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Uranium–lead systematics of phosphates in lunar basaltic regolithbreccia, Meteorite Hills 01210

Kentaro Terada a,b,c,⁎, Yu Sasaki a, Mahesh Anand c,d, Katherine H. Joy d,e,f, Yuji Sano g

a Department of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japanb Project Center of Multi-Isotope Research for Astro- and Geochemical Evolution (MIRAGE),

Hiroshima University, Higashi-Hiroshima 739-8526, Japanc Department of Earth Sciences, CEPSAR, Walton Hall, The Open University, Milton Keynes, MK7 6AA, UK

d Department of Mineralogy, The Natural History Museum, Cromwell Road, London, SW7 5BD, UKe UCL/Birkbeck Research School of Earth Sciences, University College London, Gower Street, London, WC1E 6BT, UK

f The Rutherford Appleton Laboratory, Didcot, Oxfordshire, OX11 0QX, UKg Center for Advanced Marine Research, Ocean Research Institute, The University of Tokyo, Nakano-ku, Tokyo 164-8639, Japan

Received 22 November 2006; received in revised form 20 April 2007; accepted 23 April 2007

Editor: R.W. Carlson

Available online 27 April 2007

Abstract

Chronological studies of brecciated lunar meteorites have proved difficult, because they are mixtures of materials from varioussources and the radiometric “clocks” are sometimes affected by the subsequent impact events on the Moon. Here, we report thein-situ U–Pb dating of phosphates in lunar meteorite, Meteorite Hills (MET) 01210, which is a regolith breccia consisting of low-Timare basalt clasts and mineral fragments with a minor anorthositic component. In-situ analyses of four merrillite and four apatitegrains in MET 01210, which are resistant to secondary events, resulted in a 207Pb/206Pb–204Pb/206Pb isochron age of 3904±85 Ma(95% confidence limit). This phosphate formation age, when considered as the crystallisation age of this low-Ti basalt, is similar tocrystallization ages of 3.8–3.9 Ga for unbrecciated low-Ti basalt meteorites, Asuka 881757 and Yamato 793169. This result reinforcesthe hypothesis that all these three meteorites originated from the same area on the Moon and were launched by a single impact event,consistent with the similarity of launch ages, mineralogical and geochemical signatures.© 2007 Elsevier B.V. All rights reserved.

Keywords: U–Pb dating; lunar meteorite; mare basalt; SHRIMP

1. Introduction

Lunar meteorites are valuable samples for under-standing the origin and evolution of the Moon's crust, asthey potentially provide new insights into the thermal

history of unexplored regions of the Moon. In spite oftheir scientific value, chronological studies of lunarmeteorites have been difficult, since most of them arebrecciated and consist of mixtures of materials,potentially of different origins. Therefore, age determi-nations using bulk-rock material are prohibitive, and insome cases their radiometric “clocks” might also bedisturbed by subsequent impact events. Here, we reportSensitive High-Resolution Ion MicroProbe (SHRIMP)U–Pb isotopic analyses of phosphate grains in the newly

Earth and Planetary Science Letters 259 (2007) 77–84www.elsevier.com/locate/epsl

⁎ Corresponding author. Department of Earth and Planetary SystemsScience, Hiroshima University, Higashi-Hiroshima 739-8526, Japan.Tel.: +81 824 7478; fax: +81 824 0735.

E-mail address: terada@sci.hiroshima-u.ac.jp (K. Terada).

0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.epsl.2007.04.029

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found lunar basaltic meteorite, Meteorite Hills 01210(Antarctic Meteorite Newletters, 2004; Russell et al.,2004 hereafter abbreviated as MET 01210). It is wellestablished that phosphates are one of the main carriersof U and are also resistant to secondary thermal eventsdue to relatively high closure temperatures (∼500 °C,Cherniak et al., 1991). Therefore, even if there was somedisturbance of the U–Pb systematics of the phosphates,the crystallization age could still be obtained, and adistinction can be made from the age of the secondary

event by assessment of both 238U and 235U decay series(e.g. see, Anand et al., 2003; Terada and Sano, 2004).

