T Ir anomaly, highly 

improbable and greatly strengthens the impact hypothesis. 

To date, shocked quartz grains have been identified in K-T 

boundary sediments that also contain Ir anomalies at sites 

in western North America, Europe, Asia, Haiti, and the Pacific Ocean (for example, Bohor and others, 1984; lzett 

and Pillmore, 1985; Bohor and Izett, 1986; Izett, 1990). 

The recent discovery of tektites at the K-T boundary 

in marine sedimentary rocks of the Beloc Formation of 

southern Haiti has further strengthened the impact hypothesis (lzett and others, 1990; Izett, 1991; Sigurdsson and 

others, 1991). Tektites are unusual glassy melt droplets 

formed by instantaneous fusion of terrestrial rocks during 

the hypervelocity impacts of large, extraterrestrial objects 

(asteroids or comets). The Haitian tektites, which occur at 

the paleontological K-T boundary, are accompanied by 

shocked quartz (Hildebrand and Boynton, 1990; Izett and 

others, 1990; lzett, 1991) and an iridium anomaly (Alvarez 

and others, 1982). They were the first certain tektites 

found in K-T boundary sediments and among the oldest 

tektites found on Earth. Recently, Officer and others 
2 AGES OF TEKTITES FROM CRETACEOUS-TERTIARY BOUNDARY ROCKS, BELOC FORMATION, HAITI 

(1992), Lyons and Officer (1992), and Jehanno and others 

(1992) claimed that the Haitian tektites are of volcanic, 

rather than impact, origin, but their arguments are unconvincing and ignore considerable data indicating the glass is 

different in its physical and chemical properties from any 

volcanic glass on Earth (Izett and others, 1990; Izett, 1991; 

Sigurdsson and others, 1991). Moreover, recent oxygen 

isotope analyses have shown that the Haitian tektites were 

formed from a mixture of carbonate and silicate rocks and 

preclude derivation of the glass by volcanic processes 

(Blum and Chamberlain, 1992) 

Since the discovery of the K-T tektites on Haiti, tektites accompanied by an Ir anomaly have also been found 

in K-T boundary deposits at Arroyo el Mimbral in northeastern Mexico (Smit and others, 1992) and in cores at 

DSDP sites 536 and 540 in the southeast Gulf of Mexico 

(Alvarez and others, 1992). The DSDP tektites have not 

been dated but the Mimbral tektites have an 40 Arf39 Ar age 

(65.06±0.25 Ma) compatible with that for the K-T bound-

:ary (Swisher and others, 1992). 

It now appears that the site of the impact that produced the tektites found in Haiti and Mexico was the 

Chicxulub structure on the Yucutan platform (Hildebrand 

and Boynton, 1990), which has an 40 Art39 Ar age indistin-

'guishable from those measured for the Haitian tektites 

(Swisher and others, 1992; Sharpton and others, 1992). 

The Manson structure in Iowa (Hartung and others, 1990) 

may also be of K-T age (Kunk and others, 1989). 

There seems little doubt that one or more asteroids or 

comets struck the Earth at the end of the Cretaceous Period 

and caused the deposition of impact materials worldwide 

in a K-T boundary layer that contains unusually high abundances of Ir and, in some localities, shocked minerals and 

tektites. Whether or not this impact was responsible for the 

mass faunal extinctions at the end of the Cretaceous Period 

is still a subject of lively debate. 

The Haitian tektite glasses are non-hydrated and have 

K20 contents of 0.5 percent to nearly 2.0 percent and thus 

are datable by 40 Art39 Ar methods. In 1991 we published 

two brief reports on the 40 Ar/39 Ar ages of these tektites 

(lzett and others, 1991 a, 1991 b), showing that the age of 

the tektites, and hence the age of the K-T impact that 

formed them, is 64.5±0.1 Ma. The purpose of this report is 

to provide complete documentation for these important radiometric ages and to report new data. 

DESCRIPTION OF THE TEKTITES 

The Beloc Formation of southern Haiti consists of a 

1-2-m-thick basal basaltic conglomerate overlain by a 150-

m-thick sequence of interbedded fossiliferous pelagic limestone, marl, chalk, and chert (Maurasse, 1982a, 1982b ). 

The K-T boundary occurs about 40-50 m above the base 

of the formation and is marked by a 0.5-m-thick 

volcanogenic turbidite referred to as the "K-T marker bed" 

(Izett, 1991). This K-T-marker bed consists of air-fall impact ejecta mixed with a small component of locally derived volcanogenic, biogenic, and sedimentary material 

(lzett, 1991; Maurasse and Sen, 1991 ). Latest 

Maestrichtian planktonic foraminifera occur immediately 

below and earliest Danian foraminifera occur directly 

above the turbidite (Maurasse, 1982a; Sigurdsson and others, 1991; Maurasse and Sen, 1991). An iridium abundance anomaly occurs in the upper two-thirds of the 

marker bed and extends into a 2-cm-thick marl bed that 

immediately overlies the marker bed (Maurasse, 1982a; 

Alvarez and others 1982; Frank Asaro, oral commun., 

1991). Analysis by Izett (1991) of the acid-insoluble residue (0.006 percent by weight) obtained from the turbidite 

revealed that it contains 45 percent shocked quartz and 2 

percent of shocked quartzite and metaquartzite grains 

along with locally derived volcanogenic silicates. 

The tektites, which occur within iron-rich smectite 

clay spherules that are pseudomorphs of the original tektites, have been described in detail by Izett and others 

(1990), Sigurdsson and others (1991), and Izett (1991). 

Altered tektites are abundant ( -40 percent) throughout the 

K-T marker bed but those containing relict glass are concentrated in the lower 2 em of the bed (fig. 1 ). The 

spherules are very well preserved and about 20 percent of 

them have shapes typical of small tektites, including spheroids, oblate spheroids, ellipsoids, discoids, spindles, teardrops, rods, and dumbells; the rest are irregular in shape. 

They vary in diameter from a few tenths of a millimeter to 

as much as I em. Many of the larger spherules are hollow. Similar clay spherules that may be tektite pseudomorphs occur in other K-T boundary rocks in Wyoming, 

Mexico, Alabama, Mississippi, and DSDP site 603B off 

the coast of North Carolina (see Izett, 1991, for discussion 

and references; also Pitakpaivan and Hazel, 1992). 

The Haitian clay spherules were tentatively identified 

as tektites by Maurasse ( 1982b ). The significance of his 

report, however, was largely unappreciated until 

Hildebrand and Boynton ( 1990a) positively identified the 

spherules as altered tektites on the basis of their shapes. 

Izett and others (1990) and Izett ( 1991) provided unequivocal proof that the pellets are pseudomorphs of tektites when they found that about 2 percent of the pellets in 

the lower 2 em of the K-T marker bed contain relict, unaltered glass cores whose properties are unique to tektites 

(fig. 2). 

In reflected light, most of the relict tektites appear 

dark brown to black in color but in transmitted light they 

range from pale brown to pale yellow brown; a few have 

a golden honey color. The tektites are devoid of microlites 

and crystallites, characteristically found in volcanic glass, 

but some contain spherical and almond-shaped bubbles. 

The major- and trace-element compositions of the tektites 

(lzett and others, 1990; Sigurdsson and others, 1991; Izett, 
ANALYTICAL METHODS 3 

1991; Koeberl and Sigurdsson, 1992) are broadly similar 

to those of other tektite groups, except that the Haitian tektites have lower Si (50-68 percent), Cr, Ni, Co, B, Mn, 

and Hf, and higher Fe, Ca, Na, Sc, V, Cu, Zn, Ba, Sr, Sn, 

and Ba. The honey-colored tektites have unusually high 

contents of Ca (as much as 30 percent CaO) and very high 

S (0.1-0.4 percent). The rare earth element contents are 

within the range of the other tektite groups; their Sr- and 

Nd-isotopic compositions, however, are distinct (Premo 

and Izett, 1992). Oxygen isotope analyses have shown that 

the tektites fall on a mixing line between an isotopically 

heavy (6 180=14 per mil) high-calcium sedimentary composition and an isotopically light (6 180=6 per mil) highFigure 1. Photograph of polished slab of the "K-T marker bed" 

(lzett, 1991 ), Beloc Formation, southern Haiti. The spherical 

and ovoid pellets are tektites that have been altered to smectitic 

clay. About 2 percent of the tektites have relic, unaltered glass 

cores. 

silica igneous composition, which precludes origin from a 

volcanic source (Blum and Chamberlain, 1992) and supports their origin from an impact. 

ANALYTICAL METHODS 

Several hundred tektites, ranging in size from 0.3 to 

4.5 mm, were extracted for 40 ArP9 Ar dating from a 

slabbed sample of the K-T marker bed. The tektites were 

scrubbed ultrasonically in dilute HF ( -5 percent) to remove adhering clay and then washed in distilled water. 

Most of the tektites were equant and ranged in diameter 

from about 0.7 to 1.2 mm and in mass from about 0.3 to 

1.2 mg. Sanidine crystals (-150 Jli11 in diameter) from two 

of the Z-coal bentonites (ZCB) of local usage from the 

Hell Creek locality in Garfield County, Montana 

(Baadsgaard and others, 1988), were also analyzed. One 

sample (JFL-500C) was the same material used by 

Baadsgaard and his colleagues; the other (83-0-05) was 

collected from the same locality by one of us (JDO) in 

1983. Sample 83- 0-05 is from a bentonite (sanidine-bearing only) that occurs approximately 50 em above the K-T 

boundary in the Hells Creek area, whereas sample JFL500C is from a bentonite (sanidine and biotite-bearing) 

that occurs -70 em above the K-T boundary (Carl 

Swisher, oral commun., 1983). An Ir anomaly and shocked 

quartz occurs in this section at the K-T boundary (Izett, 

1990). Sanidine and other minerals from the ZCB have 

been dated previously by the K-Ar, 40 ArP9 Ar, Rb-Sr, and 

U-Pb methods, with results ranging from about 63.5 to 

66.0 Ma (Obradovich, 1984; Baadsgaard and others, 1983, 

1988). The ZCB sample was included in the sample array 

in order to provide a direct tie between the Haitian tektites 

and a well-studied continental K-T section in Montana. 

