40Ar/39Ar Age-Spectrum and Laser Fusion Data 

for Volcanic Rocks in West Central Colorado 

by Michael J. Kunk1, Jeffrey A. Winick1, and Josette O. Stanley2 

Open-File Report 01-472 

on-line edition 

2001 

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

editorial standards or with the North American Stratigraphic Code. Any use of trade, firm, or 

pruduct 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 

1 U.S. Geological Survey, Denver, Colorado 

2 University of Northern Colorado, Greely, Colorado 
2
INTRODUCTION 

This publication contains 40Ar/39Ar data of Cenozoic basalts and sanidine 

mineral separates from basalts and tuffs from west central Colorado and is intended 

only to be a preliminary publication of these results. Also included in this report is 

information on the location and elevation of the samples. The data contained herein 

are not interpreted in a geological context, and care should be taken by users 

unfamiliar with argon isotopic data in the use of these results. No geological meaning 

is implied for any of the apparent ages presented below and many of the individual 

apparent ages are not geologically meaningful. This report is primarily a detailed 

source document for subsequent publications that will integrate this data into a 

geological context. 

SAMPLE LOCATIONS 

All of the samples in this report were collected in west central Colorado. Table 

1 is a listing of the location of all of the samples. This table includes the sample 

number, the U.S. Geological Survey 71/2’ quadrangle that the sample is located on, 

the material dated, the latitude and longitude of the sample site, and the elevation of 

the sample site, in feet above sea level. In general, the samples are listed in order of 

decreasing age, and where known in stratigraphic order, with the stratigraphically 

youngest sample in a group listed first. Stratigraphic relations between many of the 

samples are not known. Those samples that are in a known stratigraphic sequence 

are marked with an asterisk. 

METHODS 

Sample Preparation 

All of the samples were crushed, ground, and sized using 250, 180, and 150 

μm sieves. The 250-180 μm size fraction of the basalts was leached in 10% 

hydrochloric acid to remove secondary calcite. Plagioclase, olivine and other 

phenocrysts, were removed from basalts using heavy liquids and magnetic 

separation techniques. Sanidine mineral separates, from both the tuffs and from a 

small number of basalts were produced using magnetic separation, heavy liquids 

and hand picking to a purity of >99%. All samples were washed in acetone, alcohol, 

and deionized water(X3) in a Branson B-220 ultrasonic cleaner to remove dust and 

then re-sieved by hand using a 100 μm sieve. 

Amounts between 250 and 1000 mg of basalt and 50 mg of sanidine were 

packaged in copper or aluminum capsules and sealed under vacuum in quartz vials. 

The quartz vials were irradiated in several packages in the central thimble facility at 

the U.S. Geological Survey TRIGA reactor (Dalrymple and Lanphere, 1981) in Denver, 

Colorado. The neutron fluence monitor mineral used in all packages was Fish 

3
Table 1. Material dated, elevation, and location of samples. See text for explanation. 

Sample# Material 7 1/2' Elevation Latitude Longitude 

dated Quadrangle 

Flank of Mt Sopris 

K97-8-11F Sanidine Basalt 8200 39.3485 107.0667 

Dotsero 

K97-10-7K5 Basalt Dotsero 8130 39.6644 107.0076 

Castle Peak 

2ABS13-9-99* Basalt Castle Peak 11200 39.7725 106.8367 

2BBS13-9-99* Basalt Castle Peak 11200 39.7725 106.8367 

2CBS13-9-99* Basalt Castle Peak 11200 39.7725 106.8367 

1ABS13-9-99* Basalt Castle Peak 11160 39.7725 106.8372 

1CBS13-9-99* Basalt Castle Peak 11160 39.7725 106.8372 

1DBS13-9-99* Basalt Castle Peak 11160 39.7725 106.8372 

Devils Causeway 

3BBS13-9-99 Basalt Devils Causeway 10760 40.0333 107.1275 

3CBS13-9-99 Basalt Devils Causeway 10760 40.0333 107.1275 

Yarmony Mountain 

K97-8-13A1-3* Basalt McCoy 9360 39.9119 106.6494 

K97-8-13A1-4* Basalt McCoy 9360 39.9119 106.6494 

K97-8-13A1 Basalt McCoy 9200 39.9133 106.6542 

K97-8-13A2 Basalt McCoy 9200 39.9133 106.6542 

K97-8-13B2* Basalt McCoy 8920 39.9158 106.6528 

K97-8-13B3* Basalt McCoy 8920 39.9158 106.6528 

K97-8-13C2* Basalt State Bridge 6840 39.8965 106.6050 

K97-8-13C3* Basalt State Bridge 6840 39.8965 106.6050 

Gypsum 

K99-10-24A3 Basalt Gypsum 6960 39.6838 106.9675 

K99-10-24A5 Basalt Gypsum 6960 39.6838 106.9675 

Spring Valley 

K97-10-8C2 Basalt Glenwood Springs 6870 39.5150 107.2700 

K97-10-8C4 Basalt Glenwood Springs 6870 39.5150 107.2700 

Porphyry Mtn 

JS-98-13 Sanidine Burns South 6520 39.8486 106.9531 

JS-98-25 Sanidine Sugarloaf Mtn 7280 39.8211 107.0039 

JS-98-5 Sanidine Sugarloaf Mtn 7200 39.8263 107.0030 

JS-98-93 Sanidine Sugarloaf Mtn 7080 39.8321 107.0004 

JS-98-2 Sanidine Sugarloaf Mtn 7160 39.8056 107.0202 

K97-8-13D Sanidine Sugarloaf Mtn 6980 39.8215 107.0028 

JS-98-17 Sanidine Sugarloaf Mtn 7280 39.8122 107.0188 

Flat Tops 

K99-8-24A* Basalt Sweetwater Lake 11640 39.8664 107.1257 

K99-8-24B1* Basalt Sugarloaf Mtn 11320 39.8701 107.1220 

K99-8-24B3* Basalt Sugarloaf Mtn 11320 39.8701 107.1220 

K99-8-24C* Basalt Sugarloaf Mtn 11165 39.8701 107.1061 

K99-8-24D1* Basalt Sugarloaf Mtn 10920 39.8731 107.1014 

K99-8-24D2* Basalt Sugarloaf Mtn 10920 39.8731 107.1014 

K99-8-24E* Basalt Sugarloaf Mtn 10770 39.8733 107.0989 

4
Table 1. Continued. 

Sample# Material 7 1/2' Elevation Latitude Longitude 

dated Quadrangle 

Poison Creek Debris Flow 

JS-99-8A Basalt Sugarloaf Mtn 7400 39.8052 107.1152 

JS-99-8B Basalt Sugarloaf Mtn 7380 39.8056 107.0125 

JS-99-8C Basalt Sugarloaf Mtn 7320 39.8070 107.1318 

JS-99-9 Basalt Sugarloaf Mtn 7240 39.8052 107.0151 

JS-99-10 Basalt Sugarloaf Mtn 7040 39.8061 107.0034 

Sopris Bowl 

L-223 Basalt Carbondale 7340 39.3900 107.1500 

BR6(3) Basalt Basalt 6850 39.3600 107.0590 

Ba36(1) Basalt Basalt 7260 39.3590 107.0550 

Basalt Mtn.,top 

K97-8-15C2 Basalt Leon 10720 39.4106 107.0139 

K97-8-15C4 Basalt Leon 10720 39.4106 107.0139 

K97-8-15B Basalt Leon 10460 39.4097 107.0158 

L-37 Basalt Leon 10740 39.4110 107.0150 

Basalt Mtn.,west flank 

L-176 Basalt Leon 8160 39.4130 107.0660 

Basalt Mtn., north rim 

L-91A1 Basalt Leon 9480 39.4420 107.0330 

L-39 Basalt Leon 9540 39.4410 107.0390 

L-181 Basalt Leon 9540 39.4450 107.0180 

L-187D1* Basalt Leon 9720 39.4320 107.0450 

L-187B3* Basalt Leon 9600 39.4310 107.0450 

L-187A2* Basalt Leon 9560 39.4310 107.0460 

Basalt Mtn., south 

L-38A Basalt Leon 9840 39.3990 107.0020 

K97-8-11E1* Basalt Leon 9680 39.3998 107.0022 

K97-8-11E2* Basalt Leon 9600 39.3998 107.0022 

K97-8-11D3* Basalt Leon 9600 39.3998 107.0039 

K97-8-11C1* Basalt Leon 9540 39.3998 107.0039 

K97-8-11C3* Basalt Leon 9540 39.3998 107.0039 

K97-8-11B* Basalt Leon 9480 39.3998 107.0039 

K97-8-11A1* Basalt Leon 9400 39.3998 107.0039 

K97-8-11A2* Basalt Leon 9400 39.3998 107.0039 

Grand Mesa, Mesa Lakes 

K98-8-18F2* Basalt Mesa Lakes 10640 39.0419 108.0667 

K98-8-18F3* Basalt Mesa Lakes 10640 39.0419 108.0667 

K98-8-18E4* Basalt Mesa Lakes 10560 39.0467 108.0661 

K98-8-18E5* Basalt Mesa Lakes 10560 39.0467 108.0661 

K98-8-18D1* Basalt Mesa Lakes 10520 39.0489 108.0656 

K98-8-18C1* Basalt Mesa Lakes 10500 39.0503 108.0653 

K98-8-18C2* Basalt Mesa Lakes 10500 39.0503 108.0653 

K98-8-18B1* Basalt Mesa Lakes 10490 39.0508 108.0647 

K98-8-18B2* Basalt Mesa Lakes 10490 39.0508 108.0647 

K98-8-18A3* Basalt Mesa Lakes 10480 39.0514 108.0639 

K98-8-18A4* Basalt Mesa Lakes 10480 39.0514 108.0639 

K98-8-17B6* Basalt Mesa Lakes 10440 39.0525 108.0611 

K98-8-17A4* Basalt Mesa Lakes 10400 39.0528 108.0597 

5
Table 1. Continued. 

Sample# Material 7 1/2' Elevation Latitude Longitude 

dated Quadrangle 

Grand Mesa, Lands End 

K98-8-18I1* Basalt Lands End 9880 39.0247 108.2297 

K98-8-18I4* Basalt Lands End 9880 39.0247 108.2297 

K98-8-18H4* Basalt Lands End 9840 39.0244 108.2286 

K98-8-18G2* Basalt Lands End 9800 39.0239 108.2261 

K98-8-18G5* Basalt Lands End 9800 39.0239 108.2261 

Missouri Heights 

K97-8-15A3 Basalt Leon 7290 39.4214 107.0917 

K97-8-15A4 Basalt Leon 7290 39.4214 107.0917 

K97-8-12F1 Basalt Leon 6760 39.4086 107.0972 

K97-8-12F2 Basalt Leon 6760 39.4086 107.0972 

K97-8-12E1 Basalt Leon 6755 39.4086 107.0972 

K97-8-12E2 Basalt Leon 6755 39.4086 107.0972 

K97-10-8A2 Basalt Carbondale 6740 39.4219 107.1500 

K97-10-8A4 Basalt Carbondale 6740 39.4219 107.1500 

Heuschkel Park 

K97-10-8E1 Basalt Carbondale 7440 39.4353 107.2000 

K97-10-8E3 Basalt Carbondale 7440 39.4353 107.2000 

K97-10-8F3 Basalt Carbondale 7420 39.4344 107.2008 

K97-10-8F4 Basalt Carbondale 7420 39.4344 107.2008 

Red Canyon Graben 

K97-10-8B1 Basalt Glenwood Springs 6800 39.5194 107.2756 

K97-10-8B3 Basalt Glenwood Springs 6800 39.5194 107.2756 

Upper Cattle Creek 

L60 Basalt Leon 8040 39.4750 107.0610 

Spring Valley II 

K97-10-8D1 Basalt Glenwood Springs 7000 39.5194 107.2717 

K97-10-8D2 Basalt Glenwood Springs 7000 39.5194 107.2717 

McNulty Pit 

K97-8-12G Basalt Leon 7405 39.4675 107.1114 

K97-8-12H Basalt Leon 7380 39.4631 107.1089 

Griffith Pit 

L244-1 Basalt Leon 7440 39.4330 107.0830 

L244-2 Basalt Leon 7440 39.4330 107.0830 

L-244 Sanidine Leon 7440 39.4330 107.0830 

L-245 Sanidine Leon 7520 39.4339 107.0800 

Triangle Peak 

K97-8-12B1* Basalt Woody Creek 8840 39.3056 106.9153 

K97-8-12B2* Basalt Woody Creek 8840 39.3056 106.9153 

K99-8-22B2* Basalt Woody Creek 8560 39.3047 106.9167 

K99-8-22B3* Basalt Woody Creek 8560 39.3047 106.9167 

K97-8-12A2* Basalt Woody Creek 8400 39.3028 106.9167 

K97-8-22A1* Basalt Woody Creek 8400 39.3047 106.9174 

K99-8-22A4* Basalt Woody Creek 8400 39.3047 106.9174 

Willow Peak 

K97-10-7F Basalt Dotsero 8320 39.6594 107.1247 

K97-10-7G Basalt Dotsero 8275 39.6494 107.1242 

6
Canyon Tuff (FCT-3) sanidine with an age of 27.79 Ma (Kunk et al., 1985; Cebula et al 

1986) relative to MMhb-1 with an age of 519.4 ± 2.5 Ma (Alexander et al.,1978; 

Dalrymple et al., 1981). The type of container, and the geometry of samples and 

standards is similar to that described by Snee et al. (1988). 

Sample Analysis 

All basalt samples were analyzed at the U.S. Geological Survey, Denver, 

Colorado on a VG Isotopes, Ltd., Model 1200 B Mass Spectrometer fitted with an 

electron multiplier using the 40Ar/39Ar step heating method of dating. Heating for 10 

minutes per step followed a schedule of 4-12 steps per sample. Heating schedules 

were designed such that the percentage of 39Ar released per step was limited to less 

than 20% of the total released for most samples. 

Basalt samples were heated in a small volume, molybdneum-lined, low-blank 

tantalum resistance furnace similar to that described by Staudacher (1978). 

Temperature was monitored by a W5Re-W26Re thermocouple and controlled by a 

proportional, programmable controller. The furnace and the rear manifold were 

pumped between steps with a turbo molecular pump. Two isolated ion pumps 

evacuated the front manifold and the mass spectrometer flight-tube between each 

incremental step. Prior to analyzing in the mass spectrometer, the gas was purified in 

the rear manifold by a SAES ST707 Zr-V-Fe getter operated at room temperature and 

a hot Re filament. A cold finger in the rear manifold, immersed in liquid N2, was used 

to freeze out H20. Gas was equilibrated with the front manifold, then isolated and 

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

 C and a 

Ti getter operated at 3500

 C. 

An activated charcoal finger submerged in a thermally equilibrated mixture of 

dry ice and acetone in the front manifold was used to remove gasses with a 

molecular weight greater than 60 or 80 (primarily other noble gasses) prior to the 

expansion of the argon dominated gas into the mass spectrometer. The gas was 

further purified in the mass spectrometer by a second SAES ST101 active gas getter 

operated at room temperature. Its successful operation could be monitored by the 

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 analytical cycles. 40Ar, 39Ar, 38Ar, and 37Ar, peaks and their baselines, were 

measured for five 1.28 second integrations in each of the five cycles. 36Ar and its 

baselines were measured for twenty 1.28 second integrations each of the five cycles. 

After the analysis the mass spectrometer was evacuated. All phases of the sample 

heating, cleanup, equilibration, and analysis were performed under computer control. 

All sanidine samples were analyzed at the U.S. Geological Survey in Denver, 

Colorado in a MAP 216 mass spectrometer fitted with an electron multiplier using the 40Ar/39Ar-laser fusion method of dating. For each sample, 8 to 10 individual grains of 

sanidine were fused using a Synrad model 48-5-28(w) 50 watt CO2 laser. The laser 

chamber and manifold were pumped between analyses with a turbomolecular pump. 

An isolated ion pump was used to pump the mass spectrometer. Prior to mass 

spectrometer analysis the gas was purified in the manifold by two SAES ST101 Al-Zr 

7
getters, one operated at room temperature and the other at 400oC, and a hot Re 

filament. The argon rich gas was further purified by a third SAES ST 101 getter 

operated at room temperature in the flight tube of the mass spectrometer. Argon 

isotopes with masses 40 through 36 were analyzed as a function of time in six 

analytical cycles. Baselines were measured for 39Ar and 36Ar. The 36Ar baselines were 

subtracted from the 40Ar, 38Ar, 37Ar and 36Ar peaks. The 39Ar baseline was subtracted 

from the 39Ar peak to reduce the influence of the tail of the 40Ar peak. All phases of the 

sample heating, cleanup, equilibration, and analysis were performed under computer 

control. 

Isotopic Data Reduction 

All the isotopic data produced on the VG 1200b were reduced using an updated 

version of the computer program ArAr* (Haugerud and Kunk, 1988). Isotopic data from 

the MAP 216 was reduced using the computer program Mass Spec (Deino, 2001). We 

used the decay constants recommended by Steiger and Jäger (1977). The isotopic 

measurements made in the five- or six-cycle analysis had baseline values subtracted 

and then were regressed, to time zero, using standard linear regression techniques. 

These regressed peak values and associated analytical uncertainties were used in 

data reduction. For the VG 1200b, full system blanks were measured prior to the suite 

of analyses made on each sample and then subtracted from the analytical results. 

For the MAP 216, blanks were measured every fourth laser fusion analysis, and 

several of these were averaged before subtraction from the regressed peak values. 

Error estimates of the blanks were quadratically combined with the analytical errors 

and propagated through the error equations. 

Corrections for interfering reactor-produced argon isotopes from Ca, K, and Cl 

in the sample were made using the production ratios given in Dalrymple et al. (1981) 

and Roddick (1983). Errors in calculated ages or ratios include; measurement 

uncertainty in the analysis, decay factor uncertainties, uncertainties in 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 tables and figures in this report include the identification of (for the 

VG1200b) individual step ages, plateau ages, total gas ages, (for both the VG 1200b 

and the Map 216) inverse isochron ages, (for the MAP 216) laser fusion ages, and 

weighted mean ages. Total gas ages represent the age calculated from the addition 

of all of the measured argon peaks for all steps, in a single sample. The total gas 

ages 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, Sutter, and Elliot(1977) as modified by Haugerud and Kunk(1988). Laser fusion 

ages are the results of a single laser fusion analysis. Weighted mean ages are the 

weighted average of several laser fusion ages that agree within the limits of error. 

Inverse isotope correlation analysis of the analytical data to assess if nonatmospheric argon components were trapped in any samples and to calculate an 

inverse isochron age was done using the method of York (1969). 

8
RESULTS

40Ar/39Ar Data 

The 40Ar/39Ar geochronologic results in this report are presented in two different 

formats. Data within both formats are arranged in the same order as the section on 

sample locations. The first of these formats is a condensed tabular data summary 

(Table 2). These tables summarize the data contained in the subsequent, more 

detailed individual data sets. Included in this table are: the sample number, the 

material analyzed, the apparent age and its error, the percent 39Ar of the total that this 

apparent age represents(or LF for laser fusion analyses), the number of steps / total 

number of steps that this apparent age represents, and a comment listing the type of 

apparent age. 

The second format, individual data sets (Table 3), include tabular data, as well 

as two or three graphical representations of some of the age spectrum data on facing 

pages. Each sample starts with a line that gives the sample number, the sample 

weight in grams, the material analyzed, the packet and package number from the 

irradiation and the J-value used with its analytical uncertainty. The table includes; a 

letter designation for the individual argon analysis, the temperature of the step (or LF 

for laser fusion analyses), the percent of potassium derived 39ArK for each step (or 

asterisks for laser fusion analyses), the radiogenic yield (percentage of 40Ar that is 

derived from the decay of potassium), moles of 39ArK (x 10-12), a corrected 40ArR/

39ArK 

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

ratios for each step (or asterisk if the measured 38Ar or 37Ar signal measured was less 

than the detection limit of the mass spectrometer), a calculated age for the step, in 

millions of years and an estimate of the precision of each age at the 1 sigma level. 

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 second to last line in the age 

spectra data sets represents the total gas results for the sample, no analytical error is 

calculated for the age in this line because the age spectra are frequently disturbed 

and the error that could be calculated here would be meaningless. The final line in the 

age spectrum data sets indicates if there was a plateau age. If the sample has an 

age plateau, the percentage of 39Ar on the plateau, the steps on the plateau, and the 

plateau age and its precision are reported. The final line of the laser fusion data sets 

identifies the weighted mean age and its error of analyses that agree in age within 

analytical error. The number of analyses included in this weighted mean is also 

indicated. Analyses not included in the weighted mean age are italicized. Both plateau 

ages and weighted mean ages include the error in the irradiation parameter J. 

The figures associated with each age spectrum data set include three graphs. 

For basalt data the lower and larger graph on the right plots cumulative percent 39ArK 

of the individual heating steps against apparent age in millions of years. Errors in age 

are graphically displayed at the two sigma level of confidence. Also displayed are the 

apparent K/Ca ratios of each step against the cumulative 39ArK released. The graph on 

the left is the inverse isotope correlation diagram, which plots 39Ar/40Ar against 

9
36Ar/40Ar. Included in this figure are; the apparent age of the sample (calculated from 

the inverse of the X-axis intercept), the calculated initial 40Ar/36Ar ratio of the sample 

(the inverse of the y-axis intercept), the MSWD of the data (a goodness of fit indicator), 

and the points that were excluded from the calculation of the above mentioned results. 

The laser fusion data sets replace the age spectrum diagram and K/Ca 

diagram with an age probability distribution diagram, frequently referred to as an 

ideogram and two smaller graphs. In the ideogram a solid line represents the 

weighted mean age of the sample, while dashed lines represent the data with 

outliers included. Immediately above the ideogram are two smaller graphs, the lower 

of which plots the age of sample ages with error bars, and the upper graph is 

radiogenic yield. In both of these upper graphs filled circles represent data that are 

included in the weighted mean age, while open circles are used for those that were 

excluded from the weighed mean age. 

For additional information on the sample data sets see Haugerud and Kunk 

(1988) and Deino (2001) 

10
Table 2. Summary of argon isotopic results. 

Sample Apparent Age %39Ar # Steps MSWD 40Ar/36Ar Comment 

and Error (Ma) Total Steps (Initial)