MET 01210 is a 22.8 g lunar meteorite collected in2001 from the Meteorite Hills (Antarctic MeteoriteNewletters, 2004) and has been classified as a regolithbreccia consisting of low-Ti mare basalt materials with aminor anorthositic component (Patchen et al., 2005;Arai et al., 2005; Korotev and Irving, 2005; Zeigleret al., 2005; Huber and Warren, 2005; Joy et al., 2006a;Righter and Bussey, 2006; Joy et al., 2006b). Recently,

Fig. 1. Back-scattered electron images of a typical area of MET 01210-21. [a] Low-Ti basalt clasts (BC), monomineralic clasts (AN: anorthite, Pyx:pyroxene, Fa: Fayalite, mPyx: meta-stable pyroxenes) and symplectite assemblages (S) are supported in a fine-grained, glass-rich matrix. [b] Largefayalite (Fa) host of apatite (Ap) grain 21.1 (Table 1). [c] Large fayalite grain hosting merrillite (W) grain 21.5 and a small baddeleyite (B). Thefayalite grains are held in a mineral fragment (An: anorthite, Pyx: pyroxene) rich matrix (M).

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Day et al. (2006) reported the detailed petrographicaland mineralogical descriptions of basaltic clasts, anddiscussed their association with evolved (e.g. LaPazIcefield 02205; Northwest Africa 032) and ferroan (e.g.Asuka 881757; Yamato 793169) low-Ti basaltic lunarmeteorites. In order to better understand the petrogenetichistory of these meteorites, we have carried out in-situU–Pb age dating of phosphates grains in MET 01210,which in turn yields crystallization age of the basalticlithology in this meteorite. Our in-situ analyses usingthe SHRIMP installed at Hiroshima University attainhigh sensitivity at high mass resolution of ∼5800 (Sanoet al., 1999). Although recent high-precision ThermalIonization Mass Spectrometry (TIMS) analyses haveenabled the U–Pb dating from ∼10 μg phosphates (e.g.Amelin et al., 2005), our in-situ analyses have thefollowing advantages in comparison with conventionalTIMS techniques: (1) a much smaller amount of sample(usually a thin polished section) is required, (2) themineralogy of phosphates and their textural relation-ships with other minerals in the rock can be investi-gated in their natural setting, (3) U–Pb systematics in

various phases are independently investigated. Thisin-situ U–Pb dating method has been successfully ap-plied to a range of extraterrestrial phosphates includingMartian meteorites (Terada and Sano, 2004; Sano et al.,2000; Terada et al., 2003), ordinary chondrites (Teradaand Sano, 2002, 2003) and lunar meteorites (Anandet al., 2003; Terada et al., 2005; Anand et al., 2006; Teradaet al., 2006) and has enabled us to unravel the thermalevolution of the meteorite parent bodies. The primaryobjective of the present work is to investigate the U–Pbsystematics of phosphates in MET 01210, whose radio-metric age has previously not been determined. Thesecond objective was to compare the thermal, i.e., crys-tallization, history of the basaltic component in thisunique meteorite with those of other low-Ti mare basaltmeteorites, and to provide chronological constraints fordiscussion of the nature of their source regions.

2. Sample and analytical methods

Wewere allocated two standard polished thin sectionsof MET 01210 (MET 01210,21 and MET 01210,31)

Table 1Major element compositions of phosphates in thin sections MET 01210, and MET 01210,21

Grain. no. Size a Mineral P2O5 CaO FeO MgO SiO2 Al2O3 Total

(μm×μm) (%) (%) (%) (%) (%) (%) (%)

#01210,3131.1 20×30 Merrillite 39.8 53.2 1.7 0.1 0.6 b0.1 95.331.3 15×20 Apatite 39.5 55.1 0.9 b0.1 0.9 0.3 96.731.5 15×30 Merrillite 43.2 44.0 3.1 2.1 0.2 0.2 92.9

#01210,2121.1 20×30 Apatite 40.4 53.7 0.8 b0.1 0.3 0.1 95.321.2 20×80 Apatite 39.8 53.1 0.9 b0.1 0.9 0.1 94.821.4 20×20 Merrillite 39.7 52.4 1.7 b0.1 0.7 0.1 94.521.5 20×40 Merrillite 40.7 40.2 4.4 1.3 0.7 b0.1 87.3a Minor axis×major axis of individual grains.

