High-Spatial Resolution U–Pb Dating of Phosphate Minerals in Martian Meteorite Allan Hills 84001

Geochemical Journal, Vol. 48, pp. 423 to 431, 2014 doi:10.2343/geochemj.2.0319

High-spatial resolution U–Pb dating of phosphate minerals in Martian meteorite Allan Hills 84001

MIZUHO KOIKE,1** YOSHIHIRO OTA,1 YUJI SANO,1* NAOTO TAKAHATA1 and NAOJI SUGIURA2

1Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba 277-8564, Japan 2Department of Earth and Planetary Science, School of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

(Received February 15, 2013; Accepted June 4, 2014)

Phosphate minerals, which are ubiquitous in terrestrial and extra-terrestrial rocks, are important carriers of trace elements, including U and Th, which provide chronological information because of their radioactive decay. Abundance and grain sizes of the phosphates are limited in extra-terrestrial materials. Therefore, high resolution and minimally or non- destructive analytical methods must be used for age determination. For this study, we conducted U–Pb dating using a NanoSIMS for three phosphate grains from an old Martian meteorite: ALH 84001. The 238U–206Pb and 207Pb–206Pb isochron ages were found to be 3850 ± 170 Ma and 4002 ± 52 Ma, respectively, suggesting a concordant signature at approx. 4.0 Ga. A total Pb/U isochron age of 3990 ± 160 Ma is consistent with a previous SHRIMP U–Pb age of 4018 ± 81 Ma (Terada et al., 2003). Moreover, heterogeneous distributions of U are observed in these grains, which might have been preserved since igneous crystallization of the phosphates because the diffusion of U in the mineral is considerably slow.

Keywords: U–Pb dating, phosphates, NanoSIMS, Martian meteorites, ALH 84001

2004; Li et al., 2012; Zhou et al., 2013; Yin et al., 2014). INTRODUCTION This report describes our study of U–Pb dating of single Martian meteorites, which are known to show ex- phosphate grains in an extremely old Martian meteorite, tremely wide variations in chemistry, chronology, miner- Allan Hills (ALH) 84001, using a NanoSIMS 50 (Ametek alogy, and petrology (Meyer, 2012), are regarded as pro- Inc.). viding important clues to elucidate the evolution of the Martian mantle–crust, and of surface environments. Ra- SAMPLE AND ANALYTICAL METHODS diometric ages of these meteorites might provide funda- mental and valuable information about the Martian envi- In 1984, Martian meteorite ALH 84001 was collected ronment. Phosphate minerals such as apatite and merrillite from the Far Western Icefield of Allan Hills. It is unique are common in extraterrestrial materials. They are im- because of its extremely old crystallization and metamor- portant carriers of incompatible elements including REE, phic age (Previous works are presented in Table 1 except Th and U, and were used for leaching experiment (e.g., for SHRIMP data, which are presented in Table 2.) and Chen and Wasserburg, 1986). However, because the phos- complicated thermal history (Treiman, 1995, 1998). The phates are volumetrically minor and because their grain crystallization age is 4.0–4.56 Ga (Jagoutz et al., 1994, sizes are limited, minimally or non-destructive analyti- 2009; Nyquist et al., 1995, 2001; Nyquist and Shih, 2013; cal methods must be used for accurate dating, even though Bouvier et al., 2009; Lapen et al., 2010), although Ar–Ar enriched shergottites contain phosphate mineral grains up results of studies suggest several shock heating events at to several hundred micrometers. Currently, in-situ U–Th– 3.8–4.2 Ga (Ash et al., 1996; Knott et al., 1996; Turner Pb dating by SHRIMP is a major analytical method for et al., 1997; Ilg et al., 1997; Bogard and Garrison, 1999; the radiogenic system in terrestrial (Sano et al., 1999, Cassata et al., 2010). Moreover, this meteorite experi- 2006a; Lan et al., 2013) and extraterrestrial phosphates enced other chemical/physical processes including depo- (Sano et al., 2000; Terada et al., 2003; Terada and Sano, sition of secondary carbonate minerals and possibly low- temperature aqueous alteration (Wadhwa and Lugmair, 1996; Borg et al., 1999; Halevy et al., 2011; Beard et al., *Corresponding author (e-mail: [email protected]) 2013). Because ALH 84001 is a rare sample of ancient **Present address: Central Research Laboratory, Hitachi, Ltd., Martian material with age of more than 4 billion years, Kokubunji, Tokyo 185-8601, Japan. elucidating its history is crucially important to understand Copyright © 2014 by The Geochemical Society of Japan. the early days of Martian evolution, even though ex-