The age of the tektites was determined by 40 ArP9 Ar 

dating using two different U.S. Geological Survey laboratories that employ different instrumentation. Single and 

multiple tektites were analyzed with the GLM continuous 

laser system in Menlo Park, California, and two samples 

consisting of about 50-70 tektites each were analyzed using a double-vacuum resistance furnace system in Denver, 

Colorado. The methods and procedures used by these two 

laboratories are described separately below. 

The samples for this study were irradiated in six separate irradiations (table 1) in the cores of the U.S. Geological Survey TRIGA reactor (GSTR) and the Los Alamos 

National Laboratory Omega West Reactor (OWR). Irradiation procedures, methods used to correct for interfering Ar 

isotopes generated by undesirable reactions with Ca and K, 

and the GSTR flux characteristics are described in detail 

by Dalrymple and others (1981) and Dalrymple and 

Duffield (1988). 

Irradiation GLN3 contained two quartz vials. One of 

the quartz vials contained the samples used for the laser 
4 AGES OF TEKTITES FROM CRETACEOUS-TERTIARY BOUNDARY ROCKS, BELOC FORMATION, HAITI 

system analyses. These consisted of an aluminum-foil 

packet of 32 tektites and a packet of about 31 mg of 

sanidine crystals from the ZCB. These two packets were 

sandwiched between packets of monitor minerals as shown 

in figure 3. The second vial contained a packet of about 50 

tektites (67 mg), which were used for one of the two resistance furnace analyses (90G15L-1), sandwiched between 

packets of monitor minerals. 

Irradiation 0027 contained six quartz vials. In one of 

these was a single cylindrical aluminum packet (7 mm diameter by 9 mm long) of about 70 tektites (83.4 mg), 

which were used for the second resistance furnace analysis 

(90G 15L-2). The vial containing the tektites included 6 

packets of MMhb-1 ( -7 mg each) with the tektite adjacent 

to and above the fifth packet from the bottom and below 

but separated from the sixth packet of MMhb-1 by another 

sample (fig. 3). 

In irradiations 1 05 and 108, the ZCB sanidine, the tektites, and the monitor minerals were contained in a single 

stack of uniformly spaced, small aluminum cups. One cup 

in irradiation 105 contained both ZCB sanidine and tekA 

tites. In irradiation 108 the tektites were placed in the same 

cups as the sanidine monitor mineral (fig. 3). Irradiations 

J006 and 1008 consisted of a single quartz vial each containing aluminu'll-foil packets; the tektites and the ZCB 

sanidine were contained in the same packets in these two 

irradiations. 

The errors calculated for individual ages in this paper 

are estimates of the analytical precision at the 67 percent 

(1 o) level and include the estimated error in the precision 

of the irradiation parameter J but do not include possible 

errors in the values used for the ages of the monitor minerals, discussed below. Plateau ages are the weighted means 

and weighted errors (1 obest) of the plateau increments, 

where weighting is by the inverse of the variance (Taylor, 

1982). Weighted means are sometimes preferable to simple 

means because weighting takes into account the quality of 

the individual analyses. We generally report ages to a precision of four numerals, but round the grand weighted 

mean to three numerals, which is the maximum precision 

justified considering the various uncertainties involved in 

the age measurements and associated calibrations. 

B 

Figure 2. A, Photograph of clay tektite pseudomorph with relic glass core. The pseudomorph is approximately 1.8 mm in diameter. 

B, A representative selection of unaltered glass tektite cores, like those that were dated. The largest of the cores is about 4.5 mm 

long. 
ANALYTICAL METHODS 5 

Table 1. Parameters for the six irradiations that contained CONTINUOUS LASER SYSTEM (MENLO PARK) 

Haitian tektites. 

Irradiation Reacto~ Irradiation Neutron Cd 

No. time (hr) flux (nlcm2) shielding 

GLN3 GSTR 20.0 -2.2xl018 no 

DD27 GSTR 60.0 -6.6xl018 no 

105 OWR 14.3 -5.2xl018 yes 

108 OWR 14.0 -5.1xl018 yes 

JD06 GSTR 30.0 -3.3x1018 no 

JD08 GSTR 30.0 -3.3x1018 no 

a GSTR: Geological Survey TRIGA Reactor, Denver, CO. 

The GLM laser system consists of a 5W continuous 

Ar-ion laser, laser optics, a small-volume, ultra-high 

vacuum sample chamber and extraction line, an IR microscopic radiometer/thermometer, and a very sensitive, lowbackground rare gas mass spectrometer equipped with a 

Baur-Signer source and electron multiplier detector 

(Dalrymple and Duffield, 1988; Dalrymple, 1989). The laser beam, which is used to heat (or melt) the sample, is 

focused with a Galilean beam expander and directed with 

steering mirrors through the sapphire window of the 

sample chamber onto the irradiated sample (fig. 4). The 

sample chamber is moved with a motorized X-Y stage to 

bring each sample under the laser beam for heating and 

OWR: Omega West Reactor, Los Alamos National 

Laboratory, Los Alamos, NM 

0.0046 ..------------------. GLN3 

(GSTR) 

0.0045 

MMhb-1 

0.0044 

0.0043 

TCR 

0.0042 0 4 8 12 16 20 

0.0151 ....-----------------, 

0.0150 

0.0149 

0.0148 

DD27 

(GSTR) 

( 

HT 

monitors • MMhb-1 

0.0096 ..----------------, 

108 

0.0095 (OWR) 

0.0094 ZCB 

t 

0.0093 HT 

monitors = TCR 

0.0092 OL--__:___5'----1-0L------115 ___ 2_.0 

0.0070 .---------------, 

0.0069 

0.0068 }!(, 

monitors= TCR 

JD06 

(GSTR) 

0.0147 0 

0.0105 

20 40 60 80 0.0067 0 5 10 15 20 

105 

0.01 04 (OWR) 

0.0103 t 

ZCB 

0.0102 

0.0101 

0 5 10 

monitors • TCR 

15 20 

0.0070 .------:----t----------, 

0.0069 

monitors • TCR 

JD08 

(GSIR) 

0.0067 L-..--L---'---.......___........___......L..--_ __,30 

25 0 5 10 15 20 25 

Height (mm) 

Figure 3. ]-curves for the six irradiations that included Haitian tektites (HT) and ZCB sanidine samples. The monitor minerals are 

shown by open circles with the calculated standard deviations of precision of five or six measurements. TCR, Taylor Creek Rhyolite; 

FCT, Fish Canyon Tuff sanidine; MMhb-1, Minnesota hornblende; GSTR, Geological Survey TRIGA Reactor; OWR, Los Alamos 

National Laboratory Omega West Reactor. 
6 AGES OF TEKTITES FROM CRETACEOUS-TERTIARY BOUNDARY ROCKS, BELOC FORMATION, HAITI 

analysis. The gas released during heating or fusion is 

cleaned with Zr-Al and Zr-V-Fe getters and isotopically 

analyzed with the mass spectrometer. 

The irradiated Haitian tektites were placed in small 

wells in a Cu planchet; some of the tektites were weighed 

with an electromicrobalance. The planchet was placed into 

the sample chamber and the chamber and extraction line 

were evacuated and baked overnight at 300°C. The pressure in the sample chamber and extraction line after 

bakeout was <lxl o-8 torr. Procedures for the sanidine 

analyses were similar. 

Two types of 40 ArJ39 Ar analyses were done-total fusion analysis, which results in a single 40 ArP9 Ar age for 

either an individual tektite or a small number of tektites, 

and incremetal heating analyses, which results in an age 

spectrum or Ar release diagram for a single tektite. For the 

incremental heating analyses, individual tektites, ranging in 

mass from 0.9 to 1.2 mg (0.8-1.2 mm diameter), were progressively heated for 60 seconds per increment with a 

broadened (defocused) laser beam. Hall (1990) has shown 

that temperature gradients within small grains heated in 

this way are negligible. At and below 1200°C the sample 

temperature was measured with the IR radiometer. It operates in the wavelength band of 1.0 J.Uil to 5.8 J.Uil and measures temperature relative to an internal blackbody cavity 

over an area of the sample surface 35 J.Uil in diameter. The 

radiometer was malfunctioning above 1200°C so we controlled the gas release at higher temperatures by increasing 

incrementally the power setting of the laser. Because the 

radiometer is used only to help control the fractional gas 

release, the malfunction had no effect on the resulting ages 

calculated from the incremental-heating measurements. 

For the total fusion analyses, the tektites, sanidine, and 

hornblende were melted and held for about 60 seconds at 

the maximum temperature achievable, estimated to be 

about 1500°C. 

laser beam 

sample 

chamber 

1----1 

1 em 

IR 

radiometer 

objective 

to cleanup 

system__. 

Figure 4. Cross section of sample chamber, laser beam, and IR 

micr9scope used for 40 Art39 Ar dating of the single tektites with 

the Menlo Park continuous laser system. 

INTERNAL RESISTANCE FURNACE SYSTEM 

(DENVER) 

The internal resistance furnace system at Denver consists of a low blank, double-vacuum resistance furnace, an 

ultra-high vacuum extraction line, and a rare-gas mass 

spectrometer equipped with a Nier-type source and a Faraday detector. The double-vacuum resistance furnace comprises a tantalum crucible with molybdenum liner 

surrounded by an outer can cooled with a water jacket. 

