Post-crystallization reheating and partial melting of eucrite EET90020 by impact into the hot crust of asteroid 4Vesta 4.50 Ga ago

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1 Pergamon PII S (01) Geochimica et Cosmochimica Acta, Vol. 65, No. 20, pp , 2001 Copyright 2001 Elsevier Science Ltd Printed in the USA. All rights reserved /01 $ Post-crystallization reheating and partial melting of eucrite EET90020 by impact into the hot crust of asteroid 4Vesta 4.50 Ga ago AKIRA YAMAGUCHI, 1,2 G. JEFFREY TAYLOR, 1 KLAUS KEIL, 1 CHRISTINE FLOSS, 3 GHISLAINE CROZAZ, 3 LARRY E. NYQUIST, 4 DONALD D. BOGARD, 4 DANIEL H. GARRISON, 5 YOUNG D. REESE, 5 HENRY WIESMANN, 5 and CHI-Y. SHIH 5 1 Hawai i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai i at Manoa, Honolulu, Hawaii 96822, USA 2 Antarctic Meteorite Research Center, National Institute of Polar Research, Tokyo , Japan 3 Laboratory for Space Sciences and Department of Earth and Planetary Science, Washington University, St. Louis, Missouri 63130, USA 4 Planetary Sciences, NASA Johnson Space Center, Houston, Texas 77058, USA 5 Lockheed-Martin Space Mission Systems and Services Co., 2400 NASA Road 1, Houston, Texas , USA (Received November 27, 2000; accepted in revised form May 2, 2001) Abstract We performed petrologic, radiometric (Ar-Ar, Sm-Nd, and Mn-Cr ages), and ion microprobe studies of the basaltic eucrite, EET This is one of the few rare basaltic eucrites whose 39 Ar- 40 Ar age has not been reset during impact bombardment on the HED parent body 4 Ga ago and, thus, should provide a unique opportunity to study the nature of the early thermal events on its parent body (presumably asteroid 4Vesta). Hand specimen inspection shows that the rock consists of a fine-grained and a coarse-grained lithology. Microscopy indicates that the fine-grained lithology has a granulitic texture, with a coarser-grained area and a large opaque assemblage embedded in the granulitic matrix. The coarse-grained lithology has an igneous, subophitic texture. The rock has pyroxenes similar to those in type 5 eucrites (type 5 pyroxene) and experienced prolonged thermal metamorphism after rapid crystallization from a near-surface melt. However, minor mineral assemblages are unusual and suggest a complex thermal history. Tridymite occurs as large laths, irregular crystals ( 1.5 mm), and in a large assemblage of tridymite-plagioclase-pyroxene about 4 mm in size. One large opaque assemblage ( 1.4 mm in size) is a decomposition product of Cr-ulvöspinel and consists of zoned Ti-chromite, ilmenite and minor Fe-metal. The opaques are often rimmed by Fe-rich olivine and pigeonite with some chemical variations, indicating disequilibrium with the surrounding type 5 pyroxenes. Ca-phosphate with low REE abundances, and the LREE signatures observed in plagioclase suggest that melt was present during metamorphism. This indicates that EET90020 was reheated above the subsolidus temperature of eucrites of 1060 C, causing partial melting. Tridymite, Cr-ulvöspinel, and some pyroxene and plagioclase crystallized from the melt. The presence of unequilibrated phases related to opaques suggests cooling rates greater than several C/day. The reheating event was too short to destroy the exsolution textures of the type 5 pyroxenes. The temperature of the rock just before the reheating could have been 870 C, based on the two-pyroxene temperature of the type 5 pyroxenes. The absence of shock effects in plagioclases suggests that EET90020 did not experience shock events 1 5 GPa after the reheating event. EET90020 seems to have experienced the following thermal history; (1) crystallization during rapid cooling near the surface; (2) some brecciation by impact; (3) thermal metamorphism that produced type 5 pyroxene; and (4) short reheating that caused partial melting and rapid cooling. 