Geology

doi: 10.1130/0091-7613(1995)023<0021:CEAOBF>2.3.CO;2 1995;23;21-24Geology

 Michael K. Shepard, Raymond E. Arvidson, Marc Caffee, Robert Finkel and Lennox Harris Cosmogenic exposure ages of basalt flows: Lunar Crater volcanic field, Nevada  

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Cosmogenic exposure ages of basalt flows:Lunar Crater volcanic field, Nevada

Michael K. Shepard*Raymond E. Arvidson

Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri 63130Marc CaffeeRobert FinkelLennox Harris

Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, California 94550

ABSTRACT36Cl and 10Be cosmogenic exposure age data for the Black Rock basaltic lava flow,

Lunar Crater volcanic field, Nevada, imply an emplacement age of ;38 6 10 ka. 36Cl datafor the 600 ka flow north of the Lunar Crater maar are consistent with (1) an apparenterosion rate of ;3 m/m.y., (2) a model of desert pavement formation in which basalt clastseroded from the flow remain on the surface of the eolian-derived sediments that mantle theflow, and (3) the early formation of the eolian-derived sediment mantle.

INTRODUCTIONBasalt flows are abundant in the southwestern deserts and were

emplaced primarily during episodes of extension in the late Tertiaryand Quaternary periods (Wood and Keinle, 1990). The youngestflows (,100 ka) are obvious features; however, older flows arehighly altered in appearance by rubbling, the formation of an eolian-derived mantle of clay- to sand-sized sediment, and a desert pave-ment of basalt clasts (Wells et al., 1985). This paper reports pre-liminary efforts to understand quantitatively the operative surfaceprocesses in this region and to determine the role of paleoclimatein the modification process by using cosmogenic exposure datingtechniques.

Two basalt flows in the Lunar Crater volcanic field, Nevada,were sampled for this study (Fig. 1). The Black Rock flow is mor-phologically young (Scott and Trask, 1971; Kargel, 1986), yet has a350 6 50 ka K-Ar age (Kargel, 1986; K. Foland, 1993, personalcommun.). The Black Rock flow was sampled to provide anotherage estimate because exposure age dating has been successfully usedby others dating flows of similar age (Phillips et al., 1986).

The Lunar Crater flow (our unofficial designation because of itsproximity to Lunar Crater maar) is 6006 30 ka (K-Ar, Kargel, 1986;B. Turrin, 1993, personal commun.). This flow was sampled to quan-titatively study surface processes that have operated over the past600 ka. Three questions were specifically addressed: (1) What hasbeen the erosion rate of the Lunar Crater flow? (2) Did the eolianmantle form continuously and gradually or in episodic pulses tied toclimatic variations? (3) What are the mechanisms responsible forthe formation of desert pavement?

PRODUCTION OF COSMOGENIC NUCLIDESCosmogenic exposure age analysis is a relatively new dating

technique that allows a quantitative measurement of the amount oftime a rock has been exposed at or near the Earth’s surface(Yokoyama et al., 1977). The technique is based on the in-situ pro-duction of nuclides by interaction of cosmic rays with surface ma-

terials and exploits the fact that cosmic rays are almost completelyshielded by as little as a few metres of soil or rock (Yokoyama et al.,1977).

Both stable and radioactive isotopes are produced by cosmo-genic processes; however, the isotopes of interest in this study are

*Present address: Center for Earth and Planetary Studies, MRC 315,National Air and Space Museum, Smithsonian Institution, Washington,D.C. 20560.

Figure 1. Geologic sketch map of Lunar Crater volcanic field, Nevada.X—sample locations. After Ekren et al. (1972) and Snyder et al. (1972).

Geology; January 1995; v. 23; no. 1; p. 21–24; 3 figures; 2 tables. 21

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10Be (half-life 5 1500 ka) and 36Cl (half-life 5 301 ka). Both ra-dionuclides have a cosmogenic production rate that is dependentupon sample chemical composition, altitude, latitude, and depthbeneath the surface. The abundance of a cosmogenic radionuclidein a sample is decreased by radioactive decay and mechanical ero-sion. In samples subjected to steady-state erosion, production anddecay are balanced and equal after a few effective half-lives, and theabundance of the radionuclide remains constant. At this point, theproduction of the radionuclide is said to be at saturation. Moreformally,

