Potassium-argon ages of recent rhyolites of the Mono and Inyo craters, California

EARTH AND PLANETARY SCIENCE LETTERS 3 (1967) 289-298 . NORTH-HOLLAND PUBLISHING COMP., AMSTERDAM

POTASSIUM-ARGON AGES OF RECENT RHYOLITES OF THE MONO AND INYO CRATERS, CALIFORNIA G. Brent DALRYMPLE

U.S. Geological Survey, Menlo Park. California, LISA Received 22 August 1967

Twenty-two K-Ar ages were determined for sanidine samples from 10 rhyolite domes of the Mono and Inyo Craters to test the applicability of K-Ar dating to volcanic rocks of Recent age. Comparison of the results with `blank' and dosed analyses shows that radiogenic 40Ar was detected and was measured to within a factor of two or better. The estimated standard deviation of precision is 12',,", for analyses containing 5`1, or more radiogenic 4 0Ar. A statistical analysis suggests that real differences in apparent age were detected between three of the analyzed domes. The ages, which range from 6400 to 10,200 years for experiments with 5',,'~ or more radiogenic 40Ar, are, in general, consistent with ionium ages on 5 of the same samples and with glacial and 14(' evidence of age. They suggest that most of the Mono Craters volcano!; are on the order of 10,000 years old or less. The results also suggest that the problem of excess 4 0Ar may not be severe for ti-Ar dating of volcanic rocks.

1 . INTRODUCTION This work was done to test the hypothesis that the useful range of the K-Ar dating method could be extended to include rocks of Recent age . On the basis of their work with young rocks, Everndeu and Curtis [ 1 proposed that the age of 2500-year-old potassium feldspar samples could be determined with a precision of 25% (statistical parameter unspecified) . Extrapolating the results of replication experiments on older rocks, Dalrymple [21 predicted that it should be analytically possible to obtain ages on sanid ne as young as 10,000 years with a standard deviation of precision of 20°Io or better. In order to test the applicability of K-Ar dating to such extremely young rocks, samples from the Mono and Inyo Craters of California [3-8, 241 were collected for study (fig. 1). These volcanos were chosen for this investigation because (1) geologic evidence suggests that they are late Pleistocene to Recent in age, (2) some of the domes contain abundant sanidine, and (3) there was a good chance that they also could be dated by the * Publication authorized by the Director, U.S . Geological Survey .

ioniurn method 1141, which would provide an independent check on age . The Mono Craters are a north-south-trending; chain of more than two dozen rhyolite pumice cones .. domes, and coulées that extend southward from the shores of Mono Lake to U .S. Highway 395, a distance of about 10 miles . South of U.S. Highway 395 are the rhyolite domes and flows of the Inyo Craters, which have been considered to be the southern extension of the Mono Craters [3, 151 . All the domes, flows, and craters are youthful in appearance . Original volcanic features are essentially unmodified by erosion, and most of the slopes of lapilli cones appear to be unstable . Putnam [81 recognized that some of these rocks must be younger than the Tioga Glaciation (late Wisconsin) because pumice from both the Inyo and Mono Craters covers Tioga recessional moraines, and because the northernmost of the Mono Craters postdate shoreline terraces of the last highstand of Mono Lake, which occurred about 10,000 years ago according to the 14C chronology of other pluvial lakes in the Great Basin chain [9-101 . At its southwestern extremity the southernmost and largest of the coulées, called the `southern coulée' by Russell [51, appears to overlie the Tahoe Till (Wiscon-

G . B. DALRYMPLE

37°55"

-- 6GO17 ---6GO16

Crater Min --5G203

37°50'~

1

-1

~une , Loke J ,. . .

WO son Butti

Gui/ Lcke-

EXPLANAT ~ ON

c ~

Mop, A

iJ

37°45' -~

~lRhyolite domes and FlowsI

CRATERS

a

S

tea

Andesite

Ce a

CALIF Deer Min

0 il 9° 00

2

3 Mlles 1181, 55

'

Fig. l. Index map showing locations of analyzed samples. Geology from Kistler [ 11 ] and Rinehart and Huber (151 .