Flank of Mt Sopris 

K97-8-11F 35.21 ± 0.03 

35.27 ± 0.17 

Dotsero 

K97-10-7K5 23.41 ± 0.13 

23.12 ± 0.38 

23.8 

Castle Peak 

2ABS13-9-99 22.25 ± 0.26 

22.53 

2BBS13-9-99 22.59 ± 0.14 

23.12 

2CBS13-9-99 22.62 ± 0.27 

23.03 

1ABS13-9-99 23.49 ± 0.16 

23.34 

1CBS13-9-99 22.68 ± 0.15 

23.34 

1DBS13-9-99 22.91 ± 0.12 

23.27 

Devils Causeway 

3BBS13-9-99 23.05 ± 0.08 

22.61 ± 0.41 

22.89 

3CBS13-9-99 22.56 ± 0.18 

22.98 

Yarmony Mountain 

K97-8-13A1-3 22.71 ± 0.20 

22.72 

K97-8-13A1-4 22.74 ± 0.13 

22.59 ± 0.26 

22.74 

K97-8-13A1 22.95 ± 0.15 

22.96 

K97-8-13A2 22.92 ± 0.13 

23.21 ± 0.19 

23.3 

K97-8-13B2 22.90 ± 0.13 

22.65 ± 0.45 

23.09 

LF 9/10 

LF 9/10 0.4 

51.9 3/10 

78.6 5 / 7 2.543 

1 0 0 7 / 7 

64.6 3 / 7 1.999 

1 0 0 7 / 7 

82.4 4 / 8 0.722 

1 0 0 8 / 8 

97.6 6 / 8 0.54 

1 0 0 8 / 8 

1 0 0 8 / 8 9.967 

1 0 0 8 / 8 

82.1 4 / 8 1.018 

1 0 0 8 / 8 

81.1 4 / 7 1.233 

1 0 0 7 / 7 

52.2 2 / 6 

98.2 4 / 6 0.232 

1 0 0 6 / 6 

96.3 5 / 8 1.993 

1 0 0 8 / 8 

75.3 4 / 7 2.196 

1 0 0 7 / 7 

63.3 3 / 7 

63.3 3 / 7 1.476 

1 0 0 7 / 7 

1 0 0 6 / 6 6.452 

1 0 0 6 / 6 

62.7 3 / 6 

1 0 0 6 / 6 9.528 

1 0 0 6 / 6 

51.7 3 / 7 

83.9 5 / 7 0.758 

1 0 0 7 / 7 

295.5 Weighted Mean 

225.5 ± 22.4 Isochron 

295.5 Plateau 

486.4 ± 70.9 Isochron 

295.5 Total gas 

364.4 ± 19.7 Isochron 

295.5 Total Gas 

342.6 ± 5.4 Isochron 

295.5 Total Gas 

342.1 ± 16.8 Isochron 

295.5 Total Gas 

299.9 ± 1.7 Isochron 

295.5 Total Gas 

356.2 ± 7.8 Isochron 

295.5 Total Gas 

328.8 ± 4.4 Isochron 

295.5 Total Gas 

295.5 Plateau 

320.9 ± 21.5 Isochron 

295.5 Total Gas 

352.1 ±12.5 Isochron 

295.5 Total Gas 

427.2 ± 34.4 Isochron 

295.5 Total Gas 

295.5 Average 

293.0 ± 11.9 Isochron 

295.5 Total Gas 

328.1 ± 5.9 Isochron 

295.5 Total Gas 

295.5 Plateau 

324.3 ± 13.0 Isochron 

295.5 Total Gas 

295.5 Plateau 

388.3 ± 58.5 Isochron 

295.5 Total Gas 

11
Table 2. Continued. 

Sample Apparent Age %39Ar # Steps MSWD 40Ar/36Ar Comment 

and Error (Ma) Total Steps

K97-8-13B3	 22.88 ± 0.13 

22.49 ± 0.50 

23.03 

K97-8-13C2	 24.19 ± 0.13 

23.88 ± 0.36 

24.87 

K97-8-13C3	 24.06 ± 0.29 

24.36 

Gypsum 

K99-10-24A3 22.52 ± 0.08 

22.51 ± 0.22 

22.46 

K99-10-24A5 22.51 ± 0.35 

22.33 

Spring Valley 

K97-10-8C2 23.05 ± 0.33 

22.86 

K97-10-8C4 22.56 ± 0.13 

22.58 ± 0.51 

22.8 

Porphyry Mtn 

JS-98-13 20.15 ± 0.05 

20.20 ± 0.06 

JS-98-25 19.39 ± 0.05 

19.46 ± 0.05 

JS-98-5 19.44 ± 0.05 

19.31 ± 0.07 

JS-98-93 19.40 ± 0.05 

19.40 ± 0.06 

JS-98-2 19.58 ± 0.05 

19.57 ± 0.07 

K97-8-13D 19.40 ± 0.05 

19.38 ± 0.05 

JS-98-17 19.36 ± 0.05 

19.37 ± 0.06 

Flat Tops 

K99-8-24A 13.45 ± 0.16 

13.36 

K99-8-24B1 14.06 ± 0.05 

14.12 ± 0.17 

13.94 

K99-8-24B3 14.27 ± 0.07 

13.97 

56.4 3 / 7 

75.5 4 / 7 0.368 

1 0 0 7 / 7 

52.4 2 / 6 

93.7 5 / 6 1.414 

1 0 0 6 / 6 

82.5 5 / 6 0.449 

1 0 0 6 / 6 

79.4 4 / 7 

98.2 5 / 7 0.092 

1 0 0 7 / 7 

81.8 7 / 9 0.307 

1 0 0 9 / 9 

1 0 0 7 / 7 1.762 

1 0 0 7 / 7 

69.8 3 / 7 

1 0 0 7 / 7 1.137 

1 0 0 7 / 7 

LF 6 / 8 

LF 6 / 8 1.64 

LF 9/10 

LF 9/10 3.68 

LF 9/10 

LF 9/10 2.14 

LF 9 / 9 

LF 9 / 9 1.84 

LF 4 / 7 

LF 4 / 7 1.46 

LF 7 / 9 

LF 7 / 9 1.66 

LF 7 / 9 

LF 7 / 9 2.72 

85.4 5 / 8 0.856 

1 0 0 8 / 8 

5 9 3 / 9 

85.1 6 / 9 0.227 

1 0 0 9 / 9 

1 0 0 10/10 15.967 

1 0 0 10/10 

295.5 Plateau 

808 ± 260 Isochron 

295.5 Total Gas 

295.5 Plateau 

450.0 ± 40.7 Isochron 

295.5 Total Gas 

439.6 ± 45.2 Isochron 

295.5 Total Gas 

295.5 Plateau 

287.9 ± 23.2 Isochron 

295.5 Total Gas 

339.0 ± 11.9 Isochron 

295.5 Total Gas 

384.6 ± 24.6 Isochron 

295.5 Total gas 

295.5 Plateau 

414.7 ± 41.8 Isochron 

295.5 Total gas 

295.5 Weighted Mean 

243.9 ± 34.9 Isochron 

295.5 Weighted Mean 

291.5 ± 5.6 Isochron 

295.5 Weighted Mean 

387.0 ± 0.07 Isochron 

295.5 Weighted Mean 

313.0 ± 23.9 Isochron 

295.5 Weighted Mean 

318.1 ± 33.6 Isochron 

295.5 Weighted Mean 

321.1 ± 16.9 Isochron 

295.5 Weighted Mean 

286.0 ± 13.8 Isochron 

329.0 ± 22.5 Isochron 

295.5 Total Gas 

295.5 Plateau 

311.0 ± 10.2 Isochron 

295.5 Total Gas 

298.6 ± 1.1 Isochron 

295.5 Total Gas 

12
Table 2. Continued. 

Sample Apparent Age %39Ar # Steps MSWD 40Ar/36Ar Comment 

and Error (Ma) Total Steps

K99-8-24C	 14.36 ± 0.05 

13.72 ± 0.61 

14.39 

K99-8-24D1 14.91 ± 0.18 

14.3 

K99-8-24D2 15.11 ± 0.18 

14.71 

K99-8-24E 15.57 ± 0.09 

15.53 

Poison Creek Debris Flow 

JS-99-8A 15.05 ± 0.12 

14.83 

JS-99-8B 14.74 ± 0.08 

14.74 ± 0.44 

14.71 

JS-99-8C 15.09 ± 0.21 

14.98 

JS-99-9 14.48 ± 0.08 

14.51 ± 0.14 

14.53 

JS-99-10 14.87 ± 0.15 

14.85 

Sopris Bowl 

L-223 13.38 ± 0.06 

14.03 

BR6(3) 13.57 ± 0.05 

13.62 ± 0.08 

13.42 

Ba36(1) 13.81 ± 0.25 

13.5 

Basalt Mtn., top 

K97-8-15C2 10.42 ± 0.09 

9.86 

K97-8-15C4 9.97 ± 0.06 

10.33 ± 0.09 

9.99 

K97-8-15B 9.83 ± 0.05 

9.84 ± 0.15 

10.0 

L-37 9.65 ± 0.06 

9.84 

5 2 3/10 

82.5 5/10 0.653 

1 0 0 10/10 

65.5 4 / 9 1.07 

1 0 0 9 / 9 

67.2 5 / 9 1.213 

1 0 0 9 / 9 

90.5 6/10 1.924 

1 0 0 10/10 

1 0 0 7 / 7 11.409 

1 0 0 7 / 7 

50.1 2 / 8 

95.1 6 / 8 0.661 

1 0 0 8 / 8 

73.5 4 / 7 1.651 

1 0 0 7 / 7 

61.2 2 / 7 

86.9 4 / 7 2.798 

1 0 0 7 / 7 

67.5 3 / 6 1.498 

1 0 0 6 / 6 

63.5 6/12 1.744 

1 0 0 12/12 

61.5 5 / 9 

75.5 6 / 9 0.359 

1 0 0 9 / 9 

96.7 5 / 8 1.49 

1 0 0 8 / 8 

1 0 0 7 / 7 18.027 

1 0 0 7 / 7 

60.1 3 / 7 

1 0 0 7 / 7 7.033 

1 0 0 7 / 7 

75.9 3 / 6 

75.9 3 / 6 0.001 

1 0 0 6 / 6 

1 0 0 9 / 9 129.5 

1 0 0 9 / 9 

295.5 Plateau 

668 ± 261 Isochron 

295.5 Total Gas 

289.5 ± 7.9 Isochron 

295.5 Total Gas 

289.8 ± 13.9 Isochron 

295.5 Total Gas 

312.5 ± 6.6 Isochron 

295.5 Total Gas 

298.7 ± 1.06 Isochron 

295.5 Total Gas 

295.5 Plateau 

304.0 ± 31.6 Isochron 

295.5 Total Gas 

314.3 ± 9.4 Isochron 

295.5 Total Gas 

295.5 Plateau 

377.5 ± 27.8 Isochron 

295.5 Total Gas 

280.39 ± 12.6 Isochron 

295..5 Total Gas 

345.5 ± 6.8 Isochron 

295.5 Total gas 

295.5 Plateau 

285.7 ± 27.6 Isochron 

295.5 Total gas 

361.0 ± 85.5 Isochron 

295.5 Total gas 

280.2 ± 1.5 Isochron 

295.5 Total gas 

295.5 Plateau 

206.2 ± 6.7 Isochron 

295.5 Total gas 

295.5 Plateau 

292.4 ± 19.3 Isochron 

295.5 Total gas 

280.5 ± 1.0 Isochron 

295.5 Total gas 

13
Table 2. Continued. 

Sample Apparent Age %39Ar # Steps MSWD 40Ar/36Ar Comment 

and Error (Ma) Total Steps

Basalt Mtn., west flank 

L-176 9.68 ± 0.03 

9.60 ± 0.12 

9.58 

Basalt Mtn., north rim 

L-91A1 9.71 ± 0.03 

9.68 ± 0.06 

9.31 

L-39 9.75 ± 0.03 

9.73 ± 0.10 

9.42 

L-181 9.51 ± 0.02 

9.55 

L-187D1 9.75 ± 0.11 

9.47 

L-187B3 9.45 ± 0.09 

9.38 

L-187A2 9.84 ± 0.07 

9.58 

Basalt Mtn., south 

L-38A 9.93 ± 0.06 

9.66 

K97-8-11E1 10.07 ± 0.17 

10.05 

K97-8-11E2 10.20 ± 0.07 

10.03 

K97-8-11D3 11.87 ± 0.22 

11.39 

K97-8-11C1 10.18 ± 0.06 

10.20 ± 0.16 

10.14 

K97-8-11C3 10.16 ± 0.14 

10.92 

K97-8-11B 10.26 ± 0.14 

9.87 

K97-8-11A1 10.47 ± 0.21 

10.14 

K97-8-11A2 10.49 ± 0.07 

10.60 ± 0.24 

10.13 

Grand Mesa, Mesa Lakes 

K98-8-18F2 9.38 ± 0.16 

9.1 

K98-8-18F3 9.85 ± 0.03 

9.60 ± 0.38 

9.6 

56.9 4 / 8 

80.3 5 / 8 0.087 

1 0 0 8 / 8 

56.8 2 / 8 

56.8 4 / 8 0.45 

1 0 0 8 / 8 

60.9 4 / 8 

60.9 4 / 8 0.003 

1 0 0 8 / 8 

1 0 0 9 / 9 85.5 

1 0 0 9 / 9 

64.1 4 / 8 1.085 

1 0 0 8 / 8 

45.4 4 / 8 1.94 

1 0 0 8 / 8 

60.7 4 / 8 0.364 

1 0 0 8 / 8 

1 0 0 9 / 9 12.201 

1 0 0 9 / 9 

62.2 5 / 7 2.208 

1 0 0 7 / 7 

1 0 0 7 / 7 25.005 

1 0 0 7 / 7 

1 0 0 7 / 7 9.903 

1 0 0 7 / 7 

5 2 4 / 7 

68.8 5 / 7 0.053 

1 0 0 7 / 7 

1 0 0 6 / 6 0.796 

1 0 0 6 / 6 

1 0 0 7 / 7 3.441 

1 0 0 7 / 7 

51.8 4 / 7 2.255 

1 0 0 7 / 7 

57.4 5 / 8 

57.4 5 / 8 0.619 

1 0 0 8 / 8 

60.2 4 / 7 0.787 

1 0 0 7 / 7 

65.2 2 / 8 

81.2 3 / 8 1.062 

1 0 0 8 / 8 

295.5 Plateau 

332.7 ± 24.1 Isochron 

295.5 Total gas 

295.5 Plateau 

302.9 ± 14.7 Isochron 

295.5 Total gas 

295.5 Plateau 

300.4 ± 15.3 Isochron 

295.5 Total gas 

298.0 ± 0.5 Isochron 

295.5 Total gas 

296.8 ± 7.5 Isochron 

295.5 Total gas 

370.4 ± 17.5 Isochron 

295.5 Total gas 

311.3 ± 8.8 Isochron 

295.5 Total gas 

271.8 ± 0.88 Isochron 

295.5 Total gas 

313.2 ± 13.8 Isochron 

295.5 Total Gas 

293.8 ± 1.1 Isochron 

295.5 Total Gas 

285.1 ± 3.4 Isochron 

295.5 Total Gas 

295.5 Plateau 

312.6 ± 9.2 Isochron 

295.5 Total Gas 

314.2 ± 2.0 Isochron 

295.5 Total Gas 

290.4 ± 1.9 Isochron 

295.5 Total Gas 

296.5 ± 4.0 Isochron 

295.5 Total Gas 

295.5 Plateau 

295.2 ± 2.7 Isochron 

295.5 Total Gas 

297.6 ± 11.0 Isochron 

295.5 Total gas 

295.5 Plateau 

296.0 ± 23.5 Isochron 

295.5 Total gas 

14
Table 2. Continued. 

Sample Apparent Age %39Ar # Steps MSWD 40Ar/36Ar Comment 

and Error (Ma) Total Steps

K98-8-18E4	 9.81 ± 0.19 

9.56 

K98-8-18E5	 9.71 ± 0.03 

9.81 ± 0.21 

9.63 

K98-8-18D1 9.95 ± 0.28 

9.25 

K98-8-18C1 10.01 ± 0.15 

9.33 

K98-8-18C2 9.91 ± 0.19 

9.34 

K98-8-18B1	 9.96 ± 0.04 

10.35 ± 0.50 

9.94 

K98-8-18B2	 9.95 ± 0.04 

9.80 ± 0.67 

9.96 

K98-8-18A3 9.41 ± 0.30 

9.27 

K98-8-18A4 9.10 ± 0.30 

9.16 

K98-8-17B6	 9.97 ± 0.04 

10.09 ± 0.32 

10.02 

K98-8-17A4	 10.32 ± 0.13 

9.36 

Grand Mesa, Lands End 

K98-8-18I1 10.42 ± 0.15 

10.34 

K98-8-18I4 10.35 ± 0.04 

10.33 ± 0.13 

10.27 

K98-8-18H4 10.76 ± 0.24 

9.92 

K98-8-18G2 10.72 ± 0.13 

9.74 

K98-8-18G5 9.87 ± 0.08 

9.15 

Missouri Heights 

K97-8-15A3 10.63 ± 0.07 

10.19 ± 0.33 

10.49 

63.4 5 / 9 0.882 

1 0 0 9 / 9 

51.2 2 / 9 

76.8 5 / 9 2.019 

1 0 0 9 / 9 

4 1 3 / 7 0.054 

1 0 0 7 / 7 

55.7 3 / 9 0.501 

1 0 0 9 / 9 

36.2 3 / 8 0.049 

1 0 0 8 / 8 

89.7 5 / 6 

1 0 0 6 / 6 0.566 

1 0 0 6 / 6 

77.8 2 / 6 

89.7 4 / 6 0.110 

1 0 0 6 / 6 

1 0 0 8 / 8 3.636 

1 0 0 8 / 8 

47.8 4 / 6 0.450 

1 0 0 6 / 6 

75.2 2 / 6 

94.4 5 / 6 0.157 

1 0 0 6 / 6 

1 0 0 6 / 6 1.361 

1 0 0 6 / 6 

7 7 5 / 8 0.075 

1 0 0 8 / 8 

73.8 3 / 9 

92.2 7 / 9 0.311 

1 0 0 9 / 9 

36.9 3 / 7 0.673 

1 0 0 7 / 7 

1 0 0 8 / 8 3.742 

1 0 0 8 / 8 

1 0 0 7 / 7 3.111 

1 0 0 7 / 7 

9 0 4 / 5 

80.6 3 / 5 0.001 

1 0 0 5 / 5 

308.0 ± 9.1 Isochron 

295.5 Total gas 

295.5 Plateau 

303.0 ± 10.3 Isochron 

295.5 Total gas 

289.9 ± 6.5 Isochron 

295.5 Total gas 

291.4 ± 7.0 Isochron 

295.5 Total gas 

292.1 ± 13.9 Isochron 

295.5 Total gas 

295.5 Plateau 

290.1 ± 28.7 Isochron 

295.5 Total gas 

295.5 Plateau 

410.1 ± 298.3 Isochron 

295.5 Total gas 

292.3 ± 3.7 Isochron 

295.5 Total gas 

301.4 ± 5.8 Isochron 

295.5 Total gas 

295.5 Plateau 

343.8 ± 116.8 Isochron 

295.5 Total gas 

270.1 ± 3.1 Isochron 

295.5 Total gas 

288.8 ± 13.0 Isochron 

295.5 Total Gas 

295.5 Plateau 

300.7 ± 6.4 Isochron 

295.5 Total Gas 

285.7 ± 5.9 Isochron 

295.5 Total Gas 

281.8 ± 1.0 Isochron 

295.5 Total Gas 

270.7 ± 2.1 Isochron 

295.5 Total Gas 

295.5 Plateau 

300.3 ± 3.4 Isochron 

295.5 Total gas 

15
Table 2. Continued. 

Sample Apparent Age %39Ar # Steps MSWD 40Ar/36Ar Comment 

and Error (Ma) Total Steps

K97-8-15A4	 10.56 ± 0.07 

10.72 ± 0.07 

10.47 ± 0.20 

10.46 

K97-8-12F1	 10.84 ± 0.06 

10.74 ± 0.20 

10.53 

K97-8-12F2 11.07 ± 0.17 

11.06 

K97-8-12E1 10.70 ± 0.15 

10.21 

K97-8-12E2 8.96 ± 0.34 

10.11 

K97-10-8A2	 9.88 ± 0.06 

9.53 ± 0.53 

10.16 

K97-10-8A4	 9.89 ± 0.07 

9.70 ± 2.04 

10.07 

Heuschkel Park 

K97-10-8E1	 9.75 ± 0.06 

9.67 ± 0.67 

10.39 

K97-10-8E3	 10.47 ± 2.34 

10.43 

K97-10-8F3	 10.72 ± 0.12 

10.06 

K97-10-8F4	 10.28 ± 0.12 

9.76 

Red Canyon Graben 

K97-10-8B1 11.11 ± 0.23 

10.63 

K97-10-8B3 9.78 ± 0.23 

10.01 

Upper Cattle Creek 

L60 7.75 ± 0.03 

7.71 ± 0.04 

7.7 

Spring Valley II 

K97-10-8D1 7.74 ± 0.05 

7.71 ± 0.51 

7.84 

K97-10-8D2 7.79 ± 0.05 

7.59 ± 0.16 

8.01 

65.8 3 / 6 

50.6 3 / 6 

1 0 0 6 / 6 4.001 

1 0 0 6 / 6 

67.4 4 / 7 

67.4 4 / 7 0.806 

1 0 0 7 / 7 

1 0 0 7 / 7 7.325 

1 0 0 7 / 7 

70.1 4 / 7 1.243 

1 0 0 7 / 7 

1 0 0 7 / 7 7 .3 

1 0 0 7 / 7 

61.2 3 / 7 

92.6 6 / 7 2.01 

1 0 0 7 / 7 

6 1 3 / 7 

6 1 3 / 7 0.032 

1 0 0 7 / 7 

64.2 4 / 7 

73.8 5 / 7 0.144 

1 0 0 7 / 7 

58.7 5 / 8 2.212 

1 0 0 8 / 8 

1 0 0 7 / 7 4.732 

1 0 0 7 / 7 

1 0 0 8 / 8 4.091 

1 0 0 8 / 8 

1 0 0 6 / 6 4.737 

1 0 0 6 / 6 

62.6 5 / 8 1.091 

1 0 0 8 / 8 

64.6 5 / 8 

65.2 6 / 8 0.489 

1 0 0 8 / 8 

66.5 2 / 4 

1 0 0 4 / 4 0.108 

1 0 0 4 / 4 

71.6 4 / 7 

93.4 6 / 7 0.649 

1 0 0 7 / 7 

295.5 Plateau 

295.5 Plateau 

295.8 ± 2.3 Isochron 

295.5 Total gas 

295.5 Plateau 

297.0 ± 3.7 Isochron 

295.5 Total gas 

294.9 ± 1.6 Isochron 

295.5 Total gas 

319.8 ± 4.7 Isochron 

295.5 Total gas 

351.4 ± 14.7 Isochron 

295.5 Total gas 

295.5 Plateau 

399.5 ± 51.4 Isochron 

295.5 Total gas 

295.5 Plateau 

309.6 ± 163.0 Isochron 

295.5 Total gas 

295.5 Plateau 

317.0 ± 35.9 Isochron 

295.5 Total gas 

297.9 ± 67.0 Isochron 

295.5 Total gas 

272.6 ± 2.5 Isochron 

295.5 Total gas 

276.4 ± 2.8 Isochron 

295.5 Total gas 

286.1 ± 3.5 Isochron 

295.5 Total gas 

301.4 ± 5.2 Isochron 

295.5 Total gas 

295.5 Plateau 

301.5 ± 1.5 Isochron 

295.5 Total gas 

295.5 Plateau 

301.8 ± 40.9 Isochron 

295.5 Total gas 

295.5 Plateau 

338.8 ± 8.0 Isochron 

295.5 Total gas 

16
Table 2. Continued. 

Sample Apparent Age %39Ar # Steps MSWD 40Ar/36Ar Comment 

and Error (Ma) Total Steps

McNulty Pit 

K97-8-12G 3.01 ± 0.01 

2.99 ± 0.11 

2.92 

K97-8-12H 3.09 ± 0.02 

3.11 ± 0.14 

3.22 

Griffith Pit 

L244-1 2.87 ± 0.01 

2.87 ± 0.01 

2.86 ± 0.03 

2.91 

L244-2 2.84 ± 0.02 

2.90 

L-244 2.90 ± 0.02 

2.97 ± 0.02 

L-245 2.96 ± 0.02 

2.94 ± 0.03 

Triangle Peak 

K97-8-12B1 1.51 ± 0.01 

1.44 ± 0.13 

1.54 

K97-8-12B2 1.4 ± 0.21 

1.54 

K99-8-22B2 1.34 ± 0.01 

1.33 ± 0.01 

1.36 

K99-8-22B3 1.34 ± 0.01 

1.33 ± 0.01 

1.38 

K97-8-12A2 1.88 ± 0.04 

1.66 

K97-8-22A1 1.40 ± 0.01 

1.36 ± 0.04 

1.42 

K99-8-22A4 1.41 ± 0.01 

1.35 ± 0.04 

1.44 

Willow Peak 

K97-10-7F 0.310 ± 0.002 

0.26 ± 0.04 

0.389 

K97-10-7G 0.330 ± 0.003 

0.30 ± 0.08 

0.359 

59.8 7 / 9 

59.8 7 / 9 0.212 

1 0 0 9 / 9 

60.9 3 / 6 

60.9 3 / 6 0.171 

1 0 0 6 / 6 

56.8 2/10 

72.2 2/10 

1 0 0 10/10 0.201 

1 0 0 10/10 

1 0 0 9 / 9 4.006 

1 0 0 9 / 9 

LF 10/10 

1 0 0 10/10 0.87 

LF 10/10 

1 0 0 10/10 0.36 

78.7 4 / 7 

78.7 4 / 7 0.074 

1 0 0 7 / 7 

77.2 4 / 7 0.0102 

1 0 0 7 / 7 

75.7 4 / 6 

1 0 0 6 / 6 0.725 

1 0 0 6 / 6 

79.6 4 / 7 

1 0 0 7 / 7 0.956 

1 0 0 7 / 7 

39.4 3 / 7 0.231 

1 0 0 7 / 7 

72.8 4 / 6 

72.8 4 / 6 0.504 

1 0 0 6 / 6 

78.7 5 / 7 

1 0 0 7 / 7 0.748 

1 0 0 7 / 7 

63.7 4 / 6 

93.5 5 / 6 0.169 

1 0 0 6 / 6 

61.5 2 / 5 

95.9 4 / 5 1.96 

1 0 0 5 / 5 

295.5 Plateau 

296.8 ± 9.4 Isochron 

295.5 Total gas 

295.5 Plateau 

299.9 ± 21.6 Isochron 

295.5 Total gas 

295.5 Plateau 

295.5 Plateau 

306.0 ± 3.4 Isochron 

295.5 Total gas 

322.8 ± 2.2 Isochron 

295.5 Total gas 

295.5 Weighted Mean 

278.4 ± 5.8 Isochron 

295.5 Weighted Mean 

303.1 ± 10.4 Isochron 

295.5 Plateau 

447 ± 267 Isochron 

295.5 Total Gas 

443 ± 235 Isochron 

295.5 Total Gas 

295.5 Plateau 

304.5 ± 2.9 Isochron 

295.5 Total Gas 

295.5 Plateau 

307.8 ± 2.9 Isochron 

295.5 Total Gas 

290.2 ± 73.7 Isochron 

295.5 Total Gas 

295.5 Plateau 

306.7 ± 11.6 Isochron 

295.5 Total Gas 

295.5 Plateau 

315.2 ± 4.2 Isochron 

295.5 Total Gas 

295.5 Plateau 

330.7 ± 22.1 Isochron 

295.5 Total Gas 

295.5 Plateau 

302.5 ±22.5 Isochron 

295.5 Total Gas 

17
Table 3. Argon isotopic results. Age spectra or Ideograms, and isochrons are on facing page. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

Flank of Mt Soprus 

K97-8-11F; single-crystal sanidine; #19KD6, J=0.00201±0.50% 

09 LF *** 99.0 0.01516950 9.893 39.1 3.401 35.16 0.04 

03 LF *** 98.9 0.01926797 9.901 76.8 5.286 35.17 0.03 

04 LF *** 98.7 0.01891563 9.925 68.8 3.847 35.17 0.04 

10 LF *** 98.9 0.00857536 9.911 69.7 5.062 35.20 0.05 

11 LF *** 99.5 0.00779516 9.855 61.3 3.262 35.20 0.04 

05 LF *** 99.7 0.00937981 9.844 116.4 5.204 35.25 0.05 

06 LF *** 99.7 0.01451519 9.844 120.1 3.903 35.25 0.04 

12 LF *** 99.4 0.01009326 9.880 74.5 3.278 35.25 0.04 

08 LF *** 99.7 0.02092086 9.844 80.9 3.902 35.26 0.04 

07 LF * * * 96.8 0.02046475 10.080 54.1 3.978 35.06 0.04 

Weighted Mean Age n=9 35.21 0.03 

Dotsero


K97-10-7K5; 0.2539 g, whole rock basalt, #67KD6, J=0.002016±0.50% 


A 6 5 0 8 .9 41.1 0.146970 6.290 0.41 5 7 22.73 0.19 

B 7 5 0 14.5 92.1 0.239089 6.818 0.26 3 4 9 24.63 0.17 

C 8 0 0 12.2 96.6 0.200196 6.600 0.25 6 5 6 23.85 0.12 

D 8 5 0 14.3 98.3 0.235235 6.491 0.28 8 7 8 23.46 0.07 

E 9 5 0 21.8 93.8 0.358473 6.232 0.30 8 4 3 22.53 0.52 

F 1050 15.8 86.8 0.260491 6.435 0.37 2 0 0 23.25 0.14 

G 1450 12.5 60.6 0.205966 7.437 0.04 5 2 26.85 0.10 

Total Gas 100.0 84.6 1.646420 6.587 0.27 4 8 3 23.80 

Plateau Age 51.9 steps D-F 23.41 0.13 

Castle Peak


2ABS13-9-99; 0.2499 g, whole rock basalt, #49KD16, J=0.005012±0.25% 


A 6 5 0 6 .8 22.0 0.530734 2.186 2.74 3 5 19.66 0.21 

B 7 5 0 2 .7 29.6 0.209957 2.263 1.73 4 1 20.35 0.26 

C 8 5 0 11.1 45.8 0.857713 2.497 0.77 7 1 22.44 0.12 

D 9 5 0 20.6 80.2 1.598192 2.632 0.47 3 7 9 23.64 0.06 

E 1050 26.1 91.9 2.026546 2.544 0.47 1 4 8 8 22.86 0.04 

F 1150 17.8 88.7 1.384220 2.524 0.41 6 3 1 22.68 0.09 

G 1450 14.8 65.8 1.151715 2.451 0.05 1 1 5 22.02 0.06 

Total Gas 100.0 73.5 7.759077 2.507 0.62 6 0 8 22.53 

No Plateau 

18
K97-8-11F 

0.0040 92 

96 

100 

Analyses 

% Radiogenic 40Ar 

36 40 Ar/ Ar

36 

0 .07 .14 .21 .28 .35 

2ABS13-9-99 

0 .001 .002 .003 

Ar/ Ar

Ar/ Ar 39 40 

ISOCHRON AGE = 22.25 ± 0.26 Ma 

(40Ar/36Ar)i = 364.44 ± 19.65 

MSWD = 1.999 

POINTS ABCG DELETED 

* 

A 

B 

C 

D 

E 

F 

G 

0 

10 

20 

Apparent Age (Ma) 

0 100 39ArK 

.01 

.1 

1 

K/Ca 

2ABS13-9-99 

NO PLATEAU 

10 

ISOCHRON AGE = 35.27 ± 0.17 Ma 

(40Ar/36Ar)i = 222.5 ± 22.4 

MSWD = 0.40 

POINT 7 DELETED 

Ar/ Ar 39 40 

0.000 0.020 0.040 0.060 0.080 0.100 

0.0000 

0.0010 

0.0020 

0.0030 

Age (Ma)

K97-8-11F 

Weighted Mean Age = 

35.21 ± 0.03 Ma 

n = 9 

Relative Probability 

26 28 30 32 34 36 38 

0 

Apparent Age (Ma) 

0 100 39ArK 

K/Ca 

K97-10-7K5 

PLATEAU AGE = 

23.41 ± 0.13 Ma 

10 

20 

.01 

.1 

1 

ISOCHRON AGE = 23.12 ± 0.38 Ma 

(40Ar/36Ar)i = 486.44 ± 70.93 

MSWD = 2.543 

POINTS AG DELETED 

0 .001 .002 .003 

 Ar/ Ar 

Ar/ Ar 39 40 

* K97-10-7K5 

0 .03 .06 .09 .12 .15 

A 

B 

CD 

E

F 

G 

19 

RELEASED (%) 

RELEASED (%) 

40 40 36 

 40 
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

2BBS13-9-99; 0.2511 g, whole rock basalt, #51KD16, J=0.005002±0.25% 

A 6 5 0 1.3 6.3 0.088070 2.483 0.91 2 5 22.27 0.74 

B 7 5 0 1.1 19.7 0.074634 3.045 0.64 5 3 27.27 0.62 

C 8 5 0 8.6 57.6 0.565264 2.844 0.62 4 2 2 25.48 0.14 

D 9 5 0 24.3 79.2 1.601669 2.645 0.53 2 3 8 4 23.71 0.07 

E 1050 30.9 94.9 2.043657 2.552 0.56 5 1 8 3 22.88 0.03 

F 1150 18.6 96.2 1.230109 2.517 0.59 1 3 2 7 22.57 0.06 

G 1450 14.4 79.5 0.951996 2.429 0.05 1 1 4 21.79 0.14 

H 1650 0.7 62.3 0.049241 2.455 0.03 2 0 4 22.02 1.07 

Total Gas 100.0 83.6 6.604640 2.579 0.49 2 4 8 4 23.12 

No Plateau 

2CBS13-9-99; 0.2525 g, whole rock basalt, #57KD16, J=0.005021±0.25% 

A 6 5 0 1.8 11.3 0.147996 2.599 0.87 3 7 23.39 0.61 

B 7 5 0 0.5 31.8 0.044052 2.539 0.61 8 1 22.86 0.97 

C 8 5 0 6.8 65.8 0.543780 2.738 0.49 3 1 2 24.63 0.16 

D 9 5 0 18.4 86.5 1.478697 2.611 0.41 1 5 6 1 23.50 0.04 

E 1050 23.8 95.4 1.906374 2.547 0.54 2 1 0 4 22.92 0.07 

F 1150 21.5 96.8 1.727270 2.532 0.70 6 7 7 22.79 0.03 

G 1450 26.6 91.0 2.132201 2.507 0.08 1 9 6 22.57 0.04 

H 1650 0.6 72.4 0.045137 2.482 0.06 2 1 5 22.34 1.58 

Total Gas 100.0 88.8 8.025507 2.559 0.43 1 0 0 9 23.03 

No Plateau 

1ABS13-9-99; 0.2541 g, whole rock basalt, #43KD16, J=0.004968±0.25% 

A 6 5 0 1 .6 7 .2 0.119205 2.717 0.58 3 1 24.19 1.92 

B 7 5 0 1.5 29.0 0.113278 2.889 0.46 1 4 3 25.71 0.61 

C 8 5 0 9.8 69.7 0.730492 2.852 0.40 1 0 1 7 25.38 0.16 

D 9 5 0 19.5 80.6 1.455824 2.683 0.32 2 3 8 8 23.89 0.09 

E 1050 25.6 97.3 1.914283 2.595 0.43 3 2 6 0 23.11 0.03 

F 1150 21.1 97.3 1.575300 2.562 0.63 9 1 5 22.81 0.04 

G 1450 19.6 86.2 1.467021 2.531 0.07 1 3 3 22.55 0.12 

H 1650 1.3 66.7 0.100313 2.379 0.04 2 4 3 21.20 0.42 

Total Gas 100.0 86.3 7.475716 2.621 0.38 1 6 2 4 23.34 

No Plateau 

20
* A 2BBS13-9-99

1

B 

C 

D 

E F 

G 

H 

ISOCHRON AGE = 23.49 ± 0.16 Ma 

(40Ar/36Ar)

i = 299.88 

± 1.70 

MSWD = 9.967 

POINTS GH DELETED 

0 .07 .14 .21 .28 .35

39 40 Ar/ Ar

21

.01 

20

10

0

0 39ArK RELEASED (%) 100

1ABS13-9-99 

NO PLATEAU 

0 .07 .14 .21 .28 .35 

2CBS13-9-99 

0 .001 .002 .003 

Ar/ Ar 39 40 

ISOCHRON AGE = 22.62 ± 0.27 Ma 

(40Ar/36Ar)

i = 342.12 

± 16.79 

MSWD = 0.540 

POINTS AB DELETED 

* 

A 

B 

C 

D 

E F 

G 

H 

0 

10 

20 

Apparent Age (Ma) 