R5ELDC

~l 1 b!N~12 e2(l1b)t! 1 R0, (1)

where R is the ratio of a cosmogenically produced radionuclide tothe concentration of the reference isotope in the sample; E, L, andD are corrections for production due to elevation (altitude), latitude(geomagnetic field), and depth of sample, respectively; C is theproduction rate of cosmogenic radionuclide for a sample of givenchemical composition, in atoms per kilogram of sample per year andis assumed to be constant; l is the decay constant of a cosmogenicradionuclide, in yr21; b is the effective decay constant due to ero-sion, which equals sample density times erosion rate divided bycosmic-ray attenuation scale length; N is the abundance of a refer-ence nuclide in atoms per kilogram of sample; R0 is the initial ratioof the cosmogenically produced radionuclide to the reference nu-clide—prior to exposure; and t is the time of exposure (or age) ofsample, in years. The effective half-life of the cosmogenic nuclide isln[2/(l 1 b)] (Phillips et al., 1986; Lal, 1991).

From equation 1 note that increased erosion rates decrease theinferred exposure age relative to the crystallization age of the flow.Given a crystallization age less than a few effective half-lives for aspecific cosmogenic radionuclide and an exposure age, it is possibleto solve for an apparent erosion rate over the lifetime of a flow. Forolder crystallization ages, the production of a cosmogenic radionu-clide is approximately saturated in samples, and a crystallization ageis not necessary for the calculation (Lal, 1991). The apparent ero-sion rate may differ from the actual erosion-rate history because ofthe mechanics of weathering.

SAMPLE COLLECTION AND PREPARATIONFour samples were taken from the top 5–8 cm of exposed sur-

faces on the Black Rock flow. Samples were taken from relativelyflat lying original flow structures so that no geometric corrections

were necessary and to ensure a constant exposure geometry. Olivinephenocrysts were separated for 10Be analyses. Six samples were takenfrom the Lunar Crater flow, schematically illustrated in Figure 2.

Both Black Rock and Lunar Crater samples were analyzed forwhole-rock 36Cl. Sample preparation followed that of Zreda et al.(1991). Samples were crushed, wet sieved in demineralized water,dried, and sieved to ,250 mm. Prior to analysis, samples wereleached in 10% nitric acid for 2 h followed by a 12–24-h leach with18 mV demineralized water to remove secondary carbonates andmeteoric Cl. In situ Cl was separated from ;75 g of each preparedsample according to the procedures outlined by Zreda et al. (1991).

To determine the effects of the leach on rock chemistry, weduplicated major and minor chemical analyses on several leachedand unleached samples. Our results indicated that volatiles such asCa, K, Na, P, and Cl consistently decreased in the leached samples.Although much of the decrease is caused by the removal of sec-ondary carbonates, salts, and meteoric chlorine, a significant frac-tion may have been caused by the dissolution of phases like apatite.We therefore utilized the whole-rock major element analyses ofScott and Trask (1971) but were constrained to use the minor ele-ment analysis of the leached samples.

In basalt, 36Cl is produced by the spallation of Ca and K, bythermal neutron capture by 35Cl, and to a lesser extent by slownegative muon capture by Ca (Phillips et al., 1986; Zreda et al.,1991). We ignore the muon capture reaction because it is not sig-nificant in the top metre or so of the lithosphere (Zreda et al., 1991).The spallation production of 36Cl by Ca and K decreases exponen-tially with depth below the surface. We assume a cosmic-ray atten-uation scale depth of 165 g/cm3 (Nishiizumi et al., 1990; Lal, 1991).Recent experimental and theoretical work has shown that the ther-mal neutron capture reaction has a complex profile with depth,initially increasing to a maximum at 20–100 g/cm2 depth and thendecreasing, depending upon the sample composition (Dep et al.,1993). This phenomenon is still in the preliminary stages of quan-tification, and we discuss its effects on our results.

Olivine phenocrysts present in the Black Rock flow were sep-arated to analyze for 10Be, produced by spallation of Si, O, Mg, andFe. Olivine sample sizes ranged from 2.5 to 8 g. In the Lunar Craterflow, no olivine was present for 10Be analysis. Be was chemically

Figure 2. Schematic cross section of Lunar Crater flow showing majormorphologic zones and locations and exposure ages of LC samples.

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separated according to the procedures outlined by Seidl (1993). Wealso attempted to analyze the olivines for 26Al; however, youngsample age, high stable Al concentrations, and low accelerator cur-rents precluded our obtaining meaningful results.