POTASSIUM-ARGON AGES OF RECENT RHYOLITES

sin) [11 ] . This suggests a maximum age of approximately 70,000 years for the southern coulde [ 13] . The available geologic evidence does not indicate whether the Mono and Inyo Craters eruptions occurred over a period of a few years or tens of thousands ofyears . At a few Mono Craters localities, crosscutting relationships between adjacent volcanos show that the eruptions were not exactly contemporaneous, but the time differences involved could be small. All the domes, cones, and flows appear to be in similar states ofpreservation . In the writer's opinion, geomorphic evidence for large age differences between volcanos is either weak or lacking, but such differences cannot be precluded. On his map of the Mono Craters quadrangle, Kistler [ 11 ] has assigned the rhyolites of the Mono Craters to two post-Tioga age groups, the older of which consists of the domes south of the southern coulée . However, the evidence for this subdivision is not obvious, and no explanatory text accompanies the map. Putnam [8] stated that in some respects the Inyo Craters appeared to be younger than the Mono Craters, but he did not present arguments for this thesis. Charcoal in pumice that is presumed to be from an eruption of one of the Inyo Craters gives a 14C age of 1440 f 150 years, and Rinehart and Huber 1121 place the formation of the Inyo Crater Lakes (not shown on fig. 1), which occupy small explosion craters, at about 650 f 200 years on 14C evidence . Core material from a depth of 450--480 cm below the present bottom of Mono Lake gives a 14C age of about 2200 years 114, sample no. L-1158B] . Pumice was not found in the core, which suggests, but does not prove, that the main mass of the Mono Craters may be older than about 2200 years. Potassium-argon ages have been determined by Evernden and Curtis [1 ] for two of the Mono Craters domes. They report ages of 56,000, 49,000, 69,000, and 63,000 years for four replicate measurements on a sanidine sample from the dome southwest of Punch Bowl, and a single age of 5000 years for Crater Mountain . In summary, the ages of the Mono and Inyo Craters are known only within broad limits . The Mono Craters appear to be between about 2000 and 70,000 years in age, but it is not clear whether they are all of about the same age or were formed over a long period of time, as the K-Ar ages by Evernden and Curtis [ i ]

29 1

suggest . Some of the northern craters are younger than the last major glaciation . The exact age relationships of the Inyo Craters to the Mono Craters are not certain, but some of the latest Inyo Crater activity is only a few hundred years old. 2. SAMPLING AND ANALYTICAL TECHNIQUES All the major domes and coulées, except Deer Mountain, were visited. Most of them are composed primarily of glass, and separable phenocrysts are either sparse or absent. Ten of the domes, however, have abundant and easily separable sanidine phenocrysts, and 25-50 kg samples were collected from each of these domes (fig. 1). The samples were taken from outcrop in place except for 5G201, 5G204, and 6G014, which came from large blocks on the sides of the domes. The samples were crushed and sieved, and sanidine was separated from the 600 to 250 micron fraction using standard magnetic and heavy liquid techniques . The natural grain diameter of the sanidine phenocrysts ranged from about 1 to 7 mm and averaged 2-2c mm . because of its f.igh K2O content and inherently low content of atmospheric argon, sanidine is the most suitable mineral for work on such very young rocks 1."', 161 . Before the final heavy liquid concentration, the crystals were treated one or more times, as necessary, in a cold 5--10% solution of 1-1F for 10-30 minutes to remove adhering glass, and were then cleaned in distilled water with a sonic cleaner. This procedure helps to obtain a virtually pure feldspar concentrate and reduces the atmospheric argon contamination . This HF treatment has no adverse effect on feldspar age determinations [ 1 ] . In order to reduce the atmospheric contamination in the samples to a minimum, each sanidine concentrate was given another brief 1-11~ treatment and sonification within 30 minutes before it was put under vacuum on the argon extraction line. After this final HF treatment, aliquants for the potassium analyses were taken with a Jones-type microsplitter. Argon was extracted on two ultra-high-vacuum extraction lines that have atmospheric 40 Ar blanks that range from about 4 X 10-13 to 5 X 10-1 1 mole and average about 2 X 10-11 mole per analysis . Molybdenum crucibles were vacuum fired for 15--30 min-

G . B . DALRYMPLE

29 2

Table 1 Analytical data for potassium-argon age determinations on rhyolite domes of the Mono and Inyo Craters, California. All measurements on sanidine . Argon analyses