39 Ar- 40 Ar measurements show a relatively flat pattern and an age of Ga, which is consistent with rapid cooling from high temperature (event 4). Resetting of the Sm-Nd ages at Ga appears to be closely related to the remelting of Caphosphates. Rb-Sr data suggest Rb-loss from tridymite during partial melting. The resetting of the Mn-Cr age may have been related to the formation of Cr-ulvöspinels (event 4). We suggest that all these ages were reset by partial melting (event 4). We further suggest that the partial melting event (event 4) that reset the ages 4.50 Ga ago was caused by an impact into EET90020 which was part of the hot crust of 4Vesta and resulted in an increase in the temperature from the ambient temperature of 870 C to above the subsolidus temperature of eucrites of 1060 C. Copyright 2001 Elsevier Science Ltd 1. INTRODUCTION * Author to whom correspondence should be addressed (yamaguch@ nipr.ac.jp) Eucrites rank among the oldest known basalts in the solar system and are part of a suite of rocks called HED meteorites (howardites, eucrites, diogenites). The original parent body of the HED meteorites may be the asteroid 4Vesta (e.g., McCord et al., 1970; Binzel and Xu, 1993), an object with semiaxes of 289, 280, and 229 ( 5) km (Thomas et al., 1997). Eucrites display characteristics (summarized below) consistent with derivation from a relatively large body. The relatively large size of Vesta is intermediate between that of the parent asteroids of most differentiated meteorites, but smaller than planetary bodies such as the Moon and Mars. Thus, eucrites present an opportunity to study basalt generation on a body of significant size very early in solar system history, a time period for which basalts on the earth and Moon are generally not available. A striking feature of eucrites is that almost all of them are metamorphosed to different degrees, although most originally crystallized and cooled near or on the parent body surface (e.g., Reid and Barnard, 1979; Takeda and Graham, 1991; Yamaguchi et al., 1996a). This metamorphism caused Mg, Fe, and Ca

2 3578 A. Yamaguchi et al. in pyroxenes to undergo varying degrees of subsolidus diffusion to produce more limited ranges in their compositions. Many eucrites are also breccias formed by impact mixing near the surface, and these impacts disturbed the mineralogy, texture, and isotopic ages. In interpreting radiometric ages of eucrites, it is not always easy to distinguish the relative effects of internal metamorphism that reset the pyroxene compositions, from effects produced by shock heating. Nyquist et al. (1986) and Metzler et al. (1995) suggested that brecciated and thermally metamorphosed eucrites formed in contact zones between crater walls and hot impact melt sheets. Stolper (1977) and Yamaguchi et al. (1996a, 1997a) proposed that eucrites crystallized and were metamorphosed during formation of the global eucritic crust. In this model, the parent body crust developed through eruptions of lavas onto the surface, burial by successive lava flows, and reheating of the buried flows by heat from the interior. The latter process must have occurred very early (i.e., as early as 4.56 Ga ago), whereas brecciation and heating by impact could have occurred at later times. Radiometric ages of eucrites vary. Model Pb-Pb ages of and Ga for Ibitira (Chen and Wasserburg, 1985; Manhes et al., 1987) rank among the oldest ages for such meteorites. A few eucrites show evidence for the early presence of now extinct radionuclides such as 26 Al (halflife 0.7 Ma), 53 Mn (halflife 3.7 Ma), and possibly 60 Fe (halflife 0.1 Ma) (e.g., Shukolyukov and Lugmair, 1993; Srinivasan et al., 1999; Lugmair and Shukolyukov, 1998; Carlson and Lugmair, 2000). This implies that these rocks formed very soon after solar system formation. However, several Sm-Nd, U-Pb, and Rb-Sr ages of eucrites span the range of 4.40 to 4.56 Ga (e.g., Nyquist et al., 1986; Prinzhofer et al., 1992; Carlson and Lugmair, 2000). Pb-Pb ages of 4.40 to 4.48 Ga have been determined for a few cumulate eucrites (Tera et al., 1997). Because most cumulate eucrites are unbrecciated and are thought to have formed at greater depth compared to noncumulate eucrites, their younger ages may reflect persistence of substantial temperatures in the deep parent body crust at this time. The 39 Ar- 40 Ar ages of eucrites scatter widely from 3.1 to 4.4 Ga, and this has been interpreted to reflect impact metamorphism during cataclysmic bombardment, analogous to that which occurred on the moon (e.g., Bogard, 1995). Several Rb-Sr ages also indicate disturbance over the broad time period of 3.5 to 4.5 Ga. To what extent these younger ages reflect isotopic resetting from shock-impact rather than by internally derived thermal metamorphism or prolonged volcanism