423 Table 1. Summary of ALH 84001 geochronometry

Method Age (Ga) Mineral Reference 147Sm−143Nd ~4.56 WR, Opx, Msk Jagoutz et al., 1994 147Sm−143Nd 4.5 ± 0.13 WR, Opx Nyquist et al., 1995 147Sm−143Nd 4.41 ± 0.03 WR, Opx Lapen et al., 2010 147Sm−143Nd 4.57 ± 0.09 WR, Opx Nyquist and Shih, 2013 146Sm−142Nd 4.47 ± 0.04 WR, Opx Nyquist and Shih, 2013 87Rb−87Sr 4.58 ± 0.3 WR, Opx Nyquist et al., 1995, 2001# 87Rb−87Sr 3.87 ± 0.05 WR, Opx Wadhwa and Lugmair, 1996 87Rb−87Sr 1.4 ± 0.1 Crb, Msk Wadhwa and Lugmair, 1996 87Rb−87Sr 3.92 ± 0.02 Crb Borg et al., 1999 Borg and Drake, 2005 87Rb−87Sr 4.34 ± 0.23 WR, Opx, Msk Beard et al., 2013* 87Rb−87Sr 3.95 ± 0.02 WR, Opx, Msk Beard et al., 2013* Chr, Crb 176Lu−176Hf 4.09 ± 0.03 WR, Opx, Chr Lapen et al., 2010 39Ar−40Ar ~3.6 Crb Knott et al., 1996 39Ar−40Ar 4.0 ± 0.1 Msk Ash et al., 1996 39Ar−40Ar 4.07 ± 0.04 Msk Ilg et al., 1997 39Ar−40Ar 4.1 ± 0.2 Msk Bogard and Garrison, 1999 39Ar−40Ar 3.93 ± 0.12 Msk Turner et al., 1997 39Ar−40Ar 4.16 ± 0.04 Msk Cassata et al., 2010 39Ar−40Ar 1.16 ± 0.11 Opx Cassata et al., 2010 232Th−208Pb 2.93 ± 0.41 WR, Opx, Phs Jagoutz et al., 2009 207Pb−206Pb 4.05 ± 0.09 Crb Borg et al., 1999 Borg and Drake, 2005 207Pb−206Pb 4.07 ± 0.10 WR, Opx Bouvier et al., 2009 207Pb−206Pb 4.04 ± 0.14 First leachate steps Bouvier et al., 2009 207Pb−206Pb 4.12 WR, Opx Jagoutz et al., 2009

Simplified notations are: WR = whole rock, Opx = orthopyroxene, Crb = carbonates, Msk = maskelynite, Chr = chromite, Phs = phosphates. *:Remark. The carbonate isochron for ALH 84001 has been recalculated using the new decay constant; previously this isochron was calculated using an 87Rb decay constant of 1.402 × 1011 yr–1, as proposed by Minster et al. (1982) and produced an age of 3924 +/– 19 Ma using an Isoplot regression (Borg and Drake, 2005). The Nyquist et al. (1995) and Wadhwa and Lugmair (1996, 1997) isochron ages have not been recalculated using this new decay constant because data, analytical details, and standard analyses were not reported in these abstracts, preventing a detailed assessment of the impact of different decay constants. #:Remark. Nyquist et al. (2001) described: Here, as elsewhere in this paper, we use the value of the 87Rb decay constant recommended by Minster λ × 11 –1 et al. (1982); i.e., 87 = 1.402 10 yr . tremely old brecciated Martian meteorites (NWA 7034/ in close association with plagioclase glass (Pg). These 7475/7533) have been found recently (Agee et al., 2013; signatures resemble those described in Greenwood et al. Humayun et al., 2013). (2003), although carbonate cannot be found close to the The samples used for this study were two polished phosphate. Chemical compositions of the phosphates, thick sections of ALH 84001 (Fig. 1a), which were stud- orthopyroxene, and plagioclase glass were measured us- ied previously for hydrogen isotopes in carbonates and ing SEM-EDS. They are listed in Supplementary Table maskelynite (Sugiura and Hoshino, 2000). Back-scattered S1. The atomic Na/Ca ratio of phosphates is approxi- electron images of the samples were obtained prelimi- mately ten, which is consistent with the chemical formula narily using a secondary electron microprobe with an of merrillite (Mer), (Ca18(Mg,Fe)2Na2(PO4)14). Their energy dispersive spectrometer (SEM-EDS) to locate crystallographic features were analyzed using electron phosphate grains. Three phosphate grains with sizes of backscatter diffraction (EBSD) and were identified as 50–100 µm were found (Fig. 1b). It is well documented merrillite. The sections were polished again, gold-coated, that orthopyroxene is the dominant mineral in ALH 84001 and then baked at approx. 100°C overnight in the air-lock (Meyer, 2012). Phosphate grains appear to be surrounded to reduce absorbed water, a potential contributing factor by orthopyroxene (Opx) in the BSE images. They are also for hydride interference.