Within the outer can and around the crucible are two tungsten heating elements that are shielded from the outer can 

by two heat shields. The volume containing the heating 

elements is evacuated to less than lxio-7 torr. The crucible 

is on-line with the extraction line. A furnace controller 

provides power to the heating elements and can drive the 

furnace to the specified temperature in two minutes. The 

temperature of the crucible is monitored by a thermocouple whose feedback to the controller ensures that the 

set temperature during any heating step is maintained to 

within ±1 OOC. The gas released during each heating step is 

cleaned with Zr-Al, Zr-V-Fe, and Ti getters, and with molecular sieve dessicant. The extraction system includes 

three compartments and gas is transferred between compartments by freezing on charcoal fingers with liquid nitrogen. After cleaning, the Ar is expanded into the mass 

spectrometer and analyzed. 

The packets containing the irradiated Haitian tektites 

were placed in a glass side-arm located above the crucible. 

For both analyses, the furnace, sidearm, and extraction line 

were evacuated and baked for six hours at 260°C. 

Both experiments done in the resistance furnace were 

incremental heating analyses during which each sample 

was progressively heated for 20 minutes per heating step. 

After each heating step the furnace was turned off for 10 

minutes while the gas released during that step was transferred from the furnace section to the getter section of the 

extraction line. 

MONITOR MINERALS 

· 40 ArJ39 Ar dating is a relative method and so requires 

the use of one or more neutron fluence monitor minerals, 

or standards, whose ages are known. Aliquants of the 

monitor minerals are irradiated alongside the unknown 

samples and their Ar ratios measured by the same procedures. The 40 Art39 Ar ratios of the monitor minerals are 

used along with their known age to calculate a conversion 

efficiency factor, J, which is a measure of the fraction of 

39K converted to 39 Ar by the fast neutron reaction 

39K(n,p)39 Ar. J is then used in the age equation to calculate the age of the unknown samples. The calibration of 

the monitor minerals, therefore, has a direct effect on the 

accuracy of the 40 Arf39 Ar ages calculated for the unknown 
RESULTS 7 

samples. New methods and instrumentation developed 

over the past few years for 40 ArP9 Ar dating have resulted 

in the ability to acquire age data with a precision that exceeds the accuracy of the age of any fluence monitor mineral currently available. In addition, there is not universal 

agreement on the ages used for the monitor minerals, and 

different laboratories, including Menlo Park and Denver, 

sometimes use slightly different values (ages) for the same 

monitor mineral. Thus we feel that it is important to describe the monitor minerals used in this study and the values adopted for their age in some detail. 

Three different fluence-monitor minerals were used in 

our irradiations: Fish Canyon Tuff sanidine, sanidine from 

the Taylor Creek Rhyolite, and MMhb-1 hornblende (fig. 

3). MMhb-1 hornblende is used worldwide as an 

interlaboratory 40 ArP9 Ar standard (Alexander and others, 

1978; Samson and Alexander, 1987). Fish Canyon Tuff 

sanidine is used by some laboratories as an interlaboratory 

standard, whereas Taylor Creek Rhyolite sanidine is used 

by the Menlo Park laboratory as an intralaboratory standard. 

In the Menlo Park laboratory, the primary standard 

mineral is SB-3, a biotite from a rock with an age of 

162.9±0.8 Ma (Lanphere and others, 1990). The age of 

SB-3 is based on 24 JSO measurements by flame photometry and 11 measurements of the radiogenic 40 Ar content 

by isotope dilution using 38 Ar tracers that can be tied to 

absolute air calibrations. The ages for Fish Canyon Tuff 

(27.55±0.10 Ma; Lanphere and others, 1990) and Taylor 

Creek Rhyolite (27.92±0.04 Ma; Duffield and Dalrymple, 

1990) sanidine crystals were determined at Menlo Park by 

40 ArJ39 Ar methods (multiple measurements) using SB-3 as 

the neutron fluence monitor mineral. Menlo Park has also 

obtained a value of 27 .55±0.12 Ma for Fish Canyon Tuff 

biotite by 40 ArP9 Ar incremental heating (Lanphere and 

others, 1990). The Menlo Park group has measured a con:-

ventional K-Ar age of 513.9±2.3 Ma for MMhb-1 hornblende based on 16 K20 measurements by flame 

photometry and 5 measurements of the radiogenic 40 Ar 

content by isotope dilution using the 38 Ar tracers described 

above. The Menlo Park group has also measured the age 

of MMhb-1 by 40Arf39 Ar techniques using SB-3 as the 

monitor mineral and the mean of the 19 ages is 514.7± 

2.5Ma, which is not significantly different from the value 

of 513.9 Ma measured by K-Ar techniques. Thus, the ages 

used for SB-3, MMhb-1, Taylor Creek Rhyolite sanidine, 

and Fish Canyon Tuff sanidine in the Menlo Park laboratory constitute an internally consistent set of values. 

The published "international" mean for MMhb-1 is 

520.4±1.7 Ma (Samson and Alexander, 1987). This age is 

based on data from 18 laboratories, including the USGS 

laboratories at Menlo Park and Denver. The K and Arvalues for MMhb-1 are not, however, known with either the 

precision or accuracy needed for a good 40 ArP9 Ar standard. The range of the mean values obtained by the various laboratories is 3.3 percent for K and 6.5 percent for 

radiogenic 40 Ar. In addition, the Menlo Park data included 

in the international mean age for MMhb-1 contain some 

incorrect data. 

Because the published "best" age for MMhb-1 is 

based in part on some incorrect values and because of the 

wide range in the values used to calculate this age, the 

workers in the Menlo Park laboratory have elected to use 

the value of 513.9 Ma for MMhb-1. This is done to preserve the self-consistent set of monitor mineral ages within 

the Menlo Park laboratory rather than to artificially adjust 

the measured values of all of the other Menlo Park monitor minerals, whose ages were determined independently 

of MMhb-1, simply in order to bring them into conformity 

with the published mean age of MMhb-1. Foland and others (1986) have also found a 40 ArP9 Ar age of 513.5 Ma 

for MMhb-1 based on comparison with their 

intralaboratory standard biotite. 

The age of MMhb-1 was also measured in the Denver 

laboratory with a result of 515.3 Ma on the basis of two 

K20 and two Ar measurements (Samson and Alexander, 

1987). Denver personnel have not measured the age of 

Taylor Creek Rhyolite sanidine but obtained a conventional K-Ar age of 28.5±0.8 Ma for Fish Canyon Tuff 

sanidine (Steven and others, 1967). The Denver laboratory 

normally uses the published mean value of 520.4 Ma for 

MMhb-1 and adjusts the ages of Fish Canyon Tuff and 

Taylor Creek Rhyolite upward from the Menlo Park values 

to account for the difference between 513.9 Ma and 520.4 

Ma for MMhb-1. 

For the purposes of this paper, we have calculated all 

ages measured in both Menlo Park and Denver using the 

monitor mineral values measured by the Menlo Park laboratory. Conversion of a 40 ArP9 ~r age for an unknown 

sample from one monitor value to another can be done using 

where t

1

=original age, t

2=converted age, tm 1=monitor age 

used for ti' tm2=monitor age used for t

2, and A. (decay 

constant)=5.543x1o- 10 yr- 1• 

Although the conversion is not linear, the middle Tertiary to K-T boundary ages in this paper may be converted, 

without significant error, from a MMhb-1 value of 513.9 

Ma to one of 520.4 Ma by multiplying the ages by 1.0144. 

RESULTS 

LASER TOTAL FUSION ANALYSES 

The results of 53 laser total fusion analyses of single 

tektites, ranging (of those weighed) in mass from 0.28 to 
8 AGES OF TEKTITES FROM CRETACEOUS-TERTIARY BOUNDARY ROCKS, BELOC FORMATION, HAITI 

0.92 mg, and of small groups (2-3) of tektites are given in 

table 2 and figure 5. The analyses all have high yields of 

radiogenic 40 Ar, typically above 98 percent, and estimated 

standard deviations of precision of 0.6-0. 7 percent, which 

includes the estimated error in J. The K/Ca ratios of the 

measured tektites, as calculated from the 39 Arf37 Ar ratios, 

range from 0.14 to 0.36, and thus the dated tektites represent a range of compositions (fig. 6). We did not, however, 

analyze any of the high Ca (honey-colored) tektites, which 

have K/Ca ratios as low as 0.02 (Sigurdsson and others, 

1991; Izett, 1991). 

The total range of the calculated 40 Arf39 Ar ages is 

from 63.34±0.40 Ma to 66.30±1.33 Ma, or 4. 7 percent. 

Excluding the greatest age, which appears anomalous (fig. 

5), the remaining 52 40 ArP9 Ar ages have a mean and standard deviation of 64.42±0.41 Ma and a weighted mean and 

standard error of the mean ( abest) of 64.42±0.06 Ma. The 

rather large range of ages is not entirely due to analytical 

error. Much of it is due to biases in the irradiation parameter, J, caused at least in part by using two different reactors and five different irradiations. The tektite ages from 

irradiation JD06, for example, are biased toward younger 

values relative to the ages from the other irradiations (fig. 

8 

0 

63.2 63.6 64.0 64.4 

0 

63.2 63.6 64.0 64.4 

Z-coal bentonite 

sanidine 

64.8 65.2 65.6 66.0 66.4 

Haitian tektites 

64.8 65.2 65.6 66.0 66.4 

-«JAr/YIAr age (Ma) 

lrrodlotton number: ~ 105 [J.D06 

.GlN3 ~ 108 O.DOS 

Figure 5. Histogram of 40 Art39 Ar laser system total fusion ages 

on 53 single Haitian tektites and on 28 samples of the Z-coal 

bentonite (ZCB) sanidine. The arithmetic means and standard 

deviations of the two data populations are shown by the filled 

circles with bars. The two ages >66 Ma are not included in the 

means. 