EXPOSURE AGE ANALYSISBlack Rock Flow

36Cl analysis of the Black Rock samples (Table 1) gives anaverage exposure age of 33.9 6 6.7 ka. The 10Be analyses (Table 2)are concordant with the 36Cl results at the 1s level with an averageexposure age of 43.8 6 11.4 ka. The average exposure age of theBlack Rock flow is thus 38.1 6 9.7 ka, an order of magnitude lessthan the K-Ar–based crystallization age of 350 ka (Kargel, 1986).The concordance of the 36Cl and 10Be exposure ages (consideringtheir different half-lives) and maintenance of identifiable flow fea-tures argue for minimal erosion. Consequently, the depth-depend-ent production rate of 36Cl due to spallation or thermal neutroncapture should not significantly affect these results.

In addition to relatively pristine morphology, there are otherindications that the Black Rock exposure ages are more accuratethan the K-Ar ages. The phenocrysts of olivine observed throughoutthe flow are relatively fresh and only altered around the rims. Arvid-son et al. (1993) demonstrated a relation between the age of basaltflows in the Cima volcanic field and their reflectance as observed byLandsat Thematic Mapper (TM). The same relation appears to bevalid for all dated flows in the Lunar Crater volcanic field, except theBlack Rock flow (Shepard, 1994). The atmosphere-corrected re-flectance of the Black Rock flow (TM band 5) is 0.066 radiance-factor units, corresponding to an age of ;30 ka. This value is sig-nificantly different from the K-Ar age but in excellent agreementwith our exposure-age average.

Lunar Crater FlowK-Ar dating of the Lunar Crater flow gives an average crystal-

lization age of 600 6 30 ka (Kargel, 1986, B. Turrin, 1993, personalcommun.). Several lines of evidence lead us to conclude that theLunar Crater flow is accurately dated. The reflectance of the LunarCrater flow in TM band 5 is 0.151 radiance-factor units, which cor-responds to an age of 650 ka (Arvidson et al., 1993). Morphologi-cally, the flow is similar in appearance to flows dated at 500–800 kain the Cima volcanic field. We consider it unlikely that these flows(from different lava fields) all contain excess argon, especially insuch consistent abundances. Table 1 lists the analyzed Lunar Cratersamples, chemical compositions, 36Cl/35Cl ratios, and correspondingexposure ages.

All Lunar Crater samples were collected within 50 m of oneanother, and we assume that all are from the Lunar Crater flow. Thisassumption is based upon the proximity of samples to one another,the consistent morphology of the sample area, and lack of other flowsources. Unfortunately, the highly altered condition of these sam-

ples and the subsequent leaching process make it difficult to supportthis assumption with geochemical comparisons.

Sample LC-13 was taken from a protruding knob on the orig-inal flow to estimate the flow’s erosion rate. This sample sat ;1 mabove the surrounding pavement and is not thought to have beencovered by sediment. Figure 3 shows a plot of the expected buildupof 36Cl in LC-13 vs. time. The data point is plotted at the measured36Cl/35Cl ratio for LC-13 and the K-Ar crystallization age of theflow, with 1s uncertainties. The various curves are the 36Cl/35Clratios that would be measured from samples subject to various ap-parent erosion rates. The best fit is for an apparent erosion rate of3.5 m/m.y., implying a total of;2m of relief eroded over the lifetimeof the flow. Even if the K-Ar age of 600 ka is 300 ka different fromthe crystallization age, the 36Cl production rate is still saturated andthe apparent erosion rate remains the same. Thermal neutron cap-ture accounts for only 11% of 36Cl production in this sample (Ta-ble 1); therefore, we can safely ignore the depth-dependent pro-duction profile of this mechanism.

Next, consider the exposure ages of samples LC-6, -9, -10, and-11. These samples were all 10–15 cm and taken to study the for-mation of desert pavement. In boulder-sized clasts, the exponentialattenuation (interior to the boulder) of 36Cl production by spalla-tion will be balanced by an increase in 36Cl production by thermalneutron capture (Dep et al., 1993). The two mechanisms will tendto result in a constant production profile—i.e., erosion of pavementsamples will not affect our analyses. Note that samples LC-9 andLC-10 are essentially saturated. Given the uncertainties present inthese analyses, saturation occurs after ;2 36Cl half-lives—i.e., ex-posure ages $600 ka cannot be differentiated. Therefore, LC-9 andLC-10 support an age of 600 ka or older for the Lunar Crater flow.