is an issue of considerable importance in evaluating the heat source on the HED parent body. Combined investigations using petrology, mineralogy, geochemical, and isotopic studies are required to resolve this issue. Textural evidence and the old 39 Ar- 40 Ar plateau age of EET90020 suggest that this rock is one of those rare noncumulate eucrites that escaped impact bombardment on the HED parent body 4 Ga ago (Yamaguchi et al., 1996b; 1997c; Bogard and Garrison, 1997). Therefore, EET90020 provides a rare opportunity to study the nature of the early thermal environment without interference from major thermal effects due to later impact metamorphism. We therefore conducted mineralogical, petrological, geochemical and isotopic studies of the meteorite. Brief earlier reports of studies of EET90020 are given by Phinney et al. (1993), Yamaguchi et al. (1996b; Table 1. Modal compositions of the coarse-grained (,18) and finegrained (,28) lithologies of EET90020 (in vol. %) PTS (,18) PTS (,28) Eucrite mean* Pyroxene (2.6) Plagioclase (2.1) Silica (1.5) Ilmenite (0.3) Chromite tr (0.2) Troilite 0.2 nd 0.3 (0.3) Fe-metal tr tr tr Fe-rich olivine nd 0.5 Phosphate 0.1 tr 0.2 (0.1) Weathering Area (mm 2 ) One analyzed step 0.1 mm. * Delaney et al. (1984), average of 22 basaltic eucrites. tr trace ( 0.1 vol. %); nd not detected. Figures in parentheses give 1 of analyses. 1997c), Nyquist et al. (1997), Bogard and Garrison (1997), and Floss and Crozaz (1997, 1998). Specific goals of our study are to date the time of the thermal events recorded in isotopic systems as well as to understand the mineralogy and petrology of EET90020 and the geologic history of the HED parent body before the period of impact bombardment. 2. SAMPLES AND ANALYTICAL PROCEDURES Hand specimen inspection shows that EET90020 (Original mass: g, Antarctic Meteorite Newsletter, 1991) consists of two clearly distinguishable lithologies that appear to occupy distinct portions of the rock: One lithology is fine-grained, the other is coarse-grained. Microscopy of polished thin sections shows that the coarse-grained lithology has an igneous, subophitic texture, whereas the fine-grained lithology has a granulitic texture. In hand specimen, several large vugs are visible that branch deeply into the rock. Most of these vugs occur in the fine-grained lithology or at the boundary between the two lithologies. Most are completely lined with a black glass 1 to 2 mm thick that appears to be a part of the melted fusion crust forced into these interior cavities (Bogard and Garrison, 1997). Two polished thin sections (PTSs) were made for all petrographic studies and electron microprobe and ion microprobe analyses, one from the fine-grained lithology (,28) and one from the coarse-grained lithology (,18). Note that PTS,28 also contains a coarse-grained area with diffuse boundaries with the fine-grained, granulitic matrix into which it is embedded. It also contains a large opaque assemblage. However, PTS,28 is dominated by the fine-grained lithology that occupies 90 vol. % of the section. Subsamples,22 (coarse-grained) and,26 (finegrained), taken adjacent to the samples used for preparing PTSs, were used for the 39 Ar- 40 Ar analyses. All 147 Sm- 143 Nd, 146 Sm- 142 Nd, 87 Rb- 87 Sr, and 53 Mn- 53 Cr analyses were of subsample,24 (coarse-grained) Petrology and Geochemistry The PTSs were examined optically and with scanning electron microscope. Minerals were analyzed with a Cameca Camebax electron probe microanalyzer (EPMA) operated at an accelerating voltage of 15 kev and beam current of 20 na for pyroxene, ilmenite, and spinel, and 10 na for plagioclase and silica phases. An accelerating voltage of 20 kev and beam current of 20 na were used for metallic Fe, Ni, and troilite. Natural and synthetic phases of well-known compositions were used as standards, and data were corrected using a ZAF program. V 2 O 5 contents of spinels were corrected using a technique described by Snetsinger et al. (1968). The modal compositions (Table 1) were determined optically by point counting except for Ca-phosphate, which was estimated from PK scanning beam images obtained with the EPMA. Compositions of minerals are shown in Tables 2 4. Abundances of the REE and selected other trace elements were determined using the Washington University modified Cameca IMS-3f