5). Ages from irradiation 108 have a wider range of values 

than those from the other irradiations, which is probably 

due, at least partly, to imprecision in determining the ]-

values for that irradiation (fig. 3). The tektites analyzed in 

irradiations 105, 108, JD06, and JD08 were small fragments with high surface/volume ratios, the best material 

having been used for irradiation GLN3. Some of the dispersion in the ages may also be due to slight alteration of 

16 

14 

12 

~ 10 

c 

Q) 

J 

0" 8 Q) 

..... 

u.. 

6 

4 

2 

0 

0.1 0.2 0.3 0.4 

K/Ca 

Figure 6. K/Ca ratios for the 53 laser system total fusion 

measurements on individual tektites, as calculated from the 

39 Ar/37 Ar ratios. 
RESULTS 9 

Table 2. 40 ArP9 Ar laser total fusion age data on single Haiti tektites and Z-coal bentonite sanidine. 

Sample Weighta 40Arf39Arb 37Arf39AJb 36Arf39AJb 40AfraJJC 39Areac 36Areac Aged 

No. (mg) (%) (%) (%) (Ma) 

Haiti tektites 

(Irradiation GLN3, J=0.004376) 

90G15K-5 n.d. 8.544 3.4411 0.001722 97.1 0.2 53.8 64.50 ± 0.43 

90G15K-6 n.d.(2) 8.990 2.8409 0.003196 91.9 0.2 23.9 64.19 ± 0.39 

90G15K-7 n.d. 8.728 3.3032 0.002443 94.6 0.2 36.4 64.19 ± 0.47 

90Gl5K-8 0.316 8.369 1.3805 0.000618 99.0 0.1 60.1 64.32 ± 0.39 

90G15K-9 0.376 8.334 1.4455 0.000597 99.2 0.1 65.2 64.14 ± 0.38 

90G15K-10 0.49 8.325 1.4264 0.000379 99.9 0.1 100.0 64.55 ± 0.41 

90G15K-11 0.279 8.377 1.6067 0.000697 99.0 0.1 62.0 64.35 ± 0.41 

90G15K-12 0.33 8.467 2.0101 0.000980 98.4 0.1 55.2 64.66 ± 0.39 

90Gl5K-13 0.548 8.374 1.4377 0.000686 98.8 0.1 56.4 64.24 ± 0.38 

90Gl5K-14 0.452 8.346 2.7090 0.000752 99.8 0.2 96.9 64.70 ± 0.39 

90G15K-15 0.514 8.370 2.9835 0.000885 99.6 0.2 90.7 64.77± 0.39 

90G15K-16 0.772 8.409 1.5404 0.000643 99.1 0.1 64.5 64.67 ± 0.38 

90G15K-17 0.537 8.373 1.5350 0.000695 98.9 0.1 59.4 64.28 ± 0.38 

90G15K-18 0.39 8.369 1.6373 0.000769 98.7 0.1 57.3 64.15 ± 0.38 

90G15K-19 0.548 8.383 1.6028 0.000565 99.4 0.1 76.3 64.69 ± 0.38 

90G15K-20 0.759 8.694 2.7776 0.001927 95.9 0.2 38.8 64.75 ± 0.38 

90G15K-21 0.602 8.344 1.5262 0.000428 99.8 0.1 96.0 64.65 ± 0.38 

90G15K-22 0.496 8.363 1.4912 0.000607 99.2 0.1 66.1 64.37 ± 0.38 

90G15K-23 0.436 8.355 2.0282 0.000617 99.6 0.1 88.4 64.63 ± 0.38 

90GI5K-24 0.631 8.360 1.5753 0.000432 99.9 0.1 98.1 64.80 ± 0.38 

90Gl5K-25 0.923 8.371 1.5931 0.000640 99.1 0.1 66.9 64.42 ± 0.38 

90G15K-26 0.748 8.375 1.4649 0.000541 99.4 0.1 72.8 64.59 ± 0.37 

90G15K-27 0.411 8.342 1.3645 0.000558 99.2 0.1 65.8 64.24 ± 0.38 

weighted mean ± abest = 64.48 ± 0.08 

mean ±a= 64.47 ± 0.22 

(Irradiation 105, J=O.O 1 0398) 

90G15K-28 0.410 3.573 2.001 0.000830 97.6 0.1 66.3 64.35 ± 0.44 

90G15K-29 0.117 3.610 1.875 0.000722 98.2 0.1 71.4 65.42 ± 0.62 

90G15K-30 0.212 3.576 2.302 0.001114 96.0 0.2 56.8 63.34 ± 0.40 

90GI5K-31 0.241 3.539 1.522 0.000551 98.8 0.1 76.0 64.50 ± 0.56 

90G15K-32 0.371 3.594 2.057 0.000900 97.2 0.1 62.9 64.45 ± 0.38 

weighted mean± obest = 64.25 ± 0.20 

mean±a= 64.41 ±0.74 

(Irradiation 105, J=0.010452) 

90G15K-33 0.173 3.529 1.857 0.000598 99.2 0.1 85.4 64.91 ± 0.42 

90GI5K-34 0.280 3.519 1.378 0.000548 98.5 0.1 69.2 64.26 ± 0.38 

90G15K-35 0.390 3.589 2.381 0.001010 97.0 0.2 64.8 64.59 ± 0.42 

90G15K-36 0.292 3.530 2.011 0.000761 98.2 0.1 72.6 64.29 ± 0.43 

90G15K-37 0.221 3.669 2.592 0.001374 94.6 0.2 51.9 64.40 ± 0.56 

weighted mean± abest = 64.49 ± 0.19 

mean±a= 64.49 ± 0.27 

individual tektites, but the dated tektites optically appear 

unaltered and we have no way to quantitatively evaluate 

the magnitude of this effect, if any. None of these factors 

affect the mean tektite age appreciably, but they do affect 

the dispersion and hence the precision of the mean age. 

Our analyses show no significant difference in age between the ZCB sanidines from JFL-500C and 83-0-05, 

and we treat them as a single data set even though the two 

bentonites are separated stratigraphically by 20 em. The 28 

measurements from the two ZCB sanidine samples (table 

2; fig. 5) have a mean and standard deviation of 

64.75±0.33 Ma and a weighted mean and standard error 

(obest) of 64.77±0.07 Ma (value of 66.52 Ma excluded), 

which is not significantly different from the mean of the 

tektite total fusion ages at the 95 percent level of confidence. As with the tektites, there are apparent biases between irradiations-irradiation 108, for example, is biased 

toward higher ages relative to irradiations GLN3 and 
10 AGES OF TEKTITES FROM CRETACEOUS-TERTIARY BOUNDARY ROCKS, BELOC FORMATION, HAITI 

Table 2. Continued. 

Sample Weight a 40Arf39Arb 37Arf39Arh 36Arf39Arb 40Arral 39Arcac 36Arcac Aged 

No. (mg) (%) (%) (%) (Ma) 

Haiti tektites 

(Irradiation 108, J=0.009452) 

90G15K-38 n.d. 4.055 2.321 0.001196 95.9 0.2 53.4 65.20 ± 0.37 

90G15K-39 n.d. 3.905 1.433 0.000532 98.9 0.1 74.1 64.74 ± 0.62 

90G15K-40 n.d. 3.917 2.240 0.000713 99.2 0.2 86.4 65.17 ± 0.93 

90G15K-41 n.d. 4.104 2.579 0.001615 93.4 0.2 43.9 64.33 ± 0.66 

90G15K-42 n.d. 3.976 2.393 0.000725 99.4 0.2 90.7 (66.30 ± 1.33) 

weighted mean± abest = 64.96 ± 0.27 

mean± a= 64.86 ± 0.41 

(Irradiation 108, J=0.009495) 

90G15K-43 n.d. 3.858 1.990 0.000775 98.2 0.1 70.6 63.84 ± 0.82 

90G15K-44 n.d. 3.891 1.924 0.000749 98.3 0.1 70.6 64.42 ± 0.35 

90G15K-45 n.d. 3.868 3.068 0.000964 99.1 0.2 87.5 64.57 ± 1.01 

90G15K-46 n.d. 3.932 2.179 0.000755 98.8 0.1 79.4 65.41 ± 0.42 

90G15K-47 n.d. 3.920 2.867 0.001170 97.1 0.2 67.4 64.15 ± 0.79 

weighted mean± abest = 64.67 ± 0.24 

mean± a= 64.48 ± 0.59 

(Irradiation JD06, J=0.006862) 

90G15R-1 1.017 5.369 1.667 0.000725 98.1 0.1 53.6 64.13 ± 0.40 

90G15R-2 1.089 (2) 5.348 2.000 0.000930 97.4 0.1 50.1 63.44 ± 0.45 

90G15R-3 1.343 (2) 5.393 2.157 0.000914 97.7 0.1 55.0 64.17 ± 0.38 

90G15R-4 1.066 5.386 1.814 0.000923 97.2 0.1 54.2 63.76 ± 0.40 

90G15R-5 1.468 (2) 5.400 2.052 0.000905 97.6 0.1 52.8 64.20 ± 0.38 

90G15R-6 1.187 (2) 5.429 1.832 0.001007 96.8 0.1 42.4 63.99 ± 0.40 

weighted mean± abest = 63.97 ± 0.16 

mean± a= 63.95 ± 0.30 

(Irradiation JD08, J=0.006910) 

90G15R-7 0.352 (2) 5.893 2.660 0.002708 89.9 0.2 26.4 64.94 ± 0.48 

90G15R-8 0,277 (3) 5.442 2.303 0.001297 96.2 0.2 47.8 64.17 ± 0.46 

90G15R-9 0.388 (2) 5.545 1.691 0.001525 94.1 0.1 29.8 64.00 ± 0.45 

90G15R-10 0.379 (3) 5.401 2.145 0.001137 96.8 0.1 50.7 64.09 ± 0.46 

weighted mean ± abest = 64.28 ± 0.23 

mean± a= 64.30 ± 0.43 

Z-coal bentonite sanidine 

(Irradiation GLN3, J=0.004404) 