Table 2U concentrations and 238U/206Pb, 207Pb/206Pb, and 204Pb/206Pb ratios in phosphates for MET 01210

Spot no. Mineral U 238U/206Pb 207Pb/206Pb 204Pb/206Pb

(ppm)

#31.1 Merrillite 45 0.5605±0.1209 0.4130±0.0275 0.001146±0.000224#31.3 Apatite 9 0.4032±0.3570 0.4476±0.0927 0.001878±0.001315#31.5 Merrillite 38 0.8432±0.1584 0.4101±0.0182 0.000752±0.000175#21.1 Apatite 48 0.9913±0.3163 0.3731±0.3536 0.000652±0.000169#21.2.1 Apatite 55 1.0746±0.0792 0.3988±0.0234 0.000338±0.000115#21.2.2 Apatite 62 0.8521±0.1988 0.4000±0.0111 0.000241±0.000061#21.4 Merrillite 24 0.9564±0.2418 0.3907±0.0180 0.000644±0.000209#21.5 Merrillite 141 0.7976±0.1082 0.3980±0.0042 0.001363±0.000154

Uncertainties assigned to the isotopic and elemental ratio are 1σ estimated by counting statistics and calibration.Uncertainties of concentration are ±30% estimated by repeated measurements of standard apatite, PRAP.The Pb isotopic compositions have not been corrected for initial Pb.

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by the Meteorite Working Group. These thin sectionscontained samples with vesicular fusion crusts andwere composed of an immature breccia sample withrock (b4 mm) and mineral fragments (b1 mm) fusedin a fine-grained crystalline and glass matrix. Thebreccia is dominated by low-Ti mare basalt materialand has only a minor anorthositic component (Joy et al.,2006a).

We initially obtained back-scattered electron (BSE)images, followed by measurements of mineral composi-tions using an Electron Probe Micro Analyzer (EPMA;JEOL JCMA-733II), in order to identify the location andmineralogy of phosphates (Fig. 1). As about half of thegrains were too small for the 10 μm spot analysis bySHRIMP, four grains from MET 01210,21 and threegrains from MET 01210,31, which were large enoughand free from inclusion and cracks, were selected forthis study. In the present study, the phosphate grainsanalyzed by the SHRIMP technique were present main-ly as matrix components associated with large fayalitegrains (see Fig. 1-b and c). This association of phosphateand fayalite grains is typical of mare basalts in whichthese mineral phases occur as mesostasis components.We consider that because of their textural and miner-alogical characteristics, during brecciation of mare ba-salts, the mesostasis areas are more prone to fracturingand breakage compared to other silicate-oxide mineralassemblages. This selective disaggregation probablyexplains the occurrence of isolated phosphate andfayalite grains in the matrix of brecciated mare basaltmeteorites such as MET 01210. The mineral species,sizes, and major element compositions of selectedphosphate grains are listed in Table 1. Oxide totals ofsome grains are low because of the assumed contribu-tion of un-analyzed elements such as F, Cl, Ce2O3 andNd2O3, although their abundances are qualitativelyconfirmed.

After EPMA analysis, we polished the thin sectionsslightly using 0.25 μm diamond paste. Then, follow-ing cleaning to minimize surface contaminant Pb, wegold-coated them to prevent charging of the samplesurface during SHRIMP analyses. In order to furtherreduce the already minute x−1PbH+ interference on thexPb+ peaks, the thin sections were evacuated in thesample lock overnight. Before the actual analysis, theprimary ion beam was rastered over the sample sur-face for 3 min in order to remove any possible surfacecontaminants.