The pavement exposure ages appear to be most consistent withthe pavement-formation hypothesis of McFadden et al. (1987).Their model proposes that pavements are erosion fragments thataccumulate and remain on the surface of the eolian-derived sedi-ment mantle. Samples LC-9 and LC-10 represent clasts that erodedfrom the initial flow structure onto the surface of the mantle. Theyremained there, unburied, for the life of the flow and therefore haveexposure ages consistent with the flow age. This interpretation im-plies that the Lunar Crater flow mantle formed quickly. SamplesLC-6 and LC-11 (exposure ages;240 ka and;220 ka, respectively)

Figure 3. Plot of buildup of 36Cl/35Cl for sample LC-13. Upper curve isfor no erosion. Data point is measured 36Cl/35Cl for LC-13 and K-Arcrystallization age of LC flow. Error bars are 1s for exposure age andcrystallization age. Lower curves show apparent erosion rates (withuncertainties) necessary to cause observed exposure age.

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represent later additions to the pavement caused by continued ero-sion of exposed flow outcrops. Their younger exposure ages are dueto their initially shielded positions underneath 1 m or more of over-lying basalt. Although four samples preclude rigorous testing of thisinterpretation, it is favored over other pavement hypotheses—i.e.,deflation and upward migration (cf. Cooke andWarren, 1973), bothof which require burial and subsequent exhumation of clasts andwould predict all pavement exposure ages to be less than the flowage.

Finally, consider the sample LC-3. This sample was taken todetermine the timing of mantle formation. Beneath the 1-m-thickmantle, the spallation production rate of 36Cl is ;1/e lower than atthe surface on the basis of a measured soil bulk density of 1.5 g/cm3

and pavement;3–5 cm thick. In contrast, thermal neutron flux (andtherefore production from 35Cl) at this depth is approximatelyequivalent to that at the surface (Dep et al., 1993). If the mantleformed immediately after extrusion, we would expect an exposureage (a minimum age) of;300 ka. If the mantle formed recently, wewould expect an exposure age (a maximum age) of 600 ka. Themeasured exposure age of 121 6 18 ka is well below the expectedminimum age. The most likely explanation for this discrepancy isthat we have overestimated the 36Cl production. One source of errormight have arisen because we measured the soil bulk density duringan arid period. The soil would have a higher bulk density duringwetter climatic regimes as water filled pore space and, therefore,would have a higher attenuation factor. Perhaps more significantly,water is an extremely effective neutron thermalizer. A moist soilwould shift the thermal neutron flux peak higher in the profile andwould significantly reduce the flux at depth.

Despite the uncertainties, the magnitude of LC-3’s exposureage suggests that the mantle formed early in the history of the flow.This interpretation is consistent with the implication drawn fromsamples LC-9 and LC-10 above. Paleohydrologic data from OwensValley, California (a few hundred kilometers distant), record a pe-riod of arid climate from about 600 to 300 ka followed by moisterconditions (Smith, 1984; Jannik et al., 1991). The climatic record istherefore consistent with our interpretation of relatively rapid eo-lian deposition early in the history of the Lunar Crater flow.

SUMMARYThe Black Rock flow, Lunar Crater volcanic field, Nevada, is

dated at 38.1 6 9.7 ka by using 36Cl and 10Be cosmogenic nu-clides—an order of magnitude younger than K-Ar dating indicates.This lends support to the possibility of excess argon in the flow.Dating of exposures of the 600 ka Lunar Crater flow (also in theLunar Crater volcanic field) provides an apparent basalt erosionrate of 3.5 m/m.y. since that flow was emplaced. Exposure ages ofclasts from the Lunar Crater flow desert pavement range from sat-uration ($600 ka) to 224 6 43 ka. These observations support amodel of desert pavement formation in which clasts are derivedfrom topographic highs and are maintained at or near the surface ofaccumulating mantle sediment. Exposure ages from several samplessuggest that the Lunar Crater flow was quickly mantled by eolian-derived sediment. This conclusion is consistent with the arid paleo-hydrologic regime at the time of the extrusion of the Lunar Craterflow.

ACKNOWLEDGMENTSSupported in part by National Aeronautics and Space Administration

Geology Program grant 1358 to Arvidson. We thank F. Phillips, R. Couture,D. Kremser, M. Seidl, J. Koenig, and M. Glascock for analyses; K. Folandand B. Turrin for K-Ar age data; and F. Podosek, J. Morris, A. Gillespie, F.Phillips, J. Luhr, and anonymous reviewers for useful comments and dis-cussions.

REFERENCES CITEDArvidson R. E., and nine others, 1993, Characterization of lava-flow degra-

dation in the Pisgah and Cima volcanic fields, California, using LandsatThematic Mapper and AIRSAR data: Geological Society of AmericaBulletin, v. 105, p. 175–188.