K20 pércent Sample number

(1)

(2)

Sample weight (grams)

(10-12 mole)

40A rrad X 100 40, Artotal

0 .1219 0.1114

24 .4 9 .3

7,700 t 400 6,900 ±1,000

0.1'397

8.4

8,700 ±1,400

0.1090 0.0972 0 .0461 0 .1090 0.1102 0.1005 0.1822 0.1437

6 .4 7 .0

6,800 ± 1,400 6,100 1,200

1 .0 5 .0 6 .1 6 .3 5 .7 7 .1 3 .8

2,900 2,800 6,800 2,000 6,900 t 1,600 6,300 ± 1,400 11,500 i 2,900 9,000 ± 1,800

3 .0 2 .0 4 .6

6,200 ± 3,100 4,700 3,400 12,400 i 3,900

-4 .3

4 .2

12,400 ±4,200 3,900 t 1,300

12 .3 1 .8 10.0 5 .06 11 .6

8,400 ± 2,400 9,100 t 1,000

10 .75 10 .90

10 .68 10 .90

20.620 20.058

10.90 10.80 10.60

10.87 10.80 10.80

22 .485 20 .454 19 .835

56203

10 .73

10 .83

5G204

10 .70

10.80

10.74

10.78

10.81 10.90 110 .93 10 .80

10.86 10.91 10.99 10 .84 10 .94

22 .997 23 .538 24 .290 22 .1E t

10 .68 12 .03 10 .70

23 .462 22.084 21 .847

0.1291 0.0619 0.1643

24 .722 25 .389

0.1327 0.1485

56201 66005 56202

6GO06 6GO07

1

6GO08 6GO14

10 .90 11 .09

6GO16

10 .62 110 .61 10 .65

6GO 17

10.72 11 .03

11 .10

10 .78

1

20.986 21 .664 20 .834 21 .260 23 .297 24 .476

21 .212 23 .909

Calculated age (years)

40Ar rad/gram

0.1750 0 .0997 0 .0769 0 .1984 0 .2008 O .Or32

10,900 ±4,200

8,200 i 900 3,700 f 3,000 10,400 t 1,400

?, e = 0.585 X: 10-10 yr 1 , X0 = 4,72 X 10-10 yt 1 , 40K/K = 1 .19 X 10 -4 mole/ mole . The (±) figures are estimates of the precision a t the two-thirds confidence level ; see text for details .

utes to over 15000C before the sample was loaded . Sample's were preheated for 12-24 hours at 200300oC. Fusion was by induction heatin ;, a molecular sieve was used for water removal, and CuO and Ti foil were used for clean-up of reactive gases. The CuO and Ti were outgassed at operating temperatures before the start of the extraction. The step-by-step extraction procedures are similar to those described in detail by Evernden and Curtis [ 1 } . Mass analyses were made by static technique with a Reynolds-type mass spectrometer . The undesirable effects of mass spectrometer "memory' 1171, which can lead to serious errors in analyses of very young, rocks, viere minimized by reserving the machine for only these analyses until the work was completed,

and by purgings of the analyzer tube with a 38 Ar ion beam before starting the mass determinations . Mass discrimination was checked with atmospheric Ar before and after the samples were run on the spectrometer, and appropriate discrimination corrections were made. These corrections were from 0 to 0.6% for two mass units. The problems of dating these young rocks arose entirely from the argon analyses, and the potassium determinations posed no special problems . Analyses were by flame photometry with a Baird KY-2 photometer using a lithium internal standard. The chemistry was done by the method described by Shapiro and Brannock [ 181 .

POTASSIUM-ARGON AGES OF RECENT RHYOLITES

3. RESULTS The analytical data and the calculated (or apparent) ages for the Mono and luyo Craters samples are presented in table 1 . Radiogenic 40Ar percentages range from 1 to 24, and all the apparent ages are young. Two or three analyses were done on most of the samples, and the agreement between repeat determinations appears to be good where the radiogenic 40Ar percentage is 5% or more. However, the apparent ages sho+ald not be taken at face value. In the next few paragraphs, four questions that are relevant to the meaning of the ages in table I will be discussed. (I) Are the Ar analyses significantly different from zero, i.e., has radiogenic 40Ar been detected? (2) How accurate are the argon analyses, i.e., to what degree do they represent the `true' argon content of the analyzed minerals? (3) What is the precision (reproducibility) of the apparent ages, and are there real differences in radiogenic 40Ar content between samples? (4) What significance, if any, do the apparent ages have? Damon [ 191 has pointed out that it is virtually impossible to measure an age which is exactly zero. Because of random analytical errors, ages on rocks of zero age should have approximately a normal distribution about zero . In addition, systematic errors might

