40Ar/39Ar geochronology of igneous rocks in the Taylor 

Mountains and Dillingham quadrangles in SW Alaska 

by Alexander Iriondo1,2, Michael J. Kunk1, and Frederic H. Wilson3 

1 U.S. Geological Survey, Denver, Colorado 

2 University of Colorado at Boulder, Boulder, Colorado 

3 U.S. Geological Survey, Anchorage, Alaska 

Open-File Report 03-421 

2003 

This report is preliminary and has not been reviewed for comformity with U.S. Geological Survey 

editorial standards or with the North American Stratigraphic Code. 

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement 

by the U.S. Government. 

U.S. Department of the Interior 

U.S. Geological Survey 
INTRODUCTION 

This publication contains 40Ar/39Ar geochronology data for 53 mineral and rock analyses 

from 48 igneous rocks collected in the Taylor Mountains and Dillingham 1:250,000-scale 

quadrangles in southwestern Alaska (Figures 1 & 2). Wilson and others (2003) present detailed 

geologic descriptions of these igneous samples. This open file report includes a summary table 

containing information on the rock type, geographic location, mineral dated, type of age, age and 

analytical error (Table 1). The analyses were conducted at the U.S. Geological Survey (USGS) 

Argon Thermochronology Laboratory in Denver, Colorado. The results presented here are 

intended only as a preliminary publication of these geochronologic studies; the data are not 

interpreted in a geologic context. Therefore, users unfamiliar with argon isotopic data should use 

these results carefully. This report is primarily a detailed source document for subsequent 

scientific publications and maps that will integrate this data into a geologic context. 

METHODS 

Sample Preparation 

All of the igneous rocks were crushed, ground, and sized using 250, 180, and 150 μm 

sieves (60, 80, and 100 mesh respectively). Mineral separates of hornblende, muscovite, biotite, 

K-feldspar, and plagioclase were produced using magnetic separation, heavy liquids and hand 

picking to achieve a purity of >99%. We used the largest size fraction possible of the target 

mineral that is free of inclusions from other mineral phases. Basalt, andesite, and rhyolite samples 

were processed through heavy liquids and or magnetic separation to remove phenocrysts from the 

volcanic matrix. The resulting volcanic matrix samples were leached at room temperature with 

10% HCl to remove any traces of secondary calcite. All samples were washed in acetone, alcohol, 

Page 2 of 32
Figure 1. Location of dated rocks in the Taylor Mountains and Dillingham quadrangles, Alaska.

Gray areas correspond to outcrops of Late Cretaceous Early Tertiary igneous rocks.

01AWS-41

biotite

60.41 ± 0.06 Ma

00AWS-22

biotite

66.55 ± 0.07 Ma

01AH-83

muscovite

67.18 ± 0.28 Ma

01AH-85A

hornblende

75.22 ± 0.32 Ma

Area of Figure 2

00AWS-39

biotite

61.86 ± 0.44 Ma

AL00

plagioclase

41.62 ± 0.18 Ma

00AWS-13A

biotite

00AWS-15A 65.81 ± 0.06 Ma

biotite

67.94 ± 0.04 Ma

00AWS-12

biotite

84.49 ± 0.05 Ma

00AWS-31A

biotite

64.45 ± 0.11 Ma

99AWS-17

biotite

67.69 ± 0.04 Ma

99AM-309C

biotite

68.80 ± 0.07 Ma

0 5 10 15

0 5 10 15

Miles

Kilometers

N

KEMUK

biotite

86.05 ± 0.08 Ma

Alaska

61°

159°

61°

156°

59°

156°

59°

159°

Area of Figure 1

Page 3 of 32
156.75° 156° 

60° 60° 

01AM-109A 

biotite 

59.25 ± 0.05 Ma 

00AWS-16 

biotite 

59.69 ± 0.05 Ma 

01AWS-61 

biotite 

59.34 ± 0.03 Ma 

01AWS-78 

biotite 

60.90 ± 0.04 Ma 

01AWS-59 

biotite 

61.65 ± 0.05 Ma 

01AWS-39 

hornblende 

biotite 

60.07 ± 0.25 Ma 

60.93 ± 0.65 Ma 

01AH-70 

K-spar 

58.40 ± 0.20 Ma 

01AH-80 

K-spar 

58.30 ± 0.20 Ma 

01AWS-55 

volcanic matrix 

53.71 ± 0.61 Ma 

01AWS-49 

volcanic matrix 

47.95 ± 1.39 Ma 

01AH-67 

hornblende 

61.30 ± 0.20 Ma 

00AWS-37 

biotite 

63.09 ± 0.05 Ma 

01AH-63 

biotite 

65.26 ± 0.06 Ma 

01AWS-48 

biotite 

65.72 ± 0.04 Ma 

01AH-64 

biotite 

66.49 ± 0.07 Ma 

01AH-69 

biotite 

64.42 ± 0.04 Ma 01AH-65 

hornblende 

60.60 ± 0.30 Ma 

00AWS-35A 

biotite 

61.67 ± 0.05 Ma 

01AWS-50 

biotite 

61.11 ± 0.07 Ma 

01AWS-2 

hornblende 

63.77 ± 0.27 Ma 

01AH-59 

biotite 

65.50 ± 0.05 Ma 01AH-62 

biotite 

65.75 ± 0.03 Ma 

01AWS-46 

biotite 

66.24 ± 0.08 Ma 

01AWS-67B 

plagioclase 

45.10 ± 0.19 Ma 01AWS-24 

biotite 

61.70 ± 0.06 Ma 

01AWS-60 

volcanic matrix 

44.38 ± 0.38 Ma 

01AWS-57 

biotite 

61.57 ± 0.06 Ma 

00AWS-38 

biotite 

61.15 ± 0.04 Ma 

01APC-18 

biotite 

61.28 ± 0.05 Ma 

01APC-20 

biotite 

61.30 ± 0.05 Ma 

01APC-23 

biotite 

61.64 ± 0.05 Ma 

01APC-25 

biotite 

61.26 ± 0.05 Ma 

01AWS-80 

biotite 

61.63 ± 0.05 Ma 

01AWS-64 

hornblende 

64.20 ± 0.27 Ma 

N 

0 1 2 3 4 

Miles 

Kilometers 

0 1 2 3 4 5 

01AWS-67 

volcanic matrix 

44.47 ± 0.41 Ma 

59.5° 59.5° 

156.75° 156° 

Figure 2. Close-up view with location of dated rocks in the Taylor Mountains and Dillingham 

quadrangles, Alaska. Gray areas correspond to outcrops of Late Cretaceous to 

Early Tertiary igneous rocks. 

Page 4 of 32
and triply deionized water in a Branson B-220 ultrasonic cleaner to remove dust-sized particles 

and then re-sieved by hand. 

Aliquots of sample were packaged in copper capsules and sealed under vacuum in quartz 

tubes. The samples were then irradiated for 16 hours (irradiation package KD25) in an aluminum 

container in the central thimble facility at the TRIGA reactor (GSTR) at the USGS in Denver, 

Colorado. The monitor mineral used was Fish Canyon Tuff (FCT-3) sanidine with an age of 27.79 

Ma (Kunk and others, 1985; Cebula and others, 1986) relative to MMhb-1 with an age of 519.4 ± 

2.5 Ma (Alexander and others, 1978; Dalrymple and others, 1981). The type of container and the 

geometry of samples and standards is similar to that described by Snee and others (1988). 

Sample Analysis 

The samples were analyzed using the 40Ar/39Ar step-heating method of dating using a VG 

Isotopes Ltd., Model 1200B Mass Spectrometer fitted with an electron multiplier. The samples 

were heated for 10 minutes per step on a schedule of four to seventeen steps per sample. The 

number and temperature of heating steps was selected to limit the percentage of gas released to 

less than 20 percent per step for most samples. Additional biotite aliquots were also melted in a 

single heating step to produce total fusion ages. 

Heating of samples was done in a small volume, molybdenum-lined, low blank tantalum 

furnace similar to that described by Staudacher and others (1978). The temperature was monitored 

by a W5Re-W26Re thermocouple and controlled by a proportional programmable controller. The 

furnace and the rear manifold of the extraction system were pumped between steps with a turbo 

molecular pump. Two isolated ion pumps evacuated the front manifold and the mass spectrometer 

tube between each incremental step. The gas to be analyzed was purified in the first manifold by a 

SAES ST707 Zr-V-Fe getter operated at room temperature and by a hot tungsten filament to break 

Page 5 of 32
up hydrocarbons. Gas was equilibrated with the second manifold with an empty cold finger in the 

first manifold at LN2 temperature to trap water and other condensibles, then isolated and cleaned 

in the front manifold with a SAES ST101 Al-Zr getter operated at 4000

 C and with a Ti getter 

operated at 3500

 C. 

An activated charcoal finger submerged in an equilibrated mixture of dry ice and acetone 

was used to remove gasses with a molecular weight greater than 60 or 80 (primarily other noble 

gasses) prior to expansion of the argon dominated gas into the mass spectrometer. A second SAES 

ST101 active gas getter operated at room temperature further purified the argon-rich gas in the 

mass spectrometer. Its successful operation could be monitored by the consistent drop in counts of 

mass 44 (dominated by CO2) after the first gas analysis cycle. Argon isotopes with masses 40 

through 36 and CO2 (mass 44) were analyzed as a function of time in five analysis cycles. 40Ar, 

39Ar, 38Ar, 37Ar, and 36Ar peaks and their baselines, were measured for five-second integrations in 

each of the five cycles. All phases of the sample heating, cleanup, equilibration and sample 

analysis were performed under computer control. 

Isotopic Data Reduction 

All argon isotopic data were reduced using an updated version of the computer program 

ArAr* (Haugerud and Kunk, 1988), using the decay constants recommended by Steiger and Jäger 

(1977). The net peak measurements made in the five-cycle analysis were regressed using standard 

linear regression techniques to time zero. Sample blanks measured before the analyses were 

subtracted from the regressed results for 40Ar, 39Ar, 37Ar and 36Ar. Error estimates of the blanks 

were quadratically combined with the regression errors and propagated through the error 

equations. 

Page 6 of 32
Corrections for interfering reactor-produced argon isotopes from Ca, K, and Cl in the 

sample were made using the production ratios given in Dalrymple and others (1981) and Roddick 

(1983). Errors in calculating ages or ratios include the measurement errors in the analysis, decay 

factor uncertainties, measured atmospheric or calculated initial 40Ar/36Ar ratios, the irradiation 

parameter J, the production ratios of the various reactor induced argon producing reactions, the 

initial 38Ar/36Ar ratio, and the age of the monitor (Haugerud and Kunk, 1988). 

