U.S. Department of the Interior 

US. Geological Survey

K-Ar Ages and Paleomagnetic Directions from the

Lathrop Wells Volcanic Center, Southwestern Nevada:

An Evaluation of Polycyclic Volcanism

by

Brent D. Turrin

Lamont-Doherty Earth Observatory 

Columbia University 

Palisades, NY 10964

Duane E. Champion, and Robert J. Fleck

U.S. Geological Survey

345 Middlefield Road,

Menlo Park, CA 94025

Garniss H. Curtis, and Robert E. Drake 

Berkeley Geochronology Center

2455 Ridge Road 

Berkeley, CA 94709

Open-File Report 98-44

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, product, or firm names is for descriptive purposes only, 

_______and does not imply endorsement by the U.S. Government._______
Abstract

Paleomagnetic data suggest that there may be two eruptive units at the Lathrop Wells volcanic 

center, Nevada. The measured directions of remanent magnetization for the two units are 51.5° 

inclination, 2.2° declination, and 51.4° inclination, 354.5° declination. The difference in the 

paleomagnetic field directions of 4.7°, is significant at the 99.98 percent confidence level. The 

observed angular difference between the two mean directions suggests a minimum age difference 

between the eruptive events of about 100 years. Weighted means of K-Ar ages on these two units 

(Ql3 = 131 ± 10 ka; Qls = 115 ± 12 ka), though reversed from the mapped stratigraphic 

succession, are not significantly different at the 95 percent confidence level. Based on statistical 

analysis, the difference in age between the flows is less than or equal to approximately 30 k.y. 

Moreover, the K-Ar ages from the flow units Qls and Qls are consistent with the reported uranium 

series dating, uranium-trend chronology, and ^Cl and 3He exposure ages of the surficial units in 

the Yucca Mountain region. 

INTRODUCTION

Yucca Mountain, located near the southwest boundary of the Nevada Test Site (NTS), is 

being evaluated to determine its suitability for a high-level radioactive-waste repository. One 

aspect of the site characterization studies at the potential radioactive waste repository at Yucca 

Mountain is an evaluation of the hazards associated with possible future volcanic activity (Crowe 

and Carr, 1980; Crowe et al., 1983). The most recent nearby volcanic activity occurred 10 to 20 

km southwest of Yucca Mountain, in the Crater Flat area (Fig. 1). The youngest volcanic edifice 

in the Yucca Mountain area is the Lathrop Wells cone.

The chronology of these volcanic events and the estimates of eruptive volumes through 

time are essential to assessing the volcanic hazard probability and making a consequence analysis 

for the potential repository at Yucca Mountain. Moreover, an important part of assessing the 

volcanic hazards during the required isolation period (104 yr) of the high-level radioactive waste is 

understanding the past behavior of the volcanic activity of the Yucca Mountain area.
The chronology of the Lathrop Wells cinder-cone complex, the youngest volcanic center in 

the Crater Flat-Yucca Mountain region is being evaluated as part of the site characterization studies 

of Yucca Mountain. Recent studies indicate that some cinder cone complexes in the southwest 

United States are polycyclic (the products of multiple eruptive cycles) with recurrence intervals of 

103 to 105 years (Turrin and Renne, 1987; Renault et al, 1988). The Lathrop Wells cinder-cone 

complex and the Crater Flat volcanic field are being examined for polycyclic behavior. This 

evaluation includes paleomagnetic studies, soil and geomorphic studies, and isotopic dating (K-Ar, 

^Ar/^Ar, 3He/4He surface exposure dating, and uranium disequilibrium dating). The purpose of 

this paper is to present and summarize the results of K-Ar studies of samples collected from lava 

flows of the Lathrop Wells volcanic center. 

GEOLOGIC SETTING

The Lathrop Wells volcanic center is located in the southern part of the Great Basin in 

Nevada, approximately 140 km northwest of Las Vegas, Nev. (Fig. 1). The Great Basin has been 

the site of extensive Cenozoic igneous activity that reflects the spatial and temporal changes in the 

tectonic setting of western North America (Armstrong et al., 1969; McKee, 1971; Christiansen 

and Lipman, 1972; Lipman et al., 1972; Noble, 1972; Lipman, 1980). The most recent volcanism 

within the Great Basin has been dominated by transitional subalkaline (hypersthene normative) to 

alkaline undersaturated (nepheline normative) mafic lavas associated with crustal rifting (Leeman 

and Rodgers, 1970; Luedke and Smith 1978a, 1978b, 1981,1982,1983; Lipman, 1980; McKee 

and Noble, 1986). K-Ar ages from these basaltic volcanic fields indicate that there has been 

intermittent basaltic volcanism for the last 8 million years (Crowe and Carr, 1980; Turrin and 

Dohrenwend, 1984; Turrin et al., 1985a; Turrin et al., 1985b; and Turrin and Gillespie, 1986).

Locally, the Nevada Test Site area consists of a number of coalesced caldera complexes that 

were active during the Miocene (14 to 8 Ma) (Byers et aL, 1976; Christiansen et al., 1977). Since 

then volcanism has continued in the form of small isolated subalkaline (hypersthene normative) to 

alkaline undersaturated (nepheline normative) basaltic volcanic fields (Vaniman and Crowe, 1981; 

Vaniman et al., 1982).
Geology of the Lathrop Wells Volcanic Center

The Lathrop Wells cone and flow complex overlies Quaternary alluvial sediments and the 

Topopah Spring Tuff (?), Miocene ash-flow and air-fall tuff units. Generally, the flows are dense 

basalt with vesicular tops and bottoms, and exhibit block flow and aa flow morphologies. The 

basalt is mostly holocrystaline and sparsely porphyritic with phenocrysts of olivine and 

microphenocrysts (?) of plagioclase in a medium- to fine-grained pilotaxitic groundmass of 

plagioclase, olivine, clinopyroxene (Ti-augite), opaque minerals (Fe-Ti oxides), interstitial glass, 

and apatite (Vaniman and Crowe, 1981; Vaniman et al., 1982). Olivine phenocrysts may exhibit 

thin rims of iddingsite (?), and the plagioclase and clinopyroxene phenocrysts appear unaltered. 

Xenoliths of the underlying Topopah Spring Tuff are commonly found in the lava flows and scoria 

deposits. A calculated mean volume of 0.03 percent of tuff fragments from scoria deposits at four 

localities in the Lathrop Wells scoria cone was reported by Crowe et al. (1983).

Based on paleomagnetic and field data, we have divided the eruptive events at the Lathrop 

Wells volcanic center into two separate periods of activity. The sequence of volcanic events is as 

follows, from oldest to youngest:

1. The development of a northwest-southeast-trending fissure complex composed of local 

vents of irregular scoria mounds and agglutinate (unit Qss, Fig. 2) accompanied by 

eruption of small block and aa flows (unit Qls, Fig. 2). The last phases of this eruption 

produced the main scoria cone. These units have similar directions of remanent 

magnetization which are statistically different from the directions obtained from geologic 

units of the next event

2. Subsequent volcanic activity occurred along a small, poorly-exposed vent system,

producing a lava flow unit (Qls, Fig. 2). The main body of this Qls lava flow flowed to the 

east and south around the older flow and vent complex.

A third period of volcanic activity was suggested by Renault et al., (1988) and Wells et al. 

(1988,1990), who described a complex sequence of buried soils and air-fall tephra (?), eolian and 

cone apron deposits, exposed in the quarry area. Wells et al. (1990) suggested that the lapilli-size 

tephra deposits in the soil profiles represent air-fall units that have been modified by the filling of 

the voids by eolian material. Wells et al. (1992) and Crowe et al. (1992) reported a 

thermoluminescence (TL) age of 9.9 ± 0.7 ka for these deposits. However, the volcanic origin of
these deposits has been questioned (Turrin et al., 1991,1992; Turrin and Champion, 1991; 

Champion, 1991; Whitney and Shroba, 1991).