29 3

shift the midpoint of a group of analyses away from zero. Ip order to test whether or not the Mono and Inyo Craters results are significantly different from zero, seven `blank' argon determinations were made . In these experiments, the argon extraction and spec-trometry were done as usual, but without a mineral! sample (table 2). In one experiment, 11 grams of pyrex were added to increase the atmospheric 40Ar content. The results of the seven blanks have a standard deviation of 0.56 X 10-12 mole radiogenic 40Ar about a value close to zero. They are compared with the Mono and Inyo Craters analyses in fig. 2. All but two of the latter are significantly different from the blank determinations at the 95% confidence level. From these data, it is concluded that radiogenic 40Ar has been detested in most of the sanidine from the Mono and Inyo Craters. The accuracy of the argon analyses was checked by experiments in which a known dose of radiogenic 40 Ar was introduced (table 2). This was done by analyzing an older (,= 4 X 10 5 years) sanidine in an amount which would give the desired dose. In addition, Pyrex was introduced in different amounts to provide varying amounts of atmospheric contamination. Two sizes of dose, 1 X 10-12 mole and 2 X 1(:) -12

Table 2 Analytical data for blank (no sample) and dosed argon determinations . 40Ar

Experimental conditions Dose (10-1 2 moles 40Arrad)

Pyrex (grams)

Atmospheric (10-1 2 moles)

Percent radiogenic

0 0 0 0 0 0 0 1.00 1.00 1.00 1 .00 2.00 2.00 2.00

0 0 0 0 0 0 1P .9 0 1 .0 3.2 5.4 1 .0 3.2 7.6

23 .8 41 .8 18.9 26 .5 9.3 49.9 249.0 34 .5 63 .5 76 .6 101 .0 48.2 80.4 151.0

2.8 1 .2 0.4 0.2 -7 .3 0.8 -0.3 4.3 1.7 1.8 0.8 4.2 3.8 1 .6

M asured radiogenic (10-1 2 moles) 0.800 0.565 0.080 0.072 -0.641 0.405 -0.616 1.689 1.161 1.450 0.386 2.230 3.327 2.462

mean = 0.095 std. dev . = ±0.562

mean= 1 .297 std. dev. = ±0.34.8 mean = 2.673 std. dev . = ±0-578

G. B. DALRYMPLE

294

O Dosed

Q No sample A Pyres

Y O E

V EN ,\\\\

s

,BOX

MEN Eff ,

\\\gws

2.0 Measured ßrß per analysis. Id

,

3 .C moll

70

v

I

Group mean and 95%

- ~confidence level 20-

v

4.0

5 .0

Y C W O

Lo-

v 0-

No af

Soeldiee of Mm Cratas

Fig. 2. Histogram comparing Mono and Inyo Craters analyses with 'blank' analyses, demonstrating that radiogenic 40Ar was detected in most of the sanidine samples.

mole, were used, and the line along which perfect analyses would plot falls within the 95% confidence limits of each dose group (fig . 3). Too few dosed runs were done to provide any meaningful statistics on lowlevel accuracy, but two things are apparent: (1) as the dose is increased, the amount of radiogenic 40 Ar calculated from the analyses also increases, and (2) none of the analy&,s on dosed runs a:e off by as much as a factor of two. The Mono and :nyo Craters sanidine analyses are probably more accurate than the dosed runs because (1) the average amount of atmospheric 40Ar contamination in the dosed runs is approximately twice that in the sanidine analyses, and (2) in more than 75% of the sanidine analyses, the total radioger ;c 40 Ar content was greater than the largest dose used. In many of the sanidine measurements, these two factors combine to give higher radiogenic 40 Ar percentages, which, in turn, should result in more accurate and more precise, analyses. The limited data suggest that most of the Mono and Inyo Craters argon determinations are probably accurate to better than a factor of two. Estimates of the precision (reproducibility) of the age analyses were made in two independent ways. First; the standard deviation of precision was estimated for each analysis using the method formulated by Cox and Dalrymple [201 . The estimates are given in Viable 1,. They average 32% of the calculated age for