The data table (Table 2) and 40Ar/39Ar diagrams presented in this report (Figures 3 to 12) 

include the identification of individual step ages, total gas ages, plateau ages, average ages, total 

fusion ages and inverse isochron ages. 

An individual step age represents the apparent age obtained for a single temperature step 

analysis. Total gas ages represent the age calculated from the addition of all of the measured argon 

peaks for all steps in a single sample and are roughly equivalent to conventional K-Ar ages. No 

analytical precision is calculated for total gas ages. Plateau ages were determined using the 

definition of Fleck and others (1977) as modified by Haugerud and Kunk (1988). Average ages are 

calculated in the same manner as a plateau age but fail the definition of Fleck and others (1977). A 

total fusion age represents the age calculated from totally melting the sample in a single step. 

Inverse isotope correlation analysis of the analytical data to assess if non-atmospheric argon 

components were trapped in any samples and to calculate an inverse isochron age was done using 

the method of York (1969). For additional information on the sample data reduction procedure see 

Haugerud and Kunk (1988). 

Page 7 of 32
RESULTS 

40Ar/39Ar Geochronology Data 

The 40Ar/39Ar geochronology results are presented in age-spectra diagrams that plot the 

cumulative percent 39ArK of the steps against apparent age in millions of years (Figures 3 to 12). 

The precision estimate used to construct the error boxes of each step is displayed at the 95% 

confidence level (2 sigma). The upper, smaller graph plots the apparent K/Ca ratio of each step 

against cumulative 39ArK released. 

In addition, data are presented in tabular form (Table 2). The table starts with a line that 

gives the sample number, the material analyzed, and the J-value with its analytical uncertainty, the 

sample weight in milligrams, and the packet and package number from the irradiation. The table 

includes the temperature of the step, the percent of potassium derived 39ArK for each step, the 

radiogenic yield (percentage of 40ArR that is derived from the decay of potassium), moles of 39ArK, 

a corrected 40ArR/

39ArK ratio from which the age can be directly calculated, apparent K/Ca, and 

K/Cl ratios for each step, a calculated apparent age for the step (in millions of years), and an 

estimate of the precision of each age at the 98% confidence level (1 sigma). The sample precision 

includes estimates of the errors that are unique to a single sample and can be used only for 

comparisons with other steps of the same sample. This error estimate does not include the error in 

"J". The last line in the table represents the total gas results for the sample. Note that no analytical 

error is calculated for the age in this line. If the sample has a plateau age, the percentage of 39Ar on 

the plateau, the steps on the plateau, and the plateau age and its precision are indicated. 

We have calculated isochron ages using inverse-isotope correlation diagrams that plot 

39Ar/40Ar against 36Ar/40Ar (Figures 3 to 12). When reporting isochron ages we include the 

apparent age of the sample (calculated from the inverse of the x-axis intercept), the calculated 

Page 8 of 32
initial 40Ar/36Ar ratio of the sample (the inverse of the y-axis intercept), the MSWD a goodness of 

fit indicator of the data (Mean Standard of Weighted Deviates), the number of steps used in the 

age regression, and the percentage of 39Ar they represent. 

ACKNOWLEDGEMENTS 

We would like to thank Roy George for his assistance preparing mineral separates and 

Jeffrey Winick for his help running samples and reducing data. 

REFERENCES 

Alexander, E. C., Jr., Mickelson, G. M., and Lanphere, M. A., 1978, Mmhb-1: a new 40Ar/39Ar 

dating standard, in Zartman, R. E., ed., Short papers of the fourth international conference, 

geochronology, cosmochronology, and isotope geology: U.S. Geological Survey Open-File 

Report 78-701, p. 6-8. 

Cebula, G. T., Kunk, M. J., Mehnert, H. H., Naeser, C. W., Obradovich, J. D., and Sutter, J. F., 

1986, The Fish Canyon Tuff: A potential standard for the 40Ar/39Ar and fission track dating 

methods: Terra Cognita, v. 6, n. 2, p. 140. 

Dalrymple, G. B., Alexander, E. C., Lanphere, M. A., and Kraker, G. P., 1981, Irradiation of 

samples for 40Ar/39Ar dating using the Geological Survey TRIGA reactor: U.S. Geological 

Survey Professional Paper 1176, 55 p. 

Fleck, R. J., Sutter, J. F., and Elliot, D. H., 1977, Interpretation of discordant 40Ar/39Ar age spectra 

of Mesozoic tholeiites from Antarctica: Geochimica et Cosmochimica Acta, v. 41, p. 15-

32. 

Page 9 of 32
Haugerud, R. A., and Kunk, M. J., 1988, ArAr*, a computer program for reduction of 40Ar-39Ar 

data: U.S. Geological Survey, Open-File Report 88-261, 68 p. 

Kunk, M. J., Sutter, J. F., and Naeser, C. W., 1985, High-precision 40Ar/39Ar Ages of sanidine, 

biotite, hornblende, and plagioclase from the Fish Canyon Tuff, San Juan Volcanic Field, 

South-central Colorado [abs.]: Geological Society of America Abstracts with Programs, v. 

17, p. 636. 

Roddick, J. C., 1983, High precision calibration of 40Ar-39Ar standards: Earth and Planetary 

Science Letters, v. 47, p. 887-898. 

Snee, L. W., Sutter, J. F., and Kelly, W. C., 1988, Thermochronology of economic mineral 

deposits: Dating the stages of mineralization at Panasqueira, Portugal, by high precision 

40Ar/39Ar age spectrum techniques on muscovite: Economic Geology, v. 83, p. 335-354. 

Staudacher, Thomas, Jessberger, E. K., Dorflinger, D., and Kiko, J., 1978, A refined ultrahighvacuum furnace for rare gas analysis: J. Phys. E: Sci. Instrum. v. 11, p. 781-784. 

Steiger, R. H., and Jäeger, Emil, 1977, Subcommission on geochronology: Convention on the use 

of decay constants in geo- and cosmo-chronology: Earth and Planetary Science Letters, v. 

36, p. 359-363. 

Wilson, F. H., Hudson, T. L., Grybeck, Donald, Stoeser, D. B., Preller, C. C., Bickerstaff, Damon, 

Labay, Keith, and Miller, M. L., 2003, Preliminary geologic map of the northeast 

Dillingham quadrangle (D-1, D-2, C-1, and C-2), Alaska: U.S. Geological Survey, OpenFile Report 03-105, 13 p. 

York, Derick, 1969, Least squares fitting of a straight line with correlated errors: Earth and 

Planetary Science Letters, v. 5, p. 320-324. 

Page 10 of 32
Table 1. Summary table for dated igneous rocks from the Taylor Mountains and Dillingham quadrangles in SW Alaska. 

Sample Rock type Latitude Longitude Mineral Type of age Age Error Descnumber deg. N deg. W (Ma) (Ma) 00AWS-12 Syenite 59.461 158.199 biotite total fusion 84.49 ± 0.05� Okstukuk Hills at VABM 

or syenite having very cograined very fresh biotite clinopyroxene. 

00AWS-13A Granodiorite 59.304 158.121 biotite total fusion 65.81 ± 0.06 East side Muklung Hills. 

granodiorite containing abiotite. Biotite slightly cfair shape. 

00AWS-15A Biotite granite 59.329 158.301 biotite total fusion 67.94 ± 0.04 Main part, Muklung Hill

appearing biotite granite and abundant K-feldspar

00AWS-16 Dacite tuff 59.675 156.605 biotite total fusion 59.69 ± 0.05 Stuyahok Hills. Hornble

Contains excellent fresh bloaded with biotite inclus

00AWS-22 Granodiorite 60.041 156.820 biotite total fusion 66.55 ± 0.07 Northeast end of Sleitat M

hornblende granodiorite(Fm. Excellent biotite, po

00AWS-31A Biotite granite 59.703 157.987 biotite total fusion 64.45 ± 0.11 Small cirque of northeast 

Medium-grained biotite grinds, slightly vuggy. Go

00AWS-35A Quartz monzodiorite 59.778 156.134 biotite total fusion 61.67 ± 0.05� Large tor of biotite granit

pegmatitic phase containClinopyroxene is commoclinopyroxene. 

00AWS-37 Quartz monzonite 59.789 156.088 biotite total fusion 63.09 ± 0.05 Similar rock to above (35

fresh and secondary hornPlagioclase is sericitized. 

00AWS-38 Granite 59.606 156.159 biotite total fusion 61.15 ± 0.04 Granite, biotite is fine-gr

also contains clinopyroxeNo hornblende. 
Table 1. Summary table for dated igneous rocks from the Taylor Mountains and Dillingham quadrangles in SW Alaska (continued). 

Sample Rock type Latitude Longitude Mineral Type of age Age Error Descnumber deg. N deg. W (Ma) (Ma) 00AWS-39 Granodiorite 59.386 156.535 biotite isochron 61.86 ± 0.44� Isolated pluton containin

hornblende, fair amount o

01AH-59 Quartz monzonite 59.914 156.182 biotite total fusion 65.50 ± 0.05� Outcrops of medium-grai

equigranular biotite granoeuhedral biotite books. 

01AH-62 Quartz monzonite 59.900 156.198 biotite total fusion 65.75 ± 0.03� Medium-grained, medium 

granodiorite - small clean sulfides present? 

01AH-63 Hypabyssal dacite 59.868 156.165 biotite total fusion 65.26 ± 0.06� Biotite granodiorite, scre

locally, scattered xenolithsegregation within the gr

01AH-64 Quartz monzonite 59.822 156.201 biotite total fusion 66.49 ± 0.07� Medium-grained, equigra

granodiorite. Euhedral sm

01AH-65 Diorite 59.793 156.143 hornblende isochron 60.60 ± 0.30� Diorite complex. Fine- to 

gray diorite with some amlong. 

01AH-67 Quartz monzodiorite 59.802 156.028 hornblende plateau 61.30 ± 0.20� Medium dark greenish gr

seriate to porphyritic quagranodiorite. Few K-spar 

01AH-69 Quartz diorite 59.804 156.155 biotite total fusion 64.42 ± 0.04� Fine- to medium-grained

biotite granodiorite. 