An alternative interpretation (Turrin and Champion, 1991 and Turrin et aL 1991,1992) is 

that these deposits are derived from the adjacent cone slope. These deposits of sand, silt, and 

lapilli-size tephra supported in a matrix of eolian sand and silt occur immediately adjacent to the 

main cinder cone and overlie the scoria unit (Qss). Granulometry data on material from the basal 

portions of several of these "tephra11 deposits show that they contain 30 to 50 percent 

quartzofeldspathic eolian sand and silt (Turrin et al., 1992). This large proportion of eolian sand 

and silt cannot be explained by infiltration processes from overlying eolian units and argues against 

a volcanogenic origia 

Age Constraints on the Lathrop Wells Volcanic Center

Uranium-thorium and uranium-trend ages provide some constraints on the age of the 

Lathrop Wells volcanic center. Uranium-thorium dates, on massive laminated stalactitic calcrete 

from beneath the lava flow (unit Qls), and one on a thin, soft, porous carbonate layer from the base 

of a loess deposit that overlies scoria deposits (unit Qss) near the quarry, yield ages of 345 (+180, 

-70) ka, 345 (+180, -71) ka, and 25 ± 10 ka, respectively (Szabo et al., 1981). Szabo et al. 

(1981) considered the age on the loess layer to be a minimum age, because soil carbonates 

generally are not isotopically closed systems with respect to uranium. In addition, there is some 

time interval between eruption of a lava flow, and the accumulation of eolian material on the flow 

surface. Likewise, the stalactitic calcrete reflects a minimum age, but given favorable conditions 

can be considered an age for the cementation of the deposit

The principal source of error in uranium-thorium dating lies in the assumption of a closed 

system (Curtis, 1981). In fact, it has been shown that uranium-thorium ages are almost always too 

young when compared to other isotopic dating systems. Slate (1985) and Slate et al. (1991) 

compared uranium-thorium ages on soil carbonates from soils formed on K-Ar dated basaltic lava 

flows that range in age from 140 to 870 ka in the Pinacate volcanic field, northwestern Sonora,
Mexico. Uranium-thorium ages were consistently younger than the K-Ar ages by as much as an 

order of magnitude.

Uranium-trend dating does not require the isotopic system to be closed (Rosholt, 1977, 

1980). This method consists of determining an isochron from a given alluvial unit by analyzing 

four to nine samples covering various soil horizons. The results ideally define a linear isochron in 

which the slope increases with increasing age of the alluvium for a given half-period of the flux 

controlling the migration of uranium in the alluvial deposits (Rosholt, 1977,1980). An empirical 

model, calibrated by several horizons dated by other methods, compensates for different climatic 

and environmental conditions.

Uranium-trend ages on Quaternary stratigraphic units in the Yucca Mountain region provide 

additional constraints on the Lathrop Wells volcanic center (Fig. 3). Middle and upper Pleistocene 

alluvial, fluvial, and eolian deposits, the upper part of unit Q2c of Swadley et al. (1984), locally 

overlie and contain reworked cinders from the Lathrop Wells volcanic center. Swadley et al. 

(1984) reported;

1. Uranium-trend ages from the upper part of unit Q2c were 270 ± 30 ka and 240 ± 50 ka.

2. Stratigraphic relationships, however, indicate that unit Q2c is time-transgressive and the 

upper part of Q2c is equivalent to unit Q2s, which yields uranium-trend ages from 480 ± 

90ka to!60±90ka.

3. Unit Q2s is older than unit Q2b, which yields uranium-trend ages that range from 145 ± 25 

kato!60±18ka.

The uranium-thorium age on the calcrete from beneath the lava flow unit (Qla) requires that 

the unit be younger than 345 (+180, -70) ka. Moreover, if the uranium-trend ages and correlations 

between the Quaternary stratigraphic units in the Yucca Mountain region as proposed by Swadley 

et al. (1984) are correct, then the age of the major eruptive phase of the Lathrop Wells volcanic 

center is bracketed between 145 ± 25 and 270 ± 30 ka.

Cosmogenic 3He and 36C1 exposure ages provide minimum ages for the Lathrop Wells 

volcanic center. Volcanic bombs from the cinder cone rim (unit Qss, Fig. 2) yield cosmogenic 3He 

exposure ages of 22 ± 4 to 57 ± 7 ka (Poths and Crowe, 1992; Crowe et al., 1992; Poths et al., 

1994) and 36C1 exposure ages of 68 ±5.7 ka to 83 ±9.2 ka (Zreda et al., 1993). In addition, Zreda
et al. (1993) also report 36C1 exposure ages of 78 ±4.6 to 96 ±4.5 ka form volcanic bombs found 

on alluvial surfaces adjacent to the Lathrop Well volcanic center. Cosmogenic exposure ages from 

the lava flows yield similar ages as the volcanic bomb data. The 3He exposure ages from both lava 

flow units (Ql3 and Qls) yield overlapping ages that range from 61 ±6 ka to 100 ±9 ka (Poths et 

al., 1994). 36C1 exposure ages from the lava flow unit Ql5 are concordant with the ^He ages and 

range from 73 ±6.8 ka to 93 ±7.2 ka. The cosmogenic exposure age data do not provide any 

evidence for multiple eruptions nor do they support a Holocene age for the Lathrop Wells volcanic 

center. 

PALEOMAGNETIC STUDIES

Directions of remanent magnetization have been obtained from outcrops of the Lathrop Wells 

cinder cone and flow complex to investigate the possibility that the complex was formed by 

multiple eruptive events. The geomagnetic field is not constant in direction or intensity at any 

given location through time, but instead undergoes a secular variation at a rate of about 4° per 

century (Champion and Shoemaker, 1977). Thus, in any given area, volcanic rocks produced by 

two or more eruptive events that are different in age by more than a few decades usually do not 

record the same direction of magnetization. If units have the same directions of remanent 

magnetization, then the hypothesis that they were formed at the same time may not be precluded. 

If they have significantly different directions of remanent magnetization, they cannot have erupted 

at the same time. Given the rate of secular variation, it is possible to suggest the minimum time 

required to produce the observed directional differences. Therefore, paleomagnetic studies of 

eruptive units offer a simple and direct means of determining the number of eruptive events within 

a given volcanic field. 

Methods

Methods used in the paleomagnetic studies largely followed established procedures 

(McElhinny, 1973), except that a suncompass was used exclusively for core orientation. Core 

samples were taken in the field using a hand-held, gasoline-powered, diamond coring drill. Lava 

flows are generally sampled in cross section in paleomagnetic studies, but a young lava flow and
cone, such as Lathrop Wells, has few cross-sectional exposures. Exposures of massive flow 

interiors along the eroded eastern margin of the lava flow (unit Qls) provided the best sampling 

opportunity for that unit (Fig. 2). The older(?) scoria unit (Qss) was sampled in part from quarried 

exposures of rootless flow and spatter layers, but also in some cases from the tops and margins of 

naturally eroded volcanic constructs.

All specimens were progressively demagnetized in an alternating field to remove secondary 

remanent magnetizations due to viscous magnetic (VRM) and lightning-derived (IRM) 

components. Ideally, samples treated in this way should show a univectoral decay toward the 

origin on a vector-component diagram. Even with this treatment some specimens had unremovable 

secondary magnetic components probably due to lightning strikes. The stable endpoint remanent 

directions were averaged by site using conventional Fisherian statistical techniques. 

Results

Generally, well-grouped site mean directions were obtained after modest levels of alternating 

field demagnetization. With the exception of site B8179, which had severe lightning problems, 

most sites have a low percentage of rejected cores (see Table 1). The mean directions are well 

defined and present a coherent pattern on an equal-area net (Fig. 4A). Figure 4A shows all site 

means, with enclosing circles of 95 percent confidence for the lava and scoria units (Qls and Qss). 