JFL-500C n.d. (m) 8.365 0.0060 0.000345 98.7 0.0 0.5 64.42 ± 0.42 

JFL-500C n.d. (m) 8.318 0.0062 0.000138 99.4 0.0 1.2 64.52 ± 0.38 

JFL-500C n.d. (m) 8.337 0.0065 0.000110 99.5 0.0 1.6 64.73 ± 0.38 

weighted mean± abest = 64.57 ± 0.23 

mean± a= 64.56 ± 0.16 

(Irradiation 105, J=0.010322) 

JFL-500C n.d.(m) 3.585 0.0066 0.000192 98.4 0.0 0.9 64.50 ± 0.44 

JFL-500C n.d.(m) 3.574 0.0068 0.000095 99.2 0.0 2.0 64.82 ± 0.38 

JFL-500C n.d.(m) 3.586 0.0081 0.000142 98.8 0.0 1.6 64.79 ± 0.38 

JFL-500C n.d.(m) 3.570 0.0063 0.000105 99.1 0.0 1.7 64.69 ± 0.46 

JFL-500C n.d.(m) 3.591 0.0083 0.000195 98.4 0.0 1.2 64.60 ± 0.43 

JFL-500C n.d.(m) 3.586 0.0064 0.000050 99.5 0.0 3.5 65.27 ± 0.39 

weighted mean ± abest = 64.80 ± 0.17 

mean ±a= 64.79±0.27 
Table 2. Continued. 

Sample 

No. 

Wcighta 

(mg) 

(Irradiation 105, J=0.010452) 

JFL-500C n.d:(m) 

JFL-500C n.d.(m) 

JFL-500C n.d.(m) 

JFL-500C n.d.(m) 

JFL-500C n.d.(m) 

(Irradiation 108, J=0.009474) 

JFL-500C n.d.(m) 

JFL-500C n.d.(m) 

JFL-500C n.d.(m) 

JFL-500C n.d.(m) 

JFL-500C n.d.(m) 

(Irradiation JD06, J=0.006862) 

83-0-05 0.192(12) 

83-0-05 0.147(9) 

83-0-05 0.177(8) 

83-0-05 0.113(8) 

83-0-05 0.145(9) 

(Irradiation JD08, J=0.006910) 

83-0-05 0.188(10) 

83-0-05 0.074(5) 

83-0-05 0.062(5) 

83-0-05 0.083(6) 

83-0-05 0.076(6) 

3.526 

3.525 

3.525 

3.523 

3.533 

3.899 

3.926 

3.980 

4.026 

3.908 

5.350 

5.341 

5.349 

5.524 

5.344 

5.401 

5.358 

5.421 

5.514 

5.355 

Haiti tektites, all data 

(n=52) 

Z-coal bentonite sanidine, all data 

(n=28) 

RESULTS 

Z-coal bentonite sanidine 

0.0069 0.000070 99.4 

0.0061 0.000058 99.5 

0.0064 0.000073 99.3 

0.0063 0.000023 99.8 

0.0077 0.000099 99.1 

weighted mean ± O'best = mean ±a= 

0.0064 0.000093 99.2 

0.0073 0.000054 99.5 

0.0076 0.000290 97.8 

0.00066 0.000451 96.6 

0.00740 0.000149 98.8 

weighted mean± O'best = mean ±a= 

0.00588 0.0001704 99.0 

0.02839 0.0001024 99.4 

0.02687 0.0001423 99.2 

0.02752 0.0001688 99.1 

0.00546 0.0001330 99.2 

weighted mean ± O'best = mean± 0' = 

0.01809 0.000296 98.2 

0.00532 0.000366 97.8 

0.00523 0.000542 96.9 

0.00547 0.000816 95.5 

0.03895 0.000306 98.2 

weighted mean± O'best = mean± 0' = 

weighted mean ± O'best = 

mean± 0' = 

std. error of mean = 

weighted mean ± O'best = 

mean± 0' = 

std. error of mean = 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

2.7 

2.9 

2.4 

7.7 

2.1 

64.90 ± 0.17 

1.9 

3.7 

0.7 

0.4 

1.4 

65.20±0.17 

0.8 

6.5 

4.4 

3.8 

1.0 

64.50 ± 0.19 

1.6 

0.4 

0.3 

0.2 

3.4 

64.49 ± 0.19 

Aged 

(Ma) 

11 

64.86 ± 0.37 

64.91 ± 0.37 

64.84 ± 0.37 

65.07 ± 0.38 

64.83 ± 0.37 

64.90 ± 0.10 

64.95 ± 0.42 

65.59 ± 0.37 

65.32 ± 0.35 

65.31 ± 0.48 

64.82 ± 0.36 

65.20 ± 0.31 

64.43 ± 0.37 

64.57 ± 0.38 

64.53 ± 0.40 

(66.52 ± 0.42) 

64.48 ± 0.38 

64.50 ± 0.06 

64.96 ± 0.38 

64.18 ± 0.42 

64.31 ± 0.46 

64.46 ± 0.44 

64.39 ± 0.43 

64.46 ± 0.30 

64.42 ± 0.06 Ma 

64.42 ± 0.41 Ma 

0.07Ma 

64.77 ± 0.07 Ma 

64.75 ± 0.33 Ma 

0.06 Ma 

a Number of tektites or crystals in parentheses where n> 1 ~ n.d., not detennined~. m, multiple crystals, number not determined 

b Corrected for 37 Ar decay (half-life=35.1 days) and 39 Ar decay (half-life= 269 years). 

c Subscripts: rad, radiogenic~ K, potassium-derived~ Ca, calcium derived. 

d Decay constants: A.E=0.581x10-10yr-1, A.8=4.692x10-10yr- 1. Errors assigned to individual ages are estimates of the standard deviation 

of analytical precision. Weighted means and crbest are weighted by the inverse of the variance (Taylor, 1982). Value in() not 

included in means. Reactor corrections for irradiation GLN-3: e6Arf37Ar)c8

=0.000251 ± 8, e9Arf37Ar)ca = 0.000671 ± 13, 

(

40Arf39

Ar)K = 0.0285 ± 47. Reactor corrections for irradiations 105 and 108: e6Arf37Ar)ca=0.000275 ± 3, e9Arf37Ar)ca = 

0.000675 ± 3, (

40Arf39Ar)K = 0.00240 ± 2. Reactor corrections for irradiations JD0-6 and JD0-8: e6

Arf37Ar)ca=0.000269 ± 2, 

e9Arf37Ar)ca = 0.000670 ± 5, (

40Arf39

Ar)K = 0.0091 ±54. 
12 AGES OF TEKTITES FROM CRETACEOUS-TERTIARY BOUNDARY ROCKS, BELOC FORMATION, HAm 

Table 3. Means and standard deviations of 40 ArP9 Ar laser total-fusion ages on Z-coal bentonite sanidine and 

single Haitian tektites (HT), in Ma. 

[Where IIIT-ZCBI>CV, the means are significantly different at the 95% level of confidence.] 

Irradiation liT N ZCB N IIIT-ZCBI cv 

GLN3 64.47 ± 0.22 23 64.56 ± 0.16 3 0.09 0.20 

105 64.45 ± 0.52 10 64.83 ± 0.21 11 0.38 0.34 

108 64.65 ± 0.53 9 65.20 ± 0.31 5 0.55 0.44 

JD06 63.95 ± 0.30 6 64.50 ± 0.06 4 0.55 0.25 

1008 64.30 ± 0.43 4 64.46 ± 0.30 5 0.16 0.49 

All data 64.42 ± 0.41 52 64.75 ± 0.33 28 0.33 0.17 

CV, critical value. 

JD08-but these biases probably do not have an appreciable effect on the mean age calculated from all 28 measurements. 

Taken at face value, the statistics indicate that the 

mean age of the tektites and the mean age of the ZCB 

sanidine is significantly different at the 95 percent level of 

confidence (table 3). As discussed above, however, the 

two sets of data do not represent homogeneous populations. Comparing the means of the tektites and the ZCB 

sanidine obtained from the individual irradiations makes 

the apparent age differences less clear. The means from 

irradiations GLN3 and JD08 are not significantly different 

at the 95 percent level of confidence, whereas those from 

irradiations 105, 108, and JD06 are (table 3). Thus, although there is a tendency in our data for higher ages from 

the ZCB sanidine, we think that it is primarily due to imprecision in determining the neutron-efficiency factor, J, 

for the different irradiations and different sample positions 

and do not think that it is geologically significant. 

LASER INCREMENTAL HEATING ANALYSES 

Incremental heating analyses were done with the laser 

system on four single tektites weighing from 0.87 to 1.24 

mg (table 4). All four of the age spectra have plateaus over 

96 percent or more of the 39 Ar released and show no evidence that the tektite glass has been subjected to post-solidification alteration or thermal disturbance (fig. 7). The 

K/Ca ratios of the various gas fractions, as calcuated from 

the 39 ArP7 Ar values, are uniform over the full range of 

39 Ar released (fig. 8), indicating that the tektite glass is an 

unaltered, homogeneous phase. The plateau ages range 

from 64.35±0.35 Ma to 64.45±0.35 Ma and have a 

weighted mean of 64.38±0.18 Ma, which is not different 

from the weighted mean of the total fusion ages 

(64.42±0.07 Ma) at the 95 percent level of confidence. 

The isochron ages calculated from the incremental heating 

data are concordant with the age spectrum plateau ages, 

have 40 Arf36 Ar intercepts indistinguishable from the 

atmospheric value, and have good statistical fits (table 6; · 

fig. 9). 