01AH-70 Alaskite 59.837 156.351 k-spar average 58.40 ± 0.20� Rubble crop of orange w

interstitial graphic textureProbably dueterically alte

01AH-80 Alaskite 59.805 156.377 k-spar average 58.30 ± 0.20� Aplitic granite with green

vug fillings. Locally, vugand K-spar. 
Table 1. Summary table for dated igneous rocks from the Taylor Mountains and Dillingham quadrangles in SW Alaska (continued). 

Sample Rock type Latitude Longitude Mineral Type of age Age Error Descnumber deg. N deg. W (Ma) (Ma) 01AH-83 Muscovite granite 60.410 156.069 muscovite plateau 67.18 ± 0.28 Probably 10+ foot wide m

equigranular muscovite g

01AH-85A Granodiorite 60.279 156.080 hornblende average 75.22 ± 0.32 Rubble crop of medium-g

medium-light gray biotite Both biotite and hornblen

01AM-109A Dacite tuff 59.691 156.531 biotite total fusion 59.25 ± 0.05 Rubble crop of fine- to m

granodiorite. Biotie looks mm. 

01APC-18 Granite 59.606 156.223 biotite total fusion 61.28 ± 0.05 Pegmatitic biotite granite01APC-20 Granite 59.604 156.254 biotite total fusion 61.30 ± 0.05 Biotite granite. 01APC-23 Granite 59.589 156.240 biotite total fusion 61.64 ± 0.05 Granite. 01APC-25 Granite 59.579 156.240 biotite total fusion 61.26 ± 0.05 Hornblende biotite granit01AWS-2 Quartz diorite 59.891 156.251 hornblende plateau 63.77 ± 0.27 Pike Creek. Massive, dar

grained hornblende-bearirock. 

01AWS-24 Granite 59.607 156.321 biotite total fusion 61.70 ± 0.06 Biotie-hornblende granod

phenocrysts. Good fresh xenoliths. 

01AWS-39 Granodiorite 59.726 156.204 biotite isochron 60.93 ± 0.65 Isolated knoll in Stuyaho

coarse-grained hornblend

hornblende plateau 60.07 ± 0.25 

01AWS-41 Granite 60.045 157.085 biotite total fusion 60.41 ± 0.06 Sleitat pluton, tin-bearing 01AWS-46 Quartz monzonite 59.898 156.193 biotite total fusion 66.24 ± 0.08 Granodiorite having quite 

epidote, little biotite. 

01AWS-48 Quartz monzonite 59.842 156.171 biotite total fusion 65.72 ± 0.04 Coarse-grained biotite-ho01AWS-49 Andesite 59.762 156.011 volc. matrix isochron 47.95 ± 1.39 Basalt plug. 
Table 1. Summary table for dated igneous rocks from the Taylor Mountains and Dillingham quadrangles in SW Alaska (continued). 

Sample Rock type Latitude Longitude Mineral Type of age Age Error Descnumber deg. N deg. W (Ma) (Ma) 01AWS-50 Quartz diorite? 59.769 156.133 biotite total fusion 61.11 ± 0.07 Biotite granite, south edg01AWS-55 Basalt 59.791 156.298 volc. matrix isochron 53.71 ± 0.61 Columnar jointed basalt.
01AWS-57 Granite 59.618 156.122 biotite total fusion 61.57 ± 0.06 Dark colored, medium to 

biotite quartz monzonite cm K-spar phenocrysts. 

01AWS-59 Granite 59.581 156.332 biotite total fusion 61.65 ± 0.05 Yellowish biotite granite 

Another mafic could be O

01AWS-60 Basalt 59.601 156.323 volc. matrix isochron 44.38 ± 0.38 Incredibly fresh columna

01AWS-61 Dacite tuff 59.608 156.395 biotite total fusion 59.34 ± 0.03 Fine-grained porphyritic 

phenocrysts of very fresh aphanitic tuffaceous(?) m

01AWS-64 Granite 59.629 156.196 hornblende plateau 64.20 ± 0.27 Coarse- to medium-grain

K-spar phenocrysts to 2 c

01AWS-67 Basalt 59.614 156.311 volc. matrix isochron 44.47 ± 0.41 Basalt flow - fresh, spars

01AWS-67B Rhyolite 59.614 156.311 plagioclase plateau 45.10 ± 0.19 Dacite tuff. 01AWS-78 Granite 59.566 156.263 biotite total fusion 60.90 ± 0.04 Stuyahok Hills pluton. H

granodiorite with large 2and common mafic xeno

01AWS-80 Granite 59.633 156.193 biotite total fusion 61.63 ± 0.05 Stuyahok Hills pluton. H

granodiorite with large 2and common mafic xeno

99AM-309C Granite? 61.014 156.031 biotite total fusion 68.80 ± 0.07 Hook pluton. Biotite gra

intergrown biotite and hofeldspar. 

99AWS-17 Monzodiorite 60.926 156.732 biotite total fusion 67.69 ± 0.04 Hoholitna pluton. Biotite 

monzonite intruding Kusbiotite, hornblende secon

AL00 Rhyolite 59.005 156.044 plagioclase average 41.62 ± 0.18 Maroon and white bande

grained wholy crystallinesection, remnants appear 

KEMUK Pegmatitic ultramafic 59.720 157.700 biotite total fusion 86.05 ± 0.08 Kemuk iron-PGE prospe

sample, hole number 6, dSec. 24(?) T. 5 S., R. 50 W
1000

Apparent Age (Ma) Apparent Age (Ma) 

100 

Inverse-isotope correlation diagram 

10 

140 0.00024 

120 

100 

0.00016 

80 

60 

0.00008 

40 

20 

0

00AWS-12 

biotite 

Syenite 

K/Ca 

#32KD25 

A B C D E F G 

Average Age = 85.05 ± 0.36 Ma 

H 

Average Age 

I J 

0 

0 20 40 60 80 100 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 

A 

B 

C 

FGE

HDI

J 

Isochron Age = 82.01 ± 6.03 Ma 

[40Ar/36Ar]i = 5133.15 ± 6232.71 

MSWD = 0.004 

Steps E through J with 75.5% of 39ArK 

#32KD25 

00AWS-12 

biotite 

Syenite 

Cumulative % 39ArK Released 39Ar/40Ar 

1000 

100 

10 

1 

A 

B 

EDCF GH

I KJ 

Isochron Age = 61.86 ± 0.44 Ma 

[40Ar/36Ar]i = 463.68 ± 32.37 

MSWD = 0.552 

Steps C through K with 90.0% of 39ArK 

#18KD25 

00AWS-39 

biotite 

Granodiorite 

Inverse-isotope correlation diagram 100 

90 0.003 

80 

70 

60 0.002 

50 

40 

30 

00AWS-39 

biotite 

Granodiorite 

K/Ca 

#18KD25 

A 

B C D E F G 

Average Age = 62.25 ± 0.26 Ma 

H I J K 

Average Age 

0.001 

20 

10 

0 0 

0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.10 

Cumulative % 39ArK Released 39Ar/40Ar 36Ar/40Ar 36Ar/40Ar 

Figure 3. 40Ar/39Ar age spectra and inverse-isotope correlation diagrams for samples 00AWS-12 and 01AWS-39 

Page 15 of 32
Apparent Age (Ma) Apparent Age (Ma)

3 

2 

1 

Inverse-isotope correlation diagram 

100 

01AH-65 

hornblende 

Diorite 

K/Ca 

#88KD25 

A 

Average Age = 60.70 ± 0.20 Ma 

B C 

D J E F G H I 

Average Age 

0 

90 0.0010 

80 

70 0.0008 

60 

50 

0.0006 

40 

30 

20 0.0004 

10 

0 0.0002 0 20 40 60 80 100 0.07 0.08 0.09 0.10 0.11 

B C 

D 

E 

F 

G 

H 

I 

J 

A 

01AH-65 

hornblende 

Diorite 

Isochron Age = 60.60 ± 0.30 Ma 

[40Ar/36Ar]i = 310 ± 6 

MSWD = 3.9 

Steps F through J with 61.4% of 39ArK 

#88KD25 

Cumulative % 39ArK Released 39Ar/40Ar 

0.8 

0.4 

Inverse-isotope correlation diagram 100 

01AH-67 

hornblende 

Quartz Monzodiorite 

K/Ca 

#89KD25 

A-H 

Plateau Age = 61.30 ± 0.20 Ma 

I J K L M N O 

Plateau Age 

0 

0.0016 90 

80 0.0014 

70 

0.0012 

60 

0.0010 50 

40 0.0008 

30 0.0006 

20 

0.0004 10 

0 0.0002 0 20 40 60 80 100 0.06 0.07 0.08 0.09 0.10 

B 

C 

D 

E F 

G H 

I 

J 

K 

L

M 

N 

O 

A 

#89KD25 

01AH-67 

hornblende 

Quartz Monzodiorite 

Isochron Age = 61.20 ± 0.20 Ma 

[40Ar/36Ar]i = 302 ± 2 

MSWD = 1.9 

Steps F through O with 95.9% of 39ArK 

Cumulative % 39ArK Released 39Ar/40Ar 36Ar/40Ar 36Ar/40Ar 

Figure 4. 40Ar/39Ar age spectra and inverse-isotope correlation diagrams for samples 01AH-65 and 01AH-67 

Page 16 of 32
Apparent Age (Ma) Apparent Age (Ma)

120 

80 

40 

Inverse-isotope correlation diagram 

100 

01AH-70 

k-feldspar 

Alaskite 

K/Ca 

#83KD25 

A 

Average Age = 58.40 ± 0.20 Ma 

B C D E F G H I J K L M N O 

Average Age 

0 

0.0005 

90 

80 

0.0004 

70 

60 0.0003 

50 

40 0.0002 

30 

20 0.0001 

10 

0 0 

Isochron Age = 57.80 ± 0.20 Ma 

[40Ar/36Ar]i = 380 ± 11 

MSWD = 2.0 

Steps E through N with 77.6% of 39ArK 

01AH-70 

k-feldspar 

Alaskite 

#83KD25 

B 

C 

D 

E

F G 

H J I 

K 

L 

M 

N 

O 

A 

0 20 40 60 80 100 0.100 0.104 0.108 0.112 0.116 0.120 

Cumulative % 39ArK Released 39Ar/40Ar 

50 

40 

30 

Inverse-isotope correlation diagram 

100 

01AH-80 

k-feldspar 

Alaskite 

#81KD25 

CD G 

Average Age = 58.30 ± 0.20 Ma 

E J O 

Average Age 

F H I K L M N 

K/Ca 

B 

A

20 0.00042 

90 

80 0.00034 

70 

60 0.00026 

50 

40 0.00018 

30 

20 0.00010 

10 

0 0.00002 

0 20 40 60 80 100 0.102 0.106 0.110 0.114 0.118 0.122 

A 

B 

C 

D E 

F

G

H 

I 

J 

KL

N M 

O 

Isochron Age = 58.10 ± 0.20 Ma 

[40Ar/36Ar]i = 305 ± 19 

MSWD = 1.5 

Steps D through M with 72.7% of 39ArK 

#81KD25 

01AH-80 

k-feldspar 

Alaskite 

Cumulative % 39ArK Released 39Ar/40Ar 36Ar/40Ar 36Ar/40Ar 

Figure 5. 40Ar/39Ar age spectra and inverse-isotope correlation diagrams for samples 01AH-70 and 01AH-80 