The directions group in two masses close to one another. At first glance, these data seem nearly 

identical; however, the mean of site mean directions for each of the two geologic units define two 

separate directions of magnetization that differ by 4.7° (Fig. (4B). A statistical comparison of these 

mean directions shows them to be different at the 99.98 percent confidence level (see McFadden 

and Jones, 1981). At least two eruptive events are thus demonstrated for the Lathrop Wells 

volcanic center.

It is not possible to uniquely establish the time interval between these two directions of 

magnetization from paleomagnetic data alone. However, based on paleomagnetic studies of Sunset 

Crater in Arizona (Champion, 1980), the angular difference of 4.7° suggests that the time 

difference between the eruptive events is approximately 100 years.
K-Ar STUDIES 

Methods

All argon analyses in this report are by isotope dilution using procedures described by 

Dalrymple and Lanphere (1969). Potassium analyses are by flame photometry using a lithium 

internal standard following procedures described by Carmichael et aL (1968) or Ingamells (1970). 

The estimated analytical precision of the data is calculated using standard methods of error 

propagation, as described in Taylor (1982) . Reported analytical precision is one standard 

deviation (lo). 

Results

The data (Tables 2-5) exhibit significant variability between both individual rock samples 

and splits of the same rock sample. The error estimates given with each individual analysis 

consider only sources of analytical error. They do not address sample inhomogeneity, thus the 

dispersion in the data set (standard deviation) does not necessarily represent the analytical 

precision. A more reliable estimate of precision is probably derived from the standard error of the 

mean (SEM) of replicate ages. Weighted means (x^), in the tables, are calculated by multiplying 

the individual analysis (Xi) by its weighting factor, the reciprocal of the variance (l/o^), using the

following equation:

_ j= 1

xbest ~

I = 1

The uncertainty of the weighted average is obtained by pooling the variances and is calculated 

using the following equation:

i =

The value obtained from pooling the variance essentially equates to the standard error of the mean. 

See Taylor (1982) for a more complete description of using weighted means to obtain a single 

"best" estimated average and uncertainty for a data set Generally, the preferred age given for any

8
set of data is the weighted average because the more precise analyses are given more weight in the 

determination of the average age.

The data presented in Table 2 were obtained in 1978 as part of the initial volcanic hazard 

studies for the Yucca Mountain Project Three separate analyses of sample TSV-1, from the lava 

flow unit (Qls), collected from the stream-eroded eastern flow margin (Fig. 2), yielded ages of 

336 ± 28,38 ± 19, and 3 ± 75 ka. The arithmetic mean and standard deviation of sample TSV-1 

is 126 ± 183 ka with a SEM of 106 ka. The weighted average obtained from the sample TSV-1 is 

127 ± 15 ka. Sample TSV-129, a welded spatter collected from the summit crater of the cinder 

cone at Lathrop Wells, yielded an age of 670 ± 400 ka. A K-Ar age of 300 ± 100 ka was reported 

for this same sample (TSV-129) by Marvin et al. (1989) who also reported an age of 230 ± 40 ka 

for sample TSV-283, from the interior of a volcanic bomb collected on the west side of the summit 

of the Lathrop Wells cone.

In 1979 samples were again collected from the Lathrop Wells volcanic center as part of a 

test to evaluate the overall precision of K-Ar dating of late Cenozoic basalts by different 

laboratories (Sinnock and Easterling, 1983). Six samples from the Lathrop Wells center were 

collected from the block flow unit (Ql5, Fig. 2) (Scott Sinnock, Sandia National Laboratory, oral 

commun., 1989). In that study, no attempt was made to minimize the sample heterogeneity. The 

six samples were collected from one site within a radius of 20 to 30 m. The samples were 

physically split into three nominally equal pieces and sent to three separate K-Ar laboratories for 

analyses - University of California, Berkeley; Laboratory of Isotope Geochemistry, Department of 

Geosciences, University of Arizona; and Geochron Laboratories Division, Kruger Enterprises 

Inc., Cambridge, Mass. Although the results are not identified with the laboratory, the Berkeley 

laboratory chooses to identify its results as laboratory B in Table 3.

The K-Ar ages on the Lathrop Wells volcanic center in the Sinnock and Easterling (1983) 

report ranged from 1.1 Ma to -30 ka (table 2.3). The arithmetic means, standard deviation, and 

standard error of the mean, calculated from the data in Sinnock and Easterling (1983), are

Laboratory A: 730 ± 190 ka, SEM = ±80 ka
Laboratory B: 80 ± 80 ka, SEM = ±30 ka (Be±eley);

Laboratory C: 570 ± 90 ka, SEM = ±40 ka.

The weighted averages of these data are 660 ± 30 ka, 100 ± 20 ka, and 530 ± 40 ka, respectively. 

The results from laboratory B (Berkeley) are statistically younger (at the 95 percent confidence 

level) than both laboratory A and laboratory C. However, the original Berkeley data (Table 4) 

show slight discrepancies with those reported in the Sinnock and Easterling (1983) report in that:

1. Sample 1 should have an arithmetic mean of 80 ka, not 120 ka.

2. Sample 5 should have an arithmetic mean of 115 ka, not 125 ka.

In addition, the standard deviations reported by Sinnock and Easterling (1983) were 

apparently obtained by arithmetically averaging those reported for the individual measurements. 

We feel that is not the correct way to have handled these data. Granted, calculating a standard 

deviation for a sample number of two (n = 2) is also not the correct way to handle the data either, 

because it is possible to underestimate the standard deviation. For example, sample 4 (Table 4) 

yields ages of 90 ± 30 ka and 70 ± 30 ka. The arithmetic mean of these numbers is 80 ka, with a 

standard deviation of ± 10 ka. The correct way to calculate a standard deviation for these data 

would be to pool the variances using the equation of Taylor (1982). The pooled standard deviation 

for sample 4 is ± 21 ka.

The corrected results from the Berkeley data, given in Table 4, yield an arithmetic mean and 

standard deviation of 68 ± 106 ka for all analyses, with a SEM of ± 31 ka. The weighted mean of 

the data is 97 ± 13 ka.

In 1986, samples were collected at five different sites from the lava flows at the Lathrop 

Wells volcanic center (Fig. 2). Argon extractions and isotopic analyses were performed at the 

Berkeley Geochronology Center, Berkeley, Calif. (BGC). Samples 1-86 through 4-86 are from 

the lava flow unit (Qls). Sample 5-86 is from block flow unit (Qls) and is inferred to underlie or 

laterally merge into the scoria unit (Qss).

The K-Ar analytical data for both flow units (Qls and Qls) are presented in Table 5. The 

data for the two units range from 37 ka to 571 ka and from 110 ka to 337 ka, respectively. The

10
arithmetic mean, weighted average, standard deviations, and the SEM for each site is given for 

three replicate analyses in Table 5. For the lava flow unit Qls, however, sample number NNTS 

#3-86 appears to be consistently and anomalously old relative to the other 3 sites. This sample has 

a profound effect on the arithmetic mean, standard deviation and SEM. If sample NNTS #3-86 is 

included, the arithmetic mean is 214 ± 170 ka with a SEM of ± 49 ka. If NNTS #3-86 is rejected 

the values drop to 125 ± 53 ka, SEM of ± 18 ka, respectively. Because the 3 individual 

determinations for sample NNTS #3-86 each have large uncertainties, they have little effect on the 

weighted average. The weighted average age of unit (Qls) including sample NNTS #3-86 is 137 ± 

13 ka; excluding sample NNTS #3-86, the weighted average is 135 ± 13 ka. There are no obvious 

outliers in the data set for the block flow unit (Qls). The arithmetic mean for this unit is 188 ± 80 

ka with a SEM of ± 33 ka (Table 5). The weighted mean for the block flow from this data set is 

176 ± 60 ka.