0 100 39Ar K 

.01 

.1 

1 

K/Ca 

2CBS13-9-99 

NO PLATEAU 

.1 

1 

K/Ca 

1ABS13-9-99 A 

.003 

* 

0 .07 .14 .21 .28 .35 

0 .001 .002 .003 

Ar/ Ar 39 40 

ISOCHRON AGE = 22.59 ± 0.14 Ma 

(40Ar/36Ar)

i = 342.58 

± 5.42 

MSWD = 0.722 

POINTS ABGH DELETED 

B 

C 

D 

E F 

G 

H 

0 

10 

20 

Apparent Age (Ma) 

0 100 39Ar K 

.01 

.1 

K/Ca 

2BBS13-9-99 

NO PLATEAU 

RELEASED (%)

RELEASED (%)

36 40 Ar/ Ar 36 40 Ar/ Ar

.001 .002 

36 40 Ar/ Ar

Apparent Age (Ma) 

0 
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

1CBS13-9-99; 0.2526 g, whole rock basalt, #45KD16, J=0.004955±0.25% 

A 6 5 0 1 .1 5 .4 0.079663 2.552 0.55 1 5 22.67 2.13 

B 7 5 0 2.6 31.7 0.191162 3.186 0.50 1 1 0 28.26 0.29 

C 8 5 0 11.0 68.2 0.798846 2.837 0.41 9 1 9 25.18 0.06 

D 9 5 0 21.7 81.0 1.580856 2.663 0.31 2 4 2 0 23.65 0.06 

E 1050 27.6 98.1 2.010648 2.584 0.43 4 0 5 1 22.96 0.02 

F 1150 21.9 97.2 1.597952 2.555 0.66 1 2 3 5 22.70 0.03 

G 1450 12.7 80.6 0.928116 2.520 0.05 1 1 8 22.38 0.20 

H 1650 1.5 68.5 0.106542 2.465 0.03 3 0 5 21.90 0.44 

Total Gas 100.0 85.5 7.293785 2.628 0.40 2 0 3 5 23.34 

No Plateau 

1DBS13-9-99; 0.2507 g, whole rock basalt, #47KD16, J=0.005014±0.25% 

A 7 5 0 1.6 20.1 0.119643 3.143 0.59 6 6 28.20 0.62 

B 8 5 0 7.1 58.4 0.515526 2.757 0.50 5 2 6 24.77 0.10 

C 9 5 0 20.9 76.6 1.526516 2.661 0.39 2 2 2 2 23.91 0.07 

D 1050 27.0 96.7 1.965768 2.568 0.48 4 7 9 0 23.08 0.05 

E 1150 26.1 97.6 1.902586 2.539 0.75 1 0 9 3 22.82 0.04 

F 1450 16.1 84.9 1.174649 2.506 0.06 1 3 6 22.53 0.11 

G 1650 1.2 65.5 0.083896 2.286 0.03 3 1 4 20.56 0.47 

Total Gas 100.0 86.5 7.288584 2.589 0.46 2 1 0 6 23.27 

No Plateau 

Devils Causeway•

3BBS13-9-99; 0.2528 g, whole rock basalt, #53KD16, J=0.005021±0.25% 


A 7 5 0 1 .3 8 .5 0.120318 2.443 0.35 2 1 21.99 1.56 

B 8 5 0 0.5 27.1 0.047567 2.465 0.51 8 4 22.19 1.44 

C 9 5 0 9.8 64.6 0.929466 2.634 0.88 5 4 7 23.70 0.17 

D 1050 28.2 81.5 2.662790 2.565 1.05 2 3 9 1 23.08 0.03 

E 1150 24.0 94.6 2.261122 2.551 0.69 12498 22.96 0.05 

F 1450 36.2 93.0 3.416112 2.500 0.11 3 7 7 22.51 0.04 

Total Gas 100.0 85.9 9.437375 2.543 0.59 3 8 6 0 22.89 

Plateau Age 52.2 steps D-E 23.05 0.08 

22
0 .07 .14 .21 .28 .35 

1DBS13-9-99 

0 .001 .002 .003 

Ar/ Ar 39 40 

ISOCHRON AGE = 22.91 ± 0.12 Ma 

(40Ar/36Ar)i = 328.80 ± 4.36 

MSWD = 1.233 

POINTS AFG DELETED 

* 

A 

B 

C 

D E 

F 

G 

0 

10 

20 

Apparent Age (Ma) 

.01 

.1 

1 

K/Ca 

0 .07 .14 .21 .28 .35 

1CBS13-9-99 

0 .001 .002 .003 

Ar/ Ar 39 40 

ISOCHRON AGE = 22.68 ± 0.15 Ma 

(40Ar/36Ar)i = 356.15 ± 7.78 

MSWD = 1.018 

POINTS ABGH DELETED 

* A 

B 

C 

D 

E 

F 

G 

H 

0 

10 

20 

Apparent Age (Ma) 

.01 

.1 

1 

K/Ca 

K/Ca 

3BBS13-9-99 

.003 

* 

A 

1CBS13-9-99 

NO PLATEAU 

0 39ArK RELEASED (%) 100 

1DBS13-9-99 

NO PLATEAU 

0 39ArK RELEASED (%) 100•

10•

1•

3BBS13-9-99 

PLATEAU AGE = 

23.05 ± 0.08 Ma 

36 40 Ar/ Ar 36 40 Ar/ Ar

ISOCHRON AGE = 22.61 ± 0.41 Ma 

(40Ar/36Ar)i = 320.87 ± 21.5 

MSWD = 0.232 

POINTS AB DELETED 

B 

C 

D 

E 

F 

0 .07 .14 .21 .28 .35•

39 40• Ar/ Ar

23

.01 

20•

10•

0•

0 39ArK RELEASED (%) 100•

.001 .002 

36 40 Ar/ Ar

Apparent Age (Ma) 

0 
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

3CBS13-9-99; 0.2557 g, whole rock basalt, #55KD16, J=0.005021±0.25% 

A 650 0.6 13.4 0.050410 2.840 0.99 5 1 25.55 1.05 

B 750 1.6 27.9 0.129114 2.453 0.66 9 5 22.08 0.46 

C 850 7.7 69.9 0.613966 2.723 0.46 354 24.49 0.14 

D 950 18.3 87.4 1.458746 2.614 0.38 1600 23.52 0.03 

E 1050 23.3 96.5 1.852745 2.551 0.53 1883 22.96 0.06 

F 1150 22.2 97.0 1.763517 2.523 0.67 617 22.71 0.03 

G 1450 24.8 91.0 1.973679 2.492 0.08 176 22.44 0.04 

H 1650 1.4 82.1 0.111875 2.442 0.06 316 21.98 0.44 

Total Gas 100.0 89.7 7.954052 2.554 0.42 946 22.98 

No Plateau 

Yarmony Mountain


K97-8-13A1-3; 0.2457 g, whole rock basalt, #45KD6, J=0.002044±0.5% 


A 6 5 0 6.0 45.4 0.310004 5.258 0.57 4 0 19.29 0.13 

B 7 5 0 14.7 91.4 0.754508 6.462 0.43 2 5 8 23.67 0.06 

C 8 0 0 14.2 96.1 0.726790 6.350 0.48 7 6 5 23.26 0.05 

D 8 5 0 17.7 97.9 0.907084 6.289 0.62 1 3 6 8 23.04 0.07 

E 9 5 0 28.8 96.0 1.479775 6.226 0.75 9 6 7 22.81 0.04 

F 1050 12.4 85.6 0.636649 6.128 0.67 2 2 9 22.46 0.10 

G 1450 6.2 36.6 0.319621 5.914 0.04 3 3 21.68 0.16 

Total Gas 100.0 87.6 5.134431 6.199 0.58 6 9 9 22.72 

No Plateau 

K97-8-13A1-4; 0.2553 g, whole rock basalt, #47KD6, J=0.002044±0.5% 

A 6 5 0 6.4 39.1 0.297760 6.019 0.60 2 9 22.06 0.38 

B 7 5 0 7.5 74.1 0.350008 6.504 0.38 1 0 9 23.82 0.19 

C 8 0 0 9.1 87.7 0.423050 6.377 0.42 3 3 7 23.36 0.12 

D 8 5 0 13.6 93.6 0.630119 6.327 0.51 7 0 8 23.18 0.05 

E 9 5 0 30.9 94.4 1.433290 6.101 0.65 7 6 9 22.36 0.15 

F 1050 17.5 92.0 0.812618 6.237 0.65 2 0 5 22.85 0.07 

G 1450 15.0 58.7 0.694011 6.110 0.07 5 0 22.39 0.25 

Total Gas 100.0 82.8 4.640856 6.207 0.50 4 1 8 22.74 

No Plateau 

24
1•

.1•

.01•

.003 

3CBS13-9-99 

ISOCHRON AGE = 22.56 ± 0.18 Ma 

(40Ar/36Ar)

i = 352.07 

± 12.47 

MSWD = 1.993 

POINTS ABH DELETED 

* 

A 

B 

C 

D 

E F 

G 

H

Apparent Age (Ma) K/Ca 

3CBS13-9-99 

NO PLATEAU 

0•

0 39ArK RELEASED (%)•

1•

.1•

.01•

20•

10•

K97-8-13A1-3 

NO PLATEAU 

.003 0 .001 .002 .003 0 .001 .002 

36 40 Ar/ Ar 36 40 Ar/ Ar

20•

10•

0 .07 .14 .21 .28 .35 

39 40 Ar/ Ar

ISOCHRON AGE = 22.71 ± 0.20 Ma 

(40Ar/36Ar)

i = 427.17 

± 34.44 

MSWD = 2.196 

POINTS AFG DELETED 

* K97-8-13A1-3 

A 

B 

C 

DE 

F 

G 

K/Ca K/Ca Apparent Age (Ma) 

0 .03 .06 .09 .12 .15 0•

39 40 0 39ArK RELEASED (%) 100•

Ar/ Ar 

ISOCHRON AGE = 22.59 ± 0.26 Ma 

(40Ar/36Ar)

i = 292.95 

± 11.89 

MSWD = 1.476 

POINTS ABCD DELETED 

* K97-8-13A1-4 

A 

B 

C 

D E 

F 

G 

1•

.1•

.01•

20•

10•

0 .03 .06 .09 .12 .15 0•

39 40 0 39ArK RELEASED (%) 100•

K97-8-13A1-4 

NO PLATEAU 

Ar/ Ar 

25 

.001 .002 

36 40 Ar/ Ar

Apparent Age (Ma) 

0 

100
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

K97-8-13A1; 0.2510 g, whole rock basalt, #41KD6, J=0.002044±0.5% 

A 650 8.2 66.8 0.436586 6.629 0.50 144 24.28 0.19 

B 750 20.6 97.4 1.092345 6.408 0.53 819 23.47 0.08 

C 800 19.2 99.7 1.017301 6.315 0.67 1631 23.14 0.09 

D 850 19.4 99.4 1.030644 6.232 0.80 1794 22.84 0.09 

E 950 24.2 95.5 1.284809 6.075 0.68 704 22.26 0.14 

F 1050 8.4 89.1 0.443336 6.069 0.51 168 22.24 0.21 

Total Gas 100.0 94.5 5.305021 6.265 0.64 1026 22.96 

No Plateau 

K97-8-13A2; 0.2485 g, whole rock basalt, #43KD6, J=0.002044±0.5% 

A 6 5 0 7.4 74.5 0.332383 6.534 0.46 1 5 8 23.93 0.28 

B 7 5 0 20.7 99.2 0.928723 6.717 0.52 5 6 1 24.60 0.14 

C 8 0 0 19.1 98.9 0.854331 6.299 0.68 1 3 1 7 23.08 0.11 

D 8 5 0 19.6 99.3 0.876641 6.253 0.81 1 2 6 1 22.91 0.10 

E 9 5 0 24.1 97.4 1.078620 6.238 0.65 4 6 5 22.86 0.08 

F 1050 9.2 87.7 0.411229 6.071 0.47 1 0 4 22.25 0.18 

Total Gas 100.0 95.8 4.481927 6.358 0.63 7 4 7 23.30 

Plateau Age 62.7 steps C-E 22.92 0.13 

K97-8-13B2; 0.2448 g, whole rock basalt, #49KD6, J=0.002044±0.5% 

A 6 5 0 8.0 46.1 0.381751 6.349 0.60 3 3 23.26 0.70 

B 7 5 0 16.3 85.9 0.776770 6.578 0.42 1 9 8 24.09 0.16 

C 8 0 0 15.9 94.2 0.757449 6.367 0.53 5 4 3 23.33 0.13 

D 8 5 0 17.2 97.3 0.816998 6.283 0.69 9 4 2 23.02 0.13 

E 9 5 0 22.9 96.8 1.090217 6.249 0.73 7 4 9 22.90 0.11 

F 1050 11.6 89.0 0.554222 6.189 0.63 1 8 5 22.68 0.18 

G 1450 8.0 57.1 0.382916 5.936 0.04 4 4 21.76 0.26 

Total Gas 100.0 86.5 4.760323 6.303 0.56 4 8 0 23.09 

Plateau Age 51.7 steps D-F 22.90 0.13 

26
K97-8-13A1 

ISOCHRON AGE = 22.95 ± 0.15 Ma 

(40Ar/36Ar)

i = 328.13 

± 5.93 

MSWD = 6.452 

NO POINTS DELETED 

* 

B 

CD 

E 

F 

A 

1 

K/Ca 

.1 

K97-8-13A1 

NO PLATEAU 

0 .001 .002 .003 

36 40 Ar/ Ar

Apparent Age (Ma) 20 

10 

0 .03 .06 .09 .12 .15 0 

39 40 0 39ArK RELEASED (%) 

Ar/ Ar 

K97-8-13A2 

ISOCHRON AGE = 23.21 ± 0.19 Ma 

(40Ar/36Ar)

i = 324.27 

± 13.04 

MSWD = 9.528 

NO POINTS DELETED 

A 

B C 

D 

E 

F 

Ar/ Ar 

ISOCHRON AGE = 22.65 ± 0.45 Ma 

(40Ar/36Ar)

i = 388.31 

± 58.48 

MSWD = 0.758 

POINTS AG DELETED 

* K97-8-13B2 

A 

B

C 

DE 

F 

G 

1 

.1 

.01 

20 

10 

0 .03 .06 .09 .12 .15 0 

39 40 Ar/ Ar

27 

1 

.1 

20 

10 

0 .03 .06 .09 .12 .15 0 

39 40 0 39ArK RELEASED (%) 

K97-8-13A2 

PLATEAU AGE = 

22.92 ± 0.13 Ma 

.001 .0006 .0008 

Apparent Age (Ma) K/Ca 

36 40 Ar/ Ar

0 .0002 .0004 

K97-8-13B2 

PLATEAU AGE = 

22.90 ± 0.13 Ma 

0 39ArK RELEASED (%) 100 

.002 .003 

Apparent Age (Ma) K/Ca 

36 40 Ar/ Ar

0 

100

100
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

K97-8-13B3; 0.2451 g, whole rock basalt, #51KD6, J=0.002044±0.5% 

A 6 5 0 5.2 81.5 0.256991 6.511 0.42 1 4 6 23.85 0.18 

B 7 5 0 15.4 96.4 0.753703 6.562 0.44 6 3 7 24.03 0.06 

C 8 0 0 15.4 98.5 0.756118 6.344 0.53 1 2 6 0 23.24 0.08 

D 8 5 0 18.1 99.3 0.888746 6.266 0.66 1 6 8 2 22.96 0.05 

E 9 5 0 26.6 98.3 1.306554 6.226 0.73 9 0 7 22.82 0.06 

F 1050 11.6 89.5 0.571139 6.212 0.63 1 6 8 22.76 0.13 

G 1450 7.6 50.2 0.373277 5.802 0.05 4 2 21.27 0.12 

Total Gas 100.0 92.7 4.906528 6.284 0.56 8 6 9 23.03 

Plateau Age 56.4 steps D-F 22.88 0.13 

K97-8-13C2; 0.2510 g, whole rock basalt, #53KD6, J=0.002044±0.5% 

A 7 5 0 6.3 64.7 0.292872 7.331 0.48 3 2 5 26.83 0.27 

B 8 0 0 8.7 86.7 0.409192 7.094 0.56 1 5 3 0 25.97 0.16 

C 8 5 0 15.5 90.0 0.727400 6.877 0.58 2 7 7 4 25.18 0.15 

D 9 5 0 32.2 93.4 1.505135 6.614 0.62 2 8 9 6 24.22 0.06 

E 1050 20.2 99.9 0.946211 6.578 0.72 1 1 6 2 24.09 0.11 

F 1450 17.1 93.7 0.798479 6.941 0.10 1 4 2 25.42 0.07 

Total Gas 100.0 91.8 4.679289 6.790 0.53 1 7 7 6 24.87 

Plateau Age 52.4 steps D-E 24.19 0.13 

K97-8-13C3; 0.2483 g, whole rock basalt, #55KD6, J=0.002044±0.5% 

A 750 9.9 89.4 0.462514 6.954 0.49 1196 25.46 0.24 

B 800 11.3 94.0 0.529474 6.851 0.57 3046 25.09 0.14 

C 850 13.5 93.3 0.632363 6.746 0.65 2959 24.71 0.08 

D 950 28.6 99.6 1.336369 6.639 0.73 3168 24.31 0.05 

E 1050 19.2 99.3 0.896782 6.531 0.93 694 23.92 0.06 

F 1450 17.5 90.4 0.815645 6.422 0.14 109 23.53 0.03 

Total Gas 100.0 95.4 4.673147 6.650 0.61 1922 24.36 

No Plateau 

28
1 

ISOCHRON AGE = 22.49 ± 0.50 Ma .1

(40Ar/36Ar)i = 808.65 ± 260.56 

MSWD = 0.368 

POINTS AFG DELETED 

* K97-8-13B3 

A 

B CE D

F 

G 

.01 

20 

10 

0 .03 .06 .09 .12 .15 0 

39 40 Ar/ Ar

ISOCHRON AGE = 23.88 ± 0.36 Ma 

(40Ar/36Ar)i = 449.96 ± 40.69 

MSWD = 1.414 

POINTS A DELETED 

* K97-8-13C2 

A 

B C D 

E F 

K97-8-13B3 

PLATEAU AGE = 

22.88 ± 0.13 Ma 

0 39ArK RELEASED (%) 

.00024 .00036 0 .001 .002 .003 0 .001 .002 .003 

Apparent Age (Ma) K/Ca 

36 40 Ar/ Ar 36 40 Ar/ Ar

1 

K97-8-13C2 

PLATEAU AGE = 

24.19 ± 0.13 Ma 

0 

ISOCHRON AGE = 24.06 ± 0.29 Ma 

(40Ar/36Ar)i = 439.57 ± 45.24 

MSWD = 0.449 

POINTS F DELETED 

K97-8-13C3 A 

B 

C 

D

E 

F 

Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca 

.1 

20 

10 

0 .03 .06 .09 .12 .15 0 39ArK 0 RELEASED (%) 100 39 40 Ar/ Ar

1 

.1 

20 

10 

K97-8-13C3 

NO PLATEAU 

36 40 Ar/ Ar

.00012 

0 39 40 Ar/ Ar

29 

39ArK RELEASED (%) 100 0 .03 .06 .09 .12 .15 0 

100
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

Gypsum


K99-10-24A3; 0.2543 g, whole rock basalt, #5KD16, J=0.004842±0.25% Ä

A 7 5 0 0.8 41.8 0.077165 2.950 0.46 6 9 25.58 0.79 

B 8 5 0 5.2 83.3 0.516331 2.592 0.38 7 3 8 22.50 0.11 

C 9 5 0 21.9 96.1 2.176509 2.584 0.48 4 1 0 9 22.43 0.04 

D 1050 30.5 98.3 3.026168 2.591 0.69 2 5 2 1 22.50 0.03 

E 1150 21.8 97.5 2.163751 2.599 0.83 4 8 0 22.56 0.03 

F 1450 18.8 82.9 1.870458 2.566 0.09 9 0 22.28 0.06 

G 1650 1.0 72.2 0.099360 2.414 0.07 1 2 0 20.96 0.48 

Total Gas 100.0 93.3 9.929742 2.588 0.54 1 8 3 0 22.46 

Plateau Age 79.4 steps B-E 22.52 0.08 

K99-10-24A5; 0.2507 g, whole rock basalt, #10KD16, J=0.004916±0.25% 

A 7 5 0 0.5 22.7 0.055067 4.918 0.49 3 1 43.10 5.38 

B 8 5 0 3.1 67.4 0.358234 2.756 0.55 3 3 1 24.27 0.09 

C 9 0 0 6.0 92.0 0.700330 2.662 0.58 2 7 1 1 23.46 0.07 

D 9 5 0 13.8 96.8 1.598455 2.608 0.58 158864 22.98 0.05 

E 1000 24.3 98.3 2.816462 2.563 0.65 12697 22.59 0.02 

F 1050 18.0 98.3 2.080229 2.528 0.84 1 8 6 6 22.28 0.02 

G 1150 16.1 97.5 1.858641 2.499 1.00 3 9 8 22.03 0.04 

H 1450 17.0 88.5 1.964586 2.335 0.10 9 4 20.59 0.06 

I 1650 1.3 60.5 0.145320 2.270 0.08 1 1 4 20.02 0.41 

Total Gas 100.0 94.1 11.577324 2.534 0.62 25614 22.33 

No Plateau 

Spring Valley


K97-10-8C2; 0.2433 g, whole rock basalt, #77KD7, J=0.002690±0.50% Ä

A 6 5 0 4.0 76.4 0.092216 5.287 0.26 1 6 0 25.48 0.47 

B 7 5 0 9.1 99.6 0.210957 4.947 0.27 1 1 9 8 23.85 0.21 

C 8 0 0 10.5 98.8 0.244813 4.762 0.32 15322 22.96 0.54 

D 8 5 0 15.1 100.0 0.351750 4.794 0.42 * * * 23.11 0.12 

E 9 5 0 32.1 98.2 0.746850 4.660 0.57 * * * 22.47 0.06 

F 1050 20.0 99.3 0.466519 4.754 0.64 2 1 4 4 22.92 0.11 

G 1450 9.2 87.2 0.213714 4.539 0.04 7 9 21.89 0.22 

Total Gas 100.0 97.0 2.326819 4.742 0.45 2 1 6 4 22.86 

No Plateau 

30
K/Ca 

1 

.1 

.01 

ISOCHRON AGE = 22.51 ± 0.22 Ma 

(40Ar/36Ar)i = 287.89 ± 23.24 

MSWD = 0.092 

POINTS AG DELETED 

* 

A 

B 

CE D

F 

G 

K99-10-24A3 

K99-10-24A3 

PLATEAU AGE = 

22.52 ± 0.08 Ma 

.002 .003 0 .001 .002 .003 

36 40 Ar/ Ar

Apparent Age (Ma) 20 

10 

0 .07 .14 .21 .28 .35 0 

39 40 0 39ArK RELEASED (%) 100 

Ar/ Ar

ISOCHRON AGE = 22.51 ± 0.35 Ma 

(40Ar/36Ar)i = 338.98 ± 11.9 

MSWD = 0.307 

POINTS HI DELETED 

* K99-10-24A5 

A 

B 

C 

D EFG 

H 

I 

1 

.1 

.01 

20 

10 

0 .07 .14 .21 .28 .35 .42 0 

K99-10-24A-5 

NO PLATEAU 

39 40 0 39ArK RELEASED (%) 100 

Ar/ Ar 

ISOCHRON AGE = 23.05 ± 0.33 Ma 

(40Ar/36Ar)i = 384.58 ± 24.63 

MSWD = 1.762 

NO POINTS DELETED 

K97-10-8C2 A 

BC

D 

E 

F

G 

1 

.1 

.01 

20 

10 

0 .04 .08 .12 .16 .2 0 

K97-10-8C2 

NO PLATEAU 

39 40 0 39ArK RELEASED (%) 100 

Ar/ Ar 

31 

Apparent Age (Ma) K/Ca 

36 40 Ar/ Ar

.001 0 .0006 .0008 

Apparent Age (Ma) K/Ca 

36 40 Ar/ Ar

.0002 .0004 0 
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

K97-10-8C4; 0.2423 g, whole rock basalt, #79KD7, J=0.002673±0.50% 

A 6 5 0 2.8 71.0 0.066874 5.722 0.28 1 5 1 27.38 1.09 

B 7 5 0 9.6 100.0 0.232322 5.052 0.29 4 9 3 0 24.20 0.36 

C 8 0 0 12.0 100.0 0.288667 4.874 0.30 * * * 23.35 0.25 

D 8 5 0 18.9 97.9 0.454298 4.640 0.41 * * * 22.24 0.18 

E 9 5 0 35.6 98.8 0.856890 4.719 0.61 * * * 22.61 0.09 

F 1050 15.3 99.6 0.368620 4.747 0.77 7 3 2 0 22.75 0.21 

G 1450 5.8 83.9 0.140800 4.515 0.02 5 0 21.64 0.46 

Total Gas 100.0 97.4 2.408471 4.759 0.49 1 6 0 3 22.80 

Plateau Age 69.8 steps D-F 22.56 0.13 

Porphyry Mtn 

JS-98-I3; single-crystal sanidine, #127KD12, J=0.0020185±0.25% 

0 1 LF * * * 98.1 0.02975949 5.554 53.3 2.218 20.11 0.03 

0 3 LF * * * 97.8 0.02520680 5.558 62.7 2.476 20.13 0.04 

0 9 LF * * * 99.0 0.01512566 5.560 38.8 2.980 20.13 0.04 

0 8 LF * * * 98.8 0.02109378 5.567 24.4 2.648 20.16 0.04 

0 7 LF * * * 99.2 0.02529005 5.576 60.6 2.135 20.19 0.03 

0 6 LF * * * 98.7 0.02540607 5.581 47.9 2.782 20.21 0.03 

04 LF *** 98.8 0.03651664 5.597 46.3 2.008 20.27 0.03 

02 LF *** 96.7 0.01790485 5.650 51.1 1.771 20.46 0.04 

Weighted Mean Age n=6 20.15 0.05 

JS-98-25; single-crystal sanidine, #124KD12, J=0.0020199±0.25% 

1 0 LF * * * 97.8 0.01261630 5.311 66.2 2.504 19.25 0.06 

0 1 LF * * * 96.8 0.01474393 5.356 55.0 1.841 19.41 0.04 

0 8 LF * * * 87.6 0.01985658 5.357 52.5 2.496 19.42 0.05 

0 6 LF * * * 97.2 0.04757085 5.359 58.8 2.285 19.42 0.03 

0 4 LF * * * 98.5 0.02302597 5.366 57.5 2.037 19.45 0.04 

0 2 LF * * * 97.6 0.04135148 5.372 60.8 2.183 19.47 0.02 

0 9 LF * * * 98.4 0.02607659 5.376 62.3 2.456 19.49 0.03 

0 5 LF * * * 97.6 0.01849182 5.378 50.4 1.934 19.49 0.04 

0 7 LF * * * 97.7 0.02889287 5.385 72.5 2.325 19.52 0.03 

03 LF *** 40.2 0.01705468 5.483 35.9 1.524 19.87 0.15 

Weighted Mean Age n=9 19.39 0.05 

32
1 

.1 

.01 

ISOCHRON AGE = 22.58 ± 0.51 Ma 

(40Ar/36Ar)i = 414.66 ± 41.79 

MSWD = 1.137 

NO POINTS DELETED 

* K97-10-8C4 

B 

A 

CEDF 

G 

K97-10-8C4 

PLATEAU AGE = 

22.56 ± 0.13 Ma 

.002 .003 

K/Ca 

36 40 Ar/ Ar

Apparent Age (Ma) 20 

10 

0 .04 .08 .12 .16 .2 0 

39 40 0 39ArK RELEASED (%) 100 

Ar/ Ar

 0.0040 JS-98-13 

ISOCHRON AGE = 20.20 ± 0.06 Ma 

(40Ar/36Ar)i = 243.9 ± 34.9 

MSWD = 1.64 

POINTS 4,2 DELETED 

* 

104 

100 

96 

92 

0.0030 

0.0020 

0.0010 

0.0000 

0.00 0.04 0.08 0.12 0.16 

39 40 16 18 20 22 24 26 28 30 

JS-98-13 

Weighted Mean Age = 

20.15 ± 0.05 Ma 

n = 6 

Relative Probability 

Analyses 

% Radiogenic 40Ar 

Ar/ Ar Age (Ma) 