Page 17 of 32
10000

Apparent Age (Ma) Apparent Age (Ma) 

1000 

Inverse-isotope correlation diagram 

100 

01AH-83 

muscovite 

Muscovite Granite 

K/Ca 

#84KD25 

Plateau Age = 67.17 ± 0.28 Ma 

ABC D E F G H I J K L 

Plateau Age 

100 

90 

80 0.0006 

70 

60 

0.0004 50 

40 

30 

0.0002 

20 

10 

0 0 

0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.10 

A 

B 

C 

D 

E 

F

GH I

J 

K 

L 

Isochron Age = 67.16 ± 0.31 Ma 

[40Ar/36Ar]i = 311.63 ± 11.14 

MSWD = 0.377 

Steps A through L with 100% of 39ArK 

#84KD25 

01AH-83 

muscovite 

Muscovite Granite 

Cumulative % 39ArK Released 39Ar/40Ar 

10 

1 

0.1 

A 

B 

DCEFL GKI

JH

01AH-85a 

hornblende 

Granodiorite 

Isochron Age = 74.33 ± 1.57 Ma 

[40Ar/36Ar]i = 423.62 ± 215.83 

MSWD = 0.116 

Steps F through L with 82.0% of 39ArK 

#92KD25 

Inverse-isotope correlation diagram 100 

90 

0.003 

80 

70 

60 0.002 

50 

40 

30 0.001 

20 

10 

0

01AH-85a 

hornblende 

Granodiorite 

K/Ca 

#92KD25 

Average Age = 75.22 ± 0.32 Ma 

Average Age 

A

B 

CD E F G H I J K L 

0 0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 

Cumulative % 39ArK Released 39Ar/40Ar 36Ar/40Ar 36Ar/40Ar 

Figure 6. 40Ar/39Ar age spectra and inverse-isotope correlation diagrams for samples 01AH-83 and 01AH-85A 

Page 18 of 32
10 

01AWS-2 

hornblende 

Quartz Diorite 

K/Ca 

#93KD25 

A 

B 

C 

D E F G 

Plateau Age = 63.77 ± 0.27 Ma 

H I J K 

Plateau Age 

1 

0.1 

Inverse-isotope correlation diagram 

0.01 

A 

B C 

D 

E 

F 

G 

H 

I 

J

K 

Isochron Age = 63.97 ± 1.75 Ma 

[40Ar/36Ar]i = 281.9 ± 108.07 

MSWD = 0.057 

Steps F through K with 70.7% of 39ArK 

#93KD25 

01AWS-2 

hornblende 

Quartz Diorite 

100 

90 

80 

0.0003 

Apparent Age (Ma) 

70 

60 

50 

40 

30 

36Ar/40Ar 

0.0002 

0.0001 

20 

10 

0 0 

0 20 40 60 80 100 0 0.02 0.04 0.06 0.08

39Ar/ Cumulative % 40Ar 39ArK Released 

Figure 7. 40Ar/39Ar age spectrum and inverse-isotope correlation diagram for sample 01AWS-2 

Page 19 of 32

0.10 
10

Apparent Age (Ma) Apparent Age (Ma) 

1 

Inverse-isotope correlation diagram 

100 

01AWS-39 

hornblende 

Granodiorite 

K/Ca 

#86KD25 

A B C D E F G 

Plateau Age = 60.07 ± 0.25 Ma 

H I 

Plateau Age 

0.1 

90 0.00042 

80 

70 

60 0.00028 

50 

40 

30 0.00014 

20 

10 

0 0 

0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.10 

A 

B 

C 

D 

E 

F 

G 

H 

I 

Isochron Age = 60.27 ± 1.30 Ma 

[40Ar/36Ar]i = 278.42 ± 108.22 

MSWD = 0.101 

Steps E through I with 71.1% of 39ArK 

#86KD25 

01AWS-39 

hornblende 

Granodiorite 

Cumulative % 39ArK Released 39Ar/40Ar 

1000 

100 

10 

Inverse-isotope correlation diagram 

100 

01AWS-39 

biotite 

Granodiorite 

K/Ca 

#4KD25 

A B C D E F G 

Isochron Age = 60.93 ± 0.65 Ma 

H I J 

Plateau Age 

Plateau Age = 61.38 ± 0.26 Ma 

1 

90 0.00048 

80 

70 

60 0.00032 

50 

40 

A 

B 

C D 

E 

F 

G 

H

I J

Isochron Age = 60.93 ± 0.65 Ma 

[40Ar/36Ar]i = 623.01 ± 145.82 

MSWD = 0.064 

Steps C through J with 90.1% of 39ArK 

#4KD25 

01AWS-39 

biotite 

Granodiorite 

30 0.00016 

20 

10 

0 0 

0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.10 

Cumulative % 39ArK Released 39Ar/40Ar 36Ar/40Ar 36Ar/40Ar 

Figure 8. 40Ar/39Ar age spectra and inverse-isotope correlation diagrams for sample 01AWS-39 

Page 20 of 32

0.12 
1

Apparent Age (Ma) Apparent Age (Ma) 

Inverse-isotope correlation diagram 

100 

01AWS-49 

volcanic matrix 

Andesite 

K/Ca 

#71KD25 

A 

B C D E F G 

Isochron Age = 47.95 ± 1.39 Ma 

H 

Average Age 

I 

Average Age = 50.88 ± 0.22 Ma 

0.1 

90 0.0006 

80 

70 

60 0.0004 

50 

40 

30 0.0002 

20 

10 

0 0 

0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 

A 

B 

CDE F

G 

H 

I 

Isochron Age = 47.95 ± 1.39 Ma 

[40Ar/36Ar]i = 958.75 ± 218.93 

MSWD = 1.172 

Steps B through G with 75.6% of 39ArK 

#71KD25 

01AWS-49 

volcanic matrix 

Andesite 

Cumulative % 39ArK Released 39Ar/40Ar 

10 

1 

Inverse-isotope correlation diagram 100 

01AWS-55 

volcanic matrix 

Basalt 

K/Ca 

#67KD25 

A B C D E F G 

Isochron Age = 53.71 ± 0.61 Ma 

H 

Average Age 

Average Age = 54.76 ± 0.23 Ma 

0.1 

90 0.003 

80 

70 

60 0.002 

50 

40 

30 0.001 

20 

10 

0 0 

0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.10 0.12 

A 

B CDEFG 

H 

01AWS-55 

volcanic matrix 

Basalt 

Isochron Age = 53.71 ± 0.61 Ma 

[40Ar/36Ar]i = 731.13 ± 140.91 

MSWD = 1.223 

Steps B through G with 62.0% of 39ArK 

#67KD25 

Cumulative % 39ArK Released 39Ar/40Ar 36Ar/40Ar 36Ar/40Ar 

Figure 9. 40Ar/39Ar age spectra and inverse-isotope correlation diagrams for samples 01AWS-49 and 01AWS-55 

Page 21 of 32
1 

0.1 

Inverse-isotope correlation diagram 

100 

01AWS-60 

volcanic matrix 

Basalt 

K/Ca 

#69KD25 

A B C D E F G 

Isochron Age = 44.38 ± 0.38 Ma 

Average Age 

Average Age = 45.11 ± 0.19 Ma 

0.01 

0.0003 

90 

80 

70 

0.0002 

60 

50 

40 

0.0001 30 

20 

10 

0 0 

0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 0.15 

A 

B 

C 

D 

EF 

G 

Isochron Age = 44.38 ± 0.38 Ma 

[40Ar/36Ar]i = 525.65 ± 37.23 

MSWD = 0.564 

Steps A through F with 69.7% of 39ArK 

#69KD25 

01AWS-60 

volcanic matrix 

Basalt 

Cumulative % 39ArK Released 39Ar/40Ar 

1 

0.1 

Inverse-isotope correlation diagram 

100 

01AWS-64 

hornblende 

Granite 

K/Ca 

#87KD25 

ABC D E F G 

Plateau Age = 64.20 ± 0.27 Ma 

H I J 

Plateau Age 

0.01 

90 0.00048 

80 

70 

60 0.00032 

50 

40 

30 0.00016 

20 

10 

0 0 

0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.10 

A 

B 

C 

D

E

F 

G

H 

I

J 

Isochron Age = 64.00 ± 2.01 Ma 

[40Ar/36Ar]i = 306.19 ± 106.08 

MSWD = 0.123 

Steps E through J with 85.1% of 39ArK 

#87KD25 

01AWS-64 

hornblende 

Granite 

Cumulative % 39ArK Released 39Ar/40Ar 

Apparent Age (Ma) Apparent Age (Ma) 

36Ar/40Ar 36Ar/40Ar 

Figure 10. 40Ar/39Ar age spectra and inverse-isotope correlation diagrams for samples 01AWS-60 and 01AWS-64 

Page 22 of 32
1

Apparent Age (Ma) Apparent Age (Ma) 