DISCUSSION

K-Ar ages for the Lathrop Wells cinder cone and flow complex range from -170 ka to 1.1 

Ma (Tables 2-5). The dispersions in the K-Ar data are in part due to two factors:

1. Large atmospheric argon corrections. The 40Ar extracted from a sample consists 

primarily of two components, atmospheric 40Ar and radiogenic 40Ar (40Ar*). The 

atmospheric ^Ar, equal to 295.5 times the 36Ar, is subtracted from the total 40Ar by 

measuring the 36Ar component which is all from atmospheric contamination. The 

errors introduced by subtracting the large atmospheric ^Ar from the total measured 

4^Ar increase exponentially as the residuals between the two numbers diminish. 

Therefore, errors can be quite large when the percent ^Ar* is small (see Dalrymple and 

Lanphere, 1969, p. 104, for a more complete discussion).

2. The fact that some bias toward anomalously old ages may be introduced by the 

occurrence of extraneous ^Ar incorporated into rocks and minerals by several means. 

As defined in Dalrymple and Lanphere, (1969, p. 121), argon (^Ar) incorporated into 

the material being dated by any process other than in situ radioactive decay of ^K is

11
called excess ^Ar. Radiogenic ^Ar* produced within the rock or mineral grains by 

4°K before the event being dated is referred to as inherited ^Ar. Radiogenic ^Ar that 

was produced before a metamorphic or thermal event and that has remained in the 

rock/mineral system is called inherited metamorphic ^Ar. Argon can also be inherited 

by incorporating older contaminating material into the sample being dated, either by 

natural processes or during the laboratory processing of the sample. This is referred to 

as inherited contamination ^Ar.

The possible effects of excess argon from contaminating xenoliths from the underlying 

tuffs, not addressed by either the Environmental Assessment (U.S. Department of Energy, 1986) 

or Sinnock and Easterling (1983), are discussed below and also by Turrin and Champion (1991) 

and Turrin et al. (1991,1992). Crowe et al. (1983) report the occurrence of 0.032 percent tuff 

fragments, by volume, in the Lathrop Wells scoria deposits. This equates to 0.06 percent tuff 

xenoliths by volume (0.054 percent by weight; tuff density = 2.7 g/cm3; basalt density = 3.0 

g/cm3) in the Lathrop Wells lava flows, assuming densities of 1.5 gm/cm3 and 3.0 g/cm3 for the 

scoria and lava flows, respectively. The tuff xenoliths are most likely from the Topopah Spring 

Member of the Paintbrush Tuff. The Topopah Spring Member was K-Ar dated by Marvin et al. 

(1970) at 13.2 Ma. Potassium analyses from the Topopah Spring Member range from 4.7 to 6.0 

percent K2O (Lipman et al., 1966). The effect of contaminating grains can be estimated using the 

formula

ti = 13 + ft2 x K2/Ki;

where ti is the apparent age of sample, t2 is the apparent age of the contaminant, t3 is the true age 

of sample, f is the fraction of contaminant, KI is the potassium content of the uncontaminated 

sample, and K2 is the potassium content of the contaminant (Dalrymple and Lanphere, 1969, p. 

142). A conceptual uniform 0.06 percent tuff contamination would add an additional 22 ka to the 

K-Ar ages of the lava flows, provided that none of the radiogenic ^Ar released from the xenoliths 

escaped out of the lava flow, an unlikely scenario. Even though the samples were screened for 

obvious visible contamination, such contamination was retrospectively seen in sample NNTS #3-

12
86 (Table 4). The contamination of this sample by tuff xenoliths is identified by the atomic K/Ca 

ratios (K/Ca = 0.49 x ^Arx/^Arca see Dalrymple et al., [1981]) and by their analytically distinct 

older ages (Turrin et al. 1991,1992; Turrin and Champion, 1991).

Zero age residuals for the two Berkeley argon-extraction/mass spectrometer systems (Table 

6) are obtained by fusing sealed capillary pipettes that contain 0.2 to 1 microliters of air in the 

argon-extraction system. The air samples are processed and the AT isotopic ratios are measured hi 

the exact same manner as unknown samples. An age is calculated from these data based on an 

approximate potassium content and the average weight of basalt sample used for a typical analysis. 

By definition these calculated ages should be zero, because the air pipette samples do not contain 

any radiogenic 40Ar*.

Age resolution for an unknown basalt sample is given by these residual data. Residual ages 

for the two argon-extraction/mass spectrometer systems used (BC and GORT) have an arithmetic 

mean of 15 ± 70 ka and -34 ± 117 ka, with a SEM of 31 ka and 52 ka, respectively. However, the 

weighted average of the residual ages of 22 ± 15 ka for the BC system and 8 ± 13 ka for the 

GORT system, is the more significant value because the better analyses are given more weight hi 

the determination of the average zero age residuals. The zero-age residual data indicate that there is 

no statistically significant bias in the atmospheric 4° AT corrections for either of the extraction/mass 

spectrometer systems. In fact, the combined zero age residual data from the two systems 

(arithmetic mean -10 ± 94 ka; SEM 30 ka; weighted average 14 ± 10 ka) indicate that any sample 

randomly analyzed several times (n> 5) with approximately 1.5 percent K+ and using about 7.4 g 

of material would have an instrumental standard error of about 10 to 20 ka.

Combining the reported analytical precision of the weighted averages for the two flow units 

and the zero-age residuals, a 95 percent confidence interval of 30 ka can be obtained using the

following equation from Dalrymple and Lanphere (1969, p. 120):

1/2 

C.V. = 1.960x(a2/n!+a2/n2)

13
This value (30 ka) is the minimum difference in age between the two flows that would be 

detectable at the 95 percent confidence level, provided that each sample is measured at least five 

times.

As previously noted, the uranium-trend ages and the proposed correlations between the 

Quaternary stratigraphic units in the Yucca Mountain region bracket the age of the eruptive phases 

of the Lathrop Wells volcanic center between 145 ± 25 ka and 270 ± 30 ka (Swadley et al., 1984). 

These results and the ^Ar/^Ar age data reported by Turrin and coworkers (Turrin and 

Champion, 1991; Turrin et al. 1991,1992), indicating a potential for contamination from the 

underlying tuff, suggest that the K-Ar ages obtained from laboratory A (660 ± 30 ka) and 

laboratory C (530 ± 40 ka) must be rejected. In addition, because zero-age residuals are not 

reported in either the Sinnock and Easterling (1983) or U.S. Department of Energy (1986) report, 

the reported K-AT data from laboratory A and laboratory C cannot be tested for any possible 

systematic errors originating from the atmospheric 4^Ar corrections. For these reasons the data 

from these laboratories are excluded in the subsequent analysis.

The accepted K-Ar data are (Table 7) 21 analyses from the block flow unit (Qls) and cinder 

complex that yield an arithmetic mean age of 150 ± 165 ka and a SEM of ± 39 ka. The weighted 

average of the flow unit (Qls) sample set is 115 ± 12 ka. All 15 samples from the lava flow unit 

(Qb) yield an arithmetic mean of 196 ± 170 ka, a SEM of ± 44 ka, and a weighted average of 133 

± 10 ka. If the 3 age determinations of the contaminated sample (NNTS #3-86) are deleted from 

the data set, the arithmetic mean is 126 ± 90 ka and the SEM is ± 26 ka. Again, because the 3 

individual determinations for sample NNTS #3-86 have large uncertainties, they have little effect 

on the weighted average. The weighted average without sample NNTS #3-86 is!31±10ka. 

None of the ages just discussed are statistically different at the 95 percent confidence level. The KAr ages from the two flow units Qls are entirely consistent with the uranium-trend chronology and 

stratigraphy of the surficial units in the Yucca Mountain region (Swadley et al. 1984).