We focused an approximately 1 nA O2− primary beam

with acceleration voltage of 10 kV to sputter an area∼10 μm in diameter on the phosphates. Then, we ex-tracted the positive secondary ions at 10 kV and finally

detected a signal by electron multiplier. The mass reso-lution was set to 5800 at 208Pb for U–Pb analyses.The magnet was cyclically peak-stepped from mass 159(40Ca2

31P16O3+) to mass 254 (238U16O+), including

background, all Pb isotopes, and mass 238 for 238U. Wedetected no significant isobaric interferences in thismass range for the phosphates (e.g., the mass peak of159Tb (158.925 AMU) was clearly separated from thatof 40Ca2

31P16O3+ (158.884 AMU) at the mass resolu-

tion of 5800). The abundance ratio of 238U to 206Pbwas obtained from the observed 238U+/206Pb+ ratiousing the following empirical relationship between the206Pb+/238U+ and 238U16O+/238U+ ratios of the standardapatite (PRAP) derived from an alkaline rock of the

Fig. 2. Conventional 238U/204Pb–206Pb/204Pb isochron diagram (top) and235U/204Pb–206Pb/204Pb isochron diagram (bottom) for MET 01210. Er-rors are one sigma. Least-square fits using theYorkmethods (York, 1969)give a 238U/206Pb age of 3899±500 Ma with (206Pb/204Pb)initial=289±160 and 235U/207Pb age of 3882±130Ma with (207Pb/204Pb)initial=121±60 Ma, respectively (95% confidence limit). The calculation was madeusing Isoplot/Ex (Ludwig, 2001).

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Prairie Lake circular complex in the Canadian Shield(1156±45 Ma (2σ)(Sano et al., 1999);

206Pb=238U ¼ A� ð206Pbþ=238UþÞsample

ð238U16Oþ=238UþÞasample

whereA, (206Pb+/238U+)sample and (238U16O+/238U+)sample

denote a constant, the observed secondary 206Pb+/238U+

and 238U16O+/238U ratios of the sample, respectively. Weobtained the constant A and α by repeated measurementof the standard apatite. U concentrations were calibratedfrom 238U16O+/40Ca2

31P16O3+ and the U contents

(196 ppm) of PRAP. Further experimental details of theU–Pb analysis and the calibration of the data can be foundin (Sano et al., 1999, 2000). The observed 238U/206Pb,207Pb/206Pb and 204Pb/206Pb ratios of phosphates arelisted in Table 2.

3. Results

We first calculated the “conventional” 238U/204Pb–206Pb/204Pb and 235U/204Pb–207Pb/204Pb isochron ageswith error correlations using the York methods (York,1969), where the error correlations are defined as theratios of the relative errors of 206Pb/204Pb and 238U/204Pbor 207Pb/204Pb and 235U/204Pb, respectively. Althoughthese ratios are not listed in Table 2, they can be calcu-lated from the given ratios assuming 238U/235U=137.88.Fig. 2 shows “conventional” isochron diagrams, wherethe linear regressions are calibrated using Isoplot/EX(Ludwig, 2001). Analyses of four merrillites and three

apatites give a 238U/206Pb age of 3899±500 Ma with(206Pb/204Pb)initial =289±160 and 235U/207Pb age of3882±130 Ma with (207Pb/204Pb)initial=121±60, respec-tively (95% confidence limit; MSWD=0.62 and 0.80).

Next, we calculated the “inverse” 204Pb/206Pb–207Pb/206Pb isochron age, assuming the error correlationsare zero. The data for four merrillites and three apatites isexpressed by 207Pb/206Pb=A×204Pb/206Pb+B, whereA=1±11 and B=0.399±0.011 at the 1 sigma level,giving a 207Pb/206Pb isochron age of 3905±85 Ma(95% confidence limit; MSWD=0.28, Fig. 3). Thus, theobserved 207Pb/206Pb, 238U/206Pb and 235U/207Pb iso-chron ages agree well with each other within uncertain-ties, suggesting that the formation age of phosphates inMET 01210 is ∼3.9 Ga.