0.1 

Inverse-isotope correlation diagram 

100 

01AWS-67 

volcanic matrix 

Basalt 

K/Ca 

#73KD25 

A 

B C D E F G 

Isochron Age = 44.47 ± 0.41 Ma 

H 

I 

Average Age 

Average Age = 45.47 ± 0.19 Ma 

0.01 

90 

0.003 

80 

70 

60 0.002 

50 

40 

30 0.001 

20 

10 

0 0 

0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 0.15 

A 

B 

C 

DE

GFH 

I 

Isochron Age = 44.47 ± 0.41 Ma 

[40Ar/36Ar]i = 465.12 ± 43.87 

MSWD = 0.556 

Steps C through H with 73.7% of 39ArK 

#73KD25 

01AWS-67 

volcanic matrix 

Basalt 

Cumulative % 39ArK Released 39Ar/40Ar 

10 

1 

100 

01AWS-67b 

plagioclase 

Rhyolite 

K/Ca 

#77KD25 

A B C D E F G 

Plateau Age = 45.10 ± 0.19 Ma 

H I J K L M 

Plateau Age N 

0.1 

A 

B 

C 

D 

E 

F 

G 

H 

I

J 

K 

L 

M 

N 

Isochron Age = 44.80 ± 0.76 Ma 

[40Ar/36Ar]i = 391.36 ± 268.45 

MSWD = 0.166 

Steps C through L with 86.5% of 39ArK 

#77KD25 

01AWS-67b 

plagioclase 

Rhyolite 

Inverse-isotope correlation diagram 

90 

80 

70 0.00008 

60 

50 

40 

0.00004 

30 

20 

10 

0 

0 20 40 60 80 100 0 

0 0.03 0.06 0.09 0.12 0.15 

Cumulative % 39ArK Released 39Ar/40Ar 36Ar/40Ar 36Ar/40Ar 

Figure 11. 40Ar/39Ar age spectra and inverse-isotope correlation diagrams for samples 01AWS-67 and 01AWS-67B 

Page 23 of 32
1 

Inverse-isotope correlation diagram 

100 

AL00 

plagioclase 

Rhyolite 

K/Ca 

#79KD25 

A B C D E F G 

Average Age = 41.62 ± 0.18 Ma 

H 

Average Age 

I J K L MN 

0.1 

90 0.0006 

80 

70 

60 0.0004 

50 

40 

30 0.0002 

20 

10 

0 0 

0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 0.15 

A 

B

C 

D 

E 

F 

G 

H 

I 

J 

K 

L 

M 

N 

Isochron Age = 41.62 ± 0.32 Ma 

[40Ar/36Ar]i = 298.51 ± 27.8 

MSWD = 0.255 

Steps C through N with 85.2% of 39ArK 

#79KD25 

AL00 

plagioclase 

Rhyolite 

Cumulative % 39ArK Released 39Ar/40Ar 

10000 

1000 

100 

A 

B 

C 

D 

E

F 

G

H 

Isochron Age = 85.24 ± 1.26 Ma 

[40Ar/36Ar]i = 585.17 ± 122.48 

MSWD = 0.454 

Steps A through H with 100% of 39ArK 

#21KD25 

KEMUK 

biotite 

Mafic Pegmatite 

Inverse-isotope correlation diagram 

140 

120 

0.00018 

100 

80 0.00012 

60 

40 0.00006 

20 

0

KEMUK 

biotite 

Mafic Pegmatite 

K/Ca 

#21KD25 

A B C D E F G 

Average Age = 86.32 ± 0.36 Ma 

H 

Average Age 

0 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 

Cumulative % 39ArK Released 39Ar/40Ar 

Apparent Age (Ma) Apparent Age (Ma) 

36Ar/40Ar 36Ar/40Ar 

Figure 12. 40Ar/39Ar age spectra and inverse-isotope correlation diagrams for samples AL00 and KEMUK 

Page 24 of 32
Table 2. 40Ar/39Ar geochronology data for igneous rocks in SW Alaska 

Step Temp. %39Ar Radiogenic 39Ark 

40Ar* Apparent Apparent Apparent Error 

°C of total Yield (%) (Moles) 39Ark K/Ca K/Cl Age (Ma) (Ma) 

00AWS-12 biotite J = 0.003848 ± 0.35% wt = 27.8mg #32KD25 

A 850 2.7 92.8 7.99E-14 12.968 56.3 137.0 87.84 ± 0.28 

B 900 5.0 97.1 1.50E-13 12.771 226.3 161.0 86.54 ± 0.09 

C 950 8.0 98.9 2.40E-13 12.607 243.0 169.0 85.46 ± 0.06 

D 1000 8.7 99.6 2.62E-13 12.554 280.6 173.0 85.11 ± 0.11 

E 1050 7.3 99.8 2.19E-13 12.640 215.4 167.0 85.68 ± 0.08 

F 1100 7.4 99.5 2.22E-13 13.177 75.8 159.0 89.23 ± 0.10 

G 1150 10.4 99.6 3.11E-13 13.045 53.2 164.0 88.36 ± 0.07 

H 1200 18.8 99.7 5.62E-13 12.600 125.4 120.0 85.41 ± 0.05 

I 1250 17.0 99.8 5.09E-13 12.545 126.1 147.0 85.05 ± 0.06 

J 1350 14.6 99.8 4.37E-13 12.485 28.3 176.0 84.65 ± 0.05 

Total Gas 100.0 99.3 2.99E-12 12.681 132.9 154.8 85.95 

50.4% of 39ArK gas released in steps 1200 through 1350 Average Age = 85.05 ± 0.36 

00AWS-12 biotite (total fusion) J = 0.003837 ± 0.35% wt = 5.0mg #33KD25 

A 1450 100 98.2 5.29E-13 12.497 51.06 102.0 84.49 ± 0.05 

00AWS-13A biotite (total fusion) J = 0.003878 ± 0.35% wt = 4.6mg #62KD25 

A 1450 100 97.4 4.88E-13 9.581 60.98 27.0 65.81 ± 0.06 

00AWS-15A biotite (total fusion) J = 0.003854 ± 0.35% wt = 4.7mg #44KD25 

A 1450 100 96.2 5.05E-13 9.957 56.7 20.0 67.94 ± 0.04 

00AWS-16 biotite (total fusion) J = 0.003856 ± 0.35% wt = 4.3mg #46KD25 

A 1450 100 94.6 3.94E-13 8.724 115.78 33.0 59.69 ± 0.05 

00AWS-22 biotite (total fusion) J = 0.003779 ± 0.35% wt = 4.7mg #25KD25 

A 1450 100 96.7 5.19E-13 9.944 204.51 58.0 66.55 ± 0.07 

00AWS-31A biotite (total fusion) J = 0.003759 ± 0.35% wt = 3.1mg #13KD25 

A 1450 100 93.7 2.22E-13 9.675 42.93 13.0 64.45 ± 0.11 

00AWS-35A biotite (total fusion) J = 0.003753 ± 0.35% wt = 5.5mg #9KD25 

A 1450 100 96.1 5.15E-13 9.266 112.79 20.0 61.67 ± 0.05 

00AWS-37 biotite (total fusion) J = 0.003851 ± 0.35% wt = 5.1mg #39KD25 

A 1450 100 92.7 4.78E-13 9.240 38.81 12.0 63.09 ± 0.05 

00AWS-38 biotite (total fusion) J = 0.003784 ± 0.35% wt = 4.8mg #23KD25 

A 1450 100 94.6 5.04E-13 9.112 66.73 17.0 61.15 ± 0.04 

00AWS-39 biotite J = 0.003766 ± 0.35% wt = 28.4mg #18KD25 

A 750 2.8 47.9 7.63E-14 8.131 25.7 17.0 54.41 ± 0.41 

B 850 7.2 86.0 1.96E-13 9.653 151.0 20.0 64.42 ± 0.13 

C 900 6.6 96.0 1.81E-13 9.521 201.1 21.0 63.55 ± 0.10 

D 950 5.0 95.1 1.36E-13 9.467 161.2 21.0 63.20 ± 0.15 

E 1000 3.3 93.9 9.06E-14 9.568 101.8 21.0 63.86 ± 0.20 

F 1050 3.3 93.3 8.94E-14 9.595 35.1 21.0 64.04 ± 0.18 

G 1100 5.1 89.9 1.39E-13 9.888 11.5 19.0 65.96 ± 0.15 

H 1150 8.5 93.1 2.32E-13 9.793 5.4 19.0 65.33 ± 0.10 

I 1200 15.3 97.4 4.18E-13 9.502 14.7 19.0 63.43 ± 0.06 

J 1250 26.2 99.3 7.15E-13 9.300 84.4 21.0 62.10 ± 0.05 

K 1350 16.8 99.5 4.59E-13 9.283 164.8 22.0 61.99 ± 0.03 

Total Gas 100.0 94.8 2.73E-12 9.434 90.5 20.2 62.98 

58.3% of 39ArK gas released in steps 1200 through 1350 Average Age = 62.25 ± 0.26 

00AWS-39 biotite (total fusion) J = 0.003765 ± 0.35% wt = 4.0mg #19KD25 

A 1450 100 89.5 3.57E-13 9.367 21.1 18.0 62.53 ± 0.04 

Page 25 of 32
Table 2. 40Ar/39Ar geochronology data for igneous rocks in SW Alaska (continued). 

Step Temp. %39Ar Radiogenic 39Ark 

40Ar* Apparent Apparent Apparent Error 

°C of total Yield (%) (Moles) 39Ark K/Ca K/Cl Age (Ma) (Ma) 