14
SUMMARY

We believe the data presented in Table 7 to be the best synthesis of the numerous 

conventional K-Ar age determinations from the Lathrop Wells volcanic center. Possible effects of 

contamination by the tuff xenoliths and the possibility of systematic errors in the atmospheric argon 

correction have been considered. The weighted averages of the K-Ar ages from the block flow 

unit (Qls) and lava flow unit (Qls) (115 ± 12 ka and 133 ± 10 ka, respectively) indicate an older 

age for the most recent period of volcanic activity at Lathrop Wells than suggested by Wells et al. 

(1990). These ages are concordant at the 95 percent confidence level with the 4QAr/39Ar results 

presented in Turrin and Champion (1991) and Turrin et al. 1991,1992). Moreover, these K-Ar 

ages are consistent with the geochronology (Szabo et al., 1981) and stratigraphy (Swadley et al., 

1984) of the surficial units in the Yucca Mountain region.

Paleomagnetic data indicate that there may be two statistically differentiable eruptive units, 

block flow unit (Qls) and the lava flow unit (Qls), occurring at the Lathrop Wells volcanic center. 

Their mean directions of remanent magnetization are 51.5° inclination 2.2° declination, and 51.4° 

inclination 354.5° declination, respectively. The angular difference in the directions of 4.7° is 

significant at the 99.98 percent confidence level. The angular difference between the two mean 

paleomagnetic directions suggests a minimum age difference between the eruptive events of about 

100 years. Given the reported analytical precision of the weighted average K-Ar ages of the two 

units and the zero-age residuals of the spectrometers of approximately ± 10 ka, the statistical 

difference in age between Q15 and Q13 is less than or equal to approximately 30 ka.

15
REFERENCES CITED

Armstrong, R.L., Ekren, E.B., McKee, E.H., and Noble, D.C., 1969, Space-time relations of 

Cenozoic silicic volcanism in the Great Basin of the western United States, Am. J. Sci., v. 

267, p. 478-490.

Byers, P.M., Jr., Carr, W.J., Orkild, W.D., Quinlivan, W.D, and Sargent, K.A., 1976, Volcanic 

suites and related cauldrons of the Timber Mountain Oasis Valley Caldera Complex, southern 

Nevada, U.S. Geol. Surv. Prof. Pap. 919, 70 pp.

Carmichael, I.S.E., Hample, J., and Jack, R.N., 1968, Analytical data on the U.S. Geological 

Survey standard rocks, Chem. Geol., v. 3, p. 59-64.

Champion, D.E., 1980, Holocene geomagnetic secular variation in the western United States: 

Implications for global geomagnetic field, U.S. Geol. Surv. Open-File Rep. 80-824,326 pp.

Champion, D.E., 1991, Volcanic episodes near Yucca Mountain as determined by paleomagnetic 

studies at Lathrop Wells, Crater Flat, and Sleeping Butte, Nevada, in Proc. 3rd Ann. Intl. 

Conf., High Level Radio. Waste Mang. (Amer. Nucl. Soc., LeGrange, 111.), 1, p. 61-67.

Champion, D.E., and Shoemaker, E.M., 1977, Paleomagnetic evidence for episodic volcanism on 

the Snake River Plain, in Planetary Geology Field Conference on the Snake River Plain, 

Idaho, Oct 1977, NASA Tech. Memo. 78,436, p. 7-9.

Christiansen, R.L., and Lipman, P.W., 1972, Cenozoic volcanism and plate tectonic evolution of 

the western United States H: Late Cenozoic, Phil. Trans. Roy. Soc. London, Ser. A, v. 271, 

p. 249-284.

Christiansen, R.L., Lipman, P.W., Carr, W.J., Byers, P.M., Jr., Orkild, P.P., and Sargent, 

K.A., 1977, Timber Mountain-Oasis Valley caldera complex of southern Nevada, Geol. Soc. 

Am. Bull., v. 88, p. 943-959.

Crowe, B.M., and Carr, W.J., 1980, Preliminary assessment of the risk of volcanism at a 

proposed nuclear waste repository in the southern Great Basin: U.S. Geol. Surv. Open-File 

Rep. 80-357, 15 pp.

Crowe B., Self, S., Vaniman, D., Amos, R., and Perry, F., 1983, Aspects of potential magmatic 

disruption of a high level radioactive waste repository in southern Nevada, J. Geol., v. 91, p. 

259-276.

Crowe, B., Morley, R., Wells, S., Geissman, J., McDonald, E., McFadden, L., Perry, F., 

Murrel, M., Poths, J., and Forman S., 1992, The Lathrop Wells Volcanic Center: Status of 

Field and Geochronology Studies," in Proc. 3rd Ann. Intl. Conf., High Level Radio. Waste 

Man., Amer. Nucl. Soc., LeGrange, 111., v. 2, p. 1997-2013.

Curds, G.H., 1981, A guide to dating methods for the determination of the last time of movement 

of faults: U.S. Nucl. Reg. Comm., Rep. NUREG/CR-2382, 314 pp.

Dalrymple, G.B., and Lanphere, M. A., 1969, Potassium-Argon Dating, San Francisco, W.H. 

Freeman, 258 pp.

16
Dalrymple, G.B, Alexander, C.E., Jr., Lanphere, M.A., and Kraker, G.P., 1981, Irradiation of 

samples of ^Ar/^Ar dating using the Geological Survey TRIGA reactor, U.S. Geol. Surv. 

Prof. Pap. 1176,55pp.

Ingamells, C.O., 1970, Lithium metaborate flux in silicate analysis, Anal. Chim. Acta, v. 52, p. 

323-334.

Leeman, W.P., and Rodgers, J.W., 1970, Late Cenozoic alkali-olivine basalts of the Basin-Range 

Province, U.S.A., Cont Min. Petrol., v. 25, p. 1-24.

Lipman, P.W., 1980, Cenozoic volcanism in the Western United States: Implications for

continental tectonics, in Studies in geophysics: Continental tectonics, Washington, Natl. Acad. 

Press, p. 161-174.

Lipman, P.W., Christiansen, R.L., and O'Connor, J.T., 1966, A compositionally zoned ash-flow 

sheet in Southern Nevada, U.S. Geol. Surv. Prof. Pap. 534-F, 47 pp.

Lipman, P.W., Prostka, H.J., and Christiansen, R.L., 1972, Cenozoic volcanism and platetectonic evolution of the Western United States I: Early and middle Cenozoic, Phil. Trans. 

Roy. Soc. London, Ser. A, v. 271, p. 217-248.

Luedke, R.G., and Smith, R.L., 1978a, Map showing distribution, composition, and age of Late 

Cenozoic volcanic centers in Arizona and New Mexico, U.S. Geol. Surv. Misc. Inv. Ser. Map 

1-1091 A, scale 1:1,000,000.

1978b, Map showing distribution, composition, and age of late Cenozoic volcanic centers

in Colorado, Utah, and southwestern Wyoming, U.S. Geol. Surv. Misc. Inv. Ser. Map I1091B, scale 1:1,000,000.

____ 1981, Map showing distribution, composition, and age of late Cenozoic volcanic centers 

in California and Nevada, U.S. Geol. Surv. Misc. Inv. Ser. Map I-1091C, scale 1:1,000,000.

___ 1982, Map showing distribution, composition, and age of late Cenozoic volcanic centers 

in Oregon and Washington, U.S. Geol. Surv. Misc. Inv. Ser. Map I-109ID, scale 

1:1,000,000.

____ 1983, Map showing distribution, composition, and age of late Cenozoic volcanic centers 

in Idaho, western Montana, west-central South Dakota, and northwestern Wyoming, U.S. 

Geol. Surv. Misc. Inv. Ser. Map I-1091E, scale 1:1,000,000.