424 M. Koike et al. Table 2. 238U–206Pb ages and 207Pb–206Pb ages for single grain dating and multi grain dating

238U−206Pb 207Pb−206Pb Total U/Pb Age (Ma) Age (Ma) Age (Ma) Grain #1 3970 ± 460 4010 ± 180 3830 ± 470 Grain #2 3610 ± 840 3760 ± 530 4220 ± 180 Grain #3 3550 ± 860 3920 ± 160 3770 ± 540 Multi-grain 3850 ± 170 4002 ± 52 3990 ± 160

SHRIMP* 3700 ± 440 4022 ± 96 4018 ± 81

*Terada et al. (2003). Error assinged to the U–Pb and Pb–Pb ages is two sigma, while total U/Pb is 95% confidential level.

206 + 238 16 + × 238 16 + 238 16 + 2 An apatite extracted from an alkaline rock of Prairie ( Pb / U O ) = a ( U O2 / U O ) + b.(1) Lake circular complex in Ontario, Canadian Shield, PRAP, with age of 1155 ± 20 Ma (Sano et al., 2006a) was used In that equation, a and b are constants determined by cor- 206 + 238 16 + 238 16 + 238 16 + as the standard apatite. The average U concentration of relation between Pb / U O and U O2 / U O PRAP is 196 ppm (Sasada et al., 1997). The U–Pb dating of the standard. More details about the calculation are was conducted using a NanoSIMS 50 installed at the At- given elsewhere (Takahata et al., 2008). Application of mosphere and Ocean Research Institute of The Univer- this method to terrestrial phosphate is currently underway sity of Tokyo. An approx. 10 nA 16O– primary ion beam (Sano et al., 2014). Uranium concentrations of analyzed with a spot diameter of approx. 15 µm is focused on the spots are obtained by comparing the measured 238UO+/ sample surface. Each spot is sputtered preliminarily for 5 43Ca+ ratios of the sample against those of the standard min to eliminate surface residual contaminants. Positive (statistical error obtained by repeated analyses is approx. secondary ions are extracted with an accelerating volt- 30% at the 2-sigma level). age of 8 kV. For 238U–206Pb dating, 43Ca+, 204Pb+, 206Pb+, After 238U–206Pb measurements, 207Pb–206Pb ages on 238 16 + 238 16 + U O and U O2 are collected simultaneously with the same spots were determined using single-collector a dual-collector-combined multi collection system. The mode, where the magnet was cyclically peak-stepped observed background and 204Pb count rates are, respec- through 204Pb+, 206Pb+, and 207Pb+. That process required tively, 0.001–0.002 cps and 0.006–0.04 cps. Although approx. 1 h, the sum of 100 cycles and waiting time, to 204Pb abundance of the phosphates is so low that identifi- attain statistically sufficient counts. The pit depth was cation of the 204Pb peak on mass spectra images is diffi- possibly less than 3 µm after the measurement, which is cult, no isobaric interference was found in this mass range markedly smaller than the spot diameter (approx. 15 µm). or the mass range over 206Pb and 207Pb with mass resolu- The most stable and best analytical data were obtained in tion of approx. 4100 at 1% peak height (Sano et al., the condition (Ireland, 2004). 2006b). One analysis session takes 10 min to yield statistically sufficient counts. RESULTS For SIMS measurements, secondary U ions are extracted mainly as monoxide and dioxide, although Pb is Eleven spots on Grain #1 were analyzed, as were seven emitted almost entirely as atomic ions. Calibrations must spots each on Grains #2 and #3 (Fig. 1b and Supplemen- derive true U/Pb ratios of samples from the secondary tary Table S2). Results obtained for U concentrations 238U/ ion counts. For SHRIMP measurements, 238U/206Pb ra- 206Pb, 207Pb/206Pb, and 204Pb/206Pb ratios are presented in tios are calculated from an empirical power law relation Table S2. The 238U/206Pb ratios of the samples were cal- between 206Pb+/238U + and 238U16O+/ 238U + ratios culated through the described quadratic equation (1) with (Hinthorne et al., 1979; Williams, 1998; Sano et al., 1999). the best-fit values of constants a and b of 0.14 ± 0.02 and Regarding NanoSIMS, the relative ratios of secondary –0.04 ± 0.03 (Supplementary Fig. S1). A few data (1-1 + + + UO2 :UO :U for phosphates are around 10:10:1 (Sano and 2-3) were discarded because the pit overlapped the et al., 2006b). Empirically, the relation between 206Pb+/ cracks of the merrillite grains. 238 16 + 238 16 + 238 +16 + U O and U O2 / U O ratios is useful for cali- bration. We applied a quadratic relation that was derived 238U–206Pb, 207Pb–206Pb, and total U/Pb isochron ages originally for zircon dating (Takahata et al., 2008). of multi-grain We first calculated the “multi-grain age”, a conven-