4. Discussion and remarks

In this study, the analyzed phosphates occur inbreccia matrix associated with fayalite grains, taken tobe remnants of the disaggregated mesostasis assem-blages of low-Ti basalts, clasts of which dominate theMET 01210 meteorite. Therefore, in this study, we willtreat the analyzed phosphates as being directly related tothe formation of low-Ti basalt clasts.

Chronological investigations of numerous samplesof mare basalt and related pyroclastic deposits collectedby the Apollo and Luna missions have been reportedby numerous authors (for summaries, see Nyquist andShih, 1992; Nyquist et al., 2001). In general, the high-Tibasalts (N6 wt.% TiO2) from the Apollo 11 and Apollo17 sites are relatively old, generally from 3.5 to 3.9 Ga.In contrast, low-Ti (1.5–6 wt.% TiO2) mare basaltsamples are generally younger in the range of 3.1 to3.4 Ga, although some exceptions exist. The aluminouslow-Ti mare basalts have ages of 3.9 to 4.2 Ga (Tayloret al., 1983), and the very-low-Ti basalts (b1.5 wt.%TiO2) are much younger (3.2–3.3 Ga for LUNA 24).The age of about 3.9 Ga for MET 01210 observed forphosphates in this study is much older than the reportedages for most Apollo low-Ti mare basalts (3.1–3.4 Gafor Apollo 15 and 3.1–3.3 Ga for Apollo 12). Thus, ourresult clearly demonstrates that the thermal activityrecorded in MET 01210 is quite different from thoseof the regions of the Moon explored by the Apollomission, and indicate protracted low-Ti basalt volca-nism on the Moon during a period of about 800 Mafrom 3.1 to 3.9 Ga. If the age data for other low-Ti marebasalt meteorites are considered, then low-Ti basaltvolcanism on the Moon seems to have spanned morethan a billion years from 3.9 Ga to 2.9 Ga ago (Anandet al., 2006).

Fig. 3. An inverse Pb/Pb isochron diagram for MET 01210.Uncertainties are portrayed at the 1σ level. Analyses of four merrillitesand four apatites give a 207Pb/206Pb age of 3905±85 Ma (95%confidence limit; error correlation=0). The calculation was madeusing Isoplot/Ex (Ludwig, 2001).

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Recently, Day et al. (2006) reported detailed pe-trographical and mineralogical descriptions of basalticclasts in MET 01210, and discussed its association withevolved (e.g. LaPaz Icefield 02205; Northwest Africa032) and ferroan (e.g. Asuka 881757; Yamato 793169)low-Ti basaltic lunar meteorites, and suggested a possi-bility of similar mare source regions for these meteor-ites. Especially for the ferroan basaltic lunar meteorites,Arai et al. (2005) and Righter and Bussey (2006) alsopointed out the similarities of bulk compositions, tex-ture, and pyroxene compositions among MET 01210and Asuka 881757 and Yamato 793169. Furthermore,Arai et al. (2005) concluded that MET 01210 is launch-paired with Asuka 881757 and probably Yamato793169. Recent studies of the cosmic ray exposurehistories of these meteorites also support this scenario,because the 10Be and 36Cl 4π exposure ages of 0.95±0.13 and 0.90±0.18 Myr for MET 01210 are quitesimilar to those of the 10Be ejection ages of Asuka881757 (0.8±0.2 Myr) and Yamato 793169 (0.9±0.2 Myr) (Nishiizumi et al., 1992, 2006). From thepoint of view of chronological studies, the associa-tion with evolved non-brecciated basaltic meteoritesis ruled out, because the phosphate formation age ofLAP 02205 (2.9 Ga Anand et al., 2006) and the bulkAr–Ar age of NWA 032 (2.8 Ga, Fagan et al., 2002;Fernandes et al., 2003) are significantly different fromthat of MET 01210.