Cooke, R. U., and Warren, A., 1973, Geomorphology in deserts: London,B. T. Batsford Ltd., 394 p.

Dep, L., Elmore, D., Lipschutz, M., Vogt, S., Phillips, F. M., and Zreda,M. G., 1993, Depth dependence of cosmogenic neutron-capture pro-duced 36Cl in a terrestrial rock [PRIME Lab Report PL9305]: WestLafayette, Indiana, Purdue University, 14 p.

Ekren, E. B., Hinrichs, E. N., and Dixon, G. L., 1972, Geologic map of theWall quadrangle, Nye County, Nevada: U.S. Geological Survey Mis-cellaneous Geologic Investigations Map I-719, scale 1:48,000.

Jannik, N. O., Phillips, F. M., Smith, G. I., and Elmore, D., 1991, A 36Clchronology of lacustrine sedimentation in the Pleistocene Owens Riversystem: Geological Society of America Bulletin, v. 103, p. 1146–1159.

Kargel, J. S., 1986, The geochemistry of basalts and mantle inclusions fromthe Lunar Crater Volcanic Field, Nevada; petrogenic and geodynamicimplications [MS. thesis]: Columbus, Ohio State University, 375 p.

Lal, D., 1991, Cosmic ray labeling of erosion surfaces: In situ nuclide pro-duction rates and erosion models: Earth and Planetary Science Letters,v. 104, p. 424–439.

McFadden, L. D., Wells, S. G., and Jercinovich, M. J., 1987, Influences ofeolian and pedogenic processes on the origin and evolution of desertpavements: Geology, v. 15, p. 504–508.

Nishiizumi, K., Klein, J., Middleton, R., and Craig, H., 1990, Cosmogenic10Be, 26Al, and 3He in olivine from Maui lavas: Earth and PlanetaryScience Letters, v. 98, p. 263–266.

Phillips, F. M., Leavy, B. D., Jannik, N. O., Elmore, D., and Kubik, P. W.,1986, The accumulation of cosmogenic chlorine-36 in rocks: A methodfor surface exposure dating: Science, v. 231, p. 41–43.

Scott, D. H., and Trask, N. J., 1971, Geology of the Lunar Crater VolcanicField, Nye County, Nevada: U.S. Geological Survey Professional Pa-per 599-I, 22 p.

Seidl, M., 1993, Form and process in channel incision of bedrock [Ph.D.thesis]: Berkeley, University of California, 163 p.

Shepard, M. K., 1994, Application of cosmogenic exposure age dating andremote sensing observations to studies of surficial processes [Ph.D. the-sis]: Saint Louis, Missouri, Washington University, 163 p.

Smith, G. I., 1984, Paleohydrologic regimes in the southwestern Great Basin,0–3.2 m.y. ago, compared with other long records of ‘‘global’’ climate:Quaternary Research, v. 22, p. 1–17.

Snyder, R. P., Ekren, E. B., and Dixon, G. L., 1972, Geologic map of theLunar Crater quadrangle, Nye County, Nevada: U.S. Geological SurveyMiscellaneous Geologic Investigations Map I-700, scale 1:48,000.

Wells, S. G., Dohrenwend, J. C., McFadden, L. D., Turrin, B. D., andMahrer, K. D., 1985, Late Cenozoic landscape evolution on lava flowsurfaces of the Cima volcanic field, Mojave Desert, California: Geo-logical Society of America Bulletin, v. 96, p. 1518–1529.

Wood, X., and Keinle, Y., 1990, Volcanoes of North America: Cambridge,United Kingdom, Cambridge University Press, 354 p.

Yokoyama, Y., Reyss, J., and Guichard, F., 1977, Production of radionu-clides by cosmic rays at mountain altitudes: Earth and Planetary ScienceLetters, v. 36, p. 44–50.

Zreda, M. G., Phillips, F. M., Elmore, D., Kubik, P. W., Sharma, P., andDorn, R. I., 1991, Cosmogenic chlorine-36 production rates in terres-trial rocks: Earth and Planetary Science Letters, v. 105, p. 94–109.

Zreda, M. G., Phillips, F. M., Kubik, P. W., Sharma, P., and Elmore, D.,1993, Cosmogenic 36Cl dating of a young basaltic eruption complex,Lathrop Wells, Nevada: Geology, v. 21, p. 57–60.

Manuscript received June 8, 1994Revised manuscript received October 6, 1994Manuscript accepted October 17, 1994

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