100 

60 

0.0030 20 

0.0020 

0.0010 

JS-98-25 

Weighted Mean Age = 

19.39 ± 0.05 Ma 

n = 9 

Relative Probability 

Analyses 

% Radiogenic 40Ar 

0.0000 

JS-98-25 

ISOCHRON AGE = 19.46 ± 0.05 Ma 

(40Ar/36Ar)i = 291.5 ± 5.6 

MSWD = 3.68 

POINT 3 DELETED 

* 

16 18 20 22 24 26 28 30 

0.00 0.04 0.08 0.12 0.16 Age (Ma) 39 40 Ar/ Ar

33 

.001 0 

36 40 Ar/ Ar 36 40 Ar/ Ar
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

JS-98-5; single-crystal sanidine, #122KD12, J=0.0020261±0.25% 

1 0 LF * * * 98.4 0.01945620 5.315 57.4 1.861 19.32 0.04 

0 9 LF * * * 98.3 0.00836388 5.321 46.4 2.770 19.34 0.08 

0 7 LF * * * 97.8 0.01867534 5.321 69.5 2.091 19.35 0.04 

0 4 LF * * * 97.4 0.00997227 5.336 54.7 2.758 19.40 0.06 

0 8 LF * * * 98.6 0.01491417 5.350 60.5 2.159 19.45 0.05 

0 3 LF * * * 98.3 0.01311439 5.354 60.1 2.095 19.46 0.05 

0 6 LF * * * 98.3 0.02921246 5.358 57.2 2.297 19.48 0.03 

0 2 LF * * * 95.5 0.00643894 5.366 3 . 1 1.968 19.51 0.11 

0 5 LF * * * 96.4 0.02140796 5.371 47.3 1.935 19.53 0.04 

01 LF *** 98.1 0.01856865 7.914 28.1 2.014 28.70 0.05 

Weighted Mean Age n=9 19.44 0.05 

JS-98-93; single-crystal sanidine, #125KD12, J=0.0020252±0.25% 

0 8 LF * * * 98.0 0.03750574 5.329 58.4 2.280 19.37 0.03 

0 9 LF * * * 95.8 0.01853065 5.331 49.3 1.855 19.37 0.04 

0 3 LF * * * 98.6 0.02277688 5.340 63.5 2.013 19.40 0.04 

0 4 LF * * * 98.0 0.03259294 5.340 59.3 1.880 19.40 0.03 

0 6 LF * * * 98.4 0.04435245 5.342 55.8 2.059 19.41 0.02 

0 2 LF * * * 97.5 0.03143046 5.342 56.3 2.094 19.41 0.03 

0 1 LF * * * 97.5 0.03166982 5.349 74.0 2.079 19.44 0.03 

0 7 LF * * * 97.8 0.04517250 5.350 55.3 2.376 19.44 0.03 

1 0 LF * * * 97.8 0.04668219 5.358 80.6 1.800 19.47 0.03 

Weighted Mean Age n=9 19.40 0.05 

JS-98-2; single-crystal sanidine, #121KD12, J=0.002037±0.25% 

0 3 LF * * * 98.4 0.01124012 5.348 85.0 2.189 19.55 0.06 

0 8 LF * * * 97.9 0.02118851 5.361 57.3 2.042 19.60 0.03 

0 4 LF * * * 95.9 0.02014006 5.366 90.3 1.790 19.61 0.04 

0 9 LF * * * 97.6 0.01979390 5.368 72.2 2.714 19.62 0.04 

02 LF *** 97.9 0.01080192 5.396 52.2 2.614 19.72 0.06 

06 LF *** 95.7 0.02500567 5.399 53.2 1.657 19.73 0.03 

07 LF *** 98.5 0.02872340 5.407 57.0 1.907 19.76 0.03 

Weighted Mean Age n=4 19.58 0.05 

34
JS-98-5 

ISOCHRON AGE = 19.31 ± 0.07 Ma 

(40Ar/36Ar)i = 387.0 ± 33.7 

MSWD = 2.14 

POINT 1 DELETED 

0.00 0.04 0.08 0.12 0.16 

39 40 Ar/ Ar

JS-98-93 

ISOCHRON AGE = 19.40 ± 0.06 Ma 

(40Ar/36Ar)i = 313.0 ± 23.9 

MSWD = 1.84 

NO POINTS DELETED 

0.00 0.04 0.08 0.12 0.16 

39 40 Ar/ Ar

JS-98-2 

ISOCHRON AGE = 19.57 ± 0.07 Ma 

(40Ar/36Ar)i = 318.1 ± 33.6 

MSWD = 1.46 

POINTS 2,6,7 DELETED 

0.00 0.04 0.08 0.12 0.16 

39 40 Ar/ Ar

JS-98-5 

Weighted Mean Age = 

19.44 ± 0.05 Ma 

n = 9 

Relative Probability 

Analyses 

% Radiogenic 40Ar 

16 18 20 22 24 26 28 30 

Age (Ma) 

100 

96 

92 

JS-98-93 

Weighted Mean Age = 

19.40 ± 0.05 Ma 

n = 9 

Relative Probability 

Analyses 

% Radiogenic 40Ar 

16 18 20 22 24 26 28 30 

Age (Ma) 

100 

96 

92 

JS-98-2 

Weighted Mean Age = 

19.58 ± 0.05 Ma 

n = 4 

Relative Probability 

Analyses 

% Radiogenic 40Ar 

16 18 20 22 24 26 28 30 

Age (Ma) 

100 

96 

92 

0.0024 

0.0020 

40 Ar/ Ar 36 40 Ar/ Ar 36 40 Ar/ Ar 36 

0.0016 

0.0012 

0.0008 

0.0004 

0.0000 

0.0032 

0.0024 

0.0016 

0.0008 

0.0000 

0.0032 

0.0024 

0.0016 

0.0008 

0.0000 

35
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

K97-8-13D; single-crystal sanidine, #128KD12, J=0.0020265±0.25% 

0 3 LF * * * 97.5 0.01658627 5.323 56.7 3.171 19.36 0.05 

0 2 LF * * * 98.5 0.02708563 5.326 76.6 2.302 19.37 0.03 

0 4 LF * * * 98.2 0.02258068 5.328 74.6 1.862 19.37 0.03 

0 7 LF * * * 98.0 0.01667124 5.331 61.7 1.555 19.38 0.04 

0 1 LF * * * 98.5 0.02799637 5.343 81.9 2.097 19.43 0.03 

0 6 LF * * * 98.7 0.02287640 5.351 70.4 2.371 19.46 0.03 

0 9 LF * * * 94.8 0.02179100 5.356 69.0 2.288 19.48 0.04 

05 LF *** 61.2 0.02319670 5.402 54.3 1.842 19.64 0.09 

10 LF *** 97.9 0.01520618 5.475 80.3 1.825 19.91 0.05 

Weighted Mean Age n=7 19.40 0.05 

JS-98-17; single-crystal sanidine, #123KD12, J=0.002023±0.25%


0 2 LF *** 93.7 0.01504055 5.306 94.5 2.390 19.26 0.05


0 1 LF *** 97.6 0.01407073 5.310 62.4 1.816 19.28 0.05


1 0 LF *** 98.2 0.01354763 5.326 53.3 2.007 19.33 0.05


0 3 LF *** 98.2 0.03379408 5.329 59.4 2.477 19.34 0.03


0 7 LF *** 93.0 0.00824804 5.343 55.2 2.390 19.39 0.09


0 9 LF *** 98.5 0.01397419 5.347 63.7 2.366 19.41 0.04


0 4 LF *** 97.4 0.01799024 5.361 60.4 2.100 19.46 0.05


08 LF *** 98.9 0.01475063 5.388 49.5 2.180 19.56 0.05


06 LF *** 97.9 0.01798889 5.501 36.0 2.283 19.96 0.04


Weighted Mean Age n=7 19.36 0.05


Flat Tops


K99-8-24A; 0.2542 g, whole rock basalt, #61KD16, J=0.004792±0.25% 


A 6 5 0 2 . 1 35.9 0.354061 1.379 1.26 4 5 11.88 0.30 

B 7 5 0 1 . 3 74.2 0.215774 1.614 0.94 8 6 13.90 0.22 

C 8 5 0 10.8 93.1 1.842960 1.607 0.97 5 5 5 13.84 0.03 

D 9 5 0 35.4 97.7 6.048987 1.563 1.27 2 9 8 6 13.46 0.01 

E 1050 38.0 98.2 6.502008 1.549 1.22 2 9 3 0 13.34 0.01 

F 1150 9 . 6 93.7 1.646590 1.518 0.80 3 9 0 13.07 0.03 

G 1450 2 . 5 69.5 0.431775 1.426 0.03 5 0 12.29 0.29 

H 1650 0 . 3 64.1 0.055754 1.561 0.03 1 3 3 13.45 2.15 

Total Gas 100.0 94.6 17.097909 1.551 1.13 2 2 7 2 13.36 

No Plateau 

36
K97-8-13D 

ISOCHRON AGE = 19.38 ± 0.05 Ma 

(40Ar/36Ar)i = 321.1 ± 16.9 

MSWD = 1.66 

POINTS 5,10 DELETED 

Ar/ Ar 39 40 

0.00 0.04 0.08 0.12 0.16 

0.0000 

0.0008 

0.0016 

0.0024 

0.0032 

Age (Ma) 

K97-8-13D 

Weighted Mean Age = 

19.40 ± 0.05 Ma 

n = 7 

Relative Probability 

Analyses 

% Radiogenic 40Ar 

50 

70 

90 

110 

16 18 20 22 24 26 28 30 

Ar/ Ar 39 40 

0 

10 

20 

Apparent Age (Ma) 

0 100 39Ar K 

K/Ca 

K99-8-24A 

NO PLATEAU 

.1 

1 

10 

.01 

ISOCHRON AGE = 13.45 ± 0.16 Ma 

(40Ar/36Ar)i = 329.01 ± 22.46 

MSWD = 0.856 

POINTS AFG DELETED 

0 .001 .002 .003 

Ar/ Ar 36 40 

* K99-8-24A 

0 1 .2 .3 .4 .5 .6

A 

B 

C 

DE 

F 

G 

H 

JS-98-17 

ISOCHRON AGE = 19.37 ± 0.06 Ma 

(40Ar/36Ar)i = 286.0 ± 13.8 

MSWD = 2.72 

POINTS 6,8 DELETED 

Ar/ Ar 39 40 

0.00 0.04 0.08 0.12 0.16 

0.0000 

0.0010 

0.0020 

0.0030 

* 

Age (Ma) 

JS-98-17 

Weighted Mean Age = 

19.36 ± 0.05 Ma 

n = 7 

Relative Probability 

Analyses 

% Radiogenic 40Ar 

90 

95 

100 

105 

16 18 20 22 24 26 28 30 

. RELEASED (%)

36 40 Ar/ Ar 36 40 Ar/ Ar

37
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

K99-8-24B1; 0.2544 g, whole rock basalt, #63KD16, J=0.00498±0.25% 

A 6 5 0 0 . 5 3 . 1 0.033262 1.407 0.29 1 9 12.59 1.82 

B 7 5 0 0 . 7 34.8 0.045636 1.729 0.42 7 2 15.47 0.89 

C 8 5 0 8 . 2 82.9 0.527724 1.627 0.37 7 5 8 14.56 0.09 

D 9 0 0 17.2 94.0 1.105773 1.594 0.48 8 0 1 8 14.26 0.05 

E 9 5 0 26.2 96.4 1.682793 1.575 0.61 * * * 14.09 0.03 

F 1000 23.0 95.9 1.479033 1.566 0.60 154543 14.02 0.03 

G 1050 9 . 8 91.6 0.626563 1.574 0.36 3 8 7 4 14.09 0.11 

H 1150 6 . 5 76.6 0.415212 1.521 0.24 6 2 3 13.62 0.15 

I 1450 7 . 9 41.5 0.508454 1.331 0.02 5 3 11.92 0.15 

Total Gas 100 .0 87.8 6.424450 1.558 0.47 37444 13.94 

Plateau Age 59.0 steps E-G 14.06 0.05 

K99-8-24B3; 0.2560 g, whole rock basalt, #67KD16, J=0.005002±0.25% 

A 6 5 0 0 . 8 5 . 6 0.056362 1.789 0.30 2 3 16.07 0.99 

B 7 5 0 1 . 9 58.0 0.126460 1.761 0.40 1 5 1 15.82 0.15 

C 8 5 0 14.5 88.7 0.975749 1.638 0.39 1 2 1 4 14.72 0.07 

D 9 0 0 16.2 94.3 1.087197 1.590 0.50 8 8 2 4 14.29 0.04 

E 9 5 0 24.1 95.4 1.620543 1.564 0.56 90046 14.05 0.02 

F 1000 16.9 92.8 1.137080 1.557 0.49 8 8 3 9 14.00 0.03 

G 1050 8 . 0 85.7 0.537016 1.563 0.32 2 1 7 4 14.05 0.09 

H 1150 7 . 5 65.1 0.502960 1.503 0.25 6 8 6 13.51 0.06 

I 1450 9 . 3 40.0 0.628144 1.310 0.02 5 8 11.78 0.40 

J 1650 0 . 7 39.6 0.048142 1.496 0.02 9 4 13.45 1.06 

Total Gas 100.0 83.7 6.719653 1.554 0.41 25050 13.97 

No Plateau 

K99-8-24C; 0.2502 g, whole rock basalt, #69KD16, J=0.005002±0.25% 

A 6 5 0 1 . 0 7 . 4 0.147778 1.054 1.01 2 5 9.48 0.62 

B 7 5 0 2 . 7 71.4 0.393486 1.560 0.89 1 0 1 14.02 0.20 

C 8 5 0 14.1 93.6 2.042751 1.680 0.83 3 1 3 15.09 0.05 

D 9 0 0 16.5 96.6 2.385731 1.624 0.89 7 4 5 14.59 0.02 

E 9 5 0 19.9 97.0 2.888016 1.602 1.00 9 3 9 14.40 0.02 

F 1000 19.4 96.7 2.804347 1.598 0.99 8 6 2 14.36 0.02 

G 1050 12.7 94.3 1.833187 1.593 0.97 4 5 0 14.32 0.02 

H 1150 10.5 79.1 1.522958 1.571 1.13 1 1 5 14.12 0.05 

I 1450 2 . 4 50.5 0.349837 1.488 0.04 3 3 13.38 0.35 

J 1650 0 . 8 46.3 0.115759 1.478 0.04 7 0 13.29 0.29 

Total Gas 100.0 91.0 14.483850 1.601 0.93 5 9 4 14.39 

Plateau Age 52.0 steps E-G 14.36 0.05 

38
1 

.1 

.01 

ISOCHRON AGE = 14.12 ± 0.17 Ma 

(40Ar/36Ar)

i = 310.95 

± 10.17 

MSWD = 0.227 

POINTS AHI DELETED 

* A K99-8-24B1

B 

C 

D

EF G 

H 

I 

39 40 Ar/ Ar

1 

.1 

.01 

20 

10 

K99-8-24B1 

PLATEAU AGE = 

14.06 ± 0.05 Ma 

0 39ArK RELEASED (%) 100 

K99-8-24B3 

NO PLATEAU 

0 39ArK RELEASED (%) 100 

K99-8-24C 

PLATEAU AGE = 

14.36 ± 0.05 Ma 

.001 0 .003 

K/Ca 

.003 0 .001 .002 

36 40 Ar/ Ar

K/Ca Apparent Age (Ma) 20 

10 

0 

0.1.2 .3.4.5 .6 

ISOCHRON AGE = 14.27 ± 0.07 Ma 

(40Ar/36Ar)

i = 298.63 

± 1.07 

MSWD = 15.967 

* K99-8-24B3 A 

B 

C 

D

E

F 

G 

H 

J 

I

0.1.2.3.4.5 .6 

39 40 Ar/ Ar

ISOCHRON AGE = 13.72 ± 0.61 Ma 

(40Ar/36Ar)

i = 667.52 

± 260.82 

MSWD = 0.653 

POINTS ABHIJ DELETED 

* K99-8-24C A 

B 

C 

DEFG 

H 

I 

J 

0 .1 .2 .3 .4 .5 .6 

39 40 Ar/ Ar

.001 .002 

36 40 Ar/ Ar

K/Ca Apparent Age (Ma) 

0 

10 

1 

.1 

.01 

20 

10 

.002 .003 

Apparent Age (Ma) 

36 40 Ar/ Ar

0 

0 39ArK RELEASED (%) 100 

0 

39
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

K99-8-24D1; 0.2529 g, whole rock basalt, #71KD16, J=0.004952±0.25% 

A 6 5 0 9.6 17.0 0.406562 1.550 1.99 3 7 13.80 0.31 

B 7 5 0 9.2 39.3 0.388145 1.475 1.22 5 2 13.13 0.12 

C 8 5 0 20.2 60.1 0.853642 1.652 0.29 9 3 14.70 0.09 

D 9 0 0 12.7 84.6 0.539847 1.698 0.15 3 2 9 15.10 0.18 

E 9 5 0 14.1 90.3 0.597526 1.678 0.14 5 7 4 14.93 0.16 

F 1000 11.3 87.0 0.477557 1.635 0.17 4 1 0 14.55 0.15 

G 1050 7.2 83.5 0.305512 1.643 0.22 1 7 8 14.62 0.14 

H 1150 9.5 75.7 0.402099 1.544 0.18 1 1 2 13.74 0.16 

I 1450 6.2 46.1 0.263897 1.406 0.01 3 9 12.52 0.49 

Total Gas 100.0 66.8 4.234787 1.607 0.45 2 2 2 14.30 

No Plateau 

K99-8-24D2; 0.2545 g, whole rock basalt, #73KD16, J=0.004945±0.25% 

A 6 5 0 5 .1 9 .6 0.205692 1.689 1.49 2 6 15.00 0.53 

B 7 5 0 6.3 36.7 0.252269 1.466 0.92 4 6 13.04 0.36 

C 8 5 0 17.0 65.0 0.686659 1.676 0.25 1 2 6 14.89 0.11 

D 9 0 0 13.6 87.4 0.549631 1.731 0.16 4 9 6 15.38 0.07 

E 9 5 0 14.2 91.4 0.571596 1.702 0.16 6 6 6 15.12 0.11 

F 1000 13.2 89.9 0.531320 1.667 0.20 4 7 5 14.81 0.09 

G 1050 9.2 87.5 0.371074 1.680 0.28 2 0 5 14.92 0.13 

H 1150 14.0 80.7 0.566499 1.610 0.28 1 1 6 14.30 0.18 

I 1450 7.4 52.1 0.299090 1.550 0.01 3 6 13.77 0.36 

Total Gas 100.0 73.8 4.033830 1.655 0.31 2 8 8 14.71 

No Plateau 

K99-8-24E; 0.2501 g, whole rock basalt, #77KD16, J=0.004880±0.25% 

A 6 5 0 0 .8 4 .7 0.099087 1.707 1.05 3 4 14.97 0.70 

B 7 5 0 0.8 21.4 0.100131 1.724 1.11 1 0 3 15.12 0.56 

C 8 5 0 5.2 54.6 0.673651 1.852 0.85 6 2 5 16.23 0.10 

D 9 0 0 8.0 92.1 1.045956 1.821 0.97 8 4 4 0 15.96 0.07 

E 9 5 0 16.2 96.9 2.109848 1.806 1.04 * * * 15.83 0.02 

F 1000 20.4 98.3 2.657556 1.772 1.17 * * * 15.53 0.03 

G 1050 20.9 98.6 2.724175 1.756 1.24 8 4 3 0 15.39 0.02 

H 1150 19.9 98.2 2.602550 1.757 1.30 1 0 3 9 15.40 0.02 

I 1450 5.9 84.0 0.773400 1.686 0.07 8 8 14.78 0.09 

J 1650 2.0 78.1 0.261932 1.689 0.05 1 9 5 14.81 0.32 

Total Gas 100.0 92.8 13.048286 1.771 1.07 2 6 8 6 15.53 

No Plateau 

40
10 

1 

.1 

.01 

ISOCHRON AGE = 14.91 ± 0.18 Ma 

(40Ar/36Ar)

i = 289.46 

± 7.91 

MSWD = 1.07 

POINTS ABHIJ DELETED 

* K99-8-24D1 

A 

B 

C 

D 

EF G 

H 

I 

0 .1 .2 .3 .4 .5 

39 40 Ar/ Ar

* 0 .1 .2 .3 .4 .5 

39 40 Ar/ Ar

ISOCHRON AGE = 15.11 ± 0.18 Ma 

(40Ar/36Ar)

i = 289.76 

± 13.93 

MSWD = 1.213 

POINTS ABHI DELETED 

K99-8-24D2 

A 

B 

C 

D 

EFG 

H 

I 

ISOCHRON AGE = 15.57 ± 0.09 Ma 

(40Ar/36Ar)

i = 312.46 

± 6.56 

MSWD = 1.924 

POINTS ABIJ DELETED 

* A K99-8-24E

B 

C 

D 

E 

FH G

I 

J 

K99-824D1 

NO PLATEAU 

K99-8-24D2 

NO PLATEAU 

0 39ArK RELEASED (%) 

K99-8-24E 

NO PLATEAU 

Apparent Age (Ma) Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca 

.002 .003 0 .001 .002 .003 

36 40 Ar/ Ar

20 

10 

0 

0 39ArK RELEASED (%) 

10 

1 

.1 

.01 

20 

10 

0 

10 

1 

.1 

.01 

20 

36 40 Ar/ Ar

.001 0 .001 

K/Ca 

.002 .003 

36 40 Ar/ Ar

10 

0 

0 

0 .1 .2 .3 .4 .5 .6 0 39ArK RELEASED (%) 100 

39 40 Ar/ Ar

41 

100

100
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

Poison Creek Debris Flow


JS-98-8A; 0.2503 g, whole rock basalt, #11KD16, J=0.004832±0.50% Ä

A 6 5 0 7.6 29.5 0.466539 1.777 1.74 2 3 15.43 0.09 

B 7 5 0 5.0 52.9 0.307309 1.760 1.04 3 3 15.28 0.21 

C 8 5 0 15.5 80.4 0.946718 1.816 0.48 1 4 4 15.76 0.07 

D 9 5 0 28.0 95.1 1.709992 1.761 0.39 1 1 5 8 15.29 0.09 

E 1050 17.8 92.2 1.087620 1.708 0.30 6 1 4 14.82 0.05 

F 1150 7.2 85.5 0.438130 1.648 0.19 2 6 3 14.31 0.23 

G 1450 18.9 82.5 1.154003 1.521 0.05 1 0 2 13.21 0.21 

Total Gas 100.0 82.1 6.110311 1.708 0.45 4 9 7 14.83 

No Plateau 

JS-99-8B; 0.2550 g, whole rock basalt, #13KD16, J=0.004906±0.50% 

A 5 5 0 4.6 21.5 0.293058 1.520 1.60 2 4 13.40 0.24 

B 6 5 0 0.3 43.8 0.017448 1.632 1.66 2 6 14.39 1.27 

C 7 5 0 3.1 64.5 0.199188 1.765 0.81 6 7 15.56 0.20 

D 8 5 0 16.8 88.1 1.062787 1.756 0.53 3 0 2 15.47 0.08 

E 9 5 0 26.4 93.6 1.671523 1.677 0.38 9 6 5 14.78 0.07 

F 1050 23.7 92.6 1.496850 1.669 0.53 5 6 8 14.72 0.06 

G 1150 14.9 87.1 0.940119 1.631 0.29 1 7 8 14.38 0.15 

H 1450 10.2 61.8 0.647786 1.594 0.02 1 9 7 14.05 0.21 

Total Gas 100.0 83.8 6.328759 1.668 0.47 4 9 0 14.71 

Plateau Age 50.1 steps E-F 14.74 0.08 

JS-99-8C; 0.2537 g, whole rock basalt, #15KD16, J=0.004964±0.50% 

A 6 5 0 1 .8 7 .0 0.091830 1.729 0.60 2 0 15.42 0.73 

B 7 5 0 2.4 45.3 0.124259 1.810 0.43 7 1 16.14 0.51 

C 8 5 0 16.1 84.2 0.832998 1.765 0.30 4 4 8 15.74 0.14 

D 9 5 0 32.8 95.2 1.695717 1.696 0.30 1 9 0 1 15.12 0.06 

E 1050 22.2 93.2 1.148417 1.665 0.48 7 6 5 14.85 0.09 

F 1150 12.9 87.2 0.667749 1.612 0.36 3 4 6 14.37 0.10 

G 1450 11.8 63.3 0.611215 1.590 0.03 7 9 14.18 0.36 

Total Gas 100.0 85.4 5.172185 1.680 0.32 9 2 1 14.98 

No Plateau 

42
0 

ISOCHRON AGE = 15.05 ± 0.12 Ma 

(40Ar/36Ar)

i = 298.69 

± 1.06 

MSWD = 11.409 

NO POINTS DELETED 

* JS-98-8A 

A 

B 

C 

D E 

F G 

ISOCHRON AGE = 14.74 ± 0.44 Ma 

(40Ar/36Ar)

i = 304.02 

± 31.59 

MSWD = .661 

POINTS AB DELETED 

* JS-99-8B 

A 

B 

C 

D 

F E

G 

H 

Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca 

10 

1 

.1 

.01 

20 

10 

JS-98-8A 

NO PLATEAU 

0 39ArK RELEASED (%) 100 

JS-99-8B 

PLATEAU AGE = 

14.74 ± 0.08 Ma 

0 39ArK RELEASED (%) 100 

JS-99-8C 

NO PLATEAU 

0 39ArK RELEASED (%) 100 

.002 .003 0 .001 .002 .003 0 .001 .002 .003 

36 40 Ar/ Ar 36 40 Ar/ Ar

0 .1 .2.3 .4 .5 

39 40 Ar/ Ar

0.1 .2 .3.4 .5 .6 

39 40 Ar/ Ar

* 

0 .1.2 .3 .4 .5 .6 

39 40 Ar/ Ar

43 

ISOCHRON AGE = 15.09 ± 0.21 Ma 

(40Ar/36Ar)

i = 314.26 

± 9.39 

MSWD = 1.651 

POINTS AFG DELETED 

A JS-99-8C

B 

C 

D

E 

F 

G 

0 

10 

1 

.1 

.01 

20 

10 

0 

1 

.1 

.01 

20 

10 

0 

36 40 Ar/ Ar

.001 
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

JS-99-9; 0.2525 g, whole rock basalt, #17KD16, J=0.004968±0.50% 

A 6 5 0 1 . 5 22.3 0.206697 1.250 1.08 6 0 11.17 0.34 

B 7 5 0 4 . 7 86.3 0.637626 1.692 0.74 2 0 0 15.11 0.04 

C 8 5 0 20.9 96.8 2.836092 1.676 0.80 7 7 7 14.96 0.05 

D 9 5 0 36.2 98.0 4.911192 1.627 1.01 1 5 8 2 14.52 0.03 

E 1050 25.0 96.8 3.391000 1.621 0.93 6 1 1 14.47 0.02 

F 1150 8 . 0 77.7 1.082040 1.581 0.76 1 4 2 14.11 0.08 

G 1450 3 . 6 49.9 0.492393 1.591 0.05 6 1 14.20 0.35 

Total Gas 100 .0 92.4 13.557040 1.628 0.88 9 1 2 14.53 

Plateau Age 61.2 steps D-E 14.48 0.08 

JS-99-10; 0.2519 g, whole rock basalt, #19KD16, J=0.004966±0.50% 

A 7 5 0 3 . 4 28.2 0.198106 1.489 1.70 2 7 13.29 0.22 

B 8 5 0 6 . 6 61.7 0.384249 1.685 0.76 6 0 15.04 0.16 

C 9 5 0 22.6 89.0 1.323534 1.720 0.42 3 5 1 15.35 0.05 

D 1050 29.6 93.4 1.734575 1.672 0.40 8 5 7 14.92 0.05 

E 1150 24.2 89.9 1.414813 1.642 0.43 4 0 6 14.65 0.05 

F 1450 13.7 65.8 0.800304 1.625 0.03 1 2 4 14.50 0.16 

Total Gas 100 .0 83.5 5.855581 1.664 0.43 4 5 3 14.85 

No Plateau 

Sopris Bowl


L-223; 0.2472 g, whole rock basalt, #13KD8, J=0.002665±0.15% 


A 5 0 0 5 . 3 19.9 0.029842 2.392 3.16 4 4 11.46 0.06 

B 6 0 0 4 . 7 33.9 0.026544 2.687 1.62 6 0 12.87 0.03 

C 7 0 0 12.6 62.1 0.071074 2.972 1.03 1 7 8 14.23 0.03 

D 7 5 0 6 . 6 76.3 0.037421 2.948 1.10 6 2 6 14.12 0.04 

E 8 0 0 8 . 2 82.5 0.046499 2.905 1.27 1 3 8 5 13.91 0.01 

F 8 5 0 13.4 88.9 0.075482 2.865 1.42 2 4 5 8 13.72 0.02 

G 9 0 0 15.5 91.1 0.087321 2.852 1.48 2 6 2 8 13.66 0.01 

H 9 5 0 9 . 2 87.9 0.051700 2.845 1.51 1 5 8 1 13.63 0.02 

I 1000 10.6 87.2 0.059852 2.839 1.60 9 2 2 13.60 0.03 

J 1100 9 . 6 75.5 0.054215 2.822 1.51 4 3 3 13.51 0.02 

K 1250 2 . 6 100.0 0.014548 2.773 0.11 * * * 13.28 0.03 

L 1400 1 . 7 100.0 0.009338 2.853 0.06 * * * 13.66 0.22 

Total Gas 100 .0 77.1 0.563836 2.929 1.43 1 2 0 4 14.03 

No Plateau 

44
10 

1 

.1 

.01 

ISOCHRON AGE = 14.51 ± 0.14 Ma 

(40Ar/36Ar)

i = 377.48 

± 27.79 

MSWD = 2.798 

POINTS AFG DELETED 

* JS-99-9 

A 

B 

C D E

F 

G 

Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca 

JS-99-9 

PLATEAU AGE = 

14.48 ± 0.08 Ma 

0 

0 39ArK RELEASED (%) 100 

10 

1 

.1 

.01 

20 

10 

0 

0 39ArK RELEASED (%) 100 

JS-99-10 

NO PLATEAU 

10 

1 

L-223 

NO PLATEAU 

.002 .003 0 .001 .002 .003 

36 40 Ar/ Ar

20 

10 

0 .1 .2 .3 .4 .5 .6 

39 40 Ar/ Ar

* 0 .1 .2 .3 .4 .5 .6 

39 40 Ar/ Ar

ISOCHRON AGE = 14.87 ± 0.15 Ma 

(40Ar/36Ar)

i = 280.39 

± 12.61 

MSWD = 1.498 

POINTS ABC DELETED 

JS-99-10 

A 

B 

C 

D

E 

36 

F 

40 Ar/ Ar

.001 0 .001 

.1 

.01 

20 

10 

0 

ISOCHRON AGE = 13.38 ± 0.06 Ma 

(40Ar/36Ar)

i = 345.49 

± 6.76 

MSWD = 1.744 

POINT ABCJKL DELETED 

* L -223 

A 

B 

C 

D

E 

FG IH

J 

L K 0 .06 .12 .18 .24 .3 0 39ArK RELEASED (%) 100 

39 40 Ar/ Ar

45 

K/Ca 

.002 .003 

Apparent Age (Ma) 