3 Post-crystallization partial melting of eucrite 4.50 Ga ago 3579 Table 2. Chemical compositions (wt%) of pyroxene in various lithologies of EET Coarse-grained lithology (,18) Fine-grained lithology (,28) In large opaque in,28 pig aug bulk* pig aug pig aug SiO Al 2 O TiO FeO MnO MgO CaO Cr 2 O Total Wo En * Bulk, average of several analyses determined by broad beam (30 m). ion microprobe, according to techniques described by Zinner and Crozaz (1986). All measurements were made using an O primary beam and energy filtering at low mass resolution to remove complex molecular interferences. The resulting mass spectrum was deconvolved to remove simple molecular interferences that are not eliminated with energy filtering, to obtain concentrations of the elements K-Ca-Sc- Ti,Rb-Sr-Y-Zr, and Ba-REE, using procedures described by Floss et al. (2000). REE concentrations were determined using Si (for silicates) or Ca (for Ca-phosphates) as reference elements and SiO 2 or CaO concentrations obtained from quantitative EDS analyses. Analysis spots were carefully chosen to avoid cracks and inclusions commonly observed in minerals of eucrites, and were examined afterwards to check for contamination in the third dimension. In addition, masses diagnostic of potential contaminating phases (e.g., mass 31 for P) were continuously monitored during all analyses Ar- 40 Ar and Space Exposure Ages Samples of the coarse- and fine-grained lithologies were neutronirradiated and the 39 Ar- 40 Ar ages were determined at NASA JSC. We also analyzed an unirradiated sample of the fine-grained lithology for cosmogenic noble gases to determine the meteorite s space exposure age. In addition, we analyzed both irradiated and non-irradiated samples of the dark glass taken from one of the internal cavities. Results for the unirradiated glass sample are reported in Garrison et al. (1998) and show the presence of large concentrations of atmospheric Ar. Data for both the irradiated and unirradiated glass samples strongly indicate that the glass formed as a result of melting during atmospheric entry and not on the parent body. Consequently, the Ar-Ar data for the glass sample will not be further discussed. The EET90020 samples and four samples of hornblende NL-25 (Bogard et al., 1995) were irradiated at Brookhaven National Laboratory in a single quartz vial. Energetic neutrons convert a portion of the 39 Kto 39 Ar (half-life 269 yr) and a portion of the 40 Ca to 37 Ar (half-life 35.1 d). The hornblende samples serve both as a K-Ar age standard and as relative neutron flux monitors. Subsequent to irradiation, argon was extracted from each EET90020 sample by stepwise temperature release, and its isotopic composition was measured with a mass spectrometer. Corrections were made for extraction blanks, radioactive decay, and isotopic interferences produced during neutron irradiation. 39 Ar- 40 Ar ages are calculated using the decay constants recommended by Steiger and Jäger (1977). The irradiation constant for each EET90020 sample was determined from its known position relative to the hornblendes during irradiation. For the unirradiated EET90020 sample, cosmogenic noble gases were extracted in the 500 C and 1550 C temperature fractions and measured on a separate mass spectrometer that has not been used to analyze neutron irradiated samples Rb-Sr, Sm-Nd, and Mn-Cr Analyses Two chips weighing a total of g of 24 were processed for Rb-Sr, Sm-Nd and Mn-Cr isotopic studies at NASA JSC. Chip #1 (0.502 g) was coarsely crushed and 1/3 set aside for whole rock analysis, while the remainder was crushed finer and sieved. Frantz magnetic separation and hand picking of 60 to 100 and 100 to 200 mesh fractions gave 99% purity pyroxene and plagioclase. Small aliquots ( 10 mg) of whole rock, pyroxene, and hand-picked opaque minerals (mostly ilmenite and spinel) were used for Mn-Cr analysis. Using a heavy liquid of density 2.65 g/cm 3, a small amount ( 1mg) of tridymite was obtained for Rb-Sr analysis from the 325 mesh fraction. Chip #2 (0.447 g) was processed later and combined with separates from chip #1 to give samples for additional analyses, including unspiked aliquots for isotopic composition measurements. Rb-Sr and Sm-Nd analytical procedures for,24 are similar to those of Borg et al. (1997). Micro