Marvin R.F., Byers, P.M., Jr., Mehnert, H.H., Orkild, P.P., and Stern, T.W., 1970,

Radiometric ages and stratigraphic sequence of volcanic and plutonic rocks, southern Nye and 

western Lincoln Counties, Nevada, Geol. Soc. Am. Bull., v. 81, p. 2657-2676.

Marvin R.F., Mehnert, H.H., and Naeser, C.W., U.S. 1989, Geological Survey radiometric 

ages-compilation "C"; Part Three: California and Nevada, Isochron/West, v. 52, p. 3-11.

McElhinny, M.W., 1973, Paleomagnetism and Plate Tectonics, Cambridge, England, Cambridge 

University Press, 358 pp.

McFadden, P.L., and Jones, D.L., 1981, The fold test in palaeomagnetism, Geophys. J. R. 

Astron. Soc., v. 67, p. 53-58.

17
McKee, E.H., 1971, Tertiary igneous chronology of the Great Basin of the western United States: 

Implications for tectonic models, Geol. Soc. Am. Bull., v. 82, p. 3497-3502.

McKee, E.H., and Noble, D.C., 1986, Tectonic and magmatic development of the Great Basin of 

western United States during late Cenozoic time, Mod. Geol., v. 10, p. 39-49.

Noble, D.C., 1972, Some observations on the Cenozoic volcano-tectonic evolution of the Great 

Basin, western United States, Earth Planet Sci. Let, v. 17, p. 142-150.

Poths, J., Perry, F., Crowe, B.M., 1994, ^He surface exposure ages at the Lathrop Wells, NV., 

volcanic center, in Abst, Eighth Intl. Conf. on Geochron., Cosmochrono., and Isotope 

Geol., Lanphere, M.A., Dalrymple, G.B., and Turrin, B.D., eds.: U.S. GeoLSurv. 

Circ.1107, p. 255.

Poths, J. and Crowe, B.M., 1992, Surface exposure ages and noble gas components of volcanic 

units at the Lathrop Wells volcanic center: EOS Trans Am. Geophys. Union, v. 73, p. 610.

Renault, C.E., Wells, S.G., and McFadden, L.D., 1988, Geomorphic and pedologic evidence for 

polycyclic volcanism in late Quaternary cinder cones: Examples from Cima volcanic field, 

California and Crater Flat/Lathrop Wells volcanic field, Nevada, Geol. Soc. Amer. Absts. 

Progs., v. 20, p. 115.

Rosholt, J.N., 1977, Uranium-trend dating of alluvial deposits, in Short papers of the Fourth 

International Conference, Geochronology, Cosmochronology, Isotope Geology, U.S. Geol. 

Surv. Open-File Rept. 78-701, p. 360-362.

Uranium-trend dating of Quaternary sediments, 1980, U.S. Geol. Surv. Open-File Rept

80-1087, 34pp.

Sinnock, Scott, and Easterling, R.G., 1983, Empirically determined uncertainty in potassiumargon ages for Plio-Pleistocene basalts from Crater Flat, Nye County, Nevada, Sandia Natl. 

Labs. Rept SAND82-2441, 17 pp.

Slate, J.L., 1985, Soil-carbonate genesis in the Pinacate volcanic field, northwestern Sonora, 

Mexico, Univ. of Ariz., Tucson, M.S. Thesis, 85 pp.

Slate, J.L., Bull, W.B., Ku, T.L., Shafiqullah, Muhammad, Lynch, D.J., and Huang, Y.P., 

1991, Soil-carbonate genesis in the Pinacate volcanic field, northwestern Sonora, Mexico, 

Quat Res., v. 35, p. 400-416.

Steiger, R.H., and Jager, E., 1977, Subcommission on geochronology-convention on the use of 

decay constants in geo- and cosmochronology, Earth Planet Sci. Let, v. 36, p. 359-362.

Swadley, W.C., Hoover, D.L., and Rosholt, J.N., 1984, Preliminary report on late Cenozoic 

faulting and stratigraphy in the vicinity of Yucca Mountain, Nye County, Nevada, U.S. Geol. 

Surv. Open-File Rept 84-788, 42 pp..

Szabo, B.J., Can, W.J., and Gottschall, W.C., 1981, Uranium-thorium dating of Quaternary 

carbonate accumulations in the Nevada Test Site region, southern Nevada, U.S. Geol. Surv. 

Open-File Rept 81-119, 35 pp.

Taylor, J.R., 1982, An introduction of error analysis, the study of uncertainties in physical 

measurements, Oxford University Press, 270 pp.

18
Turrin, B.D., and Champion, D.E., 1991, ^Ar/^Ar laser fusion and K-Ar ages from Lathrop 

Wells, Nevada, and Cima, California: The age of the latest volcanic activity in the Yucca 

Mountain area; in Proc. 3rd Ann. Intl. Conf., High Level Radio. Waste Manage. (Amer. 

Nucl. Soc.), LeGrange, 111., v. 1, p. 68-75.

Turrin, B.D., and Dohrenwend, J. C., 1984, K-Ar ages of basaltic volcanism in the Lunar Crater 

volcanic field, northern Nye County, Nevada: Implications for Quaternary tectonism in the 

central Great Basin, Geol. Soc. Amer. Absts. Progs., v. 16, p. 679.

Turrin, B.D., and Gillespie, A., 1986, K-Ar ages of basaltic volcanism of the Big Pine volcanic 

field, California: Implications for glacial stratigraphy and neotectonics of the Sierra Nevada, 

Geol. Soc. Amer. Absts. Progs., v. 18, p. 777.

Turrin, B.D., and Renne, P.R., 1987, Multiple basaltic eruption cycles from single vents, Cima 

volcanic field, California; Evidence for polygenetic basaltic volcanism, Geol. Soc. Amer. 

Absts. Progs., v. 19, p. 458.

Turrin, B.D., Dohrenwend, J.C., Drake, R.E., and Curtis, G.H., 1985a, K-Ar ages from the 

Cima volcanic field, eastern Mojave desert, California, Isochron/West, v. 44,p. 9-16.

Turrin, B.D., Renne, P.R., and Dohrenwend, J.C., 1985b, Temporal trends in the chemical 

evolution of megacryst bearing, subalkaline-alkaline basaltic lavas from the Lunar Crater 

volcanic field, Nye County, Nevada, Geol. Soc. Amer. Absts. Progs., v. 17, p. 414.

Turrin, B.D., Champion, D.E., and Fleck, R.J., 1991,^Ar/^Ar Age of the Lathrop Wells 

Volcanic Center, Yucca Mountain, Nevada, Science, v. 253, p. 654-657.

Turrin, B.D., Champion, D.E., and Reck, R.J., 1992, Measuring the age of the Lathrop Wells 

volcanic center at Yucca Mountain, Response, Science, v. 257, p. 555-558.

U.S. Department of Energy, 1986, Environmental assessment Yucca Mountain site, Nevada 

Research and Development Area, Nevada, U.S. Dept of Energ. Rept. DOE/RW-0073, v. 1.

Vaniman, D.T. and Crowe, B.M., 1981, Geology and petrology of the basalts of Crater Flat: 

Applications to volcanic risk assessment for the Nevada Nuclear Waste Storage Investigations, 

Los Alamos Natl. Lab. Rept., LA-8845-MS, 67 pp.

Vaniman, D.T., Crowe, B.M., and Gladney, E.S., 1982, Petrology and geochemistry of Hawaiite 

lavas from Crater Flat, Nevada, Cont. Mineral. Petrol., v. 80, p. 341-357.

Wells, S.G., McFadden, L.D., and Renault, C., 1988, A geomorphic assessment of Quaternary 

volcanism in the Yucca Mountain area, Nevada Test Site, southern Nevada, Geol. Soc. Amer. 

Absts. Progs., v. 20,p. 242.

Wells, S.G., McFadden, L.D., Renault, C.E., and Crowe, B.M., 1990, Geomorphic assessment 

of late Quaternary volcanism in the Yucca Mountain area, southern Nevada: Implications for 

the proposed high-level radioactive waste repository. Geol., v. 18, p. 549-553.