36 40 Ar/ Ar

0 
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

BR6(3); 0.2525 g, whole rock basalt, #10KD8, J=0.002665±0.25% 

A 5 5 0 2 . 1 1 . 2 0.360901 0.232 2.53 7 1 1.12 0.72 

B 6 5 0 7 . 3 77.9 1.254430 2.794 1.59 5 9 6 13.38 0.07 

C 7 5 0 15.1 98.1 2.572864 2.971 1.67 3 1 1 4 14.23 0.06 

D 8 0 0 14.0 98.8 2.387945 2.871 2.04 5 2 8 8 13.75 0.04 

E 8 5 0 15.1 99.1 2.572864 2.832 2.42 3 5 7 0 13.57 0.03 

F 9 5 0 21.1 99.0 3.605060 2.827 2.85 1 8 3 9 13.54 0.03 

G 1050 17.4 97.7 2.982190 2.842 4.35 6 4 5 13.61 0.03 

H 1450 6 . 8 88.8 1.165304 2.831 0.39 1 7 3 13.56 0.08 

I 1650 1 . 1 78.7 0.193605 2.805 0.35 3 9 8 13.43 0.43 

Total Gas 100 .0 94.1 17.095163 2.801 2.46 2 3 0 6 13.42 

Plateau Age 61.5 steps E-I 13.57 0.05 

Ba36(1); 0.2528 g, whole rock basalt, #14KD8, J=0.002665±0.25% 

A 5 5 0 0 . 5 3 . 0 0.06944 1.991 1.55 2 3 9.55 2.26 

B 6 5 0 1 . 1 6 . 8 0.157636 1.445 1.87 7 0 6.94 0.48 

C 7 5 0 1 . 7 56.5 0.252374 2.671 1.39 3 2 5 12.80 0.21 

D 8 0 0 1 . 8 89.0 0.256555 2.919 1.27 8 0 9 13.98 0.22 

E 8 5 0 2 . 5 96.5 0.367386 2.993 1.33 1 9 3 9 14.33 0.16 

F 9 5 0 7 . 4 98.1 1.079418 2.966 1.59 34758 14.20 0.06 

G 1050 14.3 98.8 2.087815 2.889 2.02 3 0 7 2 13.84 0.03 

H 1650 70.7 96.3 10.283273 2.810 3.30 2 1 5 0 13.46 0.02 

Total Gas 100 .0 94.6 14.553897 2.818 2.85 4 6 0 7 13.50 

No Plateau 

Basalt Mtn., Top


K97-8-15C2; 0.2517 g, whole rock basalt, #63KD6, J=0.002026±0.5% 


A 6 5 0 23.6 31.9 1.016963 2.559 3.22 2 7 6 9.33 0.07 

B 7 5 0 8 . 5 58.4 0.367234 2.880 1.36 7 3 6 10.50 0.12 

C 8 0 0 9 . 9 87.3 0.426148 3.006 1.18 23430 10.95 0.11 

D 8 5 0 17.7 95.8 0.760286 2.880 1.19 * * * 10.50 0.07 

E 9 5 0 30.1 96.4 1.294893 2.700 0.84 * * * 9.84 0.04 

F 1050 6 . 7 90.6 0.286592 2.499 0.37 1 3 6 1 9.11 0.22 

G 1450 3 . 5 28.4 0.152506 1.965 0.07 6 1 7.17 0.45 

Total Gas 100.0 74.1 4.304622 2.705 1.48 2 5 4 0 9.86 

No Plateau 

46
10

ISOCHRON AGE = 13.62 ± 0.08 Ma 

(40Ar/36Ar)i = 285.65 ± 27.60 

MSWD = 0.359 

POINT ABC DELETED 

0 .001 .002 .003 

Ar/ Ar 39 40 

0 .06 .12 .18 .24 .3 .36

B 

C G DEF

H 

I 

0 

10 

20 

Apparent Age (Ma) 

0 100 39Ar K 

K/Ca 

.1 

1 

BR6(3) 

PLATEAU AGE = 

13.57 ± 0.05 Ma 

ISOCHRON AGE = 13.81 ± 0.25 Ma 

(40Ar/36Ar)i = 361.02 ± 85.47 

MSWD = 1.49 

POINT ABC DELETED 

0 .001 .002 .003 

Ar/ Ar 39 40 

* Ba36(1) 

0 .06 .12 .18 .24 .3 .36 

A B 

C 

D 

EFG

H 

0 

10 

20 

Apparent Age (Ma) 

0 100 39Ar K 

K/Ca 

.1 

10 

ISOCHRON AGE = 10.42 ± 0.09 Ma 

(40Ar/36Ar)i = 280.15 ± 1.49 

MSWD = 18.027 

NO POINTS DELETED 

0 .001 .002 .003 

Ar/ Ar 39 40 

* K97-8-15C2 

0 .07 .14 .21 .28 .35

A 

B 

C 

D E 

F 

G 

0 

10 

20 

Apparent Age (Ma) 

0 100 39Ar K 

K/Ca 

.01 

.1 

1 

10 

K97-8-15C2 

NO PLATEAU 

47 

RELEASED (%)

RELEASED (%)

RELEASED (%)

* BR6(3) A 

Ba36(1) 

NO PLATEAU 

36 40 Ar/ Ar 36 40 Ar/ Ar 36 40 Ar/ Ar
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

K97-8-15C4; 0.2467 g, whole rock basalt, #65KD6, J=0.002017±0.5% 

A 6 5 0 6 . 1 52.8 0.276923 2.204 0.83 3 1 7 8.00 0.20 

B 7 5 0 8 . 2 96.9 0.371046 2.996 0.77 4 0 2 4 10.87 0.14 

C 8 0 0 10.9 98.8 0.492889 2.897 0.97 * * * 10.51 0.09 

D 8 5 0 14.6 99.6 0.660736 2.799 1.25 * * * 10.16 0.07 

E 9 5 0 33.4 99.6 1.508646 2.755 1.48 14274 10.00 0.03 

F 1050 18.7 97.8 0.846753 2.730 1.51 7 1 3 9.91 0.06 

G 1450 8 . 0 82.0 0.360402 2.699 0.19 9 4 9.79 0.20 

Total Gas 100 .0 94.7 4.517395 2.754 1.19 5 2 5 8 9.99 

Plateau Age 60.1 steps E-G 9.97 0.06 

K97-8-15B; 0.2472 g, whole rock basalt, #61KD6, J=0.002044±0.5% 

A 7 5 0 6 . 4 96.5 0.296167 2.952 0.64 3 5 8 8 10.85 0.11 

B 8 0 0 7 . 0 99.7 0.322523 2.934 0.74 * * * 10.79 0.16 

C 8 5 0 10.7 100.0 0.495690 2.795 0.97 * * * 10.28 0.07 

D 9 5 0 35.2 99.0 1.623573 2.674 1.51 * * * 9.84 0.07 

E 1050 28.8 98.6 1.328326 2.677 1.84 4 9 3 4 9.84 0.03 

F 1450 12.0 64.8 0.551935 2.661 0.35 1 4 9 9.79 0.05 

Total Gas 100 .0 94.8 4.618214 2.721 1.30 1 6 6 7 10.00 

Plateau Age 75.9 steps D-F 9.83 0.05 

L-37; 0.2455 g, whole rock basalt, #5KD8, J=0.002657±0.5% 

A 5 0 0 0 . 5 6 . 0 0.001853 1.346 1.24 7 4 6.44 0.38 

B 6 0 0 2 . 0 26.0 0.007478 1.064 0.80 3 2 5 5.09 0.14 

C 7 0 0 10.8 81.7 0.041136 1.858 0.58 1 6 6 1 8.89 0.02 

D 8 0 0 19.9 93.9 0.075621 2.188 0.75 * * * 10.46 0.01 

E 9 0 0 28.4 95.8 0.107990 2.129 0.83 * * * 10.17 0.01 

F 1000 21.7 94.4 0.082700 2.091 0.54 * * * 10.00 0.03 

G 1100 9 . 0 86.5 0.034098 2.040 0.73 5 1 6 7 9.75 0.08 

H 1200 5 . 3 74.0 0.020243 1.930 0.62 1 0 7 7 9.23 0.02 

I 1350 2 . 5 41.5 0.009410 2.096 0.04 4 6 10.02 0.17 

Total Gas 100 .0 88.5 0.380529 2.059 0.68 7 0 8 9.84 

No Plateau 

48
10 

1 

.1 

ISOCHRON AGE = 10.33 ± 0.09 Ma 

(40Ar/36Ar)i = 206.23 ± 6.68 

MSWD = 7.033 

NO POINTS DELETED 

* K97-8-15C4 

C 

A 

B DE

F 

G 

K97-8-15C4 

PLATEAU AGE = 

9.97 ± 0.06 Ma 

Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca 

.002 .003 0 .001 .002 .003 

36 40 Ar/ Ar

20 

10 

0 

0 .07 .14 .21 .28 .35 0 39ArK RELEASED (%) 100 

39 40 Ar/ Ar

10 * 

1 

.1 

20 

10 

ISOCHRON AGE = 9.84 ± 0.15 Ma 

(40Ar/36Ar)i = 292.43 ± 19.26 

MSWD = 0.001 

POINT ABC DELETED 

K97-8-15B 

AB CE D

F 

0 .07 .14 .21 .28 .35 

39 40 Ar/ Ar

ISOCHRON AGE = 9.65 ± 0.06 Ma 

(40Ar/36Ar)i = 280.52 ± 1.04 

MSWD = 129.459 

NO POINTS DELETED 

* L-37 A 

B 

C 

D E

F 

G 

H 

I 

K97-8-15B 

PLATEAU AGE = 

9.83 ± 0.05 Ma 

0 

0 39ArK RELEASED (%) 100 

10 

1 

.1 

.01 

20 L-37 

NO PLATEAU 

36 40 Ar/ Ar

.001 0 .001 .002 .003 

K/Ca 

36 40 Ar/ Ar

Apparent Age (Ma) 10 

0 0 .08 .16 .24 .32 .4 .48 0 39ArK RELEASED (%) 100 

39 40 Ar/ Ar

49 

0 
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

Basalt Mtn., west flank


L-176; 0.2404 g, whole rock basalt, #7KD8, J=0.002650±0.25% Ä

A 5 5 0 3.4 18.7 0.294329 1.619 1.69 6 0 7.72 0.47 

B 6 5 0 11.2 44.4 0.980160 1.873 0.65 9 6 8.93 0.02 

C 7 5 0 14.9 77.0 1.298582 2.093 0.45 3 8 5 9.98 0.05 

D 8 0 0 13.6 89.7 1.189763 2.045 0.56 1 3 6 5 9.75 0.02 

E 8 5 0 14.8 93.9 1.293865 2.036 0.66 2 0 3 0 9.71 0.03 

F 9 5 0 21.2 95.0 1.850851 2.031 0.73 1 5 6 4 9.68 0.02 

G 1050 15.7 91.4 1.365971 2.023 0.83 4 2 9 9.65 0.03 

H 1450 5.1 68.6 0.447320 2.009 0.05 6 9 9.58 0.13 

Total Gas 100.0 81.3 8.720841 2.009 0.66 9 6 0 9.58 

Plateau Age 56.9 steps E-H 9.68 0.03 

Basalt Mtn., north rim


L-91A1; 0.2598 g, whole rock basalt, #4KD8, J=0.002658±0.25% Ä

A 5 5 0 8.1 23.8 0.785470 1.736 3.57 6 5 8.30 0.14 

B 6 5 0 29.4 50.5 2.860736 1.870 1.19 8 7 8.94 0.05 

C 7 5 0 18.6 83.8 1.813491 2.033 0.49 3 6 1 9.72 0.02 

D 8 0 0 14.2 95.5 1.384557 2.029 0.57 4 4 3 7 9.71 0.03 

E 8 5 0 12.7 96.8 1.236588 2.030 0.63 * * * 9.71 0.04 

F 9 5 0 11.2 95.6 1.090815 2.009 0.60 3 2 4 4 9.61 0.06 

G 1050 4.0 86.9 0.390018 1.836 0.32 4 7 4 8.78 0.09 

H 1450 1.7 55.9 0.164963 1.801 0.02 2 3 8.61 0.30 

Total Gas 100.0 73.4 9.726638 1.946 0.97 1 1 1 3 9.31 

Plateau Age 56.8 steps C-D 9.71 0.03 

L-39; 0.2641 g, whole rock basalt, #1KD8, J=0.002640±0.25% 

A 5 5 0 6.8 18.1 0.686886 1.657 3.20 5 1 7.87 0.25 

B 6 5 0 26.3 44.0 2.642945 1.925 0.99 7 8 9.15 0.05 

C 7 5 0 22.2 76.0 2.225301 2.058 0.50 3 4 9 9.78 0.03 

D 8 0 0 15.6 89.1 1.567815 2.051 0.61 2 6 7 2 9.74 0.03 

E 8 5 0 12.8 91.2 1.285568 2.051 0.69 2 8 6 1 9.74 0.04 

F 9 5 0 10.3 88.1 1.031921 2.053 0.65 1 7 8 9 9.75 0.04 

G 1050 4.3 74.3 0.434034 1.880 0.38 3 8 4 8.93 0.08 

H 1450 1.7 41.1 0.166680 1.899 0.02 2 1 9.02 0.32 

Total Gas 100.0 68.2 10.041150 1.983 0.86 1 0 8 6 9.42 

Plateau Age 60.9 steps C-F 9.75 0.03 

50
10•

1•

* L-176 

ISOCHRON AGE = 9.60 ± 0.12 Ma 

(40Ar/36Ar)

i = 332.68 

± 24.14 

MSWD = 0.087 

POINT ABH DELETED 

0 .001 .002 .003 

Ar/ Ar 39 40 

0 .09 .18 .27 .36 .45

A 

B 

C 

D 

EF

G 

H 

0 

10 

20 

Apparent Age (Ma) K/Ca 

.01 

.1 

ISOCHRON AGE = 9.68 ± 0.06 Ma 

(40Ar/36Ar)

i = 302.86 

± 14.66 

MSWD = 0.45 

POINT ABGH DELETED 

0 .001 .002 .003 

Ar/ Ar 39 40 

* L-91A1 

0 .09 .18 .27 .36 .45 

A 

B 

C 

DEF 

G 

H 

0 

10 

20 

Apparent Age (Ma) 

0 

K/Ca 

.01 

.1 

1 

10 

ISOCHRON AGE = 9.73 ± 0.10 Ma 

(40Ar/36Ar)

i = 300.41 

± 15.32 

MSWD = 0.003 

POINT ABGH DELETED 

0 .001 .002 .003 

Ar/ Ar 39 40 

* L-39 

0 .08 .16 .24 .32 .4 .48 

A 

B 

C 

DEF 

G 

H 

0 

10 

20 

Apparent Age (Ma) 

0 

K/Ca 

.01 

.1 

1 

10 

51 

L-176 

PLATEAU AGE = 

9.68 ± 0.03 Ma 

0 39ArK RELEASED (%) 100•

L-91A1 

PLATEAU AGE = 

9.71 ± 0.03 

39ArK RELEASED (%) 100•

L-39 

PLATEAU AGE = 

9.75 ± 0.03 Ma 

39ArK RELEASED (%) 100 

36 40 Ar/ Ar 36 40 Ar/ Ar 36 40 Ar/ Ar
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

L-181; 0.2445 g, whole rock basalt, #16KD8, J=0.002665±0.15% 

A 5 0 0 3 . 9 27.6 0.018893 2.033 4.15 6 7 9.75 0.04 

B 6 0 0 28.2 73.8 0.135492 1.943 4.17 9 8 9.32 0.01 

C 7 0 0 24.5 83.6 0.117687 1.998 1.83 1 3 2 9.58 0.01 

D 7 5 0 10.1 67.7 0.048502 2.069 1.01 2 4 6 9.92 0.03 

E 8 0 0 10.5 73.5 0.050587 2.094 0.73 5 1 6 10.04 0.03 

F 9 0 0 13.6 77.6 0.065402 2.047 0.50 8 9 3 9.82 0.02 

G 1000 6 . 6 65.1 0.031681 1.994 0.30 4 7 3 9.56 0.02 

H 1150 2 . 0 24.5 0.009827 1.362 0.16 1 1 4 6.54 0.23 

I 1350 0 . 3 4 . 9 0.001626 1.423 0.01 8 6.83 0.38 

Total Gas 100 .0 72.5 0.479697 1.992 2.06 2 9 8 9.55 

No Plateau 

L-187D1; 0.2581 g, whole rock basalt, #11KD8, J=0.002665±0.25% 

A 5 5 0 1 . 0 11.5 0.087052 1.553 2.06 4 1 7.45 0.47 

B 6 5 0 17.4 28.0 1.525676 1.804 1.11 6 3 8.65 0.05 

C 7 5 0 13.3 55.3 1.167446 2.031 0.52 1 6 4 9.74 0.04 

D 8 0 0 12.2 78.1 1.075403 2.073 0.58 8 4 6 9.94 0.11 

E 8 5 0 14.9 86.5 1.304363 2.032 0.64 2 2 9 6 9.74 0.04 

F 9 5 0 23.6 89.7 2.076246 2.014 0.72 3 1 6 7 9.66 0.04 

G 1050 12.8 76.3 1.121344 1.974 0.83 5 8 1 9.46 0.02 

H 1450 4 . 8 52.9 0.421887 1.918 0.05 7 9 9.20 0.09 

Total Gas 100 .0 68.2 8.779417 1.975 0.73 1 3 0 5 9.47 

No Plateau 

L-187B3; 0.2432 g, whole rock basalt, #6KD8, J=0.002656±0.5% 

A 5 5 0 28.6 36.2 2.602897 1.867 3.03 7 8 8.92 0.06 

B 6 5 0 19.8 63.4 1.797337 2.011 1.01 1 1 1 9.61 0.03 

C 7 5 0 16.2 83.8 1.474837 2.076 0.49 3 4 7 9.92 0.03 

D 8 0 0 8 . 9 92.8 0.812950 2.041 0.43 1 6 5 4 9.75 0.04 

E 8 5 0 8 . 8 94.4 0.802412 2.000 0.42 2 2 9 7 9.56 0.04 

F 9 5 0 11.4 94.0 1.037892 1.991 0.33 2 2 8 3 9.51 0.03 

G 1050 4 . 3 82.8 0.389533 1.741 0.21 5 5 8 8.32 0.07 

H 1450 1 . 9 56.1 0.170220 1.726 0.02 2 6 8.25 0.24 

Total Gas 100 .0 68.5 9.088078 1.963 1.27 7 3 7 9.38 

No Plateau 

52
10•

1•

.1•

.01•

.001•

0 

ISOCHRON AGE = 9.75 ± 0.11 Ma 

(40Ar/36Ar)

i = 296.76 

± 7.49 

MSWD = 1.085 

POINT ABGH DELETED 

* L-187D1 

A 

B 

C

D

E 

F

G 

H 

.002 .003 

ISOCHRON AGE = 9.51 ± 0.02 Ma 

(40Ar/36Ar)

i = 298.02 

± 0.49 

MSWD = 85.51 

NO POINTS DELETED 

* L-181 

A 

B 

C

D 

E 

F 

G 

H 

I 

Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca 

L-181 

NO PLATEAU 

0•

0 39ArK RELEASED (%) 100•

10•

1•

.1•

.01•

20•

10•

0•

0 39ArK RELEASED (%) 100•

L-187D1 

NO PLATEAU 

10•

1•

L-187B3 

NO PLATEAU 

36 40 Ar/ Ar

20•

10•

0 .001 0 .08 .16 .24 .32 .4•

39 40• Ar/ Ar

0 .08 .16 .24 .32 .4 .48•

39 40• Ar/ Ar

ISOCHRON AGE = 9.45 ± 0.09 Ma 

(40Ar/36Ar)

i = 370.37 

± 17.48 

MSWD = 1.94 

POINT ABGH DELETED 

* L-187B3 

A 

B 

C 

D 

EF 

G 

H 

.002 .003 0 .001 .002 .003 

36 40 Ar/ Ar

.1•

.01•

20•

10•

0• 0 .09 .18 .27 .36 .45 0 39ArK RELEASED (%) 100•

39 40• Ar/ Ar

53

36 40 Ar/ Ar

.001 
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

L-187A2; 0.2278 g, whole rock basalt, #3KD8, J=0.002635±0.25% 

A 5 5 0 3 . 9 19.1 0.244405 1.860 3.86 6 7 8.82 0.34 

B 6 5 0 30.2 32.5 1.887900 1.921 1.69 8 7 9.11 0.07 

C 7 5 0 15.9 63.5 0.993490 2.140 0.48 2 2 3 10.14 0.06 

D 8 0 0 13.1 93.0 0.819212 2.091 0.43 4 3 1 9 9.91 0.04 

E 8 5 0 14.9 96.3 0.929284 2.088 0.49 * * * 9.90 0.03 

F 9 5 0 0 . 0 0 . 0 1.049300 2.064 0.48 * * * 9.79 0.03 

G 1050 0 . 0 0 . 0 0.236290 1.864 0.29 * * * 8.84 0.15 

H 1450 0 . 0 0 . 0 0.084314 1.715 0.02 * * * 8.14 0.44 

Total Gas 100 .0 0 . 0 6.244195 2.020 0.00 * * * 9.58 

No Plateau 

Basalt Mtn., south


L-38A; 0.2272 g, whole rock basalt, #2KD8, J=0.002640±0.5% 


A 5 5 0 4 . 0 3 . 1 0.463025 0.601 1.11 6 3 2.86 0.28 

B 6 5 0 3 . 5 86.8 0.409491 1.957 0.89 4 6 2 9.30 0.10 

C 7 5 0 13.0 96.5 1.503794 2.220 0.98 1 3 7 2 10.54 0.03 

D 8 0 0 15.0 98.6 1.733807 2.161 1.12 1 5 0 6 10.26 0.03 

E 8 5 0 15.7 98.6 1.822295 2.092 1.38 1 0 9 7 9.94 0.03 

F 9 5 0 21.2 98.3 2.451300 2.061 1.57 5 5 3 9.79 0.02 

G 1050 15.1 96.3 1.745173 2.059 1.74 1 7 6 9.78 0.02 

H 1450 10.5 88.5 1.217298 2.024 0.38 1 0 2 9.61 0.04 

I 1750 2 . 0 61.1 0.236643 1.937 0.43 1 5 2 9.20 0.18 

Total Gas 100 .0 91.9 11.582826 2.033 1.23 7 5 2 9.66 

No Plateau 

K97-8-11E1; 0.2570 g, whole rock basalt, #15KD6, J=0.002023±0.50% 

A 6 5 0 19.5 18.2 1.235399 2.536 2.77 4 4 9.23 0.15 

B 7 5 0 18.4 35.0 1.166189 2.953 0.95 7 8 10.75 0.16 

C 8 0 0 9 . 1 64.2 0.578709 2.915 0.66 2 5 3 10.61 0.20 

D 8 5 0 10.2 88.7 0.650700 2.847 0.66 1 4 1 6 10.36 0.15 

E 9 5 0 24.9 93.2 1.583207 2.701 0.70 6 2 9 7 9.83 0.06 

F 1050 13.5 91.6 0.856463 2.815 0.57 1 4 9 6 10.24 0.14 

G 1450 4 . 4 51.0 0.278126 2.623 0.05 6 3 9.55 0.34 

Total Gas 100 .0 62.8 6.348793 2.762 1.09 1 9 6 6 10.05 

No Plateau 

54
10•

1•

.1•

.01 

20•

10•

0 

0 

ISOCHRON AGE = 9.84 ± 0.07 Ma 

(40Ar/36Ar)i = 311.32 ± 8.81 

MSWD = 0.364 

POINT ABGH DELETED 

* L-187A2 

A 

B 

C 

D EF 

G 

H 

ISOCHRON AGE = 9.93 ± 0.06 Ma 

(40Ar/36Ar)i = 271.81 ± 0.88 

MSWD = 12.201 

NO POINTS DELETED 

* A L-38A 

B 

C DEF G 

H

I 

Apparent Age (Ma) K/Ca 

L-187A2 

NO PLATEAU 

.002 .003 0 .001 .002 .003 0 .001 .002 .003 

36 40 Ar/ Ar 36 40 Ar/ Ar

0 .08 .16 .24 .32 .4 .48• 39ArK 0 RELEASED (%) 100•

10•

39 40• Ar/ Ar

0 .09 .18 .27 .36 .45•

39 40• Ar/ Ar

ISOCHRON AGE = 10.07 ± 0.17 Ma 

(40Ar/36Ar)i = 313.21 ± 13.81 

MSWD = 2.208 

POINTS AB DELETED 

* K97-8-11E1 

A 

B 

C 

D 

E F 

G 

L-38A 

NO PLATEAU 

K/Ca 

1•

.1•

20•

10•

0 

Apparent Age (Ma) K/Ca 

39ArK RELEASED (%) 100 

K98-8-11E1 

NO PLATEAU 

0 39ArK RELEASED (%) 100•

0 

10•

1•

.1•

.01•

20•

10•

0•

0 .06 .12 .18 .24 .3 .36•

39 40• Ar/ Ar

55

Apparent Age (Ma) 

36 40 Ar/ Ar

.001 
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

K97-8-11E2; 0.2450 g, whole rock basalt, #17KD6, J=0.002024±0.50% 

A 6 5 0 19.0 17.6 1.135353 2.593 3.14 4 4 9.44 0.16 

B 7 5 0 16.3 30.4 0.973193 2.864 1.08 6 7 10.43 0.13 

C 8 0 0 11.2 59.3 0.669607 2.966 0.68 2 0 2 10.80 0.05 

D 8 5 0 12.4 88.9 0.742195 2.869 0.67 1 3 3 6 10.45 0.04 

E 9 5 0 24.6 92.6 1.470672 2.670 0.70 4 7 3 2 9.72 0.02 

F 1050 12.1 90.6 0.722087 2.787 0.56 1 4 0 4 10.15 0.06 

G 1450 4 . 4 49.7 0.261131 2.537 0.05 5 3 9.24 0.21 

Total Gas 100 .0 61.9 5.974238 2.753 1.17 1 5 4 5 10.03 

No Plateau 

K97-8-11D3; 0.2440 g, whole rock basalt, #13KD6, J=0.002021±0.50% 

A 6 5 0 9 . 5 22.8 0.522912 2.843 0.94 5 5 10.33 0.32 

B 7 5 0 12.0 55.3 0.657775 3.583 0.44 2 1 9 13.02 0.40 

C 8 0 0 10.1 81.6 0.556511 3.678 0.45 8 6 7 6 13.36 0.45 

D 8 5 0 12.1 95.6 0.662778 3.449 0.54 * * * 12.53 0.15 

E 9 5 0 27.8 100.0 1.526195 3.196 0.65 * * * 11.62 0.07 

F 1050 17.7 80.0 0.970082 2.700 0.92 5 7 2 9.82 0.23 

G 1450 10.8 53.4 0.591863 2.694 0.12 8 3 9.80 0.16 

Total Gas 100 .0 76.3 5.488116 3.134 0.61 1 0 2 1 11.39 

No Plateau 

K97-8-11C1; 0.2492 g, whole rock basalt, #8KD6, J=0.001977±0.50% 

A 5 5 0 15.5 18.4 0.827589 2.731 3.08 4 7 9.71 0.15 

B 6 5 0 15.7 28.7 0.837340 2.740 1.16 6 2 9.75 0.19 

C 7 5 0 16.7 58.0 0.892867 2.992 0.51 1 7 1 10.64 0.12 

D 8 0 0 11.4 90.9 0.605481 2.889 0.50 1 7 9 7 10.28 0.18 

E 8 5 0 14.2 98.9 0.757499 2.879 0.59 24783 10.24 0.15 

F 9 5 0 20.3 97.9 1.079982 2.847 0.55 9 4 6 9 10.13 0.09 

G 1050 6 . 2 91.1 0.331143 2.913 0.32 8 4 8 10.36 0.40 

Total Gas 100 .0 66.9 5.331901 2.850 1.02 5 7 4 1 10.14 

Plateau Age 52.0 steps D-G 10.18 0.06 

56
10 

ISOCHRON AGE = 10.20 ± 0.07 Ma 

(40Ar/36Ar)

i = 293.77 

± 1.05 

MSWD = 25.005 

NO POINTS DELETED 

* K97-8-11E2 

A 

B 

C 

D 

E F 

G 

1 

.1 

.01 

20 

10 

0 

0 .06 .12 .18 .24 .3 .36 0 39ArK RELEASED (%) 

K97-8-11E2 

NO PLATEAU 

39 40 Ar/ Ar

Apparent Age (Ma) Apparent Age (Ma) 

.003

K/Ca 

.003 0 .001 .002 .003 0 .001 .002 

36 40 Ar/ Ar 36 40 Ar/ Ar

1 

.1 

20 

10 ISOCHRON AGE = 11.87 ± 0.22 Ma (40Ar/36Ar)i = 285.09 ± 3.37 

MSWD = 9.903 

NO POINTS DELETED 

* K97-8-11D3 

A 

B 

C 

D 

E 

F 

G 

K97-8-11D3 

NO PLATEAU 

K/Ca 

0 

0 39ArK 0 .06 .12 .18 .24 .3 RELEASED (%)