columns containing 150 L of strontium specific resins (EIChromM Sr resin) were used with 3N HNO 3 and water to purify and to elute Sr. Small columns containing 5 mlof cation exchange resin (Bio-Rad AG50X12) and 2N HCl were used to purify Rb collected from the Sr micro column washes. Rare earth elements were purified using REE specific resin (EIChromM RE resin) and techniques similar to those employed for Sr. The final separation of Sm from Nd was achieved using Bio-Rad AG50X8 (NH 4 form) and -hydroxyisobutyric acid ( -HIBA). Total procedure blanks in picograms were 17 to 21 for Rb, 26 to 27 for Sr, 2 to 3 for Sm, and 3 to 7 for Nd. Mn-Cr analyses for whole rock samples and mineral separates followed the procedures described by Nyquist et al. (1994). Total procedural blank for Cr isotopic analyses is 10 ng, which is negligible for these samples. Small aliquots ( 10%) of dissolved sample solutions were used for Cr and Mn concentration determination by the graphitefurnace atomic absorption technique. The analytical uncertainties for Mn/Cr measurements are 5%. Some samples were leached in 2N HCl before dissolution to remove adhering phosphates that otherwise might have influenced the measured Sm/Nd ratios and Nd isotopic compositions. Both leached and unleached whole rock and plagioclase samples were analyzed, but only leached samples of pyroxene were analyzed because of the effect of phosphates in lowering the Sm/Nd ratio. This effect is especially critical for pyroxene samples, which have high Sm/Nd ratios and, thus, play a very important role in determining the calculated isochron age. Mass spectrometric analysis utilized both Nd and NdO ion beams. No detectable differences were observed between these two modes of analysis for either the Ames Nd isotopic standard or for replicate analyses of unleached bulk samples Texture 3. RESULTS PTS,18 (16 10 mm), made from the coarse-grained, igneous portion has a subophitic texture and consists of lathy plagioclase ( m) (41.4 vol.%) and anhedral to granular pigeonite ( m) (53.6 vol.%),

4 3580 A. Yamaguchi et al. Table 3. Chemical compositions (wt%) of plagioclase and tridymite in various lithologies of EET Plagioclase Tridymite Coarse-grained lithology (,18) Fine-grained lithology (,28) Coarse-grained lithology (,18) Fine-grained lithology (,28) Coarse-grained area granulitic core rim core rim matrix TPP* lath granulitic matrix SiO Al 2 O FeO CaO Na 2 O K 2 O Total Or Ab * TPP Large tridymite-plagioclase-pyroxene assemblage. sharing curved boundaries (Table 1, Fig. 1). One large, irregular area ( mm) is composed of fine-grained ( 100 m) granular to stubby plagioclase and interstitial tridymite with a small amount ( 10 vol.%) of irregular pyroxene (Fig. 2a). Ca-phosphates are concentrated in a portion of this area less than several hundreds of m in size (Fig. 2b). Tridymite grains, both laths and irregularshaped grains ( mm), are abundant (4.6 vol.%). Irregularly shaped troilite ( 200 m) is scattered evenly throughout the thin section; it frequently contains rounded grayish (weathered) materials. Ilmenite and spinel ( 50 m in size) are common and a few Fe-metal grains ( 20 m) are observed. The oxide minerals are comparatively depleted in the coarse-grained lithology (PTS,18) compared to the fine-grained lithology (PTS,28) (Table 1) (see below). PTS,28 (11 6 mm) is dominated by a granulitic texture. It also contains 10 vol. % of a coarse-grained area (Fig. 1), although there are no well-defined boundaries between it and the granulitic host matrix. The texture of the coarse-grained area is similar to that of PTS,18 but slightly coarser grained and is comprised of lath-shaped plagioclase ( m) and anhedral pyroxene ( m in size). The granulitic portion is composed of anhedral scalloped pyroxenes ( m) enclosed by plagioclase granules (50 80 m in diameter) with well-developed 120 triple junctions; some plagioclases have lathy to stubby shapes. Tridymite occurs as an interstitial phase between pyroxene and plagioclase or as a massive phase ( m), peppered with small grains of plagioclase. Oxides occur as relatively large discrete grains more than several tens of m in size. Troilite mainly occurs at the boundaries of the plagioclase granules. One opaque assemblage is extremely large ( mm) and has a complex internal texture (Fig. 1, see below). PTS,28 has similar modal abundances of pyroxene and plagioclase as does PTS,18, but has less tridymite and more ilmenite and spinel (Table 1). The modal compositions of both the coarse-grained and fine-grained lithologies are roughly similar to those of other basaltic eucrites (e.g., Delaney et al., 1984). Table 4. Chemical compositions (wt%) of spinel, ilmenite, and olivine in various lithologies of EET Fine-grained lithology (,28) Coarse-grained lithology (,18) Large opaque Opaque 2 sp1 sp2 ilm sp1 sp2 ilm oliv sp1 oliv SiO Al 2 O TiO FeO MnO MgO CaO Cr 2 O V 2 O nd nd nd 0.75 nd Total nd not determined