Various radiometric ages have been reported forAsuka 881757 and Yamato 793169 based on bulk sam-ple analyses. Misawa et al. (1993) reported a Pb–Pbage of 3940±28 Ma, a U–Pb age of 3850±150 Ma, aTh–Pb age of 3820±290 Ma, a Rb–Sr age of 3840±32 Ma, a Sm–Nd age of 3871±57 Ma and a Ar–Ar ageof 3798±12 Ma for Asuka-881757. Thalmann et al.(1996) also reported a K–Ar gas retention age of 3100±600 Ma and a 244Pu–136Xe fission age of 4240±170 Ma, also indicating an old formation age. Torigoe-Kita et al. (1995) investigated the U–Th–Pb, Sm–Ndand Ar–Ar isotopic systematics of Yamato 793169,and suggested that the nearly concordant U–Pb age of3.8 Ga indicates an original formation age of 3.9 Ga, andthat the young Sm–Nd age of 3.4 Ga might be caused bysecondary events. Recently, Fernandes et al. (2005) alsoreported Ar–Ar ages for lunar mare meteorites Asuka881757 (3.69±0.07 Ga), and Yamato 793169 (3.71±0.11 Ga) with a disturbance event recorded at 430 Ma.The phosphate formation age of 3904±86 Ma obtainedfor MET 01210 in this study is consistent with oldcrystallization ages of 3.8–3.9 Ga reported for Asuka881757 and Yamato 793169 meteorites by other inves-tigators, indicating that these meteorites have the oldest

formation ages among the known low-Ti mare basalts,except for aluminous low-Ti mare basalts. The similarityof the ages of the basaltic components in MET 01210and of the two unbrecciated mare basalt meteorites(Asuka, Yamato) indicates that there is no chronologicalimpediment to the hypothesis that all these meteoritesoriginate from the same locality on the Moon. Further-more, similar cosmogenic ages for all three meteoritessuggest that they were launched by a single impactevent.

In general, the investigation of U–Pb systematicsprovides not only the radiometric ages of basalts, butalso the μ-values (=238U/204Pb) of parent magmasources, which have important implications for theevolution of lunar magma source regions. In particular,the μ-value could be used for assessing the fractiona-tion between refractory and volatile elements and couldbe an important indicator when considering large-scaleplanetary differentiation scenarios. (For a review, seePremo et al., 1999). Our result of low inclination shownin Fig. 3 implies that the μ-values of the source magmaof MET 01210 is much lower than previous studies oflow-Ti mare basalts from Apollo samples (100 to 300Tatsumoto et al., 1971) and appears to be consistent withthose of the Asuka 881757 (10±3 Misawa et al., 1993)and Yamato 793169 (21.6±3.5 Kita et al., 1995). Thus,the rough estimation of the μ-values of MET 01210also supports the hypothesis that all these meteoritesoriginated from the same magma source and werelaunched by a single impact event, although investiga-tion of U–Pb systematics of not only radiogenic phos-phates but also other minerals in MET 01210 would berequired to further confirm this interpretation.

In summary, the eight spot analyses of phosphatessuggest that MET 01210 is the 3rd lunar basaltic me-teorite that shows the oldest crystallization age of about3.9 Ga for a mare basalt, and which was derived from aprobably low μ-value and low-Ti magma source regionin the lunar mantle. Thus, our in-situ dating techniqueprovides a new insight into the thermal history ofbrecciated meteorites, unravelling chronological histo-ries that have previously not been possible.

Acknowledgments

We thank H. Ishisako, Y. Shibata, A. Kearsely forsample preparations and EPMA analyses. We are alsograteful to Dr. T. Arai from NIPR for useful discussion.This study is partly supported by the Scientific ResearchGrant of the Ministry of Education, Culture, Sports,Science and Technology (MEXT: No. 17540462). Aportion of the manuscript was prepared when K.T. was

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visiting the Department of Earth Science at the OpenUniversity by the MEXT promotion program for thehigher education. K. H. J. is funded by a UK PPARCstudentship and would like to acknowledge the help andguidance of I. A. Crawford. M.A. acknowledges supportfrom PPARC and RCUK fellowships. This contributionis an outcome of a joint project between the HiroshimaUniversity and the Open University.

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