39 40 Ar/ Ar

ISOCHRON AGE = 10.20 ± 0.16 Ma 

(40Ar/36Ar)

i = 312.58 

± 9.16 

MSWD = .053 

POINTS AB DELETED 

* K97-8-11C1 

A 

B 

C 

D 

EF 

G 

57 

10 

K98-8-11C1 

PLATEAU AGE = 

10.18 ± 0.06 Ma 

K/Ca 

1 

.1 

.001 .002 

Apparent Age (Ma) 

36 40 Ar/ Ar

20 

10 

0 

0 0 .06 .12 .18 .24 .3 .36 0 39ArK RELEASED (%) 100 

39 40 Ar/ Ar

100

100
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

K97-8-11C3; 0.2546 g, whole rock basalt, #10KD6, J=0.001999±0.50% 

A 6 5 0 32.3 29.8 1.894854 3.313 1.60 6 1 11.91 0.13 

B 7 5 0 20.7 57.5 1.213724 3.108 0.52 1 5 8 11.17 0.33 

C 8 0 0 13.4 89.7 0.785655 2.822 0.51 1 3 9 0 10.15 0.12 

D 8 5 0 13.0 98.6 0.766062 2.849 0.56 6 8 6 5 10.24 0.08 

E 9 5 0 15.5 99.8 0.908041 2.764 0.54 5 7 6 2 9.94 0.09 

F 1050 5 . 2 98.2 0.306110 2.878 0.32 7 5 3 10.35 0.35 

Total Gas 100 .0 66.9 5.874446 3.037 0.87 2 0 6 3 10.92 

No Plateau 

K97-8-11B; 0.2590 g, whole rock basalt, #6KD6, J=0.002001±0.50% 

A 6 5 0 24.3 25.8 1.306978 2.708 2.45 6 0 9.75 0.11 

B 7 5 0 18.3 43.3 0.984700 2.819 0.76 9 6 10.15 0.23 

C 8 0 0 9 . 4 73.9 0.505664 2.982 0.45 2 9 9 10.73 0.17 

D 8 5 0 11.8 93.7 0.635105 2.866 0.43 1 6 0 0 10.32 0.12 

E 9 5 0 21.0 89.6 1.133648 2.571 0.47 4 1 5 1 9.26 0.30 

F 1050 9 . 6 95.4 0.517095 2.767 0.38 1 5 9 7 9.96 0.07 

G 1450 5 . 7 66.4 0.305114 2.574 0.05 7 0 9.27 0.41 

Total Gas 100 .0 63.9 5.388304 2.742 0.96 1 2 7 9 9.87 

No Plateau 

K97-8-11A1; 0.2515 g, whole rock basalt, #4KD6, J=0.002001±0.50% 

A 6 5 0 17.7 18.1 0.846422 2.811 1.40 4 9 10.12 0.58 

B 7 5 0 12.1 44.4 0.580772 3.095 0.47 1 3 3 11.14 0.25 

C 8 0 0 8 . 4 83.5 0.400050 2.905 0.43 8 6 4 10.46 0.22 

D 8 5 0 13.6 97.3 0.653255 2.828 0.53 15033 10.18 0.21 

E 9 5 0 30.1 96.2 1.439893 2.727 0.58 13971 9.82 0.07 

F 1050 13.0 94.9 0.622533 2.813 0.36 1 8 2 7 10.13 0.11 

G 1450 5 . 1 54.7 0.243760 2.516 0.04 5 1 9.06 0.31 

Total Gas 100 .0 72.9 4.786685 2.816 0.64 6 5 9 1 10.14 

No Plateau 

58
0 

ISOCHRON AGE = 10.16 ± 0.14 Ma 

(40Ar/36Ar)

i = 314.24 

± 2.03 

MSWD = 0.796 

NO POINTS DELETED 

* K97-8-11C3 

A 

B 

C 

D E F 

ISOCHRON AGE = 10.26 ± 0.14 Ma 

(40Ar/36Ar)

i = 290.43 

± 1.86 

MSWD = 3.441 

NO POINTS DELETED 

* K97-8-11B 

A 

B 

C 

D 

E 

F 

G 

Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca 

10•

1•

.1•

20•

10•

0•

0 39ArK RELEASED (%)•

K97-8-11C3 

NO PLATEAU 

.002 .003 0 .001 .002 .003 0 .001 .002 .003 

36 40 Ar/ Ar 36 40 Ar/ Ar

0 .07 .14 .21 .28 .35•

39 40• Ar/ Ar

0 .06 .12 .18 .24 .3 .36•

39 40• Ar/ Ar

* 

0 .06 .12 .18 .24 .3 .36•

39 40• Ar/ Ar

ISOCHRON AGE = 10.47 ± 0.21 Ma 

(40Ar/36Ar)

i = 296.52 

± 3.99 

MSWD = 2.255 

POINTS EFG DELETED 

K97-8-11A1 

A 

B 

C 

DE F 

G 

K97-8-11B 

NO PLATEAU 

0 39ArK RELEASED (%) 100•

K97-8-11A1 

NO PLATEAU 

0 39ArK RELEASED (%) 100 

10•

1•

.1•

.01•

20•

10•

0 

10•

1•

.1•

.01•

20•

10•

0 

36 40 Ar/ Ar

.001 

59

100
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

K97-8-11A2; 0.2432 g, whole rock basalt, #2KD6, J=0.001999±0.50% 

A 5 5 0 8 . 0 15.4 0.356230 2.944 2.70 4 2 10.59 0.48 

B 6 5 0 12.3 23.6 0.545835 2.825 1.30 5 9 10.16 0.50 

C 7 5 0 14.5 40.2 0.643585 3.016 0.54 1 0 7 10.84 0.24 

D 8 0 0 9 . 8 76.2 0.435460 3.027 0.41 4 6 8 10.88 0.32 

E 8 5 0 12.8 94.8 0.570681 2.885 0.46 8 3 2 5 10.38 0.11 

F 9 5 0 26.4 91.6 1.175535 2.576 0.52 5 3 6 1 9.27 0.10 

G 1050 11.6 92.2 0.516625 2.824 0.36 1 2 1 4 10.16 0.10 

H 1450 4 . 6 39.8 0.204560 2.646 0.03 4 4 9.52 0.28 

Total Gas 100 .0 66.3 4.448511 2.816 0.73 2 7 0 0 10.13 

Plateau Age 57.4 steps A-E 10.49 0.07 

Grand Mesa, Mesa Lakes


K98-8-18F2; 0.2569 g, whole rock basalt, #93KD12, J=0.002023±0.25% 


A 6 0 0 20.1 28.3 0.288291 2.345 2.79 1 0 2 8.54 0.08 

B 6 5 0 9 . 9 39.7 0.141767 2.387 1.69 1 1 5 8.69 0.09 

C 7 0 0 9 . 8 47.3 0.140905 2.472 1.00 1 2 9 9.00 0.09 

D 7 5 0 9 . 8 55.9 0.140218 2.562 0.56 1 5 1 9.33 0.15 

E 8 0 0 10.2 67.9 0.146449 2.624 0.33 2 1 5 9.55 0.08 

F 8 5 0 10.1 80.3 0.144816 2.604 0.25 4 7 1 9.48 0.10 

G 1150 30.1 89.4 0.432053 2.552 0.22 4 0 1 9.29 0.05 

Total Gas 100 .0 61.7 1.434499 2.500 1.01 2 5 0 9.10 

No Plateau 

K98-8-18F3; 0.2516 g, whole rock basalt, #95KD12, J=0.002121±0.25% 

A 6 0 0 5 . 1 19.5 0.073683 2.315 3.34 7 8 8.84 0.34 

B 6 5 0 3 . 5 32.2 0.050514 2.433 4.10 1 0 8 9.29 0.27 

C 7 0 0 2 . 6 38.3 0.038105 2.412 3.82 1 0 7 9.20 0.24 

D 7 5 0 3 . 2 40.3 0.046222 2.285 2.87 1 1 7 8.72 0.19 

E 8 0 0 4 . 4 46.3 0.063999 2.404 1.97 1 2 0 9.18 0.21 

F 8 5 0 6 . 2 52.2 0.089043 2.481 1.28 1 3 2 9.47 0.22 

G 1150 59.0 74.0 0.853811 2.583 0.36 2 7 2 9.86 0.02 

H 1450 16.0 84.7 0.231094 2.462 0.06 1 8 9 9.40 0.23 

Total Gas 100 .0 66.9 1.446471 2.516 0.90 2 1 8 9.60 

Plateau Age 65.2 steps F-G 9.85 0.03 

60
ISOCHRON AGE = 9.38 

± 0.16 Ma 

(40Ar/36Ar)

i = 297.56 

± 10.99 

MSWD = 0.787 

POINT ABC DELETED 

0 .001 .002 .003 

Ar/ Ar 39 40 

* K98-8-18F2 

0 .06 .12 .18 .24 .3 .36 

A 

B 

C 

D 

E 

F 

G 

0 

10 

20 

Apparent Age (Ma) 

0 

K/Ca 

.1 

1 

10 

ISOCHRON AGE = 9.60 ± 0.38 Ma 

(40Ar/36Ar)

i = 296.01 

± 23.47 

MSWD = 1.062 

POINT ABCDE DELETED 

0 .001 .002 .003 

Ar/ Ar 39 40 

* K98-8-18F3 

0 .06 .12 .18 .24 .3 .36 

A 

B 

C D 

E 

F 

G 

H 

0 

10 

20 

Apparent Age (Ma) 

0 

K/Ca 

.01 

.1 

1 

10 

0 

10 

20 

Apparent Age (Ma) K/Ca 

.1 

1 

10 

.01 

0 .001 .002 .003 

Ar/ Ar 39 40 

ISOCHRON AGE = 10.60 ± 0.24 Ma 

(40Ar/36Ar)

i = 295.20 

± 2.71 

MSWD = .619 

POINTS FGH DELETED 

* K97-8-11A2 

0 .07 .14 .21 .28 .35

A 

B 

C 

D 

E G 

F

H 

61 

K97-8-11A2 

PLATEAU AGE = 

10.49 ± 0.07 Ma 

36 40 Ar/ Ar 36 40 Ar/ Ar 36 40 Ar/ Ar

0 39ArK RELEASED (%) 100 

K98-8-18F2 

NO PLATEAU 

39ArK RELEASED (%) 100•

K98-8-18F3 

PLATEAU AGE = 

9.85 ± 0.03 Ma 

39ArK RELEASED (%) 100•
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

K98-8-18E4; 0.2590 g, whole rock basalt, #89KD12, J=0.002022±0.25% 

A 5 5 0 1 . 1 9 . 7 0.024286 2.192 3.21 4 1 7.98 0.49 

B 6 0 0 15.9 23.1 0.348598 2.345 2.76 6 9 8.54 0.05 

C 6 5 0 10.6 28.9 0.232852 2.465 1.57 7 8 8.97 0.11 

D 7 0 0 8 . 9 35.9 0.195927 2.608 0.95 9 6 9.49 0.09 

E 7 5 0 9 . 4 48.2 0.205911 2.794 0.62 1 4 0 10.17 0.12 

F 8 0 0 10.5 63.6 0.230500 2.817 0.47 2 6 9 10.25 0.10 

G 8 5 0 10.5 76.0 0.230435 2.766 0.40 5 2 1 10.06 0.09 

H 1150 28.8 79.2 0.630721 2.675 0.36 2 6 2 9.73 0.06 

I 1450 4 . 2 61.7 0.093026 2.732 0.03 1 5 8 9.94 0.68 

Total Gas 100 .0 54.7 2.192256 2.627 0.98 2 0 7 9.56 

No Plateau 

K98-8-18E5; 0.2551 g, whole rock basalt, #91KD12, J=0.002021±0.25% 

A 5 5 0 4 . 1 15.6 0.078422 2.330 2.39 5 5 8.48 0.40 

B 6 0 0 6 . 5 25.3 0.123675 2.334 1.97 7 1 8.49 0.08 

C 6 5 0 6 . 5 29.9 0.124666 2.402 1.35 7 7 8.74 0.08 

D 7 0 0 6 . 2 37.6 0.118441 2.646 0.95 9 2 9.62 0.13 

E 7 5 0 7 . 5 46.1 0.143220 2.688 0.65 1 2 4 9.77 0.16 

F 8 0 0 8 . 5 59.7 0.163147 2.846 0.48 1 9 7 10.35 0.06 

G 8 5 0 9 . 6 69.1 0.184812 2.777 0.39 3 2 4 10.10 0.07 

H 1150 38.7 76.1 0.741925 2.669 0.34 1 8 7 9.71 0.02 

I 1450 12.5 74.2 0.238965 2.711 0.08 1 5 7 9.86 0.18 

Total Gas 100 .0 60.4 1.917273 2.647 0.64 1 6 6 9.63 

Plateau Age 51.2 steps H-I 9.71 0.03 

K98-8-18D1; 0.2485 g, whole rock basalt, #85KD12, J=0.001999±0.25% 

A 6 5 0 31.3 24.7 0.719700 2.432 2.65 9 9 8.75 0.04 

B 7 5 0 21.5 31.4 0.493604 2.573 1.28 1 1 4 9.26 0.07 

C 8 0 0 16.3 37.8 0.374236 2.681 0.78 1 3 9 9.64 0.09 

D 8 5 0 12.7 45.0 0.292649 2.710 0.49 1 8 1 9.75 0.08 

E 9 5 0 11.9 53.7 0.273742 2.719 0.25 2 3 2 9.78 0.08 

F 1050 3 . 7 53.1 0.085862 2.460 0.16 1 7 7 8.85 0.29 

G 1150 2 . 5 51.0 0.056772 2.380 0.09 4 3 8.56 0.33 

Total Gas 100 .0 36.0 2.296565 2.572 1.33 1 3 7 9.25 

No Plateau 

62
10 

1 

.1 

.01 

ISOCHRON AGE = 9.81 ± 0.19 Ma 

(40Ar/36Ar)

i = 307.97 

± 9.11 

MSWD = 0.882 

POINT ABCD DELETED 

* K98-8-18E4 

A 

B 

C 

D 

E 

F 

G H 

I 

39 40 Ar/ Ar

10 

1 

.1 

.01 

20 

10 0 0 .05 .1 .15 .2 .25 .3 

39 40 Ar/ Ar

10 

1 

.1 

.01 

20 

10 

0 0 .04 .08 .12 .16 .2 

39 40 Ar/ Ar

63 

ISOCHRON AGE = 9.95 ± 0.28 Ma 

(40Ar/36Ar)

i = 289.88 

± 6.54 

MSWD = 0.054 

POINT ABFG DELETED 

* K98-8-18D1 

A 

B 

C 

D 

E 

F

G 

K98-8-18E4 

NO PLATEAU 

39ArK RELEASED (%) 

K98-8-18E5 

PLATEAU AGE = 

9.71 ± 0.03 Ma 

0 39ArK RELEASED (%) 100 

K98-8-18D1 

NO PLATEAU 

0 39ArK RELEASED (%) 100 

.002 .003 0 .001 .002 .003 0 .001 .002 .003 0 

ISOCHRON AGE = 9.81 ± 0.21 Ma 

(40Ar/36Ar)

i = 303.00 

± 10.28 

MSWD = 2.019 

POINT ABCD DELETED 

* K98-8-18E5 

A 

B 

C 

D 

E 

F 

G 

H I 

Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca 

36 40 Ar/ Ar 36 40 Ar/ Ar

20 

10 0 0 .05 .1 .15 .2 .25 .3 0 

36 40 Ar/ Ar

.001 

100
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

K98-8-18C1; 0.2530 g, whole rock basalt, #81KD12, J=0.002000±0.25% 

A 5 5 0 4.2 13.2 0.120454 2.108 3.53 6 6 7.59 0.18 

B 6 0 0 7.2 20.0 0.207557 2.257 3.97 8 4 8.12 0.15 

C 6 5 0 8.6 24.6 0.247156 2.375 2.98 9 1 8.55 0.13 

D 7 0 0 9.3 28.9 0.266688 2.417 2.02 9 8 8.70 0.13 

E 7 5 0 10.3 35.8 0.294172 2.577 1.22 1 1 6 9.28 0.09 

F 8 0 0 11.2 46.5 0.320273 2.723 0.75 1 5 5 9.80 0.09 

G 8 5 0 12.5 62.4 0.357067 2.786 0.51 2 7 7 10.03 0.07 

H 1000 32.1 85.0 0.918125 2.768 0.32 1 0 5 4 9.96 0.04 

I 1150 4.6 64.4 0.131963 2.283 0.13 7 6 8.22 0.28 

Total Gas 100.0 53.7 2.863455 2.592 1.26 4 3 1 9.33 

No Plateau 

K98-8-18C2; 0.2498 g, whole rock basalt, #83KD12, J=0.001997±0.25% 

A 5 5 0 9.0 20.9 0.245487 2.345 2.78 8 4 8.39 0.12 

B 6 5 0 26.1 33.1 0.708935 2.482 1.79 1 0 7 8.88 0.03 

C 7 5 0 22.5 49.5 0.610951 2.687 0.85 1 5 0 9.62 0.06 

D 8 0 0 14.2 65.6 0.386791 2.751 0.51 2 5 1 9.84 0.07 

E 8 5 0 11.2 77.9 0.305726 2.764 0.35 4 6 3 9.89 0.08 

F 9 5 0 10.8 84.4 0.293053 2.756 0.24 5 9 0 9.86 0.11 

G 1050 3.6 72.1 0.097246 2.501 0.21 1 7 4 8.95 0.12 

H 1150 2.6 55.9 0.071658 2.198 0.09 4 0 7.87 0.35 

Total Gas 100.0 52.9 2.719847 2.608 1.06 2 2 8 9.34 

No Plateau 

K98-8-18B1; 0.2454 g, whole rock basalt, #77KD12, J=0.001999±0.25% 

A 6 0 0 4.6 64.4 0.101670 2.853 0.50 2 3 2 10.26 0.22 

B 7 5 0 1.0 99.7 0.021795 3.186 0.55 6 2 7 11.45 0.92 

C 8 0 0 3.6 90.5 0.079525 2.766 0.58 7 3 5 9.95 0.24 

D 8 5 0 9.1 95.7 0.201454 2.824 0.64 9 5 2 10.15 0.12 

E 1150 71.5 93.5 1.588516 2.767 0.93 4 3 4 9.95 0.02 

F 1450 10.3 56.9 0.228623 2.600 0.05 1 3 4 9.35 0.13 

Total Gas 100.0 88.6 2.221583 2.763 0.78 4 5 4 9.94 

Plateau Age 89.7 steps A-E 9.96 0.04 

64
10 

1 

.1 

ISOCHRON AGE = 10.01 ± 0.15 Ma 

(40Ar/36Ar)

i = 291.41 

± 7.01 

MSWD = 0.501 

POINTS ABCDEI DELETED 

* K98-8-18C1 

A 

B 

C 

D 

E 

F 

G 

H 

I 

K98-8-18C1 

NO PLATEAU 

.002 .003 0 .001 .002 .003 0 .001 .002 .003 0 

ISOCHRON AGE = 9.91 ± 0.19 Ma 

(40Ar/36Ar)

i = 292.12 

± 13.87 

MSWD = 0.049 

POINT ABCGHI DELETED 

* K98-8-18C2 

A 

B 

C 

D 

E 

F 

G 

H 

Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca 

36 40 Ar/ Ar 36 40 Ar/ Ar

20 

10 

0 0 .06 .12 .18 .24 .3 0 39ArK RELEASED (%) 100 

39 40 Ar/ Ar

10 

1 

.1 

.01 

20 

10 

0 0 .06 .12 .18 .24 .3 0 39ArK RELEASED (%) 100 

39 40 

K98-8-18C2 

NO PLATEAU 

Ar/ Ar

ISOCHRON AGE = 10.35 ± 0.5 Ma 

(40Ar/36Ar)

i = 290.09 

± 28.74 

MSWD = 0.566 

NO POINTS DELETED 

* K98-8-18B1 

A 

B 

C

D E

F 

1 

.1 

.01 

20 

10 

0 0 .06 .12 .18 .24 .3 .36 0 39ArK RELEASED (%) 100 

39 40 

K98-8-18B1 

PLATEAU AGE = 

9.96 ± 0.04 Ma 

Ar/ Ar

65 

36 40 Ar/ Ar

.001 
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

K98-8-18B2; 0.2426 g, whole rock basalt, #79KD12, J=0.001999±0.25% 

A 7 0 0 1.8 87.8 0.036969 2.933 0.48 4 6 8 10.55 0.26 

B 7 5 0 3.9 91.2 0.079864 2.815 0.55 8 1 3 10.12 0.15 

C 8 0 0 8.1 95.2 0.165294 2.824 0.61 1 0 9 3 10.16 0.09 

D 8 5 0 12.4 95.7 0.253386 2.760 0.70 1 4 6 3 9.93 0.07 

E 1150 65.4 94.5 1.338771 2.767 0.92 3 5 5 9.95 0.01 

F 1450 8.5 57.6 0.173436 2.678 0.04 1 4 0 9.63 0.14 

Total Gas 100.0 91.3 2.047720 2.768 0.77 5 5 3 9.96 

Plateau Age 77.8 steps D-E 9.95 0.04 

K98-8-18A3; 0.2444 g, whole rock basalt, #73KD12, J=0.001966±0.25% 

A 6 0 0 26.4 23.9 0.379421 2.580 2.60 1 0 3 9.13 0.14 

B 6 5 0 10.7 25.7 0.153799 2.507 2.02 1 0 7 8.87 0.16 

C 7 0 0 11.2 27.8 0.160656 2.540 1.30 1 1 4 8.99 0.19 

D 7 5 0 12.3 31.5 0.177127 2.679 0.77 1 1 8 9.48 0.21 

E 8 0 0 12.2 34.7 0.175132 2.682 0.43 1 2 8 9.49 0.08 

F 8 5 0 10.6 41.4 0.151986 2.805 0.24 1 4 4 9.92 0.18 

G 1000 14.1 54.5 0.203505 2.699 0.09 2 4 1 9.55 0.22 

H 1150 2.6 43.3 0.037822 2.140 0.04 2 3 7.58 0.36 

Total Gas 100.0 33.4 1.439448 2.621 1.23 1 3 1 9.27 

No Plateau 

K98-8-18A4; 0.2522 g, whole rock basalt, #75KD12, J=0.001965±0.25% 

A 650 50.0 30.9 0.557246 2.556 1.32 112 9.04 0.04 

B 750 22.8 35.9 0.253710 2.671 0.48 125 9.44 0.06 

C 800 11.9 42.0 0.132701 2.635 0.25 149 9.32 0.12 

D 850 7.2 52.6 0.080574 2.685 0.14 197 9.49 0.24 

E 950 6.0 65.8 0.066510 2.570 0.08 354 9.09 0.22 

F 1050 2.2 51.4 0.024322 2.010 0.07 193 7.11 0.98 

Total Gas 100.0 37.5 1.115063 2.590 0.82 142 9.16 

No Plateau 

66
1• * K98-8-18B2 

ISOCHRON AGE = 9.41 ± 0.30 Ma 

(40Ar/36Ar)

i = 292.25 

± 3.71 

MSWD = 3.636 

NO POINTS DELETED 

0 .001 .002 .003 

Ar/ Ar 39 40 

* K98-8-18A3 

0 .04 .08 .12 .16 .2 

A B C D E 

F 

G 

H 

0 

10 

20 

Apparent Age (Ma) 

0 100 39Ar K 

K/Ca 

.01 

.1 

1 

10 

K98-8-18A3 

NO PLATEAU 

ISOCHRON AGE = 9.80 ± 0.67 Ma 

(40Ar/36Ar)

i = 410.01 

± 298.29 

MSWD = 0.110 

POINTS AF DELETED 

0 .001 .002 .003 

Ar/ Ar 39 40 

0 .06 .12 .18 .24 .3 .36

A B 

C

D

E 

F 

0 

10 

20 

Apparent Age (Ma) 

0 100 39Ar K 

K/Ca 

K98-8-18B2 

PLATEAU AGE = 

9.95 ± 0.04 Ma 

.01 

.1 

0 

10 

20 

Apparent Age (Ma) 

0 100 39Ar K 

K/Ca 

.01 

.1 

1 

10 

K98-8-18A4 

NO PLATEAU 

ISOCHRON AGE = 9.10 ± 0.30 Ma 

(40Ar/36Ar)

i = 301.42 

± 5.80 

MSWD = 0.450 

POINTS AF DELETED 

0 .001 .002 .003 

Ar/ Ar 39 40 

* K98-8-18A4 

A 

B 

C 

D 

E 

F 

0 .05 .1 .15 .2 .25 

67 

RELEASED (%)

RELEASED (%)

RELEASED (%)

36 40 Ar/ Ar 36 40 Ar/ Ar 36 40 Ar/ Ar
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

K98-8-17B6; 0.2524 g, whole rock basalt, #71KD12, J=0.001967±0.25% 

A 7 0 0 1 . 9 87.8 0.041287 2.922 0.48 5 0 1 10.34 0.34 

B 7 5 0 3 . 9 91.9 0.085089 2.861 0.55 7 2 0 10.13 0.20 

C 8 0 0 7 . 6 97.2 0.166968 2.919 0.59 1 2 8 0 10.33 0.09 

D 8 5 0 11.5 98.6 0.251892 2.886 0.65 2 3 4 0 10.21 0.05 

E 1150 69.6 96.7 1.529098 2.819 1.10 9 3 8 9.98 0.02 

F 1450 5 . 6 54.5 0.122355 2.683 0.03 9 2 9.50 0.50 

Total Gas 100 .0 94.2 2.196689 2.830 0.91 1 0 6 1 10.02 

Plateau Age 75.2 steps E-F 9.97 0.04 

K98-8-17A4; 0.2488 g, whole rock basalt, #67KD12, J=0.001967±0.25% 

A 6 5 0 52.8 38.1 0.790993 2.558 2.55 1 1 5 9.06 0.07 

B 7 5 0 22.6 53.5 0.338339 2.711 0.77 1 4 5 9.60 0.09 

C 8 0 0 10.5 65.1 0.157142 2.819 0.34 1 8 9 9.97 0.08 

D 8 5 0 6 . 1 76.3 0.090751 2.846 0.17 2 9 1 10.07 0.26 

E 9 5 0 5 . 6 81.0 0.083766 2.700 0.10 4 6 5 9.56 0.36 

F 1050 2 . 6 71.3 0.038252 2.570 0.09 2 4 3 9.10 0.46 

Total Gas 100 .0 49.9 1.499243 2.646 1.57 1 6 3 9.36 

Plateau Age 1 0 0 . 0 steps A-F 9.49 0.03 

Grand Mesa, Lands End


K98-8-18I1; 0.2529 g, whole rock basalt, #105KD12, J=0.002037±0.25% 


A 6 5 0 5 . 5 27.5 0.084735 2.213 0.48 1 1 1 8.11 0.17 

B 7 5 0 8 . 1 84.7 0.124004 3.002 0.37 8 7 6 11.00 0.14 

C 8 0 0 9 . 4 92.8 0.144004 2.950 0.39 2 6 7 7 10.81 0.19 

D 8 5 0 12.7 93.2 0.194933 2.828 0.43 4 1 3 2 10.36 0.12 

E 9 5 0 26.2 95.0 0.400871 2.867 0.48 1 6 0 6 10.51 0.04 

F 1050 19.2 91.2 0.294493 2.824 0.63 5 5 7 10.35 0.04 

G 1150 12.4 83.4 0.189604 2.830 0.55 1 1 2 10.37 0.14 

H 1450 6 . 4 33.0 0.098575 2.723 0.05 1 1 4 9.98 0.38 

Total Gas 100 .0 83.8 1.531219 2.823 0.47 1 4 0 4 10.34 

No Plateau 

68
10 

1 

.1 

.01 

ISOCHRON AGE = 10.09 ± 0.32 Ma 

(40Ar/36Ar)

i = 343.82 

± 116.83 

MSWD = 0.157 

POINT F DELETED 

* K98-8-17B6 

A 

B 

C

D

E 

F K98-8-17B6 

PLATEAU AGE = 

9.97 ± 0.04 Ma 

.002 .003 0 .001 .002 .003 

Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca 

36 40 Ar/ Ar

20 

10 

0 0 .06 .12 .18 .24 .3 .36 0 39ArK RELEASED (%) 100 

39 40 Ar/ Ar

ISOCHRON AGE = 10.32 ± 0.13 Ma 

(40Ar/36Ar)

i = 270.11 

± 3.16 

MSWD = 1.361 

NO POINTS DELETED 

* K98-8-17A4 

A 

B 

C 

D 

E 

F 

10 

1 

.1 

.01 

20 

10 

0 .05 .1 .15 .2 .25 .3 0 

K98-8-17A4 

NO PLATEAU 

39 40 0 39ArK RELEASED (%) 100 

Ar/ Ar 

1 

.1 

.01 

20 

10 

0 

K98-8-18I1 

ISOCHRON AGE = 10.42 ± 0.15 Ma 

(40Ar/36Ar)

i = 288.78 

± 12.96 

MSWD = 0.075 

POINTS ABC DELETED 

* 

A 

B 

C D 

E 

F 

G 

H 

0 .06 .12 .18 .24 .3 0 39ArK RELEASED (%) 100 

K98-8-18I1 

NO PLATEAU 

39 40 Ar/ Ar

69 

36 40 Ar/ Ar

.001 0 .001 

K/Ca 

.002 .003 

Apparent Age (Ma) 