01AH-59 biotite (total fusion) J = 0.003864 ± 0.35% wt = 3.7mg #50KD25 

A 1450 100 97.4 3.98E-13 9.569 159.37 10.0 65.50 ± 0.05 

01AH-62 biotite (total fusion) J = 0.003839 ± 0.35% wt = 5.5mg #27KD25 

A 1450 100 97.3 4.97E-13 9.670 103.28 9.0 65.75 ± 0.03 

01AH-63 biotite (total fusion) J = 0.003765 ± 0.35% wt = 4.0mg #15KD25 

A 1450 100 96.0 3.93E-13 9.785 30.77 12.0 65.26 ± 0.06 

01AH-64 biotite (total fusion) J = 0.003868 ± 0.35% wt = 4.3mg #58KD25 

A 1450 100 93.6 4.15E-13 9.708 47.78 11.0 66.49 ± 0.07 

01AH-65 hornblende J = 0.003918 ± 0.35% wt = 9.0mg #88KD25 

A 800 6.6 69.1 1.39E-14 9.095 2.22 1.7 63.31 ± 0.27 

B 900 14.5 92.4 3.06E-14 10.036 2.71 4.4 69.74 ± 0.13 

C 1000 10.6 92.7 2.24E-14 9.744 1.93 5.1 67.75 ± 0.13 

D 1100 6.9 84.7 1.47E-14 8.601 0.27 4.4 59.93 ± 0.17 

E 1150 9.8 77.8 2.08E-14 8.563 0.09 2.7 59.67 ± 0.18 

F 1175 10.1 86.5 2.15E-14 8.785 0.14 3.3 61.2 ± 0.15 

G 1200 10.5 89.4 2.22E-14 8.78 0.33 4.8 61.16 ± 0.14 

H 1250 15.2 93.7 3.21E-14 8.725 0.64 5.9 60.79 ± 0.11 

I 1300 9 87 1.91E-14 8.693 0.45 0.8 60.56 ± 0.16 

J 1350 6.8 80.9 1.44E-14 8.803 0.14 3.3 61.32 ± 0.19 

Total Gas 62.92

68.3% of 39ArK gas released in steps 1100 through 1350 Average Age = 

01AH-67 hornblende J = 0.003918 ± 0.35% wt = 59.1mg #89KD25 

60.7 ± 0.2 

A 900 1.3 53.4 9.43E-15 9.023 0.46 2.2 62.82 ± 0.4 

B 1000 0.8 67.7 6.08E-15 8.664 0.64 1.6 60.37 ± 0.38 

C 1050 0.8 73.1 6.11E-15 8.723 0.57 1.8 60.77 ± 0.38 

D 1075 0.6 74.3 4.20E-15 8.796 0.37 2.7 61.27 ± 0.45 

E 1100 0.5 70 3.92E-15 8.446 0.2 5.1 58.88 ± 0.53 

F 1125 0.6 68.6 4.70E-15 8.683 0.12 8.3 60.5 ± 0.41 

G 1150 0.8 70.9 6.28E-15 8.877 0.11 8.9 61.83 ± 0.37 

H 1175 1.2 70.2 9.38E-15 8.897 0.12 8.1 61.96 ± 0.32 

I 1200 2.2 73.3 1.65E-14 8.894 0.13 7.9 61.95 ± 0.26 

J 1225 4.9 81.3 3.69E-14 8.832 0.12 8.2 61.52 ± 0.16 

K 1250 14.1 87.8 1.06E-13 8.8 0.12 8.2 61.3 ± 0.12 

L 1275 22.5 92.5 1.69E-13 8.793 0.12 8.1 61.25 ± 0.1 

M 1300 10.3 92.4 7.75E-14 8.801 0.13 7.9 61.31 ± 0.1 

N 1350 37.7 93.1 2.84E-13 8.801 0.12 8.6 61.31 ± 0.09 

O 1400 1.6 85.2 1.18E-14 8.751 0.11 9 60.97 ± 0.22 

Total Gas 61.31

89.5% of 39ArK gas released in steps 1225 through 1350 Plateau Age = 61.3 ± 0.2 

01AH-69 biotite (total fusion) J = 0.003749 ± 0.35% wt = 4.4mg #2KD25 

A 1450 100 97.1 3.98E-13 9.697 105.03 30.0 64.42 ± 0.04 

Page 26 of 32
Table 2. 40Ar/39Ar geochronology data for igneous rocks in SW Alaska (continued). 

Step Temp. %39Ar Radiogenic 39Ark 

40Ar* Apparent Apparent Apparent Error 

°C of total Yield (%) (Moles) 39Ark K/Ca K/Cl Age (Ma) (Ma) 

01AH-70 K-feldspar J = 0.003895 ± 0.35% wt = 4.1mg #83KD25 

A 800 1.5 84.9 5.00E-15 8.233 3.6 15.9 57.08 ± 0.27 

B 900 4.9 95.7 1.65E-14 8.593 48.6 31.7 59.53 ± 0.13 

C 950 5.1 98.2 1.71E-14 8.477 79.6 58.4 58.74 ± 0.12 

D 1000 6.4 99 2.14E-14 8.465 98.4 77.6 58.66 ± 0.1 

E 1050 7.7 98.7 2.59E-14 8.376 90.2 78.9 58.05 ± 0.1 

F 1100 7.9 99 2.66E-14 8.374 87.7 79.6 58.04 ± 0.09 

G 1150 7.6 98.6 2.57E-14 8.378 83.5 59.1 58.06 ± 0.1 

H 1200 7.1 97.9 2.39E-14 8.38 93.4 34.9 58.08 ± 0.1 

I 1215 4.3 97.6 1.44E-14 8.398 90.3 26.1 58.2 ± 0.13 

J 1250 4.8 97.2 1.61E-14 8.464 70.2 19.7 58.65 ± 0.11 

K 1275 4.9 96.8 1.66E-14 8.389 45.1 17.7 58.14 ± 0.12 

L 1300 3.9 95.2 1.32E-14 8.427 50.5 5.7 58.4 ± 0.13 

M 1350 9.6 95.5 3.23E-14 8.473 72.5 3.5 58.71 ± 0.1 

N 1450 19.8 95 6.67E-14 8.476 80.7 17.7 58.73 ± 0.09 

O 1550 4.4 91.2 1.48E-14 8.893 70.2 5.9 61.57 ± 0.14 

Total Gas 58.59 

89.1% of 39ArK gas released in steps 950 through 1450 Average Age = 58.40 ± 0.20 

01AH-80 K-feldspar J = 0.003891 ± 0.35% wt = 4.7mg #81KD25 

A 750 0.7 88.5 2.66E-15 8.253 25 11.8 57.16 ± 0.28 

B 800 1.2 94.2 4.50E-15 8.517 40.7 19.9 58.96 ± 0.2 

C 850 2 95.9 7.59E-15 8.469 39.8 30.7 58.64 ± 0.15 

D 900 2.5 96.7 9.44E-15 8.396 41 44.2 58.14 ± 0.13 

E 950 2.8 97.1 1.07E-14 8.388 40.2 53.4 58.08 ± 0.14 

F 1000 3.4 97.5 1.28E-14 8.396 45.6 69.7 58.13 ± 0.11 

G 1050 3.7 97.7 1.39E-14 8.419 40.4 68 58.29 ± 0.1 

H 1100 4.2 98 1.58E-14 8.423 39 49.4 58.32 ± 0.1 

I 1150 3.4 97.5 1.27E-14 8.427 36.6 24.5 58.35 ± 0.11 

J 1200 3.1 96.4 1.17E-14 8.404 25.7 12.2 58.19 ± 0.12 

K 1250 10.1 98.8 3.81E-14 8.392 22.2 24.3 58.11 ± 0.08 

L 1300 17.4 98.4 6.59E-14 8.38 28 4.4 58.03 ± 0.07 

M 1350 22.1 99 8.35E-14 8.412 32.9 19.3 58.25 ± 0.07 

N 1450 14.1 99 5.36E-14 8.448 32.8 25.9 58.49 ± 0.07 

O 1650 9.4 97.3 3.55E-14 8.457 30.2 19.1 58.55 ± 0.09 

Total Gas 58.26 

96.2% of 39ArK gas released in steps 900 through 1650 Average Age = 58.3 ± 0.2 

01AH-83 muscovite J = 0.003891 ± 0.35% wt = 254.0mg #84KD25 

A 850 1.2 89.8 3.93E-14 9.86 239.2 633 67.92 ± 0.32 

B 900 2.1 90.5 6.64E-14 9.844 426.4 1032 67.81 ± 0.4 

C 950 4.8 92.8 1.53E-13 9.825 1014.3 1812 67.68 ± 0.12 

D 1000 22.2 98.2 7.17E-13 9.744 2448.8 62266 67.13 ± 0.05 

E 1050 19.4 99.2 6.26E-13 9.748 2480.5 0 67.16 ± 0.04 

F 1100 10.4 98.6 3.34E-13 9.758 1876.4 0 67.23 ± 0.04 

G 1150 6.8 98.1 2.20E-13 9.746 1359.5 5789 67.14 ± 0.1 

H 1200 6.9 98 2.24E-13 9.707 1319.7 3552 66.88 ± 0.11 

I 1250 7.5 98.5 2.41E-13 9.741 1110.3 275 67.11 ± 0.07 

J 1350 11.3 98.6 3.64E-13 9.769 1664.9 449 67.3 ± 0.05 

K 1450 4.5 89 1.47E-13 9.757 1140.9 1079 67.22 ± 0.11 

L 1650 2.9 79.6 9.23E-14 9.753 578.5 617 67.2 ± 0.36 

Total Gas 100 97 3.22E-12 9.755 1804.5 14741.9 67.2 

91.97% of 39ArK gas released in steps 1000 through 1650 Plateau Age = 67.18 ± 0.28 

Page 27 of 32
Table 2. 40Ar/39Ar geochronology data for igneous rocks in SW Alaska (continued). 

Step Temp. %39Ar Radiogenic 39Ark 

40Ar* Apparent Apparent Apparent Error 

°C of total Yield (%) (Moles) 39Ark K/Ca K/Cl Age (Ma) (Ma) 

01AH-85A hornblende J = 0.003922 ± 0.35% wt = 180.0mg #92KD25 

A 900 1.3 72.5 4.82E-14 10.194 

B 1000 1.9 88.4 6.82E-14 10.565 

C 1050 1.8 95.9 6.57E-14 11.141 

D 1100 3.5 94.9 1.27E-13 11.208 

E 1150 9.5 95.4 3.44E-13 11.063 

F 1175 23.6 96 8.52E-13 10.916 

G 1200 18.3 97.6 6.61E-13 10.867 

H 1225 9.2 97.9 3.32E-13 10.831 

I 1250 4.7 97.1 1.71E-13 10.783 

J 1275 8.8 97.5 3.17E-13 10.837 

K 1300 13.1 97.8 4.72E-13 10.886 

L 1400 4.4 97.1 1.60E-13 10.872 

Total Gas 100 96.4 3.62E-12 10.892 

82.0% of 39ArK gas released in steps 1175 through 1400 

1.12 86 70.72 ± 0.46 

1.16 138 73.25 ± 0.32 

0.68 96 77.16 ± 0.35 

0.3 36 77.61 ± 0.12 

0.2 22 76.62 ± 0.13 

0.16 18 75.63 ± 0.11 

0.19 20 75.29 ± 0.08 

0.2 23 75.05 ± 0.16 

0.19 22 74.72 ± 0.17 

0.19 23 75.09 ± 0.06 

0.19 22 75.43 ± 0.1 

0.18 20 75.33 ± 0.2 

0.23 25.6 75.47 

Average Age = 75.22 ± 0.32 

wt = 4.5mg #29KD25 

145.53 33.0 59.25 ± 0.05 

wt = 4.6mg #41KD25 

76.13 18.0 61.28 ± 0.05 

wt = 3.7mg #54KD25 

35.8 13.0 61.30 ± 0.05 

wt = 4.8mg #42KD25 

14.07 13.0 61.64 ± 0.05 

wt = 4.5mg #31KD25 

94.84 15.0 61.26 ± 0.05 

01AM-109A 

A 1450 

01APC-18 

A 1450 

01APC-20 

A 1450 

01APC-23 

A 1450 

01APC-25 

A 1450 

01AWS-2 

A 900 

B 1000 

C 1050 

D 1100 

E 1150 

F 1175 

G 1200 

H 1225 

I 1250 

J 1275 

K 1300 

biotite (total fusion) 

