Recent additions to the list of lunar meteorites include Northwest Africa (NWA) 032 and Dhofar 081. NWA 032 is an unbrecciated basalt, found in Morocco; Dhofar 081 is a fragmented feldspathic breccia, found in Oman. Our goal is the determination of the cosmic ray exposure history of these objects. Most lunar meteorites have complex cosmic ray exposure histories, having been exposed both at some depth on the lunar surface (2{pi} irradiation) before their ejection and as small bodies in space (4{pi} irradiation) during transport from the Moon to the Earth. These exposures were then followed by residence on the Earth's surface, the terrestrial residence time. Unraveling the complex history of these objects requires the measurement of at least four cosmogenic nuclides. The specific goals of these measurements are to constrain the depth of the sample at the time of ejection from the Moon, the transit time from the time of ejection to the time of capture by the Earth, and the residence time on the Earth's surface. These exposure durations in conjunction with the sample depth on the Moon can then be used to model impact and ejection mechanisms. To investigate the complex exposure histories of lunar meteorites, we measured cosmogenic nuclides, {sup 36}Cl (half-life = 3.01 x 10{sup 5} yr), {sup 26}Al (7.05 x 10{sup 5} yr), and {sup 10}Be (1.5 x 10{sup 6} yr) in NWA 032 and Dhofar 081. The measurements of {sup 41}Ca (1.04 x 10{sup 5} yr) are in progress

Caffee, M.

Nishiizumi, K.