Because the infrared microscope is equipped for optical viewing, it was possible to observe the behavior of the 

tektite glass during incremental heating. Physical changes 

in the tektites as temperature increased are of interest and 

provide information about the tektite glass as a 40 Arf39 Ar 

geochronometer. From room temperature to about 90<YC, 

no visible change occurred in the general shape (equant) 

and color (opaque and black) of the tektites. Moreover, <1 

percent of the 39 Ar was liberated during the low temperature heating steps. At 900°C, slight changes in the surface 

reflectivity of the tektites occurred. At about 1,000°C, the 

surfaces of the tektites changed from rough to smooth and 

angular ridges separating sculptured areas disappeared, but 

the original shape and color of the tektites were unchanged. Near 1,200°C, their surfaces were completely 

smooth and their overall shapes were oviform or spherical. 

Heating above 1 ,200°C converted the tektites to perfect 

spheres although only a cumulative 10-15 percent of their 

total 39 Ar content had been expelled by that and the lower 

temperature increments. From 1,200 to 1 ,500°C, the shape 

and color of the tektites did not change. At the maximum 

temperature ( -1 ,500°C) the glass abruptly became clear 

and small patches of a black crust appeared on the surfaces 

of the glass spheres. Unlike diffusion of Ar from most 

minerals, the diffusion of Ar from the tektite glass was 

surprisingly slow even at very high (>1,200°C) temperatures, despite the fact that the glass was soft enough to be 

reshaped into perfect spheres by surface tension. This indicates that unaltered tektite glass remains a closed system to 

Ar even at low and moderate temperatures and is, therefore, an excellent 40 ArJ39 Ar geochronometer. 
RESULTS 13 

Table 4. Analytical data for 40ArP9Ar laser age spectra on single K-T tektites from Haiti. 

Temp. a 40Arf39Ar 37Arf39Afh 36Arf39Ar 40Arradc 39Arcac 36Arcac 39Ar Aged 

(oC) (%) (%) (%) (% oftota1) (Ma) 

90G15K-1, 0.870 mg (Irradiation GLN-3, J=0.004376) 

600 107.7 4.361 0.3101 15.2 0.3 0.4 0.03 125. ± 175 

650 19.09 2.179 0.0442 32.5 0.1 1.3 0.03 48.4 ± 201 

700 15.46 1.855 0.0230 57.0 0.1 2.2 0.05 68.4 ± 103 

750 10.24 2.037 0.01094 69.9 0.1 5.0 0.1 55.7 ± 25 

800 8.109 1.964 0.00718 75.6 0.1 7.4 0.2 47.8 ± 16 

850 8.360 2.227 0.01380 53.2 0.1 4.4 0.3 34.8 ± 18 

900 8.179 2.268 0.00921 68.8 0.2 6.6 0.4 44.0 ± 14 

950 8.431 2.243 0.00518 83.8 0.2 11.6 0.7 55.0 ± 7.9 

1000 8.281 2.217 0.001074 98.1 0.1 55.5 1.1 63.16 ±4.8 

1050 8.335 2.259 0.000843 99.1 0.2 72.1 1.5 64.13 ± 1.91 

1100 8.306 2.294 0.000789 99.3 0.2 78.2 1.6 64.05 ± 1.79 

1140 8.225 2.338 0.000895 98.9 0.2 70.3 1.4 63.21 ± 2.03 

1200 8.230 2.366 0.000766 99.4 0.2 83.1 1.6 63.57 ± 1.74 

120X 8.364 2.327 0.000765 99.4 0.2 81.9 2.9 64.57 ± 1.91 

121X 8.243 2.358 0.000979 98.7 0.2 64.8 2.5 63.18±1.12 

122X 8.265 2.361 0.000854 99.1 0.2 74.3 4.3 63.63 ± 1.94 

123X 8.299 2.342 0.000792 99.3 0.2 79.5 4.6 64.02 ± 1.80 

124X 8.300 2.381 0.000753 99.5 0.2 85.0 6.5 64.14 ± 0.46 

125X 8.329 2.388 0.000705 99.7 0.2 91.1 12.2 64.48 ± 0.56 

126X 8.352 2.381 0.000671 99.8 0.2 95.4 18.3 64.72 ± 0.36 

127X 8.354 2.373 0.000869 99.1 0.2 73.4 17.4 64.29 ± 0.37 

128X 8.353 2.415 0.000827 99.3 0.2 78.5 15.3 64.40 ± 0.41 

129X 8.356 2.381 0.000766 99.4 0.2 83.6 4.8 64.53 ± 3.3 

130X 8.365 2.396 0.00410 87.7 0.2 15.7 1.7 57.08 ± 3.2 

131X 8.639 2.291 0.00417 87.4 0.2 13.8 0.5 58.8 ± 11 

90G 15K-2, 1.117 mg (Irradiation GLN-3, J=0.004376) 

800 10.55 2.282 0.00474 88.4 0.2 13.0 0.2 72.2 ± 11 

900 8.429 2.381 0.00209 94.8 0.2 30.6 0.5 62.1 ± 4.7 

1000 8.422 2.550 0.000864 99.3 0.2 79.4 1.2 64.93 ± 1.96 

1090 8.372 2.551 0.000950 99.0 0.2 72.2 2.1 64.36 ± 2.15 

1200 8.334 2.567 0.000701 99.9 0.2 98.6 4.3 64.64 ± 1.59 

121X 8.306 2.561 0.000889 99.2 0.2 77.4 3.4 63.99 ± 1.02 

122X 8.365 2.596 0.001100 98.5 0.2 63.5 5.5 63.99 ± 2.49 

123X 8.320 2.562 0.000727 99.8 0.2 94.8 9.7 64.47 ± 0.51 

124X 8.339 2.522 0.000850 99.3 0.2 79.9 16.6 64.31 ± 0.33 

125X 8.339 2.511 0.000807 99.4 0.2 83.7 20.0 64.40 ± 0.29 

126X 8.327 2.492 0.000708 99.8 0.2 94.7 15.7 64.52 ± 0.37 

127X 8.362 2.475 0.000867 99.2 0.2 76.8 10.1 64.42 ± 0.48 

128X 8.394 2:457 0.000776 99.5 0.2 85.2 9.1 64.85 ± 0.48 

129X 8.447 2.512 0.00341 90.3 0.2 19.8 1.5 59.36±2.17 

130X 9.281 2.708 0.0302 0.1 0.2 0.2 0.1 4.4 ± 33 

90G15K-3, 1.240 mg (Irradiation GLN-3, J=0.004376) 

900 8.586 1.524 0.000606 99.2 0.1 67.6 1.2 66.10 ± 1.38 

1000 8.283 1.574 0.000478 99.7 0.1 88.7 1.6 64.11 ± 1.09 

1100 8.283 1.579 0.000510 99.6 0.1 83.3 2.9 64.03 ± 1.16 

1200 8.328 1.577 0.000858 98.4 0.1 49.4 4.7 63.59 ± 0.73 

121X 8.304 1.570 0.000512 99.6 0.1 82.5 5.0 64.19 ± 0.69 

122X 8.324 1.557 0.000456 99.8 0.1 91.9 8.1 64.46 ± 0.45 

123X 8.326 1.541 0.000444 99.8 0.1 93.5 10.0 64.49 ± 0.38 

ll4X 8.335 1.513 0.000499 99.6 0.1 81.6 11.3 64.41 ± 0.35 

ll5X 8.324 1.504 0.000503 99.5 0.1 80.4 10.8 64.31 ± 0.36 
14 AGES OF TEKTITES FROM CRETACEOUS-TERTIARY BOUNDARY ROCKS, BELOC FORMATION, HAITI 

Table 4. Continued. 

Temp. a 40Arf39Ar 37Arf39Arb 36Arf39Ar 40Arradc 39Arcac 36Arcac 39Ar Aged 

(°C) (%) (%) (%) (% oftotal) (Ma) 

126X 8.335 1.499 0.000651 99.0 0.1 61.9 9.7 64.06 ± 0.39 

127X 8.359 1.478 0.000406 99.9 0.1 98.0 9.1 64.79 ± 0.41 

128X 8.327 1.481 0.000480 99.6 0.1 82.9 7.7 64.38 ± 1.10 

129X 8.357 1.467 0.000489 99.6 0.1 80.7 7.6 64.58 ± 0.48 

130X 8.370 1.467 0.000745 98.7 0.1 53.0 8.9 64.10 ± 0.42 

131X 8.718 1.489 0.00386 88.2 0.1 10.4 1.3 59.75 ± 2.68 

90Gl5K-4. 1.082 mg (Irradiation GLN-3. J=0.004376) 

900 8.641 1.615 0.000820 98.6 0.1 53.0 2.2 66.09 ± 1.85 

1000 8.322 1.614 0.000649 99.1 0.1 66.9 2.8 64.05 ± 1.47 

1100 8.307 1.612 0.000440 99.9 0.1 98.5 4.1 64.40 ± 1.00 

1200 8.287 1.614 0.000751 98.8 0.1 57.8 5.5 63.55 ± 0.77 

121X 8.307 1.614 0.000646 99.1 0.1 67.2 5.9 63.94 ± 0.71 

122X 8.362 1.613 0.000544 99.5 0.1 79.8 8.3 64.59 ± 0.64 

123X 8.310 1.628 0.000600 99.3 0.1 73.0 14.6 64.07 ± 0.33 

124X 8.313 1.651 0.000465 99.8 0.1 95.5 13.6 64.41 ± 0.35 

125X 8.320 1.665 0.000466 99.8 0.1 96.1 11.7 64.48 ± 0.39 

126X 8.345 1.676 0.000488 99.8 0.1 92.4 10.8 64.62 ± 0.42 

127X 8.317 1.661 0.000468 99.8 0.1 95.4 15.5 64.44 ± 0.32 

128X 8.354 1.652 0.000447 99.9 0.1 99.4 4.1 64.77 ± 1.02 

129X 8.719 1.611 0.000544 82.9 0.1 8.0 0.9 56.26 ±4.6 

a Plateau increments indicated in bold type. Increments extracted above 1200°C are numbered sequentially, terminated with an 

"X", and have no temperature significance. 

b Corrected for 37 Ar decay, half-life=35.1 days. 

c Subscripts: rad, radiogenic~ K, potassimn-derived~ Ca, calcium derived. 

d Decay constants: A.E=0.58lx10·10yr-1, A.p=4.692xlo-10yr-1• Errors assigned to individual ages are estimates of the standard 

deviation of analytical precision. Reactor corrections for irradiation GLN-3: e6Arf37

Ar)ca=0.000251 ± 8, e9

Arf37

Ar)ca = 

0.000671 ± 13, (

40Arf39Arh< = 0.0285 ± 47. 