NanoSIMS U–Pb dating of ALH 84001 425 (a) (b)

Fig. 1. (a) Backscattered electron images of two sections of ALH 84001 with phosphate locations as circles. (b) Analyzed merrillite grains with spot numbers. Rough contour images of U concentrations (ppm) are also overprinted. Pg, plagioclase glass; Opx, orthopyroxene; Mer, merrillite.

tional way where all grains’ data are combined to deter- dimensional (3D) total U/Pb isochron age requires 238U/ mine a single isochron age. This age estimate is based on 206Pb, 207Pb/206Pb, and 204Pb/206Pb ratios (Wendt, 1989). the idea that all grains were formed simultaneously. The Two datasets of 204Pb/206Pb ratios were used (Table S2). 23 data points obtained from the three grains show corre- One is derived from the multi-collector mode. The other lations in both 238U–206Pb and 207Pb–206Pb inverse plots derives from the single mode with magnet scanning. We (Figs. 2a and 2b). Based on least-squares fitting using calculated the weighted mean average of 204Pb/206Pb ra- the York method (York, 1969), both 238U–206Pb and 207Pb– tios when considering the error and used the value for 3D 206Pb isochron ages were determined (Table 2). The cal- total U/Pb isochron. Concordia-constrained linear regres- culations were made using isoplot3 (Ludwig, 2003). The sion shows the age of 3990 ± 160 Ma (95% confidence 238U–206Pb and 207Pb–206Pb ages were, respectively, 3850 limit, MSWD = 1.9 in Table 2 and Supplementary Fig. ± 170 Ma (2σ, MSWD = 2.1) and 4002 ± 52 Ma (2σ, S2), although the common Pb plane intercept gives no MSWD = 1.3). The two ages are concordant at approx. precise number because the error is large. 4.0 Ga within uncertainties. Calculation of three-

426 M. Koike et al. a large error, our age results agree around 4.0 Ga, indicating that the U–Pb system in the Grain #1 is probably concordant. The 3D total U/Pb isochron ages are, respectively, 3830 ± 470 Ma (95% confidence limit, MSWD = 1.2), 4220 ± 180 Ma (95% confidence limit, MSWD = 1.2), and 3770 ± 540 Ma (95% confidence limit, MSWD = 0.21) for Grains #1, #2, and #3. They are mutually consistent within the allocated error.

Concentrations of uranium Variations of U concentrations are 1.2–3.6 ppm for three grains. The individual grains have U heterogeneity of a factor of approx. 2 (see Fig. 1b and Table S1). Com- pared to terrestrial apatite, for which typical U concentrations vary from a few parts per million to approx. 200 ppm (Sano et al., 1999), the concentrations of U in the observed merrillite grains are significantly lower and show smaller variations. The variations of U concentrations observed in individual grains are even smaller than those of grain-to-grain differences reported previously for apatite and merrillite in ALH 84001 (Terada et al., 2003). Although the variation is rather small, the observed distributions of U contents are not random but instead show some trend. The U-rich regions are, respectively, on the right side in Grain #1, the lower area in Grain #2, and the upper side in Grain #3. No simple relation exists between the U content and mineral close to the phosphate. For example, a higher U concentration is located adjacent to plagioclase glass in Grain #1, although it is lower in Grain #2.