36 40 Ar/ Ar

0 
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

K98-8-18I4; 0.2528 g, whole rock basalt, #107KD12, J=0.002037±0.25% 

A 5 5 0 3.4 6.4 0.048675 2.339 0.85 3 2 8.58 0.61 

B 6 5 0 4.3 24.3 0.061442 2.551 0.40 8 5 9.35 0.32 

C 7 5 0 9.3 48.3 0.131112 2.899 0.29 2 1 0 10.62 0.09 

D 8 0 0 9.2 69.4 0.130105 2.869 0.28 4 8 1 10.51 0.12 

E 8 5 0 13.4 84.4 0.189314 2.816 0.30 1 1 2 7 10.32 0.08 

F 9 5 0 24.9 89.3 0.353169 2.839 0.34 1 2 8 8 10.40 0.06 

G 1050 18.6 76.1 0.263843 2.785 0.41 5 7 5 10.21 0.10 

H 1150 10.8 59.2 0.152758 2.849 0.39 1 0 7 10.44 0.14 

I 1450 6.1 25.9 0.086721 2.796 0.04 1 3 0 10.25 0.59 

Total Gas 100.0 67.8 1.417139 2.803 0.35 6 6 7 10.27 

Plateau age 73.8 steps E-G 10.35 0.04 

K98-8-18H4; 0.2495 g, whole rock basalt, #103KD12, J=0.002037±0.25% 

A 5 5 0 13.4 20.4 0.203026 2.551 2.72 8 4 9.35 0.12 

B 6 5 0 21.6 27.5 0.326494 2.597 1.63 1 0 1 9.52 0.05 

C 7 5 0 25.2 33.3 0.380398 2.687 0.73 1 1 7 9.85 0.05 

D 8 0 0 14.7 42.1 0.222119 2.802 0.41 1 4 8 10.27 0.06 

E 8 5 0 11.1 56.2 0.168140 2.885 0.24 2 4 4 10.57 0.12 

F 9 5 0 11.1 75.3 0.167078 2.874 0.15 4 9 5 10.53 0.20 

G 1050 2.9 68.8 0.043287 2.633 0.10 1 8 6 9.65 0.63 

Total Gas 100.0 39.8 1.510542 2.707 1.01 1 7 1 9.92 

No Plateau 

K98-8-18G2; 0.2544 g, whole rock basalt, #97KD12, J=0.002023±0.25% 

A 5 5 0 23.8 14.4 0.446800 2.282 3.90 6 4 8.31 0.09 

B 6 0 0 2.1 15.5 0.040154 2.172 4.30 6 5 7.91 0.53 

C 6 5 0 5.9 19.2 0.110753 2.467 1.96 7 4 8.98 0.21 

D 7 0 0 7.6 24.8 0.142035 2.706 1.17 8 9 9.85 0.15 

E 7 5 0 8.6 34.7 0.161266 2.916 0.77 1 2 7 10.61 0.05 

F 8 0 0 10.2 50.6 0.190716 2.980 0.56 2 2 9 10.84 0.09 

G 8 5 0 11.0 70.0 0.205700 2.916 0.44 5 4 4 10.61 0.06 

H 1150 30.9 79.9 0.578995 2.792 0.26 3 2 3 10.16 0.05 

Total Gas 100.0 47.2 1.876419 2.675 1.48 2 2 1 9.74 

No Plateau 

70
1•

0 .001 .002 .003 

Ar/ Ar 39 40 

ISOCHRON AGE = 10.72 ± 0.13 Ma 

(40Ar/36Ar)

i = 281.78 

± 0.94 

MSWD = 3.742 

NO POINTS DELETED 

* K98-8-18G2 

0 .05 .1 .15 .2 .25 

A 

B

C 

D 

E 

F 

G 

H 

0 

10 

20 

Apparent Age (Ma) 

0 100 39Ar K 

K/Ca 

K98-8-18G2 

NO PLATEAU 

.1 

1 

10 

K98-8-18H4 

0 .001 .002 .003 

Ar/ Ar 39 40 

ISOCHRON AGE = 10.76 ± 0.24 Ma 

(40Ar/36Ar)

i = 285.68 

± 5.89 

MSWD = 0.673 

POINTS ABCG DELETED 

* 

0 .05 .1 .15 .2 .25 

A 

B 

C 

D 

E 

F 

G 

0 

10 

20 

Apparent Age (Ma) 

0 100 39Ar K 

K/Ca 

K98-8-18H4 

NO PLATEAU 

.1 

1 

10 

0 .001 .002 .003 

Ar/ Ar 39 40 

ISOCHRON AGE = 10.33 ± 0.13 Ma 

(40Ar/36Ar)

i = 300.70 

± 6.37 

MSWD = 0.311 

POINTS AB DELETED 

0 .06 .12 .18 .24 .3 

B 

C 

D 

E 

F 

G 

H 

I 

0 

10 

20 

Apparent Age (Ma) 

0 100 39Ar K 

K/Ca 

K98-8-18I4 

PLATEAU AGE = 

10.35 ± 0.04 Ma 

.01 

.1 

71 

RELEASED (%)

RELEASED (%)

RELEASED (%)

* A K98-8-18I4

36 40 Ar/ Ar 36 40 Ar/ Ar 36 40 Ar/ Ar
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

K98-8-18G5; 0.2532 g, whole rock basalt, #99KD12, J=0.002009±0.25% 

A 6 5 0 49.6 42.9 1.035476 2.456 2.19 1 2 1 8.88 0.02 

B 7 5 0 23.9 59.2 0.498375 2.577 0.75 1 4 5 9.32 0.05 

C 8 0 0 12.2 70.8 0.254045 2.681 0.37 1 8 6 9.69 0.10 

D 8 5 0 6 . 5 79.7 0.134930 2.743 0.20 2 6 2 9.92 0.24 

E 9 5 0 4 . 5 77.0 0.094023 2.520 0.12 3 2 9 9.11 0.28 

F 1050 2 . 0 74.2 0.041482 2.401 0.16 1 8 3 8.68 0.19 

G 1150 1 . 4 55.4 0.030227 2.250 0.04 2 5 8.14 0.56 

Total Gas 100 .0 54.9 2.088558 2.530 1.33 1 5 3 9.15 

No Plateau 

Missouri Heights


K97-8-15A3; 0.2497 g, whole rock basalt, #57KD6, J=0.002044±0.5% 


A 6 5 0 41.0 23.1 0.971015 2.927 2.04 2 9 10.76 0.16 

B 7 5 0 28.4 36.2 0.673331 2.853 0.56 4 9 10.49 0.16 

C 8 0 0 11.2 59.9 0.265045 2.798 0.30 1 6 5 10.29 0.37 

D 8 5 0 9 . 4 80.0 0.222899 2.988 0.25 4 1 6 10.98 0.35 

E 9 5 0 10.0 69.0 0.237055 2.492 0.19 4 0 1 9.17 0.39 

Total Gas 100 .0 40.9 2.369345 2.854 1.07 1 2 4 10.49 

Plateau Age 90.0 steps A-D 10.63 0.07 

K97-8-15A4; 0.2554 g, whole rock basalt, #59KD6, J=0.002044±0.5% 

A 6 5 0 28.1 23.5 0.688557 2.832 1.90 3 2 10.41 0.13 

B 7 5 0 22.5 30.6 0.551295 2.887 0.76 4 4 10.61 0.13 

C 8 0 0 15.1 40.5 0.369916 2.906 0.43 7 9 10.68 0.16 

D 8 5 0 12.9 53.3 0.316823 2.963 0.32 1 4 6 10.89 0.16 

E 9 5 0 16.0 54.4 0.392068 2.639 0.22 1 5 9 9.71 0.14 

F 1050 5 . 3 52.3 0.129787 2.881 0.16 8 2 10.59 0.34 

Total Gas 100 .0 38.0 2.448446 2.844 0.86 7 9 10.46 

Plateau Age 65.8 steps A-C 10.56 0.07 

Plateau Age 50.6 steps B-D 10.72 0.07 

72
10 

1 

.1 

.01

0 

ISOCHRON AGE = 9.87 ± 0.08 Ma 

(40Ar/36Ar)

i = 270.67 

± 2.06 

MSWD = 3.111 

NO POINTS DELETED 

* K98-8-18G5 

A 

B 

C 

D 

E F 

G 

ISOCHRON AGE = 10.19 ± 0.33 Ma 

(40Ar/36Ar)

i = 300.25 

± 3.36 

MSWD = 0.001 

POINT DE DELETED 

* K97-8-15A3 

A 

B 

C 

D 

E 

Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca 

K98-8-18G5 

NO PLATEAU 

0 

0 39ArK RELEASED (%) 

10 

1 

.1 

K97-8-15A3 

PLATEAU AGE = 

10.63 ± 0.07 Ma 

.002 .003 0 .001 .002 .003 0 .001 .002 .003 

36 40 Ar/ Ar 36 40 Ar/ Ar

20 

10 

0 .06 .12 .18 .24 .3 

39 40 Ar/ Ar

20 

10 

0 

0 .05 .1 .15 .2 .25 .3 0 39ArK RELEASED (%) 100 

39 40 Ar/ Ar

10 

1 

.1 

20 

10 

0 

ISOCHRON AGE = 10.47 ± 0.20 Ma 

(40Ar/36Ar)

i = 295.77 

± 2.28 

MSWD = 4.001 

NO POINTS DELETED 

* K97-8-15A4 

A 

B 

C 

D E F 

0 .04 .08 .12 .16 .2 0 39ArK RELEASED (%) 100 

K97-8-15A4 

PLATEAU AGE = 

10.56 ± 0.07 

10.72 ± 0.07 Ma 

39 40 Ar/ Ar

73 

36 40 Ar/ Ar

.001 

100
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

K97-8-12F1; 0.2552 g, whole rock basalt, #35KD6, J=0.002044±0.5% 

A 6 5 0 17.0 30.8 0.320511 2.960 0.73 6 6 10.88 0.21 

B 7 5 0 25.9 65.9 0.486521 2.826 0.45 2 4 7 10.39 0.32 

C 8 0 0 14.3 80.3 0.269813 2.948 0.30 5 4 6 10.84 0.06 

D 8 5 0 10.1 91.0 0.190696 2.963 0.24 7 2 1 10.89 0.11 

E 9 5 0 12.8 100.0 0.239906 2.874 0.23 4 3 2 10.57 0.08 

F 1050 9.0 100.0 0.168965 2.868 0.22 1 4 9 10.55 0.12 

G 1450 10.9 86.8 0.204106 2.658 0.05 5 0 9.78 0.52 

Total Gas 100.0 74.2 1.880518 2.865 0.36 3 0 1 10.53 

Plateau Age 67.4 steps A-D 10.84 0.06 

K97-8-12F2; 0.2547 g, whole rock basalt, #37KD6, J=0.002044±0.5% 

A 5 5 0 16.2 17.4 0.218451 3.068 1.27 3 4 11.28 0.20 

B 6 5 0 16.9 32.9 0.228113 2.905 0.75 5 8 10.68 0.08 

C 7 5 0 30.3 41.1 0.408029 2.762 0.38 9 9 10.16 0.31 

D 8 0 0 12.1 48.4 0.162426 3.271 0.21 1 5 0 12.02 0.14 

E 8 5 0 8.1 60.4 0.109109 3.249 0.16 6 4 8 11.94 0.38 

F 9 5 0 8.8 92.0 0.118782 3.245 0.13 8 5 2 11.92 0.17 

G 1050 7.5 94.1 0.101615 3.150 0.12 2 5 3 11.58 0.19 

Total Gas 100.0 46.8 1.346525 3.009 0.51 2 1 0 11.06 

No Plateau 

K97-8-12E1; 0.2554 g, whole rock basalt, #31KD6, J=0.002044±0.5% 

A 6 5 0 40.8 50.3 0.689925 3.169 0.81 5 11.65 0.09 

B 7 5 0 14.2 81.1 0.239677 2.950 0.27 5 10.85 0.12 

C 8 0 0 8.0 92.2 0.134884 3.015 0.17 7 11.09 0.19 

D 8 5 0 7.2 92.1 0.121634 2.848 0.15 1 0 10.47 0.26 

E 9 5 0 8.8 98.2 0.149480 2.667 0.15 1 3 9.81 0.17 

F 1050 6.5 99.4 0.110798 1.924 0.15 1 7 7.08 0.28 

G 1450 14.5 79.0 0.245565 1.794 0.08 1 1 6.60 0.26 

Total Gas 100.0 72.6 1.691963 2.777 0.43 8 10.21 

No Plateau 

74
36 40 Ar/ Ar 36 40 Ar/ Ar 36 40 Ar/ Ar

ISOCHRON AGE = 11.07 ± 0.17 Ma 

(40Ar/36Ar)

i = 294.85 

± 1.64 

MSWD = 7.325 

NO POINTS DELETED 

0 .001 .002 .003 

Ar/ Ar 39 40 

* K97-8-12F2 

0 .05 .1 .15 .2 .25 .3

A 

B 

C 

D 

E 

F G 

0 

10 

20 

Apparent Age (Ma) K/Ca 

.1 

1 

10 

ISOCHRON AGE = 10.74 ± 0.20 Ma 

(40Ar/36Ar)

i = 297.00 

± 3.67 

MSWD = 0.806 

POINTS EFG DELETED 

0 .001 .002 .003 

Ar/ Ar 39 40 

* K97-8-12F1 

0 .07 .14 .21 .28 .35 

E 

A 

B 

C 

D 

F 

G 

0 

10 

20 

Apparent Age (Ma) K/Ca 

.01 

.1 

1 

0 

10 

20 

Apparent Age (Ma) K/Ca 

.01 

.1 

1 

ISOCHRON AGE = 10.70 ± 0.15 Ma 

(40Ar/36Ar)

i = 319.77 

± 4.71 

MSWD = 1.243 

POINT EFG DELETED 

0 .001 .002 .003 

Ar/ Ar 39 40 

* K97-8-12E1 

F 0 .09 .18 .27 .36 .45 .54

A 

B 

CD 

E 

G 

75 

K97-8-12F1 

PLATEAU AGE = 

10.84 ± 0.06 Ma 

0 39ArK RELEASED (%) 100 

K97-8-12F2 

NO PLATEAU 

0 39ArK RELEASED (%) 100•

K97-8-12E1 

NO PLATEAU 

0 39ArK RELEASED (%) 100•
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

K97-8-12E2; 0.2493 g, whole rock basalt, #33KD6, J=0.002044±0.5% 

A 6 5 0 49.1 56.4 0.813113 2.884 0.98 7 10.60 0.13 

B 7 5 0 16.4 62.9 0.271154 2.627 0.22 1 1 9.66 0.10 

C 8 0 0 7 . 1 75.5 0.117424 3.035 0.14 1 7 11.16 0.53 

D 8 5 0 5 . 4 85.8 0.088622 3.069 0.13 2 0 11.28 0.24 

E 9 5 0 6 . 3 100.0 0.104586 2.980 0.12 2 0 10.95 0.32 

F 1050 4 . 8 100.0 0.078781 2.429 0.11 2 2 8.94 0.36 

G 1450 11.0 74.4 0.182747 2.010 0.05 1 2 7.40 0.09 

Total Gas 100 .0 67.2 1.656427 2.750 0.55 1 1 10.11 

No Plateau 

K97-10-8A2; 0.2470 g, whole rock basalt, #69KD6, J=0.002023±0.5% 

A 6 5 0 7 . 4 33.1 0.129690 2.586 0.32 9 2 9.41 0.31 

B 7 5 0 10.3 73.7 0.180232 3.081 0.25 2 3 8 11.21 0.20 

C 8 0 0 9 . 6 80.1 0.167708 2.905 0.24 3 7 0 10.57 0.24 

D 8 5 0 11.6 88.1 0.203250 2.911 0.25 4 5 6 10.59 0.18 

E 9 5 0 21.5 83.4 0.377153 2.689 0.26 4 0 8 9.79 0.10 

F 1050 21.4 81.6 0.374953 2.782 0.50 1 4 7 10.12 0.16 

G 1450 18.3 67.4 0.321120 2.708 0.11 6 8 9.86 0.12 

Total Gas 100 .0 75.6 1.754106 2.792 0.28 2 5 1 10.16 

Plateau Age 61.2 steps E-G 9.88 0.06 

K97-10-8A4; 0.2590 g, whole rock basalt, #71KD6, J=0.002019±0.5% 

A 6 5 0 7 . 5 27.2 0.135027 2.673 0.56 6 3 9.71 0.55 

B 7 5 0 9 . 2 47.5 0.165111 2.801 0.35 1 2 8 10.18 0.43 

C 8 0 0 10.3 62.7 0.185619 2.914 0.28 2 2 9 10.58 0.41 

D 8 5 0 12.0 74.4 0.215550 2.968 0.25 3 7 3 10.78 0.34 

E 9 5 0 20.5 74.7 0.369256 2.692 0.24 4 2 1 9.78 0.20 

F 1050 17.5 75.9 0.314341 2.734 0.38 1 9 7 9.93 0.20 

G 1450 23.0 67.4 0.414703 2.733 0.15 8 6 9.93 0.15 

Total Gas 100 .0 65.9 1.799607 2.774 0.28 2 2 5 10.07 

Plateau Age 61.0 steps E-G 9.89 0.07 

76
1 

ISOCHRON AGE = 8.96 ± 0.34 Ma 

(40Ar/36Ar)i = 351.36 ± 14.67 

MSWD = 7.3 

NO POINTS DELETED 

* K97-8-12E2 

A 

B 

C 

D 

E 

G 

F 

.1 

.01 

20 

10 

0 

0 .08 .16 .24 .32 .4 0 39ArK RELEASED (%) 100 

K97-8-12E2 

NO PLATEAU 

39 40 Ar/ Ar

1 

.1 

K/Ca 

20 

10 

0 

ISOCHRON AGE = 9.53 ± 0.53 Ma 

(40Ar/36Ar)i = 399.45 ± 51.41 

MSWD = 2.01 

POINT A DELETED 

* K97-10-8A2 

A 

B 

C 

D 

E F 

G 

0 .06 .12 .18 .24 .3 0 39ArK RELEASED (%) 100 

K97-10-8A2 

PLATEAU AGE = 

9.88 ± 0.06 Ma 

39 40 Ar/ Ar

1 

.1 

20 

10 

0 

ISOCHRON AGE = 9.70 ± 2.04 Ma 

(40Ar/36Ar)i = 309.56 ± 163.00 

MSWD = 0.032 

POINT ABCD DELETED 

* K97-10-8A4 

A 

B 

C 

D EF 

G 

0 .05 .1 .15 .2 .25 .3 0 39ArK RELEASED (%) 100 

K97-10-8A4 

PLATEAU AGE = 

9.89 ± 0.07 Ma 

39 40 Ar/ Ar

0 .003 .003 0 .001 .002 

K/Ca Apparent Age (Ma) 

36 40 Ar/ Ar

.001 .002 

Apparent Age (Ma) 

K/Ca 

36 40 Ar/ Ar

.001 0 .002 .003 

Apparent Age (Ma) 

36 40 Ar/ Ar

77
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

Heuschkel Park


K97-10-8E1; 0.2575 g, whole rock basalt, #85KD7, J=0.002651±0.50% Ä

A 6 5 0 11.9 35.7 0.189943 2.250 0.43 2 8 10.73 0.16 

B 7 5 0 14.3 70.9 0.227056 2.418 0.24 7 3 11.53 0.21 

C 8 0 0 12.9 78.0 0.205582 2.158 0.20 1 4 7 10.29 0.84 

D 8 5 0 14.6 81.2 0.232615 2.060 0.18 2 1 5 9.82 0.13 

E 9 5 0 20.4 78.3 0.324596 2.035 0.16 1 8 9 9.71 0.10 

F 1050 16.2 55.0 0.258403 2.038 0.22 6 4 9.72 0.19 

G 1450 9.6 33.1 0.153577 2.483 0.03 2 6 11.84 0.98 

Total Gas 100.0 64.4 1.591772 2.178 0.21 1 1 6 10.39 

Plateau Age 64.2 steps C-F 9.75 0.06 

K97-10-8E3; 0.2445 g, whole rock basalt, #87KD7, J=0.002653±0.5% 

A 5 5 0 19.1 23.2 0.308509 1.906 1.72 2 4 9.10 0.32 

B 6 5 0 11.0 44.8 0.176924 2.015 0.95 2 9 9.62 0.52 

C 7 5 0 20.3 56.3 0.328158 2.236 0.46 4 3 10.67 0.21 

D 8 0 0 9.5 52.5 0.153458 2.359 0.24 4 4 11.26 0.44 

E 8 5 0 8.2 54.5 0.132172 2.567 0.14 5 4 12.25 0.68 

F 9 5 0 9.7 45.7 0.156973 2.164 0.09 4 5 10.33 0.33 

G 1050 8.6 35.2 0.139226 1.840 0.11 2 8 8.78 0.88 

H 1450 13.6 39.2 0.219148 2.517 0.04 2 2 12.01 0.21 

Total Gas 100.0 43.0 1.614568 2.185 0.58 3 5 10.43 

No Plateau 

K97-10-8F3; 0.2471 g, whole rock basalt, #89KD7, J=0.002655±0.5% 

A 6 5 0 30.3 35.9 0.955707 1.970 2.20 1 0 7 9.41 0.09 

B 7 5 0 21.4 57.3 0.675493 2.130 0.60 1 7 0 10.17 0.19 

C 8 0 0 9.7 83.6 0.306610 2.294 0.36 5 1 9 10.95 0.13 

D 8 5 0 9.9 97.3 0.311034 2.265 0.33 2 5 2 7 10.81 0.14 

E 9 5 0 17.7 92.9 0.557729 2.067 0.32 9 6 5 9 9.87 0.10 

F 1050 7.1 96.0 0.222223 2.331 0.22 1 2 7 6 11.13 0.17 

G 1450 3.9 48.3 0.122786 1.908 0.03 4 9 9.12 0.32 

Total Gas 100.0 66.0 3.151582 2.105 0.94 2 1 7 0 10.06 

No Plateau 

78
1•

ISOCHRON AGE = 10.72 ± 0.12 Ma 

(40Ar/36Ar)

i = 272.57 

± 2.54 

MSWD = 4.732 

NO POINTS DELETED 

0 .001 .002 .003 

Ar/ Ar 39 40 

* K97-10-8F3 

0 .08 .16 .24 .32 .4 .48

A 

B 

C 

D 

E F 

G 

0 

10 

20 

Apparent Age (Ma) 

0 100 39Ar K 

K/Ca 

.01 

.1 

1 

10 

K97-10-8F3 

NO PLATEAU 

ISOCHRON AGE = 9.67 ± 0.67 Ma 

(40Ar/36Ar)

i = 316.92 

± 35.86 

MSWD = 0.144 

POINT AB DELETED 

0 .001 .002 .003 

Ar/ Ar 39 40 

0 .07 .14 .21 .28 .35 .42 

A 

B 

C 

D

E 

F 

G 

0 

10 

20 

Apparent Age (Ma) 

0 100 39Ar K 

K/Ca 

K97-10-8E1 

PLATEAU AGE = 

9.75 ± 0.06 Ma 

.01 

.1 

ISOCHRON AGE = 10.47 ± 2.34 Ma 

(40Ar/36Ar)

i = 297.92 

± 67.00 

MSWD = 2.212 

POINT AGH DELETED 

0 .001 .002 .003 

Ar/ Ar 39 40 

* K97-10-8E3 

0 .05 .1 .15 .2 .25 

A 

B 

C 

D E 

F 

G H 

0 

10 

20 

Apparent Age (Ma) 

0 100 39Ar K 

K/Ca 

.01 

.1 

1 

10 

K97-10-8E3 

NO PLATEAU 

79 

RELEASED (%)

RELEASED (%)

RELEASED (%)

* K97-10-8E1 

36 40 Ar/ Ar 36 40 Ar/ Ar 36 40 Ar/ Ar
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

K97-10-8F4; 0.2535 g, whole rock basalt, #91KD7, J=0.002613±0.5% 

A 6 5 0 17.2 37.8 0.475209 1.982 3.78 1 2 2 9.32 0.07 

B 7 5 0 14.0 54.2 0.385652 2.051 2.40 1 4 1 9.64 0.13 

C 8 0 0 9 . 3 61.2 0.256609 2.061 1.51 1 5 0 9.69 0.07 

D 8 5 0 7 . 3 67.6 0.200202 2.118 0.80 1 6 8 9.96 0.12 

E 9 5 0 11.2 82.1 0.308250 2.283 0.37 2 7 5 10.73 0.09 

F 1050 12.8 90.8 0.351544 2.196 0.26 8 7 3 10.32 0.32 

G 1450 24.8 88.3 0.682359 2.033 0.21 2 0 6 2 9.56 0.04 

H 1650 3 . 5 47.2 0.096164 1.811 0.02 3 3 8.52 0.13 

Total Gas 100 .0 69.0 2.755989 2.076 1.31 7 2 1 9.76 

No Plateau 

Red Canyon Graben


K97-10-8B1; 0.2444 g, whole rock basalt, #73KD6, J=0.002019±0.25% 


A 5 5 0 26.1 18.6 0.277775 2.818 2.37 2 5 10.24 0.44 

B 6 5 0 11.7 35.3 0.125077 2.648 1.27 3 8 9.62 0.23 

C 7 5 0 27.2 48.6 0.289336 2.956 0.46 4 4 10.74 0.20 

D 8 0 0 11.9 64.3 0.126513 3.094 0.21 7 3 11.23 0.27 

E 8 5 0 11.3 85.4 0.120837 3.299 0.12 2 1 7 11.98 0.25 

F 9 5 0 11.8 76.5 0.125703 2.853 0.09 4 4 2 10.36 0.48 

Total Gas 100 .0 48.6 1.065241 2.927 0.94 1 0 9 10.63 

No Plateau 

K97-10-8B3; 0.2452 g, whole rock basalt, #75KD7, J=0.002691±0.5% 

A 5 5 0 27.2 23.8 0.519927 2.005 2.08 3 5 9.71 0.09 

B 6 5 0 16.8 37.8 0.320761 2.075 0.79 3 9 10.05 0.10 

C 7 5 0 20.5 48.6 0.390496 2.094 0.28 5 7 10.14 0.08 

D 8 0 0 9 . 6 59.5 0.183677 2.033 0.16 1 0 7 9.84 0.16 

E 8 5 0 7 . 5 65.3 0.142534 1.972 0.12 1 5 7 9.55 0.25 

F 9 5 0 8 . 2 64.2 0.156677 2.062 0.10 1 7 6 9.98 0.20 

G 1050 5 . 2 58.6 0.098545 2.451 0.15 1 0 4 11.86 0.24 

H 1450 5 . 0 37.4 0.095914 2.079 0.02 2 5 10.07 0.35 

Total Gas 100 .0 43.6 1.908531 2.067 0.80 7 1 10.01 

No Plateau 

80
10 

1 

.1 

.01 

ISOCHRON AGE = 10.28 ± 0.12 Ma 

(40Ar/36Ar)

i = 276.36 

± 2.77 

MSWD = 4.091 

NO POINTS DELETED 

* K97-10-8F4 

A 

B 

C 

D 

E 

F G 

H 

K97-10-8F4 

NO PLATEAU 

.002 .003 0 .001 .002 .003 0 .001 .002 .003 0 

ISOCHRON AGE = 11.1 ± 0.23 Ma 

(40Ar/36Ar)

i = 286.09 

± 3.51 

MSWD = 4.737 

NO POINTS DELETED 

* K97-10-8B1 

A 

B 

C 

D 

E 

F 

Apparent Age (Ma) K/Ca

Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca 

36 40 Ar/ Ar 36 40 Ar/ Ar

20 

10 

0 0 .08 .16 .24 .32 .4 0 39ArK RELEASED (%) 100 

39 40 Ar/ Ar

10 

1 

.1 

.01 

20 

10 0 0 .05 .1 .15 .2 .25 0 39ArK RELEASED (%) 100 

39 40 

K97-10-8B1 

NO PLATEAU 

Ar/ Ar

ISOCHRON AGE = 9.78 ± 0.23 Ma 

(40Ar/36Ar)

i = 301.4 

± 5.22 

MSWD = 1.091 

POINT AGH DELETED 

* K97-10-8B3 

A 

B 

C 

D 

F E 

G 

H 

10 

1 

.1 

.01 

20 

10 

0 0 .06 .12 .18 .24 .3 .36 0 39ArK RELEASED (%) 100 

39 40 

K97-10-8B3 

NO PLATEAU 

Ar/ Ar

81 

36 40 Ar/ Ar

.001 
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

Upper Cattle Creek


L60; 0.2486 g, whole rock basalt, #15KD8, J=0.002665±0.25% Ä

A 550 

B 650 

750 

D 800 

E 850 

F 950 

G 1050 

H 1450 

Total Gas 

Plateau Age 

Spring Valley II 

0.5 5.6 0.044046 2.582 0.28 1 1 12.37 1.10 

12.2 50.6 1.019083 1.620 0.53 1 9 7 7.77 0.05 

24.7 82.9 2.062169 1.611 0.92 1 6 4 5 7.73 0.02 

10.1 81.1 0.839866 1.613 1.09 2 2 5 1 7.74 0.06 

7.3 86.2 0.613566 1.627 0.75 1 2 4 2 7.81 0.05 

10.4 88.6 0.866660 1.623 0.56 1 1 2 9 7.79 0.04 

13.3 89.0 1.114793 1.580 0.82 1 0 3 7 7.58 0.02 

21.5 79.4 1.796288 1.557 0.09 1 5 8 7.47 0.04 

100.0 79.3 8.356471 1.604 0.65 1 0 3 7 7.70 

64.6 steps B-F 7.75 0.03 

K97-10-8D1; 0.2586 g, whole rock basalt, #81KD7, J=0.002674±0.50% 

A 8 5 0 4.3 100.0 0.089105 1.586 1.28 * * * 7.64 0.75 

B 9 5 0 62.3 94.4 1.297887 1.607 1.12 6 2 9 3 7.74 0.06 

C 1050 26.0 82.8 0.541194 1.685 0.70 5 7 7 8.11 0.14 

D 1450 7.5 23.6 0.156380 1.635 0.03 3 9 7.87 0.47 

Total Gas 100.0 86.3 2.084566 1.628 0.94 4 0 7 1 7.84 

Plateau Age 66.5 steps A-B 7.74 0.05 

K97-10-8D2; 0.2479 g, whole rock basalt, #83KD7, J=0.002674±0.50% 

A 6 5 0 8.0 46.4 0.330176 1.904 0.46 9 9 9.16 0.19 

B 7 5 0 14.0 78.7 0.578970 1.604 0.52 7 3 6 7.72 0.14 

C 8 0 0 14.1 87.9 0.581167 1.619 0.75 2 1 6 6 7.79 0.21 

D 8 5 0 16.2 93.8 0.668949 1.678 1.00 2 6 0 7 8.08 0.13 

E 9 5 0 27.3 86.7 1.127827 1.606 1.17 1 7 6 3 7.73 0.07 

F 1050 13.9 65.4 0.575919 1.701 0.95 3 5 5 8.19 0.10 

G 1450 6.6 21.6 0.271517 1.736 0.05 5 3 8.36 0.39 

Total Gas 100.0 76.5 4.134525 1.665 0.83 1 3 7 1 8.01 

Plateau Age 71.6 steps B-E 7.79 0.05 

82

C 
10 

1 

.1 

.01 

0 

ISOCHRON AGE = 7.71 ± 0.04 Ma 

(40Ar/36Ar)i = 301.52 ± 1.52 

MSWD = 0.489 

POINT GH DELETED 

* L60 A 

B 

C D

EFG

H

ISOCHRON AGE = 7.71 ± 0.51 Ma 

(40Ar/36Ar)i = 301.78 ± 40.91 

MSWD = 0.108 

NO POINTS DELETED 

* K97-10-8D1 

A 

B 

C 

D 

Apparent Age (Ma) K/Ca 

L60 

PLATEAU AGE = 

7.75 ± 0.03 Ma 

0 

0 39ArK RELEASED (%) 100 

10 

1 

.1 

.01 

20 

K97-10-8D1 

PLATEAU AGE = 

7.74 ± 0.05 Ma 

.002 .003 0 .001 .002 .003 0 .001 .002 .003 

36 40 Ar/ Ar 36 40 Ar/ Ar

20 

10 

0 .1 .2 .3 .4 .5 .6 

39 40 Ar/ Ar

K/Ca K/Ca Apparent Age (Ma) 10 

0 0 .1 .2 .3 .4 .5 .6 .7 0 39ArK RELEASED (%) 100 

39 40 Ar/ Ar

ISOCHRON AGE = 7.59 ± 0.16 Ma 

(40Ar/36Ar)i = 338.83 ± 7.96 

MSWD = 0.649 

POINT G DELETED 

* K97-10-8D2 

A 

B 

C 

D 

E 

F 

G 

10 

1 

.1 

.01 

20 

10 

0 0 .1 .2 .3 .4 .5 .6 0 39ArK RELEASED (%) 100 

39 40 

K97-10-8D2 

PLATEAU AGE = 

7.79 ± 0.05 Ma 

Ar/ Ar

83 

36 40 Ar/ Ar

Apparent Age (Ma) 