-1 .0

10

2,0

4o, 101! mole

3,0

4.0

Meosured rodiogenic Ar

Fig. 3. Graph of radiogenic 40Ar dose versus measured radiogenic 40 Ar.

all analyses, and 19% for analyses with 5% or more radiogenic 40 Ar . The second method of estimating precision was to utilize the replicate analyses and from these to calculate a pooled estimate of the standard deviation [211 . This pooled estimate is 34% for all analyses and 12% for the analyses with 5% or more radiogenic 40Ar. From these data, it is possible to infer with 90% confidence that, on the average, o is 26--51% for the population of all ages and 8-22% for the population of ages of rocks with 5% or more radiogenic argon [211 . Note that there are only crude estimates because the pooled estimate technique is strictly applicable only for a statistical sample from a population with a common variance . This assumption is clearly not appropriate for these age determinations because of variations in the atmospheric 40Ar contamination and the quality of the experiment, both of which lead to variable errors . However, the pooled estimates of precision calculated from the replicate analyses are consistent with the averages of the estimates given in table 1 and suggest that the values in table 1 mry be reasonable estimates of analytical precision. For the present study, it is important to know whether or not differences in the apparent ages of different domes indicate real differences in 40A rrad / 40K (apparent age) or are merely due to random analytical errors . In order to claim with 95% confidence that a true difference in age between two rocks has been detected, the apparent difference must ex-

POTASSIUM-ARGON AGES OF RECENT RHYOLITE S

Geed the critical value

on each dome constitute a random sample from a population of age measurements with a common variance. As mentioned above, this assumption is not strictly valid, mainly because of variations in atmospheric 40 Ar contamination. However, it is useful to obtain an estimate of the likelihood that real differences have been detected by making estimates of the variances and applying the above test. Critical values have been calculated for the age

2

01 ,o ( - + Q l Î z , n1 ~2 ~

c.v. = 1

where Q1 and u2 are the population standard deviations and n 1 and n2 are the number of determinations on each of the two rocks [21, 22] . 'fo apply this test, it is necessary to assume that the age determinations

2483 3424 2657

3800 3722 2547

1533 1912 2391

2850 2405 2268

No

~les

i

983 2582 1408

N° _

i

I

No

-2300 2966 1188

1

1950 4574

._ .1 .

_N°__

Critical value using standard deviation from method 1

0

Yes

No

No

29 5

ro

550 2845 ï 2370 No i . .

is there a tea t difference in the; apparent ages with 95% confidence? U'es, if apparent difference 21 bli , h critical values

Fig. *. Statistical comparison of age determinations that contain 5% or more radiogenic 40Ar . A real difference in age is claimed with 95% confidence only where the apparent age difference exceeds both critical values (critical values are discussed in the text). Method 1 standard deviations are the means of the standard deviations given in table 1 . Method 2 standard deviations were calcu lated from the replicate determinations .

296

G. B. DALRYMPLE

measurements on rocks that have 5% or more radiogenic 40Ar (fig. 4). The 5% cutoff was made in order to eliminate the ages with extremely high error probabilities, thereby making the statistical samples somewhat more homogeneous. Two different methods were used to estimate the `average' standard deviation of the ages from each dome . Method 1 was simply to take, for each dome, the mean of the estimated standard deviations given in table 1 . Method 2 was to calculate a standard deviation for the ages from each dome using the replicate measurements . In the case of a pair of measurements, the standard deviation is simply the mean deviation. The second method is probably iio1 as reliable as the first because the largest number of replicates on any single dome is three, and the second method does not take into account the variability in the atmospheric contamination . The critical values calculated for all possible pairs of domes using both of the standard deviation estimates are given in table 3 along with the apparent age difference and other relevant infostnation . The detection of a real difference in age wn-s claimed only where the apparent difference exceeded both critical values. By this criterion . real differences in 40Arrad/40K (or apparent age) have been detected with 95%Q confidence in only three instances: between 5G202 and 60006; between 5G202 and 6G016 . and between 5G204 and 6G016. Note that the test indicates only Chat a true difference has been detected, not measured . It does not follow that the apparent difference is the true difference or that domes between which no difference has been detected are the same age. Wihether these differences indicate actual differences in age or are due to other factors such as excess argon, contamination, or argon toss can only be tested in one of the three cases. Evidence from aerial photographs and field inspection shows that Punch Bowl (60006 = 10,200 years) is older than the dome immediately to the northeast (5G202 = 6400 years) because the dome to the northeast was emplaced partly into the crater of Punch Bowl . The apparent ages are consistent with this observed sequence, but with only one such test, it cannot be said with confidence that the other two significant apparent differences necessarily indicate true differences in :age. The most that can be stated is that no available evidence shows that these - "tatistically significant apparent age differences are not due to real differences in age between the domes.