UNT Digital Library

U.S. Department of EnergyLawrenceLivermoreNationalLaboratoryPreprintUCRL-JC-145405Exposure Histories ofLunar MeteoritesNorthwest Africa 032 andDhofar 081.K. Nishiizumi, M.W. Caffee¯ This article was submitted to32nd Lunar and Planetary Conference, Houston, TX, March 12-16,2001April 1, 2001Approved for public release; further dissemination unlimitedDISCLAIMERThis document was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor the University of California nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately owned rights. Reference herein to any specificcommercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does notnecessarily constitute or imply its endorsement, recommendation, or favoring by the United StatesGovernment or the University of California. The views and opinions of authors expressed herein do notnecessarily state or reflect those of the United States Government or the University of California, andshall not be used for advertising or product endorsement purposes.This is a preprint of a paper intended for publication in a journal or proceedings. Since changes may bemade before publication, this preprint is made available With the understanding that it will not be citedor reproduced without the permission of the author.This report has been reproduced directly from the best available copy.Available electronically at http://www.doe.gov/bridgeAvailable for a processing fee to U.S. Department of Energyand its contractors in paper fromU.S. Department of EnergyOffice Of Scientific and Technical InformationP.O. Box 62Oak Ridge, TN 37831-0062Telephone: (865) 576-8401Facsimile: (865) 576-5728E-mail: reports@adonis.osti.govAvailable for the sale to the public fromU.S. Department of CommerceNational Technical Information Service5285 Port Royal RoadSpringfield, VA 22161Telephone: (800) 553-6847Facsimile: (703) 605-6900E-maih qrders@ntis.fedworld.govOnline ordering: http: / / www.ntis.gov / ordering.htmORLawrence Livermore National LaboratoryTechnical Information Department’s Digital Libraryhttp://www.llnl.gov/tid/Library.htmlEXPOSURE HISTORIES OF LUNAR METEORITES NORTHWEST AFRICA 032 AND DHOFAR 081. K.Nishiizumit and M. W. Caffee2, ’Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450(kuni@ssl.berkeley.edu), 2Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory,Livermore, CA 94550 (caffee 1 @ilnl.gov).Introduction: Recent additions to the list of lunarmeteorites include Northwest Africa (NWA) 032 andDhofar 081. NWA 032 is an unbrecciated basalt,found in Morocco; Dhofar 081 is a fragmented feld-spathic breccia, found in Oman. Our goal is the de-termination of the cosmic ray exposure history of theseobjects. Most lunar meteorites have complex cosmicray exposure histories, having been exposed both atsome depth on the lunar surface (2n irradiation) beforetheir ejection and as small bodies in space (4~ irradia-tion) during transport from the Moon to the Earth.These exposures were then followed by residence onthe Earth’s surface, the terrestrial residence time. Un-raveling the complex history of these objects requiresthe measurement of at least four cosmogenic nuclides.The specific goals of these measurements are to con-strain the depth of the sample at the time of ejectionfrom the Moon, the transit time from the time of ejec-tion to the time of capture by the Earth, and the resi-deuce time on the Earth’s surface. These exposuredurations in conjunction with the sample depth on theMoon can then be used to model impact and ejectionmechanisms. To investigate the complex exposurehistories of lunar meteorites, we measured cosmogenicnuclides, 36C1 (half-life = 3.01x105 yr), 26A1 (7.05x10syr), and I°Be (1.5x106 yr) in NWA 032 and Dhofar081. The measurements of 41Ca (1.04x105 yr) are progress.Sample Description: The recovered mass ofNWA 032 is ~300 g and it is covered by fusion crust.The recovered mass of Dhofar 081 is 174 g and in sizeit is ~4.5x4x4.5 cm; it is covered by fusion crust.Since some lunar meteorites, specifically CalcalongCreek, Y-791197, Y-793169, and QUE 93069/94269contain SCR (solar cosmic ray) produced nuclides [e.g. 1, 2], which indicate negligible ablation during at-mospheric entry, searching for SCR effects is an im-portant component of this study. SCR-produced nu-clides exhibit strong depth dependence so we measuredcosmogenic radionuclides in 2 sub-samples havingdifferent shielding depths for both NWA 032 and Dho-far 081. The exterior samples of both meteorites arecovered with fusion crust. Two samples of NWA 032were chipped from a 4.5x6.7 cm slab. The interiorsample was obtained from near center of the slab. Theexterior sample is covered by the fusion crust and theentire sample is less than 1.5 mm from the surface.The distance between the exterior and interior samplesof NWA 032 is 2.5 cm; for Dhofar 081 this distance is-1.5 cm.Experimental Procedures and Results: Toeliminate weathering products each sample was etchedtwice with 0.2 N HNO3 solution in an ultrasonic bathfor 15 minutes; the weight loss from this procedure is1.3-2.8%. Each sample was then dissolved in an HF-HNO3 mixture along with Be and AI carriers. Theconcentrations of Mg, A1, Ca, Mn, and Fe in the sam-ples were determined by atomic absorption spectros-copy. The AMS measurements were performed at theLawrence Livermore National Laboratory. The pre-liminary radionuclide data are shown in Table 1. Thequoted errors represent + lo AM S measurement error.NWA 032: The concentrations of l°Be in NWA032 are extremely low, indeed, among the lowestmeasured in lunar meteorites. The low concentrationsof all measured spallation products, 1°Be, 26A1, and36C1, indicate that the integrated exposure of this sam-ple to GCR is minimal. The eosmogenic nuclides ob-served in this sample could have been produced uringthe sample’s residence in the lunar surface or during itstransition time between the Moon and Earth, or a com-bination of both. With the nuclides measured to datewe can test two scenarios: production of the cos-mogen!c nuclides at depth in the lunar regolith; andproduction of the cosmogenic nuclides after ejectionfrom the lunar surface during the short transition be-tween the Moon and Earth.2~ exposure: Production rates are required to de-termine a regolith exposure age; these are typicallyobtained from comparison of the measured activities tothose in the Apollo 15 drill core, which extend to adepth of about 400 g/cm2 [3, 4]. However, the nuclideconcentrations in NWA 032 are lower than the valuesassociated with the deepest samples in the Apollo 15drill core, indicating exposure at a greater depth. Pro-duction rates for burial deeper than 400 g/cm2 werecalculated using two methods [5]. The first uses thetheoretical GCR production rate calculation