100 95.6 

biotite (total fusion) 

100 97.6 

biotite (total fusion) 

100 95.7 

biotite (total fusion) 

100 96.7 

biotite (total fusion) 

100 95.1 

J = 0.003832 ± 0.35% 

4.17E-13 8.714 

J = 0.003854 ± 0.35% 

5.20E-13 8.965 

J = 0.003869 ± 0.35% 

3.67E-13 8.933 

J = 0.003853 ± 0.35% 

4.95E-13 9.022 

J = 0.003848 ± 0.35% 

4.39E-13 8.977 

hornblende J = 0.003917 ± 0.35% wt = 150.4mg #93KD25 

4.0 92.8 6.43E-14 10.7350 1.22 68.0 74.30 ± 0.39 

4.8 91.7 7.66E-14 9.5510 0.52 59.0 66.26 ± 0.23 

4.1 91.6 6.50E-14 9.1970 0.20 36.0 63.85 ± 0.48 

7.5 92.4 1.20E-13 9.8060 0.27 35.0 67.99 ± 0.27 

8.9 94.1 1.42E-13 9.4710 0.36 40.0 65.72 ± 0.16 

5.3 91.0 8.48E-14 9.1440 0.16 29.0 63.49 ± 0.28 

9.3 92.3 1.48E-13 9.2210 0.12 30.0 64.01 ± 0.24 

12.4 93.5 1.98E-13 9.1800 0.11 25.0 63.73 ± 0.19 

10.5 94.4 1.67E-13 9.1870 0.11 34.0 63.78 ± 0.21 

12.5 93.9 1.99E-13 9.1880 0.10 33.0 63.78 ± 0.23 

20.7 93.8 3.29E-13 9.1850 0.10 33.0 63.76 ± 0.10 

Total Gas 100.0 93.3 1.59E-12 9.3390 0.21 35.2 64.81 

70.67% of 39ArK gas released in steps 1175 through 1300 Plateau Age = 63.77 ± 0.27 

01AWS-24 biotite (total fusion) J = 0.003874 ± 0.35% wt = 4.2mg #66KD25 

A 1450 100 96.7 4.29E-13 8.981 44.98 13.0 61.70 ± 0.06 

Page 28 of 32
Table 2. 40Ar/39Ar geochronology data for igneous rocks in SW Alaska (continued). 

Step Temp. %39Ar Radiogenic 39Ark 

40Ar* Apparent Apparent Apparent Error 

°C of total Yield (%) (Moles) 39Ark K/Ca K/Cl Age (Ma) (Ma) 

01AWS-39 hornblende J = 0.003883 ± 0.35% wt = 150.3mg #86KD25 

A 1000 3.3 87.3 6.78E-14 9.165 1.26 9.0 63.08 ± 0.30 

B 1050 3.1 91.4 6.36E-14 9.088 0.53 3.0 62.56 ± 0.44 

C 1100 5.6 89.4 1.14E-13 9.035 0.27 2.0 62.21 ± 0.26 

D 1150 16.9 92.4 3.46E-13 8.805 0.17 2.0 60.65 ± 0.08 

E 1175 19.9 93.9 4.07E-13 8.704 0.16 2.0 59.97 ± 0.08 

F 1200 17.6 96.1 3.60E-13 8.720 0.16 3.0 60.07 ± 0.08 

G 1225 7.0 95.1 1.42E-13 8.741 0.19 3.0 60.21 ± 0.19 

H 1250 6.1 93.0 1.25E-13 8.683 0.20 3.0 59.82 ± 0.30 

I 1300 20.5 93.9 4.19E-13 8.750 0.16 2.0 60.28 ± 0.12 

Total Gas 100.0 93.5 2.04E-12 8.780 0.22 2.7 60.48 

71.08% of 39ArK gas released in steps 1175 through 1300 Plateau Age = 60.07 ± 0.25

01AWS-39 biotite J = 0.003750 ± 0.35% wt = 17.9mg #4KD25 

A 850 3.6 85.4 6.39E-14 9.669 93.1 11.0 64.25 ± 0.23 

B 900 6.4 96.0 1.15E-13 9.387 207.5 12.0 62.41 ± 0.21 

C 950 7.9 98.6 1.42E-13 9.322 161.4 12.0 61.99 ± 0.20 

D 1000 6.5 99.0 1.16E-13 9.308 162.5 12.0 61.90 ± 0.23 

E 1050 5.2 97.9 9.42E-14 9.363 47.6 12.0 62.25 ± 0.16 

F 1100 5.9 96.0 1.06E-13 9.625 14.3 10.0 63.97 ± 0.18 

G 1150 10.9 97.1 1.96E-13 9.421 9.7 11.0 62.64 ± 0.10 

H 1200 27.4 98.9 4.93E-13 9.234 122.2 12.0 61.41 ± 0.04 

I 1250 20.3 99.4 3.66E-13 9.219 394.9 12.0 61.31 ± 0.06 

J 1350 6.0 99.2 1.07E-13 9.239 415.0 13.0 61.44 ± 0.20 

Total Gas 100.0 97.9 1.80E-12 9.318 182.7 11.7 61.96

53.7% of 39ArK gas released in steps 1200 through 1350 Plateau Age = 61.38 ± 0.26

01AWS-39 biotite (total fusion) J = 0.003745 ± 0.35% wt = 3.4mg #5KD25 

A 1450 100 95.8 3.57E-13 9.199 59.9 11.0 61.10 ± 0.09 

01AWS-41 biotite (total fusion) J = 0.003873 ± 0.35% wt = 4.9mg #64KD25 

A 1450 100 97.1 4.82E-13 8.792 182.22 23.0 60.41 ± 0.06 

01AWS-46 biotite (total fusion) J = 0.003746 ± 0.35% wt = 4.0mg #3KD25 

A 1450 100 92.9 1.98E-13 9.985 8.79 7.0 66.24 ± 0.08 

01AWS-48 biotite (total fusion) J = 0.003764 ± 0.35% wt = 3.1mg #17KD25 

A 1450 100 95.8 3.12E-13 9.856 89.06 11.0 65.72 ± 0.04 

01AWS-49 volcanic matrix J = 0.003809 ± 0.35% wt = 259.7mg #71KD25 

A 750 3.0 82.0 1.18E-13 6.658 0.90 190 45.18 ± 0.29 

B 850 9.5 94.6 3.73E-13 7.899 0.84 402 53.48 ± 0.10 

C 900 9.6 96.9 3.76E-13 7.875 0.58 567 53.32 ± 0.11 

D 950 11.9 97.2 4.64E-13 7.654 0.33 544 51.85 ± 0.10 

E 1000 11.9 97.5 4.64E-13 7.508 0.29 423 50.87 ± 0.17 

F 1050 10.1 97.5 3.97E-13 7.452 0.36 266 50.49 ± 0.12 

G 1150 22.6 97.7 8.85E-13 7.337 0.65 130 49.72 ± 0.05 

H 1450 19.4 94.6 7.59E-13 7.103 0.21 95 48.16 ± 0.05 

I 1650 1.9 92.6 7.57E-14 7.157 0.16 155 48.52 ± 0.52 

Total Gas 100.0 96.1 3.91E-12 7.442 0.47 291 50.43

75.6% of 39ArK gas released in steps 850 through 1150 Average Age = 50.88 ± 0.22 

01AWS-50 biotite (total fusion) J = 0.003761 ± 0.35% wt = 4.5mg #7KD25 

A 1450 100 91.0 4.18E-13 9.160 37.64 11.0 61.11 ± 0.07 

Page 29 of 32
Table 2. 40Ar/39Ar geochronology data for igneous rocks in SW Alaska (continued). 

Step Temp. %39Ar Radiogenic 39Ark 

40Ar* Apparent Apparent Apparent Error 

°C of total Yield (%) (Moles) 39Ark K/Ca K/Cl Age (Ma) (Ma) 