Table 3 Comparison of K-Ar and ionium ages on samples from the Mono and Inyo Craters . Sample No.

ionium ages (years)

1141

SG201

1,000 t 3,000

(60005) 5G203

1,800 ~: 2,000

6GO14 6GO16

39,300 ± 6,000 35,100 4,700 2,500

6GO17

10,500 ± 1,800

K-Ar ages (years) (this paper) 7,700 ± 400 6,900 ± 1,000 8,700 ± 1,400 2,900 t 2,800 6,800 ± 2,000 3,900 ± 1,300 8,200 ± 900 3,700 ± 3,000 10,400 ± 1,400 8,400 ± 2,400 9,100 ± 1,000

Finally, are the K-Ar ages oil these domes reasonable estimates of the ages of the domes? This question is especially important in view of the ages of about 60,000 years obtained by Evernden and Curtis [ 1 ] on the dome southwest of Punch Bowl, and in view of the subdivision of the Mono Craters by Kistler [ 11 into two age groups . Taddeucci, Broecker and Thurber [ 14] have made age determinations on five of the same samples using the ionium method on hornblende and glass (table 3). In general, the ages are roughly consistent, i.e., ages by both methods are on the order of 10,000 years or less. The exception is the one Inyo Craters sample (6G014) where the ionium age is an order of magnitude higher than the K-Ar age. There is, at present, no basis for deciding whether the ionium age or the K-Ar age is incorrect in this case, but the rough overall agLeement between the ages determined by these two different decay schemes suggests that the bulk of the Mono Craters are on the order of 10,000 years old or less. The K-Ar ages of 49,000 to 69,000 years reported by Evernden and Curtiis [ 1 ] were determined on a sample from the dome just southwest of Punch Bowl. The first sample from this dome that was analyzed in the present work (5G201) gave ages of 7700 and 6900 years. Because of this order-of-magnitude discrepancy, a second sample (6G005) was collected from a different part of the dome at a later date . This sample gave an age of 8700 years, which agrees relatively well with

POTASSIUM-ARGON AGES OF RECENT RHYOLITES

the first two measurements . It seems unlikely that the ages determined in the present study could be low by a factor of 10 because of errors in measurement for two reasons: (1) the experiments on blank and dosed runs show that the amount of radiogenic 40Ar in the sanidines has probably been correctly determined to within a factor of two or better ; and (2) the K-Ar ages are relatively consistent with each other in spite of large differences in atmospheric 40Ar contamination, with the ages determined by the ionium method (with the one exception mentioned above), and with the K-Ar age of 5000 years reported by Evernden and Curtis [ 1 ] for Crater Mountain . Could the HF treatment possibly cause the K-Ar ages to be too low by a factor of 10? This also seems unlikely for several reasons. (1) The method has been thoroughly tested on potassium feldspars [ 1 ] and no such effect has been observed. The usual tendency is for the age either to remain unchanged or to be increased because the HF removes adhering fragments of glass. In addition, this HF treatment has been used routinely by several laboratories on young potassium feldspars for several years. No such anomalous effect has been reported . (2) Such an effect would require that the HF remove 80-90% of the radiogellic 40Ar without having any appreciable effect on the K20 content (note that the K20 values in table 1 are all greater than 10%p) or on the mass or optical properties of the crystals. (3) In three of the four older ages determined by Evernden and Curtis, the samples were also treated with HF in the same way as in the present analyses . (4) The samples were not uniformly treated. They were treated with HF as few as two and as many as four times, and there is no correlation between number of treatments and age. If Kistler's [I 11 division of the Mono Craters into an older and a younger group is valid, the age difference must be small because it was not detected by the present study. In fact, the statistical evaluation of the data suggests that 6G016, from Kistler"s younger group, may be older than 5G202 and 5G204, both from his older group. Again, it seems improbable that the HF treatment could obscure any large age differences between the northern and southern Mono Craters, because samples from both groups of domes were given similar HF treatments and yet the calculated ages are all on the order of 10,(10 years or less, Any