Wells, S.G., Crowe, B.M., and McFadden, L.D., 1992, Measuring the Age of the Lathrop Wells 

Volcanic Center at Yucca Mountain, Science, v. 257, p. 555-556.

19
Whitney, J.W., and Shroba, R.R., 1991, Comment on Geomorphic Assessment of Late 

Quaternary Volcanism in the Yucca Mountain Area, Southern Nevada: Implications for the 

Proposed High-Level Radioactive Waste Repository, GeoL, v. 18, p. 661.

Zreda, M.G., Phillips, P.M., Kubik, P.W., Sharma, P., Elmore, D., 1993, Cosmogenic 36C1 

dating of a young basaltic eruption complex, Lathrop Wells, Nevada, Geology., v. 21, p. 57- 

60.

20
FIGURE CAPTIONS

Figure 1. Location map of the Crater Flat/Yucca Mountain area. Brick pattern delineates Paleozoic 

limestones; stippled pattern represents Tertiary ash-flow tuffs; chevron patterns are 

Tertiary basalt flows; light shaded areas are Tertiary and Quaternary scoria cones and 

pyroclastic deposits; dark shaded areas are Quaternary basalt flows; white area is 

Quaternary alluvium. Sawtooth line is a detachment fault, teeth toward upper plate. On 

the inset LV indicates Las Vegas, NV.

Figure 2. Generalized geologic map of the Lathrop Wells cinder cone and flow complex, showing 

locations of sampling sites of K- AT and paleomagnetic samples. Numbers correspond to 

samples listed in Tables 1 through 5. Areas enclosed with hachured outlines are scoria 

mounds composed of welded agglutinate and volcanic bombs; area enclosed with 

sawtoothed outline is crater at top of main scoria cone. Units: Qls, early basaltic lava 

flows; Qb, later basaltic lava flows; Qss, early scoria deposits; Qpsi, pyroclastic basesurge deposits; Qsu, undifferentiated scoria deposits, with some scoria mounds 

contemporaneaous with unit Qls. Unpatterned areas, Quaternary alluvium.

Figure 3. Correlations between the regional Quaternary stratigraphic units and corresponding

radiometric ages in the Yucca Mountain region as reported by Swadley et al. (1984). If 

these correlations are correct, then the age of the major eruptive phase of the Lathrop 

Wells volcanic center is bracketed between 145±25ka and 27Q±30ka.

Figure 4. Equal-area plots of (A) mean directions of remanent magnetization with circles of 95% 

confidence for paleomagnetic sites taken in Lathrop Wells volcanic units, and (B) mean 

directions of remanent magnetization with circles of 95% confidence for units Qls and 

Qss at Lathrop Wells. Unit means differ at the 99.98% confidence level.

21
Table 1 Lathrop Wells Paleomagnetic Data

Site

Location 

Lat°N Long°E

Peak Cleaning Direction 

N/No Field (mT) I D a95 k R

Unit QI3

B8017

B8119

B8131

B8143

B8155

B8167

B8179

Mean

>

36.696°

36.694°

36.694°

36.693°

36.693°

36.689°

36.684°

243.495°

243.502°

243.502°

243.503°

243.503°

243.506°

243.505°

23/24

12/12

12/12

12/12

12/12

12/12

6/12

111

20

10+

10+

10+

5+

10+

20+

47.0°

53.4°

52.1°

52.4°

50.6°

51.9°

52.6°

51.4°

355.8°

356.2°

353.6°

351.9°

355.8°

356.3°

351.5°

354.5°

1.8°

2.6°

2.7°

2.8°

3.1°

2.9°

2.7°

1.9°

286

292

266

245

196

222

634

1060

22.92319

11.96229

11.95857

11.95517

11.9438

11.9504

5.9921

6.9943

t 

Unit Qs5

B8041

B8061

B8191

B8203

B8215

Mean

36.684°

36.688°

36.683°

36.683°

36.684°

243.492°

243.494°

243.497°

243.497°

243.497°

20/20

11/12

12/12

9/12

13/15

5/5

20+

30+

10

25+

10+

51.8°

53.9°

49.6°

51.4°

50.6°

51.5°

2.2°

2.0°

0.4°

1.7°

4.7°

2.2°

1.3°

3.1°

2.2°

2.8°

1.6°

1.8°

595

222

402

331

656

1855

19.9679

10.9548

11.9726

8.9758

12.9817

4.9978

[N/No is number of cores averaged in site mean direction over number of cores taken at site. I and D are mean r

declination values, ot95 is radius of the cone of 95 percent confidence about the mean direction, k is Fisherian pr

length of site resultant vector, and VGP is virtual geomagnetic pole calculated from site location and mean reman
Table 2. Lathrop Wells K-Ar, U.S. Geological Survey, 1978

Flow 

unit

Qb

Sample 

number

TSV-1

K+ 

(wt . percent)

1.509±0.005

(mol/g x 10- 12)

0.907 

0.100 

0.012

Percent

2.4 

0.4 

0.1

Age±la(ka)

336 ±28 

38 ±19 

3±75

Best age for

Q15 TSV-129 

TSV-129*

Arith. mean ±la is 126 ± 183ka [SEM ± 106ka] 

Weighted average is 127 ± 15 ka

Arith. mean ± SEM 126 ± 106 ka

to TSV-283a 

Best age for Qls;

1.500±0.006 

1.552±0.012

1.498±0.006

1.727 3.8 670 ±400 

0.7552 1.7 300 ±100

Arith. mean ±la is 485 ± 261ka [SEM ± 185ka] 

Weighted average is 322 ± 97ka

0.5861 3.0 230 ±40

Weighted average 243 ± 37 ka

[Decay constants: Xe + Xe1 = 0.581 x 10'10 yr1 ; kp = 4.962 x lO'10 yr1 ; an 

aData from Marvin et al. (1989)

= 1.167 x 1(H; see 
Table 3. Lathrop Wells K-Ar Data as reported in Sinnock and Easterling (1983)

Percent

Sample K+% (mol/g x 10-12)

Laboratory A

Laboratory B

Age±la(ka)

1

2

3

4

5

6

1.389

1.5281

1.518

1.611

1.586

1.503

1.75

1.78

2.10

1.70

3.25

1.58

3.5

1.5

2.8

1.6

2.4

2.3

700 ±70

650 ±70

770 ±80

590 ±60

1100±300

580 ±80

1

2

3

4

5

6

1.633

1.606

1.628

1.545

1.577

1.558

0.23

0.10

-0.08

0.21

0.30

0.48

1.6

0.1

0.0

1.1

0.4

0.9

120 ±30

-10 ±290

-30 ±140

80 ±30

125 ± 180

175 ±90

Laboratory C

1

2

3

4

5

6

1.625

1.643

1.580

1.597

1.607

1.566

1.69

1.74

1.80

1.54

1.64

1.05

7.9

4.0

6.5

7.2

3.4

6.6

600± 90

610 ±160

660 ±100

560 ± 90

590 ±210

390 ± 70

[Isotope abundances for potassium were not given, and decay constants were not 

specified in the Sinnock and Easterling (1983) report. It was reported, however, that

one of the laboratories used Xe + Xe- = 0.585 x 10'10 yr1 ; Xp = 4.72 x 10'10 yr1 for 

decay constants; the other two used the accepted decay constants of Steiger and Jager 

(1977) of (Xe + Xe- = 0.581 x 10-10 yrl; X = 4.962 x 10-10 yr-1)]

24
Table 4. U.C. Berkeley K-Ar data used in Sinnock and Easterling (1983)

Flow 

unit

Q15

Sample

1

Lab 

number

3667 

3668

Sample 

weight (g)