01AWS-55 volcanic matrix J = 0.003811 ± 0.35% wt = 247.2mg #67KD25 

A 750 1.4 61.9 7.74E-14 8.342 0.38 32.0 56.46 ± 0.49 

B 850 5.7 95.5 3.17E-13 8.415 0.73 693.0 56.95 ± 0.12 

C 900 6.4 98.7 3.59E-13 8.278 0.99 13823.0 56.04 ± 0.09 

D 950 10.3 99.3 5.75E-13 8.124 1.09 0.0 55.00 ± 0.08 

E 1000 11.0 99.1 6.15E-13 8.016 0.96 18636.0 54.29 ± 0.07 

F 1050 10.5 98.9 5.88E-13 7.980 0.65 3132.0 54.05 ± 0.06 

G 1150 18.2 98.7 1.02E-12 7.970 0.67 962.0 53.98 ± 0.15 

H 1450 36.6 88.5 2.05E-12 7.696 0.26 229.0 52.15 ± 0.10 

Total Gas 100.0 94.4 5.60E-12 7.942 0.61 3558.3 53.79 

62.0% of 39ArK gas released in steps 850 through 1150 Average Age = 54.76 ± 0.23 

01AWS-57 biotite (total fusion) J = 0.003868 ± 0.35% wt = 4.6mg #52KD25 

A 1450 100 94.4 4.66E-13 8.976 23.16 23.0 61.57 ± 0.06 

01AWS-59 biotite (total fusion) J = 0.003771 ± 0.35% wt = 4.6mg #56KD25 

A 1450 100 96.2 4.76E-13 9.221 64.54 19.0 61.65 ± 0.05 

01AWS-60 volcanic matrix J = 0.003814 ± 0.35% wt = 264.0mg #69KD25 

A 850 5.4 91.4 1.92E-13 7.024 0.28 779 47.69 ± 0.18 

B 900 7.1 95.6 2.51E-13 6.857 0.44 2246 46.57 ± 0.19 

950 10.7 96.2 3.80E-13 6.711 0.59 4017 45.60 ± 0.12 

D 1000 12.8 97.5 4.55E-13 6.679 0.47 5601 45.38 ± 0.09 

E 1050 13.7 97.8 4.88E-13 6.646 0.36 2921 45.16 ± 0.08 

F 1150 20.1 98.0 7.13E-13 6.586 0.37 1460 44.75 ± 0.07 

G 1450 30.3 94.2 1.08E-12 6.247 0.08 214 42.48 ± 0.10 

Total Gas 100.0 96.0 3.56E-12 6.560 0.32 2105 44.58

57.3% of 39ArK gas released in steps 950 through 1150 Average Age = 45.11 ± 0.19 

01AWS-61 biotite (total fusion) J = 0.003849 ± 0.35% wt = 5.1mg #35KD25 

A 1450 100 96.0 4.96E-13 8.689 95.69 35.0 59.34 ± 0.03 

01AWS-64 hornblende J = 0.003882 ± 0.35% wt = 162.2mg #87KD25 

A 1000 2.5 85.2 3.83E-14 10.219 0.64 28.0 70.18 ± 0.59 

B 1050 2.5 92.1 3.86E-14 9.883 0.86 30.0 67.92 ± 0.52 

C 1100 3.3 93.0 5.11E-14 9.948 0.65 24.0 68.36 ± 0.41 

D 1150 6.6 93.7 1.01E-13 9.641 0.40 16.0 66.29 ± 0.24 

E 1175 5.5 92.0 8.47E-14 9.404 0.23 16.0 64.68 ± 0.35 

F 1200 6.6 89.5 1.02E-13 9.321 0.12 20.0 64.12 ± 0.33 

G 1225 11.2 91.2 1.73E-13 9.319 0.10 24.0 64.11 ± 0.21 

H 1250 17.9 94.1 2.76E-13 9.344 0.10 26.0 64.28 ± 0.10 

I 1300 40.1 93.9 6.19E-13 9.315 0.10 30.0 64.08 ± 0.12 

J 1400 3.9 91.7 5.98E-14 9.263 0.08 29.0 63.73 ± 0.48 

Total Gas 100.0 92.8 1.54E-12 9.403 0.18 26.1 64.68

85.14% of 39ArK gas released in steps 1175 through 1400 Plateau Age = 64.20 ± 0.27

01AWS-67 volcanic matrix J = 0.003911 ± 0.35% wt = 263.4mg #73KD25 

A 650 2.9 36.5 1.10E-13 6.454 0.24 30.0 44.97 ± 0.37 

B 750 4.0 72.3 1.52E-13 6.885 0.32 204.0 47.94 ± 0.32 

C 850 8.9 90.3 3.38E-13 6.789 0.42 1204.0 47.28 ± 0.15 

D 900 19.2 94.4 7.29E-13 6.613 0.55 3512.0 46.06 ± 0.07 

E 950 16.5 96.4 6.26E-13 6.509 0.46 4873.0 45.35 ± 0.07 

F 1000 5.6 97.8 2.13E-13 6.463 0.45 3515.0 45.03 ± 0.18 

G 1050 8.4 98.3 3.19E-13 6.542 0.39 3238.0 45.58 ± 0.11 

H 1150 15.2 97.7 5.79E-13 6.390 0.37 1516.0 44.53 ± 0.07 

I 1450 19.4 91.1 7.40E-13 5.873 0.06 191.0 40.97 ± 0.17 

Total Gas 100.0 92.2 3.81E-12 6.426 0.36 2326.2 44.78

73.7% of 39ArK gas released in steps 850 through 1150 Average Age = 45.47 ± 0.19 

Page 30 of 32

C 
Table 2. 40Ar/39Ar geochronology data for igneous rocks in SW Alaska (continued). 

Step Temp. %39Ar Radiogenic 39Ark 

40Ar* Apparent Apparent Apparent Error 

°C of total Yield (%) (Moles) 39Ark K/Ca K/Cl Age (Ma) (Ma) 

01AWS-67B plagioclase J = 0.003909 ± 0.35% wt = 262.0mg #77KD25 

A 850 2.7 97.0 8.94E-14 6.634 

B 950 7.3 97.8 2.40E-13 6.653 

C 1000 5.8 97.4 1.91E-13 6.474 

D 1050 7.5 98.2 2.47E-13 6.486 

E 1100 9.6 98.7 3.16E-13 6.501 

F 1150 11.5 98.6 3.75E-13 6.464 

G 1200 12.7 98.7 4.17E-13 6.467 

H 1250 12.2 98.9 3.98E-13 6.467 

I 1300 10.6 98.8 3.47E-13 6.462 

J 1350 7.4 98.1 2.43E-13 6.423 

K 1400 5.1 97.3 1.67E-13 6.420 

L 1450 4.0 98.1 1.31E-13 6.507 

M 1500 2.2 97.2 7.22E-14 6.462 

N 1550 1.2 97.8 4.03E-14 6.630 

Total Gas 100.0 98.3 3.27E-12 6.487 

70.0% of 39ArK gas released in steps 1000 through 1300 

01AWS-78 biotite (total fusion) J = 0.003850 ± 0.35% 

A 1450 100 96.4 3.76E-13 8.918 

01AWS-80 biotite (total fusion) J = 0.003860 ± 0.35% 

A 1450 100 96.9 4.42E-13 9.003 

99AM-309C biotite (total fusion) J = 0.003871 ± 0.35% 

A 1450 100 93.9 4.07E-13 10.043 

99AWS-17 biotite (total fusion) J = 0.003752 ± 0.35% 

A 1450 100 90.4 4.21E-13 10.191 

2.20 813.0 46.19 ± 0.32 

0.87 2302.0 46.32 ± 0.10 

0.57 1636.0 45.09 ± 0.16 

0.49 2811.0 45.17 ± 0.17 

0.46 4158.0 45.27 ± 0.08 

0.44 3964.0 45.02 ± 0.10 

0.44 3678.0 45.04 ± 0.08 

0.45 132.0 45.04 ± 0.10 

0.45 2027.0 45.01 ± 0.09 

0.41 1428.0 44.73 ± 0.16 

0.38 929.0 44.71 ± 0.15 

0.43 1828.0 45.31 ± 0.22 

0.44 1671.0 45.01 ± 0.44 

0.45 1063.0 46.16 ± 0.67 

0.53 2329.4 45.18 

Plateau Age = 45.1 ± 0.19 

wt = 3.8mg #38KD25 

142.7 13.0 60.90 ± 0.04 

wt = 4.5mg #48KD25 

52.25 17.0 61.63 ± 0.05 

wt = 4.3mg #60KD25 

48.74 23.0 68.80 ± 0.07 

wt = 4.9mg #11KD25 

37.19 17.0 67.69 ± 0.04 

AL00 plagioclase J = 0.003911 ± 0.35% wt = 204.7mg #79KD25 

A 950 5.6 94.2 8.28E-14 6.149 0.27 798.0 42.87 ± 0.32 

B 1000 9.2 97.0 1.35E-13 6.055 0.24 2864.0 42.22 ± 0.23 

C 1050 10.9 96.3 1.59E-13 5.943 0.21 2445.0 41.45 ± 0.11 

D 1100 12.4 96.8 1.82E-13 5.920 0.19 2714.0 41.30 ± 0.10 

E 1150 13.7 93.8 2.01E-13 6.000 0.17 1106.0 41.85 ± 0.10 

F 1200 13.9 97.6 2.03E-13 6.022 0.17 1587.0 42.00 ± 0.14 

G 1250 9.7 96.4 1.42E-13 6.004 0.18 64.0 41.87 ± 0.15 

H 1300 4.4 91.7 6.45E-14 5.973 0.20 201.0 41.66 ± 0.41 

I 1350 1.8 86.0 2.61E-14 5.891 0.17 126.0 41.10 ± 0.77 

J 1400 4.1 89.0 5.95E-14 5.982 0.15 565.0 41.72 ± 0.49 

K 1450 5.2 86.6 7.61E-14 5.933 0.14 877.0 41.38 ± 0.17 

L 1500 4.1 85.4 6.03E-14 5.944 0.14 694.0 41.46 ± 0.51 

M 1550 2.9 81.7 4.19E-14 5.999 0.14 475.0 41.84 ± 0.58 

N 1600 2.3 81.7 3.40E-14 6.033 0.14 503.0 42.07 ± 0.67 

Total Gas 100.0 93.8 1.47E-12 5.992 0.19 1420.6 41.79 

85.2% of 39ArK gas released in steps 1050 through 1600 Average Age = 41.62 ± 0.18 

KEMUK biotite J = 0.003783 ± 0.35% 

A 950 1.9 93.7 2.13E-14 

B 1000 7.4 95.6 8.23E-14 

C 1050 13.8 98.4 1.54E-13 

D 1100 14.6 98.7 1.64E-13 

E 1150 14.8 98.2 1.66E-13 

F 1200 15.6 97.8 1.75E-13 

G 1250 19.9 97.7 2.22E-13 

H 1350 12.1 97.5 1.35E-13 

Total Gas 100.0 97.8 1.12E-12 

wt = 10.5mg #21KD25 

13.581 180.9 143.0 90.38 ± 0.87 

13.481 608.1 169.0 89.73 ± 0.30 

13.240 932.2 184.0 88.17 ± 0.13 

13.162 934.6 183.0 87.66 ± 0.11 

13.035 648.5 181.0 86.83 ± 0.15 

12.942 1024.9 181.0 86.23 ± 0.16 

12.903 972.4 73.0 85.98 ± 0.13 

12.970 722.8 172.0 86.41 ± 0.18 

13.077 849.7 157.4 87.10 

62.3% of 39ArK gas released in steps 1150 through 1350 Average Age = 86.32 ± 0.36 

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Table 2. 40Ar/39Ar geochronology data for igneous rocks in SW Alaska (continued). 

Step Temp. %39Ar Radiogenic 39Ark 

40Ar* Apparent Apparent Apparent Error 

°C of total Yield (%) (Moles) 39Ark K/Ca K/Cl Age (Ma) (Ma) 

KEMUK biotite (total fusion) J = 0.003784 ± 0.35% wt = 5.1mg #20KD25 

A 1450 100 96.0 5.27E-13 12.913 554.3 93.0 86.05 ± 0.08 

Ages calculated assuming an initial 40Ar/36Ar = 295.5± 0.


All precision estimates are at the one sigma level of precision.


Ages of individual steps do not include error in the irradiation parameter J.


No error is calculated for the total gas age.


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