Fig. 2. Correlations between 204Pb/206Pb and 238U/206Pb ra- DISCUSSION 204 206 207 206 tios (a), and Pb/ Pb and Pb/ Pb ratios (b) of the three Evaluations of NanoSIMS U–Pb dating merrillite grains. Solid lines are regression lines calculated with Observed “multi-grain” 238U–206Pb and 207Pb–206Pb Isoplot3 (Ludwig, 2003). Dashed curves are error envelopes. Previous U–Pb study of phosphates (Terada et al., 2003) and ages by NanoSIMS are concordant: approx. 4.0 Ga within the isotopic ratios of terrestrial common Pb (Stacey and uncertainty, which agrees well with the previous SHRIMP 238 206 Kramers, 1975) and initial Pb estimated for ALH 84001 study: 3700 ± 440 Ma for U– Pb and 4022 ± 96 Ma 207 206 (Bouvier et al., 2009) are also shown. for Pb– Pb in Table 2 (Terada et al., 2003). Further- more, the errors of our results (170 Ma and 52 Ma for the 238U–206Pb and 207Pb–206Pb ages, respectively) are smaller than those of the previous work (Table 2), suggesting that 238U–206Pb, 207Pb–206Pb and total U/Pb isochron ages of NanoSIMS U–Pb dating of phosphates with low U abun- single grains dance can provide accurate and reasonably precise age The 238U–206Pb and 207Pb–206Pb isochrons for the three information. individual grains are calculated separately (Supplemen- A few differences exist in 204Pb/206Pb ratios between tary Fig. S3, Table 2). The calculated 238U–206Pb ages are, datasets of 238U–206Pb and 207Pb–206Pb measurements. For respectively, 3970 ± 460 Ma (2σ, MSWD = 3.7), 3610 ± example, in Grain #1, the ratios of spots 1–4 are discrep- 840 Ma (2σ, MSWD = 0.63), and 3550 ± 860 Ma (2σ, ant, although the other spots are mutually consistent MSWD = 0.32) for Grains #1, #2, and #3. The 207Pb– within experimental error. Similar features are observed 206Pb ages of three grains are also determined respectively in Grains #2 and #3. If a discrepancy results from con- as 4010 ± 180 Ma (2σ, MSWD = 2.2), 3760 ± 530 Ma siderable contaminations of terrestrial common Pb from (2σ, MSWD = 0.35), and 3920 ± 160 Ma (2σ, MSWD = the surface, all spots might show a similar tendency, i.e., 0.16). Although 238U–206Pb ages of Grains #2 and #3 have 204Pb/206Pb ratios of multi-collector mode (shallow) are