VACUUM FURNACE INCREMENTAL HEATING 

ANALYSES 

Incremental heating analyses were done on two bulk 

samples of tektites (67 .3 mg and 83.4 mg) using a doublevacuum resistance furnace (fig. I 0; tables 5, 6). Both of 

the resulting age spectra are flat with similar plateau ages 

of 64.65±0.22 Ma and 64 . .46±0.11 Ma over 56.5 percent 

and 74.8 percent of the 39 Ar released, respectively, and a 

weighted mean for the pair of measurements of 64.50±0.1 0 

Ma. The total-gas ages for the two analyses are 64.50±0. 79 

Ma and 64.80±0.40 Ma, respectively. Unlike the age spectra obtained with the laser system, both of the furnace age 

spectra exhibit minor excess 40 Ar in the first 5 percent of 

the 39 Ar released, which may be due to 39 Ar recoil from a 

very small amount of non-glass phases (clays?) in the bulk 

sample. The isochron ages for the plateau parts of each 

analysis are 64.98±0.10 Ma and 64.52±0.08 Ma, respectively, and have 40Arf36Ar intercepts indistinguishable 

from the atmospheric value. 

DISCUSSION 

The tektites we analyzed give 40 Arf39 Ar ages that indicate that the fresh tektite glass cores have remained undisturbed K-Ar systems since they were formed by the 

K-T impact event at the close of the Cretaceous Period. 

The weighted mean of 58 of our our 59 analyses (52 total 

fusion, 6 incremental heating, one anomalous total fusion 

value excluded) is 64.43±0.05 Ma, which is our best estimate for the age of the tektites and, therefore of both the 

K-T impact and the K-T boundary. 

Since we first reported our results (lzett and others, 

1991 a, 1991 b), three other laboratories have reported ages 

for the Haiti tektites (fig. 11). Gillot and others (1991) 

measured two conventional K-Ar ages on ground, bulk 

tektite samples using the peak-intensity method, which 

does not require the use of a 38 Ar tracer. Their results 

were concordant (64.27±0.40 Ma, 63.63±0.40 Ma) with a 

mean value of 64.0±0.35 Ma (lo). 
DISCUSSION 15 

Hall and others ( 1991) obtained flat and concordant 

age spectra on four single tektites using a continuous laser 

system and methods similar to ours. The mean of their 

four values, 64.75±0.08Ma (Is), is based on a monitor 

mineral (3gr homblende=l,071 Ma) that does not depend 

for its calibration on MMhb-1. Hall and his colleagues 

commented that a comparison of their tektite results with 

ours (lzett and others, 1991 a) and two analyses of 

""" 0 

~ 

-

<D 

0) 

0 

"0 

<D 

..... 

0 

::J 

0 

0 

u 

75 ~-----------------------------------, 

70 ~-~----

90G lSK-1 (0.870 mg) 

64.36t0.36 

65 

70 64.45+0.35 

65 

60 

70 

64.35t0.35 

65 

60 

55 ~~--~--~--~--~--~--~--~~--~ 0 20 40 60 80 1 00 

39 Ar released (%) 

Figure 7. 40 ArP9 Ar age spectra for four single Haitian tektites 

measured with the laser system. The weighted means and 

standard deviations of the plateau increments are shown for each 

age spectrum. The 1 ,200°C increment, above which the IR 

microscope was not functioning properly, is labeled. Some of 

the lowest temperature increments have been omitted from the 

age spectra because their 39 Ar content was too small to plot. 

MMhb-1 using 3gr as the monitor mineral suggest that the 

correct age for MMhb-1 may be 517 Ma, which is midway between the Menlo Park USGS value of 513.9 Ma 

and the international mean of 520.4 Ma. 

McWilliams and others (1992) recently reported a 

mean age for Haiti tektites of 64.91±0.06 Ma (lo) using 

Taylor Creek Rhyolite sanidine (27.92 Ma) as the monitor. 

They claim that this age is marginally older than their 

mean value for sanidine from three bentonite beds that lie 

immediately above the K-T boundary at three localities in 

western North America (see table below). 

Swisher and others ( 1992) reported weighted mean 

ages of 65.07±0.11 Ma and 65.01±0.08 Ma for three 40Ar/ 

39 Ar total fusion and nine 40 ArP9 Ar incremental heating 

experiments on single Haitian tektites. They also determined a weighted mean age of 65.06±0.25 Ma for 40 Ar/ 

39 Ar total fusion ages on two single tektites from the Arroyo el Mimbral K-T locality in northeastern Mexico. 

Their ages are based on a MMhb-1 value of 520.4 Ma and 

convert to values of 64.15 Ma (total fusion) and 64.09 Ma 

(incremental heating) for the Haitian tektites, and 64.14 

Ma for the Arroyo el Mimbral tektites. 

In addition to the ages for the Haitian tektites, there 

have been several other recent studies directed toward determining the age of the K-T boundary (fig. 11 ). 

Obradovich (1984) and Obradovich and Sutter (1984) reported a single 40 Ar/39 Ar age spectrum age of 66.0±0.27 

Ma (lo) for sanidine from JFL-500C, which occurs in the 

Z-coal -70 em above the K-T boundary near Hell Creek, 

Montana. Their age for the sanidine is based on a 

MMhb-1 age of 519.5 Ma and converts to an age of 65.1 

Ma for MMhb-1=513.9 Ma. 

Baadsgaard and others ( 1988; also Baadsgaard and 

Lerbekmo, 1983) measured K-Ar, Rb-Sr, and U-Pb ages 

on minerals from bentonites associated with three coal 

seams that lie stratigraphically above, but very close to, the 

0 

u 

~ 

0.4 r------------------------------------. 

90Gl5K-3 

I I II I I/' I I I II .. I •' I I I I 

••••••• rl •• 

0.3 ---·----

0.2 

90Gl5K-4 

90Gl5K-l 

90Gl5K-2 

-------

0.1 ~---I...---1----L---L----L-----L..--....1.... __ ..1..-___J'---_j 

0 20 40 60 80 100 

39Ar released (%) 

Figure 8. K/Ca diagrams for the four tektites measured with the 

laser system. The K/Ca ratios for each increment are calculated 

from the 39 ArP7 Ar ratios. 
16 AGES OF TEKTITES FROM CRETACEOUS-TERTIARY BOUNDARY ROCKS, BELOC FORMATION, HAITI 

K-T boundary. These include the Z-coal near Hell Creek, 

Montana (70 em above the K-T boundary in the upper part 

of the Z-coal), the Ferris coal near Frenchman River, 

Saskatchewan (50 em above the K-T boundary immediately above the Ferris coal), and the Nevis coal (40 em 

above the K-T boundary immediately above Nevis coal). 

They obtained the following mean ages (la errors): 

K-Ar Rb-Sr isochron U-Pb discordia 

(sanidine) (biotite) (zircon) 

Z-coal-----------· --- 64.6±0.5 63.7±0.3 63.9±0.3 

Ferris coal --------- 65.8±0.6 64.5±0.2 64.4±0.3 

Nevis coal --------- 65.8±0. 7 63.9±0.3 64.3±0.3 

Their weighted mean age for the nine values is 

64.3±0.6 Ma. The K-Ar ages were done by Ian 

McDougall, who measured an age of 524.2 Ma for 

MMhb-1 hornblende (Samson and Alexander, 1987). 

Carl Swisher has reported 40 ArP9 Ar ages of 66.1-66.2 

Ma for sanidine crystals from bentonites within the Z-coal 

(Berggren and others, 1992) using an MMhb-1 age of 

520.4 Ma, but these ages now appear to be invalid because 

of difficulties in sample preparaton (Carl Swisher, oral 

commun., 1992). 

McWilliams and others (1991a, 1991b, 1992) have reported 40 Arf39 Ar total fusion and age spectrum ages for 

sanidine crystals from splits of the same mineral concentrates used by Baadsgaard and others (1988). Their mean 

ages, which are based on the USGS Menlo Park monitor 

values, are: 

40 Art39 Ar total fusion 40 Art39 Ar age spectrum 

Z-coal---------------- 64.79±0.08 64.66±0.06 

Ferris ---------------- 64.71 ±0.06 64.56±0.08 

Nevis ----------------64.81 ±0.12 64.72±0.08 

4.0 Their weighted mean value for the three K-T localities 

0.0 

0.0 0.5 1.0 

39 Ar/ 36Ar (xl03

) 

1.5 

Figure 9. 40 ArP6 Ar vs 39 ArP6 Ar isochron diagrams for (a) one 

of the laser incremental heating measurements on single tektites, 

and (b) one of the resistance furnace incremental heating 

experiments on a bulk tektite samples. The isochron diagrams for the other incremental heating experiments are similar 

(table 5). 

is 64.71 ±0.05 Ma. 