.001 
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

McNulty Pit


K97-8-12G; 0.5114 g, whole rock basalt, #17,18KD15, J=0.000249±0.5% 


A 8 5 0 2 . 6 29.5 0.025987 6.896 2.16 3 2 3.10 0.18 

B 9 5 0 3 . 6 38.8 0.037056 6.566 1.93 3 8 2.95 0.09 

C 1050 4 . 9 42.6 0.050308 6.439 1.45 4 7 2.89 0.09 

D 1150 7 . 9 49.2 0.080609 6.691 1.00 7 5 3.00 0.04 

E 1250 7 . 5 56.3 0.076698 6.833 0.85 1 2 5 3.07 0.04 

F 1350 14.9 64.5 0.151502 6.729 0.85 2 1 6 3.02 0.03 

G 1450 18.3 66.5 0.186475 6.651 0.92 2 6 1 2.99 0.03 

H 1650 27.8 60.5 0.282898 6.393 0.88 2 3 7 2.87 0.02 

I 1750 12.4 44.6 0.126493 5.940 0.43 1 2 7 2.67 0.03 

Total Gas 100 .0 56.5 1.018026 6.512 0.93 1 8 2 2.92 

Plateau Age 59.8 steps A-G 3.01 0.01 

K97-8-12H; 0.2455 g, whole rock basalt, #39KD6, J=0.002044±0.5% 

A 7 5 0 8 . 6 89.3 0.376893 0.997 0.57 9 1 4 3.67 0.14 

B 8 0 0 11.6 100.0 0.508400 0.929 0.82 10278 3.42 0.10 

C 8 5 0 19.0 99.7 0.833551 0.895 1.23 * * * 3.30 0.10 

D 9 5 0 38.8 92.7 1.706773 0.831 1.43 * * * 3.06 0.04 

E 1050 17.9 92.0 0.787829 0.861 1.71 2 1 1 2 3.17 0.06 

F 1450 4 . 2 14.4 0.183607 0.911 0.04 3 9 3.36 0.34 

Total Gas 100 .0 91.2 4.397053 0.873 1.24 1 6 4 7 3.22 

Plateau Age 60.9 steps D-F 3.09 0.02 

Griffith Pit


L244-1; 0.2475 g, whole rock basalt, #8KD8, J=0.002665±0.15% 


A 5 0 0 0 . 1 3 . 9 0.000545 2.072 0.45 1 6 9.93 3.26 

B 6 0 0 1 . 5 20.2 0.008357 0.744 0.76 1 7 9 3.57 0.26 

C 7 0 0 19.4 82.6 0.107826 0.605 0.98 5 8 2 0 2.91 0.02 

D 8 0 0 37.4 93.3 0.208242 0.598 1.70 * * * 2.87 0.01 

E 8 5 0 25.6 92.2 0.142601 0.592 2.04 * * * 2.84 0.02 

F 9 0 0 9 . 2 86.3 0.051124 0.599 1.36 8 2 3 8 2.88 0.02 

G 1000 3 . 4 70.0 0.019157 0.616 0.68 6 4 3 2.96 0.05 

H 1100 1 . 7 47.0 0.009335 0.596 0.42 2 0 1 2.86 0.19 

I 1250 1 . 4 19.9 0.007711 0.719 0.03 2 6 3.46 0.11 

J 1400 0 . 3 10.8 0.001654 1.252 0.01 2 4 6.01 0.87 

Total Gas 100 .0 86.3 0.556552 0.606 1.52 1 9 1 3 2.91 

Plateau Age 56.8 steps C-D 2.87 0.01 

Plateau Age 72.2 steps D-E 2.87 0.01 

84
10 

1 

.1 

4 

3 

ISOCHRON AGE = 2.99 ± 0.11 Ma 

(40Ar/36Ar)

i = 296.83 

± 9.41 

MSWD = 0.212 

POINTS HI DELETED 

* K97-8-12G 

A 

B C 

D 

E 

F G 

H 

I 

Apparent Age (Ma) K/Ca 

K97-8-12G 

PLATEAU AGE = 

3.01 ± 0.01 Ma 

.001 0 .003 .003 0 .001 .002 

36 40 Ar/ Ar

2 

1 

0 

0 39ArK RELEASED (%) 

10 

1 

.1 

.01 

4 

3 

K97-8-12H 

PLATEAU AGE = 

3.09 ± 0.02 Ma 

L244-1 

PLATEAU AGES = 

2.87 ± 0.01 Ma 

2.87 ± 0.01 Ma 

0 .02 .04 .06 .08 .1 

39 40 Ar/ Ar

ISOCHRON AGE = 3.11 ± 0.14 Ma 

(40Ar/36Ar)

i = 299.87 

± 21.56 

MSWD = 0.171 

POINTS ABC DELETED 

* K97-8-12H 

A 

BC 

E 

D

F 

K/Ca 

.001 .002 

36 40 Ar/ Ar

K/Ca Apparent Age (Ma) 

2 

1 

0 

0 39ArK RELEASED (%) 

10 

1 

.1 

.01 

.001 

4 

3 

0 .2 .4 .6.8 1 1.2 

39 40 Ar/ Ar

ISOCHRON AGE = 2.86 ± 0.03 Ma 

(40Ar/36Ar)

i = 306.02 

± 3.39 

MSWD = 0.201 

NO POINTS DELETED 

* A 

B 

C 

DE 

F 

G 

H 

I 

J 

L244-1 

.002 .003 

36 40 Ar/ Ar

Apparent Age (Ma) 

2 

1 

0 0 .3 .6 .9 1.2 1.5 0 39ArK RELEASED (%) 100 

39 40 Ar/ Ar

85 

0 

100

100
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

L244-2; 0.2599 g, whole rock basalt, #12KD8, J=0.002665±0.25% 

A 5 5 0 2 . 6 18.2 0.331162 0.673 0.83 2 0 0 3.23 0.11 

B 6 5 0 4 . 8 79.1 0.622718 0.567 0.81 2 5 3 5 2.72 0.03 

C 7 5 0 25.0 93.2 3.232410 0.604 1.27 * * * 2.90 0.01 

D 8 0 0 25.1 95.9 3.250782 0.598 1.87 * * * 2.88 0.01 

E 8 5 0 22.2 96.6 2.870759 0.603 2.00 * * * 2.90 0.01 

F 9 5 0 14.4 90.4 1.865167 0.596 1.14 2 4 6 8 2.86 0.02 

G 1050 3 . 4 58.2 0.434742 0.553 0.49 2 7 3 2.66 0.10 

H 1150 1 . 7 24.3 0.219894 0.561 0.09 2 9 2.70 0.16 

I 1450 0 . 9 14.7 0.112943 1.230 0.01 3 4 5.90 0.25 

Total Gas 100 .0 88.6 12.940577 0.604 1.47 4 9 3 2.90 

No Plateau 

L-244; single-crystal sanidine, #21KD8, J=0.002612±0.45% 

0 5 LF * * * 73.3 0.00002999 0.616 74.7 1.874 2.902 0.024 

0 9 LF * * * 67.6 0.03207688 0.618 27.3 1.903 2.907 0.032 

0 7 LF * * * 85.0 0.03329507 0.619 30.8 2.698 2.916 0.020 

0 2 LF * * * 91.9 0.03329084 0.624 27.6 1.893 2.938 0.017 

1 0 LF * * * 91.2 0.03175724 0.625 32.1 2.226 2.944 0.017 

0 8 LF * * * 91.6 0.03274811 0.626 33.0 1.913 2.948 0.018 

0 6 LF * * * 90.6 0.02972324 0.628 39.1 1.969 2.955 0.017 

0 1 LF * * * 92.7 0.03064652 0.629 25.2 2.482 2.961 0.016 

0 4 LF * * * 94.5 0.03614500 0.631 52.5 2.486 2.969 0.014 

0 3 LF * * * 90.4 0.03529113 0.632 24.7 1.927 2.975 0.017 

Weighted Mean Age n=10 2.902 0.016 

L-245; single-crystal sanidine, #22KD8, J=0.002642±0.45% 

1 0 LF * * * 80.0 0.03300525 0.616 22.8 2.122 2.934 0.025 

0 5 LF * * * 76.9 0.03152267 0.620 38.9 2.296 2.953 0.028 

0 4 LF * * * 77.3 0.03103184 0.620 54.0 2.244 2.953 0.025 

0 6 LF * * * 79.0 0.03058501 0.620 29.8 2.301 2.954 0.025 

0 9 LF * * * 83.7 0.03023286 0.620 33.8 2.448 2.955 0.021 

0 1 LF * * * 80.7 0.03382442 0.624 35.0 1.907 2.970 0.024 

0 8 LF * * * 73.1 0.03458988 0.624 33.9 2.179 2.971 0.024 

0 2 LF * * * 74.6 0.03392541 0.625 43.7 2.239 2.977 0.027 

0 3 LF * * * 68.3 0.03331096 0.625 31.1 1.947 2.979 0.028 

0 7 LF * * * 82.9 0.03141027 0.626 34.3 1.890 2.983 0.025 

Weighted Mean Age n=10 2.962 0.017 

86
10 

1 

.1 

.01 

.001 

ISOCHRON AGE = 2.84 ± 0.02 Ma 

(40Ar/36Ar)i = 322.76 ± 2.24 

MSWD = 4.006 

NO POINTS DELETED 

* L244-2 

A 

B 

C DE 

F 

G 

H 

I 

L244-2 

.002 

NO PLATEAU 

.003 

K/Ca 

0 

4 

3 

2 

1 

0 0 .3 .6 .9 1.2 1.5 0 39ArK RELEASED (%) 100 

39 40 Ar/ Ar

 100 

80 

60 

0.0030 

0.0020 

0.0010 

L-244 

Weighted Mean Age = 

2.902 ± 0.016 Ma 

n = 10 

Relative Probability 

Analyses 

% Radiogenic 40Ar 

2.0 2.4 2.8 3.2 3.6 0.0000 

L-244 

ISOCHRON AGE = 2.968 ± 0.016 Ma 

(40Ar/36Ar)i = 278.4 ± 5.8 

MSWD = 0.87 

NO POINTS DELETED 

* 

0.0 0.4 0.8 1.2 1.6 Age (Ma) 39 40 Ar/ Ar

 85 

0.0032 75 

65 

0.0024 

0.0016 

0.0008 

L-245 

Weighted Mean Age = 

2.962 ± 0.017 Ma 

n = 10 

Relative Probability 

Analyses 

% Radiogenic 40Ar 

2.0 2.4 2.8 3.2 3.6 

0.0000 

0.0

L-245 

ISOCHRON AGE = 2.941 ± 0.032 Ma 

(40Ar/36Ar)i = 303.1 ± 10.4 

MSWD = 0.36 

NO POINTS DELETED 

* 

0.4 0.8 1.2 1.6 Age (Ma) 

36 40 Ar/ Ar

Apparent Age (Ma) 

.001 

36 40 Ar/ Ar 36 40 Ar/ Ar

39 40 Ar/ Ar

87
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

Triangle Peak


K97-8-12B1; 0.2563 g, whole rock basalt, #27KD6, J=0.002044±0.50% 


A 6 5 0 4 . 3 31.5 0.181492 0.590 0.67 1 6 9 2.17 0.25 

B 7 5 0 15.3 86.0 0.645888 0.428 1.25 2 5 5 1 1.58 0.06 

C 8 0 0 21.1 95.9 0.894525 0.401 1.62 756577 1.48 0.04 

D 8 5 0 22.0 96.6 0.932346 0.393 1.74 * * * 1.45 0.05 

E 9 5 0 20.3 93.6 0.857648 0.425 1.12 6 0 6 8 1.57 0.05 

F 1050 9 . 0 88.9 0.381008 0.486 0.54 6 8 9 1.79 0.11 

G 1450 8 . 0 23.2 0.339969 0.333 0.06 6 6 1.23 0.13 

Total Gas 100 .0 84.9 4.232876 0.419 1.23 161579 1.54 

Plateau Age 78.7 steps B-E 1.51 0.01 

K97-8-12B2; 0.2463 g, whole rock basalt, #29KD6, J=0.002045±0.50% 

A 6 5 0 4 . 3 41.8 0.184688 0.567 0.74 2 9 9 2.09 0.24 

B 7 5 0 17.3 79.9 0.744876 0.420 1.35 2 5 2 4 1.55 0.02 

C 8 0 0 21.4 87.5 0.920823 0.399 1.68 49108 1.47 0.01 

D 8 5 0 21.2 90.1 0.912336 0.404 1.73 626476 1.49 0.02 

E 9 5 0 17.3 84.6 0.742840 0.429 1.01 3 1 7 5 1.58 0.03 

F 1050 9 . 2 85.2 0.396200 0.494 0.53 6 5 8 1.82 0.05 

G 1450 9 . 3 24.9 0.401136 0.309 0.07 6 9 1.14 0.06 

Total Gas 100 .0 78.2 4.302899 0.416 1.22 144405 1.54 

No Plateau 

K99-8-22B2; 0.5066 g, whole rock basalt, #35,36KD15, J=0.000250±0.25% 

A 6 5 0 4 . 9 13.1 0.067285 3.728 0.70 1 4 1.68 0.10 

B 7 5 0 19.4 70.9 0.264600 3.037 1.07 5 7 1.37 0.01 

C 8 5 0 44.0 83.0 0.600905 2.967 1.04 2 5 9 1.34 0.01 

D 9 5 0 20.6 72.0 0.281274 2.971 0.70 7 6 9 1.34 0.02 

E 1050 7 . 7 58.1 0.105373 3.000 0.33 4 9 2 1.35 0.04 

F 1150 3 . 3 42.8 0.045123 2.745 0.25 1 4 1 1.24 0.09 

Total Gas 100 .0 71.7 1.364560 3.014 0.88 3 2 7 1.36 

Plateau Age 75.7 steps C-F 1.34 0.01 

88
10 

1 

.1 

.01 

ISOCHRON AGE = 1.44 ± 0.13 Ma 

(40Ar/36Ar)

i = 477.78 

± 266.93 

MSWD = 0.074 

POINTS AFG DELETED 

* K97-8-12B1 

A 

B 

C

D E 

F 

G 

K97-8-12B1 

PLATEAU AGE = 

1.51 ± 0.01 Ma 

K/Ca 

.001 0 .003 

4 

3 

2 

1 

0 

0 .5 1 1.5 2 2.5 0 39ArK RELEASED (%) 100 

39 40 Ar/ Ar

10 

1 

.1 

.01 

4 

3 

2 

1 

0 

ISOCHRON AGE = 1.40 ± 0.21 Ma 

(40Ar/36Ar)

i = 433.50 

± 234.92 

MSWD = 0.102 

POINTS AFG DELETED 

* K97-8-12B2 

A 

B 

C

D 

F E 

G 

0 .4 .8 1.2 1.6 2 2.4 0 39ArK RELEASED (%) 100 

39 40 

K97-8-12B2 

NO PLATEAU 

Ar/ Ar

ISOCHRON AGE = 1.33 ± 0.01 Ma 

(40Ar/36Ar)

i = 304.46 

± 2.92 

MSWD = 0.725 

NO POINTS DELETED 

* K99-8-22B2 

A 

B

C

D 

E 

F 

10 

1 

.1 

3 

2 

1 

0 0 .05 .1 .15 .2 .25 .3 0 39ArK RELEASED (%) 100 

39 40 

K99-8-22B2 

PLATEAU AGE = 

1.34 ± 0.01 Ma 

Ar/ Ar

89 

.003 0 .001 .002 

K/Ca Apparent Age (Ma) 

36 40 Ar/ Ar 36 40 Ar/ Ar 36 40 Ar/ Ar

.001 .002 

K/Ca Apparent Age (Ma) 

.002 .003 

Apparent Age (Ma) 

0 
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

K99-8-22B3; 0.5073 g, whole rock basalt, #37,38KD15, J=0.000250±0.25% 

A 6 5 0 4.0 11.3 0.057948 4.349 0.54 1 9 1.96 0.14 

B 7 5 0 14.0 70.7 0.205536 3.087 1.14 1 1 8 1.39 0.01 

C 8 5 0 36.6 82.0 0.535666 2.967 1.04 4 1 4 1.34 0.01 

D 9 5 0 25.8 78.4 0.377560 2.990 0.86 1 1 4 8 1.35 0.01 

E 1050 11.4 66.0 0.166395 2.938 0.54 6 5 2 1.33 0.02 

F 1150 5.9 59.3 0.087017 2.918 0.37 1 8 6 1.32 0.04 

G 1450 2.4 30.6 0.034813 3.457 0.02 4 0 1.56 0.11 

Total Gas 100.0 72.3 1.464935 3.050 0.87 5 5 1 1.38 

Plateau Age 79.6 steps C-F 1.34 0.01 

K-97-8-12A2; 0.2585 g, whole rock basalt, #23KD6, J=0.002022±0.25% 

A 6 5 0 17.4 41.6 0.615778 0.477 2.22 2 0 1.738 0.035 

B 7 5 0 36.7 69.0 1.295492 0.389 1.01 7 9 1.419 0.013 

C 8 0 0 15.8 98.4 0.559483 0.515 0.64 3 6 0 1.878 0.039 

D 8 5 0 11.6 91.5 0.411506 0.512 0.59 8 6 4 1.866 0.041 

E 9 5 0 7.5 78.6 0.264957 0.534 0.49 5 2 2 1.948 0.169 

F 1050 4.4 73.0 0.154155 0.483 0.47 2 5 9 1.759 0.116 

G 1450 6.5 44.0 0.231334 0.397 0.04 4 6 1.447 0.118 

Total Gas 100.0 70.8 3.532705 0.454 0.99 2 4 4 1.656 

No Plateau 

K99-8-22A1; 0.5042 g, whole rock basalt, #21,22KD15, J=0.000246±0.25% 

A 7 5 0 18.9 26.0 0.249662 3.581 1.33 2 1 1.59 0.02 

B 8 5 0 28.2 52.7 0.372769 3.183 0.96 6 4 1.41 0.01 

C 9 5 0 25.6 66.1 0.339092 3.160 0.73 1 7 5 1.40 0.01 

D 1050 12.3 65.9 0.163326 3.101 0.60 3 7 0 1.38 0.02 

E 1150 6.6 61.0 0.087957 3.088 0.44 2 3 6 1.37 0.04 

F 1450 8.4 42.5 0.110732 2.875 0.06 7 2 1.28 0.04 

Total Gas 100.0 52.4 1.323538 3.210 0.82 1 3 4 1.42 

Plateau Age 72.8 steps B-E 1.40 0.01 

90
ISOCHRON AGE = 1.33 ± 0.01 Ma 

(40Ar/36Ar)i = 307.83 ± 2.86 

MSWD = 0.956 

NO POINTS DELETED 

0 .001 .002 .003 

Ar/ Ar 39 40 

* K99-8-22B3 

0 .05 .1 .15 .2 .25 .3 

A 

B 

C D 

E 

F 

G 

Apparent Age (Ma) 

K 

K/Ca 

K97-8-22B3 

PLATEAU AGE = 

1.34 ± 0.01 Ma 

.1 

1 

10 

.01 

0 

1 

2 

3 

4 

0 100 39Ar 

ISOCHRON AGE = 1.36 ± 0.04 Ma 

(40Ar/36Ar)i = 306.69 ± 11.56 

MSWD = 0.504 

POINTS AF DELETED 

0 .001 .002 .003 

Ar/ Ar 39 40 

* K99-8-22A1 

0 .04 .08 .12 .16 .2 

A 

B 

CD 

E 

F 

Apparent Age (Ma) 

K 

K/Ca 

K99-8-22A1 

PLATEAU AGE = 

1.40 ± 0.01 Ma 

.1 

1 

10 

.01 

0 

1 

2 

3 

4 

0 100 39Ar 

Ar/ Ar 39 40 

ISOCHRON AGE = 1.88 ± 0.04 Ma 

(40Ar/36Ar)i = 290.2 ± 73.71 

MSWD = 0.231 

POINTS ABG DELETED 

0 .001 .002 .003 

* K97-8-12A2 

0 .4 .8 1.2 1.6 2 2.4

A 

B 

C 

D 

E 

F 

G 

Apparent Age (Ma) 

K 

K/Ca 

K97-8-12A2 

NO PLATEAU 

.1 

1 

10 

.01 

0 

1 

2 

3 

4 

0 100 39Ar 

91 

RELEASED (%) 

RELEASED (%) 

RELEASED (%) 

36 40 Ar/ Ar 36 40 Ar/ Ar 36 40 Ar/ Ar
Table 3. Continued. 

Temp 39Ar 40Ar* 39ArK 

40Ar* K/Ca K/Cl Apparent Age (Ma) 

(°C) (% Total) (%) (x 10-12 mol) 39ArK and error (±1σ) 

K99-8-22A4; 0.5029 g, whole rock basalt, #27,28KD15, J=0.000249±0.25% 

A 6 5 0 9 . 0 20.3 0.119294 4.046 1.39 1 6 1.82 0.08 

B 7 5 0 12.3 43.2 0.162061 3.444 1.44 3 0 1.55 0.06 

C 8 5 0 37.2 61.9 0.492217 3.170 0.87 8 4 1.42 0.02 

D 9 5 0 22.7 70.9 0.299614 3.115 0.63 3 5 0 1.40 0.03 

E 1050 8 . 4 59.8 0.111270 2.964 0.52 3 2 6 1.33 0.08 

F 1150 4 . 9 51.7 0.064240 2.635 0.43 2 6 7 1.18 0.14 

G 1450 5 . 6 38.0 0.073410 2.796 0.04 4 9 1.26 0.13 

Total Gas 100 .0 55.9 1.322106 3.206 0.84 1 5 9 1.44 

Plateau Age 78.7 steps C-G 1.41 0.01 

Willow Peak


K97-10-7F; 0.9900 g, whole rock basalt, #5,6,7,8KD15, J=0.000246±0.25% 


A 8 5 0 2 . 3 25.0 0.046882 0.936 0.55 3 3 1 0.415 0.064 

B 9 5 0 11.1 39.5 0.228560 0.665 0.99 1 6 3 5 0.295 0.013 

C 1050 23.9 50.8 0.491184 0.673 1.17 3 9 4 5 0.299 0.009 

D 1150 26.3 49.3 0.539548 0.702 1.17 2 9 2 0 0.312 0.006 

E 1450 29.8 35.4 0.611204 0.778 0.63 1 1 9 6 0.345 0.005 

F 1650 6 . 5 34.4 0.133900 3.127 0.07 1 1 1 1.387 0.067 

Total Gas 100 .0 42.9 2.051278 0.877 0.90 2 2 6 6 0.389 

Plateau Age 63.7 steps A-D 0.310 0.002 

K97-10-7G; 0.7467 g, whole rock basalt, #9,10,11KD15, J=0.000242±0.25% 

A 9 5 0 4 . 1 29.2 0.058113 2.684 0.45 1 8 4 1.171 0.235 

B 1050 11.5 24.4 0.163290 0.807 0.74 5 0 6 0.352 0.018 

C 1150 22.9 28.8 0.326038 0.670 1.11 9 1 1 0.292 0.009 

D 1450 53.9 25.9 0.766227 0.757 0.89 6 0 0 0.331 0.006 

E 1650 7 . 6 7 . 6 0.108177 0.771 0.28 9 0 0.337 0.095 

Total Gas 100 .0 25.2 1.421845 0.823 0.86 6 0 5 0.359 

Plateau Age 61.5 steps D-E 0.330 0.003 

92
ISOCHRON AGE = 1.35 

± 0.04 Ma 

(40Ar/36Ar)

i = 315.23 

± 4.29 

MSWD = 0.748 

NO POINTS DELETED 

0 .001 .002 .003 

Ar/ Ar 39 40 

* K99-8-22A4 

0 .04 .08 .12 .16 .2 .24 

A 

B 

C 

D 

E 

F 

G 

Apparent Age (Ma) 

K 

K/Ca 

K99-8-22A4 

PLATEAU AGE = 

1.41 ± 0.01 Ma 

.1 

1 

10 

.01 

0 

1 

2 

3 

4 

0 100 39Ar 

ISOCHRON AGE = 0.26 ± 0.04 Ma 

(40Ar/36Ar)

i = 330.65 

± 22.12 

MSWD = 0.169 

POINT F DELETED 

0 .001 .002 .003 

Ar/ Ar 39 40 

* K97-10-7F 

0 .1 2 3 4 5 .6 7 .8 

A 

B 

C D 

F 

E

Apparent Age (Ma) 

K 

K/Ca 

K97-10-7F 

PLATEAU AGE = 

0.310 ± 0.002 Ma 

.1 

1 

10 

.01 

0 100 39Ar 

0 

.5 

1 

ISOCHRON AGE = 0.30 ± 0.08 Ma 

(40Ar/36Ar)

i = 302.51 

± 22.48 

MSWD = 0.196 

POINT A DELETED 

0 .001 .002 .003 

Ar/ Ar 39 40 

* K97-10-7G 

A 

B 

C D 

E 

0 .08 .16 .24 .32 .4 0 Apparent Age (Ma) 

0 100 39Ar K 

K/Ca 

K97-10-7G 

PLATEAU AGE = 

0.330 ± 0.003 Ma 

.1 

1 

10 

.5 

1 

93 

RELEASED (%) 

..... RELEASED (%) 

RELEASED (%)

36 40 Ar/ Ar 36 40 Ar/ Ar 36 40 Ar/ Ar
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 1978: 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 40 Ar/39Ar dating using the Geological Survey TRIGA reactor: U.S. 

Geol. Survey Prof. Paper 1176, 55 p. 

Deino, A.L., 2001, Users manual for Mass Spec v. 5.02: Berkeley Geochronology 

Center Special Publication 1a, 119pp. 

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

spectra of Mesozoic tholeiites from Antarctica: Geochim. Cosmochim. Acta, v. 

41, p. 15.-32. 

Haugerud, R. A., and Kunk, M.J., 1988, ArAr*, a computer program for reduction of 40Ar39Ar data: U.S.Geol. 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: 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. 

Steiger, R.H., and Jäeger, E., 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. 

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

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

94