NanoSIMS U–Pb dating of ALH 84001 427 larger than those of single (deep). However, such is not the case. They differ spot by spot (Table S2), although a general tendency is apparent: the 204Pb/206Pb ratios are somewhat higher in 207Pb–206Pb measurements than in 238U–206Pb measurements (see Fig. S3). The origin of common Pb is more complicated, requiring some other explanation than simple terrestrial contamination on the sample surface. We again calculated the 3D total U/Pb isochron age based on the results obtained in this work (Table S1), to- gether with those reported by Terada et al. (2003). Concordia-constrained linear regression using 35 data gives the isochron age of 4015 ± 60 Ma (95% confidence limit, MSWD = 2.2). This ion microprobe age is consist- ± ent with the Pb–Pb silicate mineral age of 4074 99 Ma Fig. 3. Calculated relations between closure temperatures of obtained using the MC-ICP-MS measurement of strong U and Pb in phosphate and cooling rates of the system. Diffu- leachates and orthopyroxene residue of ALH 84001 sion radii are 10 µm (solid curve) and 50 µm (dashed curve), (Bouvier et al., 2009). It is noteworthy that three- which respectively present intra-grain diffusion and grain-to- component mixing of a terrestrial contaminant, common grain diffusion. Peak metamorphic temperature between 900°C Martian Pb, and radiogenic Pb are not expected to form a and 1200°C is shown as shaded area. linear array in the 238U/206Pb–207Pb/206Pb–204Pb/206Pb diagram (Wendt, 1989). A simple mixing relation exists between radiogenic and common Pb in this work. gle phosphate grains by NanoSIMS might provide chrono- Common-Pb plane intercept in the 3D diagram pro- logical information of much smaller spatial resolutions vides the end member with the 206Pb/204Pb = 13.1 ± 6.2 (Terada et al., 2014) than previously studied. (2σ) and 207Pb/204Pb = 13.4 ± 5.0 (2σ) with the 207Pb/ 206Pb ratio of 1.02. These initial ratios are less radiogenic Interpretation of U–Pb ages than those of the present terrestrial common Pb (Stacey Although the U–Pb system shows concordance for and Kramers, 1975), even though the errors are large. Borg multi-grain isochron, one can not simply regard them as et al. (2005) reported that the Martian meteorites appear igneous age of the host rock. From a number of textural, to be contaminated by Martian surface Pb characterized chemical and isotopic studies, ALH 84001 is regarded as by a 207Pb/206Pb ratio of at least one. They further sug- having suffered severe impact shock, consequent anneal- gested that the surface Pb is prevalent in impact glass- ing and aqueous alterations for several times (summa- rich mineral fractions. Borg and Drake (2005) reported rized in Treiman (1995, 1998)). The metamorphic tem- that the surface Pb was derived from the sulfur-rich soil perature of the ancient impact heating at 3.8–4.2 Ga (Ash in most aqueous systems. Observed common Pb in this et al., 1996; Turner et al., 1997; Bogard and Garrison, work might be related to the surface Pb. 1999; Cassata et al., 2010) is estimated as >875°C using Isochrons calculated for individual grains have larger mineral geothermometers (Treiman, 1995), but it is not errors because of low U concentrations and small varia- above 1200°C because no evidence exists of wholesale tions as well as the limited number of spots analyzed. melting (Treiman, 1998). Therefore, the peak metamor- However, their mean ages are consistent with the multi- phic temperature might be between about 900°C and grain 238U–206Pb and 207Pb–206Pb ages, indicating the U– 1200°C. Phosphates in ALH 84001 are known as igneous Pb system within an individual grain is probably concord- by a textural, chemical and oxygen isotopic study (Green- ant at approx. 4.0 Ga. These results suggest that analyses wood et al., 2003). We examined our samples by the tex- of single grains provide useful U–Pb age information. ture (Fig. 1b). Numerous cracks in merrillite minerals are Regarding the age precision, it can be regarded as rea- apparently connected with those in orthopyroxene, al- sonable considering the limited sample size (50–100 µm though plagioclase glasses have no cracks. The phosphate diameter) and small variations of U concentrations (Ta- might be generated simultaneously of orthopyroxene or ble S2). For all cases in this study, the 207Pb–206Pb ages at least before the shock heating formation of plagioclase can be determined more precisely than the 238U–206Pb age glass, suggesting its igneous origin. Burger et al. (2012) because of the old age of approx. 4.0 Ga and the system- reported that a positive relation exists between Mg# and atics of U–Pb Concordia curve where the time variation Na contents in igneous Martian merrillite. Our chemistry of 207Pb/206Pb ratio is large at 3–4 Ga. Based on these data (Table S1) are consistent with the trend. The rare results, it is noteworthy that the U–Pb dating of the sin- earth element (REE) pattern of whole-rock ALH 84001