The age data for North American and Caribbean K-T 

boundary sediments is relatively consistent considering the 

variety of laboratories and methods involved in the mea-

-0 

~ 

-(I) 

0> 

<( 

'U 

~ 0 

'5 

0 

0 

() 

75 

70 

65 

60 

70 

65 

60 

55 

90Gl5L-l (67.3 mg) 

64.65+0.22 

90Gl5L-2 (83.4 mg) 

64.46 t 0.11 

0 20 40 60 80 100 

39 Ar released (%) 

Figure 10. 40 Art39 Ar age spectra for tektite samples analyzed 

with the resistance furnace system. 
DISCUSSION 17 

Table 5. Analytical data for 40 ArP9 Ar resistance furnace age spectra on K-T boundary tektites from Haiti. 

Temp.a 40Arf39Ar 37Arf39~ 36Arf39Ar 40Arradc 39Arcac 36Arcac 39Ar Aged 

(°C) (%) (%) (%) (%of total) (Ma) 

90G15L-1, 67.3 mg (Irradiation GLN-3, 1=0.004546) 

800 36.10 1.788 0.0896 27.1 <0.1 <0.1 0.3 77.5 ± 6.5 

850 12.67 1.873 0.0166 61.2 <0.1 <0.1 0.3 62. ± 19. 

900 10.61 1.840 0.00802 79.0 <0.1 <0.1 0.6 66.6 ± 7.6 

950 10.99 1.977 0.00429 90.1 0.1 <0.1 1.1 78. ± 10 

1000 9.839 1.808 0.00704 81.6 0.1 <0.1 1.6 63.8 ± 2.1 

1050 8.594 1.698 0.00272 91.6 0.1 12.5 4.6 62.6 ± 1.1 

1100 8.307 1.747 0.00139 96.5 0.1 33.3 10.2 63.7 ± 1.6 

1150 12.38 1.729 0.0148 65.8 0.1 3.2 13.2 64.74 ± 0.50 

1200 4.247 1.779 0.00201 94.3 0.1 20.0 11.7 63.31.± 0.46 

1250 9.084 1.782 0.00361 90.0 0.1 13.3 6.1 64.9 ± 1.2 

1300 10.49 1.780 0.0084 77.1 0.1 5.3 3.5 64.27 ± 0.76 

1350 9.826 2.286 0.00617 83.2 0.2 8.7 5.9 65.00 ± 0.56 

1450 8.718 1.714 0.00319 93.2 <0.1 14.5 41.0 64.61 ± 0.26 

90G15L-2, 83.4 mg (Irradiation DD27, 1=0.015052) 

550 1.082 0.2084 0.00305 19.9 0.1 <0.1 0.1 57.± 32. 

600 6.109 1.943 0.01364 34.6 0.2 <0.1 0.1 56.± 10. 

650 11.01 1.880 0.0309 18.2 0.2 <0.1 0.1 53.± 19. 

700 218.5 2.831 0.706 4.4 <0.1 <0.1 <0.1 241. ± 69. 

750 13.61 2.055 0.0401 11.8 <0.1 <0.1 0.1 42. ± 31. 

800 5.186 2.048 0.00813 56.3 0.1 4.8 0.6 76.56 ± 0.64 

850 2.967 2.032 0.00185 86.0 0.1 33.3 1.2 67.1 ± 2.1 

900 2.857 2.074 0.00180 87.0 0.1 30.4 3.2 65.41 ± 0.82 

950 2.714 2.069 0.00120 92.8 0.1 45.8 4.9 66.24 ± 0.18 

1000 2.671 2.071 0.00133 91.0 0.1 42.3 4.8 63.99 ± 0.60 

1050 2.621 2.120 0.00101 94.7 0.1 56.1 10.0 65.29 ± 0.23 

1100 2.685 2.189 0.00134 91.3 0.1 42.7 16.4 64.51 ± 0.23 

1150 2.658 2.130 0.00128 91.7 0.1 44.0 16.2 64.16 ± 0.20 

1200 2.792 2.080 0.00168 87.9 0.1 32.4 10.0 64.56 ± 0.27 

1250 2.949 2.103 0.00226 82.7 0.1 23.9 7.8 64.21 ± 0.61 

1350 3.122 2.002 0.00275 78.7 0.1 18.9 12.8 64.70 ± 0.27 

1450 4.141 2.149 0.00622 59.6 0.1 9.2 11.6 64.91 ± 0.41 

a Plateau increments indicated in bold type. 

b Corrected for 37 Ar decay, half-life=35.1 days. 

c Subscripts: rad, radiogenic; K, potassium-derived; Ca, calcium derived. 

d Decay constants: A.E=0.581x10-1

0yr-1, A.~=4.692xi0- 0yr- . Errors assigned to individual ages are estimates ofthestandard 

deviation of analytical precision. Reactor corrections for irradiation GLN-3: e6Arf37

Ar)c:ft=0.000261 ± 2, e9

Ar/37Ar)ca = 

0.000675 ± 4, (

40Arf39Ar)K = 0.00497 ± 37. Reactor corrections for irradiation DD27: ( 6Arf37Ar)ca=0.000263 ± 2, 

e9Ar/37Ar)ca = 0.000641 ± 3, (

40Arf39Ar)K = 0.00770 ± 25. 

surements, and the difficulties of calibrations both within 

and between laboratories. When the single age reported by 

Obradovich (1984) and Obradovich and Sutter (1984) is 

converted to an MMhb-1 value of 513.9 Ma, all of the 

values fall between 64.5 Ma and 65.2 Ma or are not significantly different from an age within that range (that is, 

the age of Baadsgaard and others, 1988). Because of the 

uncertainties introduced by different sample collections, 

preparation methods, and irradiations, as well as differences between laboratory calibrations and instrumentation, 

we think that this agreement is good. We do not think that 

there is any significant difference between the tektite ages 

and the sanidine ages for the continental K-T sections despite small differences in the numerical values. Instead, we 

think that these differences are most likely due to calibration and irradiation differences and do not have any real 

geological significance. 

Our best age for the Haiti tektites, which are found at 

the K-T boundary and provide a direct age for the K-T 

bolide impact, is 64.4±0.1 Ma. If the age of Obradovich 

(1984) is converted to a MMhb-1 value of 513.9 Ma, then 

the weighted mean of the 26 ages shown in Figure 11 is 
18 AGES OF TEKTITES FROM CRETACEOUS-TERTIARY BOUNDARY ROCKS, BELOC FORMATION, HAITI 

Table 6. Summary of 40 Ar/39 Ar age spectra and isochron results on K-T boundary tektites from Haiti. 

Sam plea Weight Plateau Isochron 

No. (mg) 39Ar Ageh Agee SUMS/(n-2) 40Arf36AfC 

(%[steps]) (Ma) (Ma) 

90G15K-1 0.870 95.9[15 of25) 64.36 ± 0.36 64.00 ± 0.45 0.32 22 ± 139 

-2 1.117 98.2[12 of 15] 64.45 ± 0.35 64.25 ± 0.34 0.16 200 ± 68 

-3 1.240 97.5[13 of 15) 64.35 ± 0.35 64.27 ± 0.33 0.16 146 ± 44 

-4 1.082 96.9[11 of 13] 64.35 ± 0.35 64.30 ± 0.34 0.10 21 ± 64 

90G15L-1 67.3 56.5 [4 of 13] 64.69 ± 0.28 64.98 ± 0.20 0.05 291 ±2 

-2 83.4 74.8[6 of 17] 64.46 ± 0.11 64.52 ± 0.08 0.01 300± 1 

a Data for 90G 15K obtained with laser system, 90G 15L with resistance furnace system. 

b Weighted mean plateau age and crbest· 

c 1 cr errors. 

64.6±0.1 Ma (crbest). This value, because it is based on 

measurements made in several laboratories using several 

dating methods and includes several calibrations that are 

independent of the value chosen for MMhb-1 hornblende 

monitor, is probably the best value for the age of the K-T 

boundary and the K-T impact. 

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ref. 

BLM88 

BLM88 

BLM88 

MBB91 

MBB91 

BLM88 

BLM88 

BLM88 

MBB91 

MBB91 

084 

BLM88 

BLM88 

BLM88 

MBB91 

MBB91 

IDS91 

SGM92 

IDS91 

IDS91 

IDS91 

GJRS91 

HYS91 

MBB92 

SGM92 

SGM92 

no.& 

type 

I I I I I I I 

~ 

Nevis coal bentonite 

sanldlne C-D 

---t::r-- ISOC 

- 2 

--+-- 12TF 

... 11H 

Ferris coal bentonite 

---o-- sanldlne C-D 

--l:r- ISOC 

- 7 

.. 6 TF 

.... 1 IH 

Z-coal bentonite sanidine 

• 1 IH 

---o- C-D 

--b-- ISOC 

~ 12 '-' 

.. 11H 

... 17TF 

• 28 TF 

Arroyo el Mimbral tektites 

~ 2TF 

• 

--e-

• 

--o--

.... 

I I I 

... 

... 

• I I 

Haiti tektites 

21H 

41H 

52TF 

2 

41H 

18 TF 

3TF 

91H 

I I 

63.0 64.0 65.0 66.0 67.0 

Radiometric age (Ma) 

K-Ar Rb-Sr U-Pb 

19 

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Figure 11. Summary of age measurements on deposits related 

to the K-T boundary impact marker bed in North America and 

Haiti. Errors shown are 2o level (2 obest for weighted means) . 

The ages are plotted as reported and have not been adjusted for 

differences in monitor mineral values. BLM88, Baadsgaard and 

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McWilliams and others, 1991a, 1991b; MBB92, McWilliams 

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Sutter, 1984; IDS91, Izett and others, 1991a, 1991b, this paper; 

GJRS91, Gillot and others, 1991; HYS91, Hall and others, 

1991; SGM92, Swisher and others, 1992. IH, incremental 

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