29 7

. Table 4 Maximum excess or inherited 40Ar in Mono Craters sanidines. Data calculated wring analyses containing 510 or more radior genic 40 Ar and assumptions described in text . Sample No . 5G201 and 60005 SG202 5G203 5G204 6GO06 6GO16 6G017

Maximum excess or inherited 40Ar (10-12 mole/gram) 0.089 0.068 0.074 0.071 0 .129 0.113 0.105

argon loss due to HF should produce roughly systematic changes in apparent age. In conclusion, the K-Ar ages appear to be reasonable order-of-magnitude or better estimates of the age of the analyzed rhyolite domes, i.e., the domes are on the order of 10,000 years old or less. This is consistent with the 14 C and glacial. evidence, which indicates that at least some of the Mono and Inyo Craters are less than 10,000 years old. However, this does not preclude the possibility that there are older domes or flows in the Mono and Inyo Craters groups on which analyses were not made. Also, there is no way to exclude the possibility that some or all of the ages may have been influenced slightly by such things as excess argon, contamination, and argon loss. In fact, it is entirely possible that one of these causes is responsible for the real differences in apparent age detected between tliree of the domes. The present data can be used to calculate the amount of excess argon that would be required to produce the apparent ages in table l . The absence of rhyolite ash in the Mono Lake core suggests that most of the Mono Craters domes are more than 2200 years old. The maximum excess 40Ar in the Mono Craters sanidines (table 4) was calculated assuming a minimum age of 2200 years, using only the determinations that have 5% or more radiogenic 40Ar, and neglecting the estimated standard deviations of the measurements . The results are more than two orders of magnitude less than those summarized by Damon [231 and by Livingston . Damon, Mauger, Bennett and Laughlin

29 8

G. B. DALRYMPLE

for plutonic and metamorphic plagioclase wfeldand suggest that the problem of excess `~~Ar may spar not be serious for volcanic rocky of this type. 1251

ACKNOWLEDGEMENTS This work was done partially in cooperation with Wallace S. Broecker, Adriano Taddeucci and David L. Thurber (see ref. [141) in'an effort to check ionium dating versus K-Ar dating. Wallace S. Broecker and Marvin A. Lanphere helped in collecting the samples and in discussing the problem and results. Wilfrid A. Davis assisted with the argon analyses . K20 determinations were by Lois B. Sehlocker. REFERENCES 111 J. I.. Evernden and G. .J . Curtis, The potassium-argon dating of late Cenozoic rocks in east Africa and Italy, Cuff . Anthropol. 6 (1965) 343. 1211 G. B. Dabymple, Potassium-argon dating in Quaternary correlation, INQUA Proc. 8 (1967) in press. 131 E. B. Mayo, L. C. Conant and J. R. Chelikowsky, Southern extension of the Mono Craters, California, Am . J. Sci. 32 11936) 81 . 141 J Le Conte, On the extinct volcanoes about Lake Mono and their relation to the glacial drift, Am . J. Sci., 3rd Ser. 18 (1879) 35 . (51 1. f:. Russell, Quaternary history of Mono Valley, California, U.S . Geol . Survey 8th Ann . Rept . (1889) 261 . [61 H. Williams, The history and character of volcanic domes, Univ . Calif. Pubs . Bull . Dept. Geol. Sci. 2 1 (1932) 1. 171 W. C. Putnam, The Mono Craters, California, Geog. Rev. 28 (1938) 68 . ('8[ W. C. Putnam, Quaternary geology of the June Lake district, California, Geol. Soc. Am . Bull. 60 (1949) 1281 . ~, 91 W. S. Broecker and A. Kaufman, Radiocarbon chronology of Lake Lahontan and Lake Bonneville 11, Great Basin, Geol . Sac. Am . Bull . 76 (1965) 537.

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