428 M. Koike et al. is similar to those of other igneous Martian meteorites diffusion scale “a”, we used 10 µm (approximate spot (Meyer, 2012), again suggesting merrillite’s igneous ori- diameter) and 50 µm (grain size), respectively, for intra- gin because the phosphate is a major carrier of REE ele- grain diffusion and grain-to-grain diffusion. ments. The 207Pb–206Pb isochron age of 4002 ± 52 Ma in Figure 3 shows the calculated closure temperatures this work might show an igneous formation event. with various cooling rates of the system. The peak tem- Several studies have identified later shock events for perature of metamorphic event (900–1200°C) is shown ALH 84001 (e.g., Treiman, 1998; Cassata et al., 2010). in the diagram. Based on the heterogeneous U content The 207Pb–206Pb system in the phosphates might have not distributions in each grain, one might say that U in the been disturbed during those later events, although some phosphate has never been lost or homogenized since its 238U–206Pb isochrons of the individual grains show a igneous crystallization. These observations suggest that slightly younger age with large uncertainty (Table 2). An the cooling rate is lower than 0.4°C/year if the maximum experimental shock and heating study (Gaffney et al., peak temperature of 1200°C is adopted in the event. The 2011) reported the weakness of U–Pb system to impact rate is consistent with the rapid cooling system of 0.04– heating compared to Rb–Sr and Sm–Nd systems. Further 0.4°C/year in Shergotty phosphates (Sano et al., 2000). discussion related to the younger 238U–206Pb ages, how- No evidence exists of partial melt of phosphate minerals ever, requires additional analyses of phosphate grains in in texture (Fig. 2b). Therefore, the peak temperature might the meteorite. be less than 1000°C. However even in this condition, Pb Distributions of U in the individual grains are also might be partly lost by diffusion from the grain. Then informative, although the variations observed in the grain 238U–206Pb system can be reset somehow. After the are not so large (Fig. 1b and Table S2). The success of approx. 4.0 Ga event, the temperature might not have the single grain dating is because of these U exceeded approx. 600°C except for localized heating (in- heterogeneities and consequent production of radiogenic ferred from the Ar–Ar study; Cassata et al., 2010) be- Pb. From diffusion behaviors of U and Pb, the thermal cause the 207Pb–206Pb system has been closed for approx. history of the host rock can be discussed. More specifi- 4.0 Ga, even with the rapid cooling rate of Shergotty. If cally, the relation between peak temperatures and cool- the cooling rate is on the order of 10–5°C/year, similar to ing rates for heating events of ALH 84001 can be regu- the terrestrial sample assumed by Cherniak et al. (1991), lated by distributions of U and Pb in the observed grains. which is much smaller than Shergotty, the sample might ° The closure temperature (Tc) of a certain ion species not have been heated more than 500 C at any event since in a mineral crystal can be approximated from the fol- 4.0 Ga because Pb is not homogenized at the 50 µm scale lowing equation (Dodson, 1973). (see Fig. 3).

ER/ T = . ()2 CONCLUSIONS c 2 2 ′ ln()ARTc () D0 / a / ET The U–Pb dating of merrillite grains in ALH 84001 was conducted using a NanoSIMS. For both multi-grain Therein, R is the gas constant, A stands for the geometri- dating and single-grain dating, obtained U–Pb ages and cal constant of 55 for spherical shape, T′ denotes the cool- Pb–Pb ages are approx. 4.0 Ga. They also agree with the previous SHRIMP U–Th–Pb work, indicating that our ing rate, D0 represents the pre-exponential factor of diffusion coefficient, E is the activation energy, and a is the dating method reveals a local history of the sample. Fur- diffusion radius. Previous study of Shergotty phosphates thermore, the observed heterogeneous distributions of U used this relation for Pb diffusion and discussed the clo- content within the grains might have been preserved since sure temperature of U–Pb age (Sano et al., 2000). Diffu- its igneous formation. Calculation of the closure tempera- sion parameters of U or Pb in merrillite are not well tures of U and Pb in phosphate crystal indicates that the known. However, it might be reasonable to apply those cooling rate might be consistent with the rapid value of of apatite in this case because previous data of U–Th–Pb 0.04–0.4°C/year. The U–Pb system remained almost analyses of apatite and merrillite plot on the same regres- closed since approx. 4.0 Ga, suggesting that the tempera- sion lines (Sano et al., 2000; Terada et al., 2003), which ture has not exceeded 500°C since then. indicates diffusion behaviors of U–Th–Pb system are not significantly different between the two. Therefore, we Acknowledgments—Useful advice related to experimental apply apatite parameters to our results here. Experimen- techniques and chronological studies from Prof. H. Hiyagon, tally acquired D and E values in apatite crystal are re- Dr. T. Iizuka, and Dr. K Ichimura is greatly appreciated. We are 0 also thankful to Dr. K R. Ludwig for providing computer soft- ported, respectively, as D = 2 × 10–4 cm2/s and E = 231 0 ware: “Isoplot/Ex”. We are most grateful to Dr. K. Misawa and × –6 2 kJ/mol for Pb and D0 = 1.3 10 cm /s and E = 394 kJ/ Dr. D. J. Cherniak for valuable comments, and to Dr. Terada mol for U (Cherniak et al., 1991; Cherniak, 2005). For for kindly handling the manuscript from previous versions. This

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