as givenby the LAHET Code System (LCS) [6]. The secondmethod employs an extrapolation of the Apollo 15depth profile to below 400 g/cm2. Differences in targetchemistry were accounted for by normalizing the ob-served nuclide concentrations in the NWA 032 to thoseof the Apollo 15 core chemical composition. The ele-mental production ratios were calculated using LCS.The measured radionuelide concentrations in NWA032 correspond to exposure at depths of 660-730 g/cm2for )°Be, 520:630 g/cm2 for 26A1, and 520-540 g/cm2for 36CI. Clearly the three nuclides do not yield aunique exposure depth. Although many lunar meteor-ites do not indicate production of all radionuclides at aunique depth, the shorter half-life nuclides indicateproduction at a greater depth. This circumstancearises because of the faster decay of these nuclidesduring terrestrial residence [e. g. 1]. For NVCA 032 theshorter half-life nuclides indicate less shielding. A1-EXPOSURE HISTORIES OF LUNAR METEORITES: K. Nishiizumi and M. W. Ca(feethough we cannot rule out the possibility that somefraction of the radionuclides in NWA 032 were pro-duced in the regolith, the data are inconsistent with theproduction of most of the cosmogenic nuclides in thelunar regolith.4~ exposure: The 26AI activity in the exterior sam-ple of NWA 032 is 50 % higher than that of the inte-rior sample. If this excess 2~A! is SCR produced, it isnecessarily produced during the 4r~ irradiation inspace. The production rate of SCR-produced 26A1 isstrongly dependent on depth and the object’s size inspace. Since SCRs produce nuclides in only the upperfew cm, the preatmospheric radius of NWA 032 mustbe 4-7 cm, assuming nominal ablation of materialduring atmospheric entry. The GCR production ratesof J°13e, 26AI, and 36C1 in objects having radii of 4 and 6cm were calculated by LCS [6]. The 4g-exposure age,i.e. the transition time from the Moon to Earth, is0.042±0.008 Myr for 1°Be and 0.042:t-0.003 Myr for36C1. The 26AI ages (interior sample) are 0.064±0.007Myr for a 6 cm radius and 0.077±0.008 Myr for 4 cmradius. The longer 26A1 exposure age may be the resultof production of some 26A1 by SCRs in interior sample.Although the experimental error of l°Be is large, thel°Be exposure age is robust because t°Be is exclusivelyproduced by GCR. The excess 26A1 that was producedby SCRs is 0.5-1 dpm for the interior sample and 2.1-2.6 dpm for the exterior sample assuming a 0.042 Myrexposure age and a preatmospheric size of 4 and 6 cmin radius. The saturation values of SCR produced Z6A1for these radii are 12-25 dpm and 50-65 dpm respec-tively. The latter value is similar to the SCR produc-tion rate for 26A1 at a depth of ~1 cm in an object hav-ing a radius of 3-4 cm [7, 8]. A coherent self-consistent model for the irradiation history of NWA032 is (I) a 0.042 Myr transition time from the Moonto Earth, (2) a preatmospheric size of less than 5 radius, (3) ~1 crnof ablation during atmospheric entry,and (4) a terrestrial residence time of < 80 kyr. Theejection depth was more than 1100 g/era2 on the Moon.The measurements of cosmogenic nuclides in oppositeside of exterior sample will further constrain the expo-sure history of NWA 032. Although this scenario isconsistent with the available data other more complexscenarios can only be eliminated from considerationwith the measurement of 14C and 4’Ca.Dhofar 081: Since the chemical composition ofDhofar 081 is comparable to that of MAC88104/88105 and QUE 93069/94269, ~°Be, 26A!, and36CI averages from these samples [1] [2] are shown inTable 1 in addition to the measurements fromDh0far081. It is interesting to note that the elemental abun-dances of solar noble gases of Dhofar 081 are similarto those of MAC 88104 [9]. The t°Be, 26A1, and 36C1concentrations in both the interior and exterior samplesof Dhofar 08 ! are identical. The lack of excess 26A1 inthe exterior samples relative to the interior samplesclearly shows no SCR effects. Furthermore, the lackof a gradient in cosmogenic nuclide concentrations as afunction of depth indicates that most of the cos-mogenic nuclides were produced at some depth on theMoon during a 2g exposure geometry. This conclu-sion is supported by discordance among model 4re-exposure ages based on the three measured nuclides.Evidently, the transition time between the Moon andEarth was too short for the production of detectableamounts of long-lived nuclides. The l°Be, 26A1, and36Ci concentrations of Dhofar 081 are about midwaybetween the extremes observed in MAC 88104/88105and QUE 93069/94269. The l°Be, 26AI, 36C1 concen-trations in Dhofar 081 were normalized to Apollo 15drill core chemical composition as described above.The normaliZed nuclide concentrations correspond toexposure at 205-220 g/cm2 for l°Be, 240-260 g/cm2 for2~A1, and 280-300 g/cm2 for1306C1. 2Correctin~ for a ter-restrial age of 200 kyr, the Be, AI, and C1 are allconsistent with ejection from a depth of 190-210g/cm2. The 41Ca measurement as well as 14C will fur-ther constrain the ejection depth and provide a directmeasurement of the terrestrial age of Dhofar 081.Acknowledgment We wish to thank Walt Radom-sky for providing samples of NWA 032 and Museumof Natural History Berlin for providing samples ofDh0far 081. This work was supported by NASA grantNAG5-4992 and in part by DOE at LLNL workingunder contract W-7405-ENG-48.References: [1] Nishiizumi K. et al. (1991) GCA55, 3149-3155. [2] Nishiizumi K. et al. (1996) MAPS31, 893-896. [3] Nishiizumi K. et al. (1984) EPSL 70,157-163. [4] Nishiizumi K. et al. (1984) EPSL 70, 164-168. [5] Nishiizumi K. et al. (1999) LPS 30, CD-ROM. [6] Masarik J. and Reedy R.C. (1994) GCA58,5307-5317. [7] Reedy R.C. and Arnold J.R. (1972)JGR 77, 537-555. [8] Reedy R.C. (1987)LPS XVIII,822-823. [9] Shukolyukov Y.A. et al. (2001) LPS32,CD-ROM.Table 1. Cosmogenic radionuclide concentration (dprn/kg meteorite) in NWA 032 and Dhofar 081.Mass (mg) ~°Be Z°Al ~°C1 ~°Cl*NWA 032 (interior) 37.6 0.32±0.07 2.9±0.3 1.30±-0.04NWA 032 (exterior) 23.6 0.40±0.11 4.4±0.3 1.39±0.05Dhofar 081 (interior) 63.1 5.18±0.09 31.9±0.9 7.40±0.12Dhofar 081 (exterior) 64.9 5.38±0.09 31.2i-0.8 7.16±0.11MAC 88104/88105 average 2.25±0.250 16.4+3.2+ 3.43±0.14~QUE 93069/94269 average 11.96±0.35 ~ 72.1±3.2~ 14.0+0.9~*dpm/kg (10Ca÷Fe); "[1]; ~ [2]1.32:~--0.041.35±0.045.54±0.095.68±0.092.73±0.17I 1.8±0.8This work was performed under the auspices of the U.S. Department of Energy by the University of California,Lawrence Livermore National Laboratory under ContraCt No. W-7405-Eng-48.

Exposure Histories of Lunar Meteorites Northwest Africa 032 and DHOFAR 081

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Exposure Histories of Lunar Meteorites Northwest Africa 032 and DHOFAR 081

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