7.2064 

8.5813

%K+

1.633 ±0.03 

1.633 ±0.03

^Ar* 

(mol/gmxlO- 11)

0.035 

0.011

%Ar*

2.8 

0.5

Age ± la (ka)

120 ±20 

40±40

Q15

Q15

Q15

Q15

Q15

3697

3706

3665

3787

3655

3721

3662

3663

3659

3728

7.6553

10.4419

7.7166

9.8831

8.5546

8.995

9.4878

9.2694

8.6201

11.7424

1.577 ±0.02 

1.635 ± 0.03

1.626 ±0.01 

1.630 ±0.01

1.547 ± 0.01 

1.542 ±0.01

1.557 ± 0.01 

1.577 ±0.01

1.558 ±0.04 

1.558 ±0.04

Arith. mean ± la is 80 ± 18 ka 

Weighted average is 104 ± 18 ka)

-0.006 -0.7 -80 ±420 

0.16 0.2 60 ±160

Arith. mean ± la is -10 ± 41 ka 

Weighted average is 42 ± 150 ka)

0.032 

-0.047

0.5 

-0.5

110±110 

-170 ±160

Arith. mean ± la is -30 ± 91 ka 

Weighted average is 20 ± 91 ka)

0.023

0.018

1.3 

1

90 ±30 

70 ±30

Arith. mean ± la is 80 ± 21 ka 

Weighted average is 80 ± 21 ka)

0.01

0.05

0.1 

0.8

40 ±230 

190 ±130

Arith. mean ± la is 115 ± 113 ka 

Weighted average is 154 ± 113 ka)

0.04 0.7 150 ±100 

0.055 1.1 200 ±80

Arith. mean ± la is 175 ± 62 ka 

Weighted average is 180 ± 62 ka)

Arith. mean of ±la is 68 ± 106 ka [SEM ± 31 ka] 

Weighted average of all analyses is 97 ± 13 ka

[Decay constants: A£ + A.e- = 0.581 x lO" 10 yr 1 ; A.p = 4.962 x 10' 10 yr 1 ; and ^K/Ktotai = 1.167 x 10~4; see Steiger and 

Jager(1977)]

25
Table 5 Lathrop Wells K-Ar Data, 1986

o\

Flow Sample 

unit

Q13 NNTS #1-86

Q13 NNTS #2-86

Ql3 NNTS #3-86

Q13 NNTS #4-86

Ql5 NNTS #5-86

Lab 

number

5429 

5258 

5259

5264 

5430 

5263

5266 

5267 

5265

5378 

5431 

5269

5432 

5272 

5273 

5605 

5941-1 

5928-1

Material Sample 

weight (g)

WRAOB 8.10876 

8.12395 

8.30009

WRAOB 7.6488 

7.38628 

7.13162

WRAOB 7.81638 

7.44786 

7.92347

WRAOB 7.27209 

6.34199 

9.11042

WRAOB 7.15661 

7.65629 

9.38816 

7.01987 

5.98324 

6.21759

%K+ ^Ar* %A

(mol/gm x 10-13) 

1.556 ±0.013 5.356 2.4 

4.115 1.

1.741 0.7 

Arith. mean ±lo is 139 ± 

Weighted average i

1.496 ±0.018 3.107 1.

4.774 1.

2.387 0.

Arith. mean ±lo is 132 ± 

Weighted average i

1.459 ± 0.03 10.58 2. 

11.41 2.

14.44 3.

Arith. mean ±lo is 480 ± 

Weighted average is 

1.531 ±0.017 0.9759 0.

3.7630 1.

3.6310 1.

Arith. mean ±lo is 105 ± 

Weighted average 

1.565 + 0.016 4.124 0.5 

4.225 0.

1.576 ±0.04 5.763 0.

1.565 ±0.016 9.149 0.7 

3.137, 2.800 0.3

1.565 -I- 0.016 3.913, 5,404 0.5

Arith. mean ±lo is 188 ± 

Weighted average i

Q13 lava flow: Arith. mean ±lo is 214 ± 170 ka; [SEM ± 49 ka]; weighted average is 137 ± 13 ka

Ql3 lava flow (less NNTS #3-86): Arith. mean ±lo is 125 ± 53 ka; [SEM ± 18 ka]; weighted average is 135 ± 13 ka

Best age of Lathrop Wells Ql3 lava flow: Weighted average of 137 ± 

Best age of Lathrop Wells Qls lava flow: Weighted average of 176 ± 

[Decay constants: AC + Atf = 0-581 x 10" 10 yr1 ; Afl = 4.962 x 10'10 yr1 ; and 40K/Ktotai = 1.167 x 10-4; see Steiger and 
Table 6. Zero-age residual data from Berkeley argon extraction systems

Sample 

number

Extraction 

system Material

4°Ar* Percent 

%K+ (mol/gmxlO- 14) 4°Ar* Age ±la 

5761-3 

5758-2 

5720-2 

5723-3 

5716-2

5713B

5721

5717

5713A

5713D

BC 

BC 

BC 

BC 

BC

GORT 

GORT 

GORT 

GORT 

GORT

Air 

Air 

Air 

Air 

Air

Air 

Air 

Air 

Air 

Air

1.52

1.52

1.52

1.52

1.52

1.52

1.52

1.52

1.52

1.52

3.258

8.887

-4.670 

26.860

-24.350

0.3 

1.11

-0.3 

0.7

-0.5

12 ±2

34 ±2

18 ±7

102 ±5

-92 ±6

Arith. mean ±la is 15 ± 70 ka [SEM ± 31 ka] 

Weighted average is 22 ± 15 ka

15.440

-48.900

-0.0002 

51.660

-32.970

1.1

-1.9

-0.0 

1.3

-1.9

59 ±9

-186 ±6

0±2

82 ±2

-125 ±3

Arith. mean ±la is -34 ± 117 ka [SEM ± 52 ka] 

Weighted average is 8 ± 13 ka

Arith. mean of all samples ± la is -10 ± 94 ka [SEM ± 30 ka] 

Weighted average of all samples ±la is 14 ± 10 ka

[Decay constants: Xe + AC' = 0.581 x 10'10 yr1 ; Xp = 4.962 x 10'10 yrl; an 

see Steiger and Jager (1977)]

= 1.167 x 
Table 7. Compilation of K-Ar data from tables 2-5 for the Lathrop Wells cinder cone and flow complex

Lava flow unit (Qla) Block flow uni

Sample __ Age±la(ka) Sample

K)

00

Arithmetic mean ±lo [SEM] 

Arithmetic mean ±lo:

Weighted average:

196 ±170 [44 ka] 

126 ±90 [26 ka]*

133 ± 10 ka 

131 ± 10 ka*

199 ± 23

NNTS #1-86 153 ±96

65 ±88

120 ±66

NNTS #2-86 184 ±110

92 ±85

418 ±240

NNTS #3-86 451 ±240

571 ±360

37 ± 29

NNTS #4-86 142 ± 33

137 ±35

336 ± 28

TSV-1 38 ±19

3±75

KA3667

KA3668

KA3687

KA3706

KA3665

KA3787

KA3655

KA3721

KA3662

KA3663

KA3659

KA3728

NNTS #5

TVS 129

TVS 283

'(not including NNTS #3-86)
116°35' 116°30'

.YUCCA 

MOUNTA N

LATHROP

WELLS

CONE

FIGURE 1.

29
116 0 31'09"

36° 

41' 

51"

116°30'00"

36° 

40' 

39"

NNTS01-86

Qps1

Quarry 

site

Ql

NNTS #4-86

B8131

NNTS #5-86

NNTS #2-86 

B8119

B8143 

B8155

B8167

B8179

KILOMETERS

FIGURE 2.

30
 ff fff ifff if if if if if if if if if if if if^

if if if if if if iff if

ff * f if if if if if if if if *f*
 I I I I

B

PS5

FIGURE 4.

32