5 Post-crystallization partial melting of eucrite 4.50 Ga ago 3581 Fig. 1. Photomicrographs of the various lithologies in EET (a) Overview of the coarse-grained lithology (PTS,18), showing a subophitic texture. Note a large tridymite lath indicated by arrow (Si). (b) Tridymite-plagioclase-pyroxene assemblage in,18. (c) Overview of the fine-grained lithology (PTS,28), showing a granulitic texture. Note a large, poikiloblastic opaque (lower left). (d) Area within the fine-grained lithology showing a coarse-grained texture. Width (a) and (c) is 5.2 mm; (b) and (d), 2.6 mm. Pl plagioclase; Px pyroxene Mineralogy Pigeonite contains homogeneously distributed (001) augite lamellae and is similar to that in type 5 eucrites (hereafter referred to as type 5 pyroxene) (Takeda and Graham, 1991). There is no significant difference in the internal textures of the pyroxenes among the observed lithologies, except for pyroxenes associated with opaque assemblages (see below). The augite lamellae are thin (several m thick) and closely spaced (5 10 m wide). Occasionally, we found thick augite lamellae up to 10 m. Pyroxene is rarely fractured and has very sharp optical extinction. The pigeonite hosts (and some augite?) often contain fine cracks approximately perpendicular to the c-axis. Most pyroxenes are clear and remarkably free of inclusions (clouding; Harlow and Klimentidis, 1980). Some pyroxenes contain sparse, tiny (submicron size) dusts of oxides and larger (up to 30 m) rounded plagioclase inclusions. Except for pyroxenes in the opaques (see below), Mg/Fe ratios of the pigeonites in all lithologies are very homogeneous (Fig. 3) and are in the range of those of other basaltic eucrites (e.g., Takeda and Graham, 1991). Note that the range of pyroxene compositions in Fig. 3 is artifacts and results from the small sizes of the pyroxenes and, hence, beam overlap during EPMA analysis. An exsolved pair of augite (Wo 41.0 En 30.2 ) in pigeonite (Wo 3.1 En 35.6 ) with the widest Ca range in the coarse-grained lithology indicates an equilibration temperature of 870 C, using the Kretz (1982) Ca-thermometer. Cores of the lathy to stubby plagioclases in both,18 and,28 are generally cloudy, containing numerous fine ( 10 m), needle-like to subrounded inclusions of mainly pyroxenes, rarely silica minerals, and more rarely oxides, troilite, and Fe-metal. Plagioclases in the fine-grained lithology, on the other hand, are very clear with no inclusions. The compositional range is very limited (Table 3, Fig. 4). Plagioclase in,18 shows normal igneous zoning from Ab , but Or zoning is not detectable. K 2 O contents are uniformly low and vary between wt.% (Fig. 4). The granular plagioclases have also similar compositional ranges but zoning is not apparent. Plagioclases generally show sharp optical extinction. However, some show very weak mottled extinction ( 5 misorientation) and are composed of several domains with similar crystallographic orientation. EET90020 contains 5 vol.% of minor minerals (Table 1). The most abundant minor mineral is tridymite; it occurs as a massive interstitial phase between pyroxene and plagioclase grains, and as lathy to irregular discrete crystals. Tridymite crystals are mottled, composed of irregular domains ( m), with similar crystallographic orientation ( 10 misorientation). The mineral contains significant amounts of im-

6 3582 A. Yamaguchi et al. Fig. 2. Backscattered electron image (BEI) of a tridymite-plagioclase-pyroxene assemblage in the coarse-grained lithology of EET90020 (PTS,18). (a) The assemblage consists of stubby to rounded plagioclase (Pl; medium grey) and interstitial tridymite (Si; dark grey). Pyroxene (Px) is light grey. (b) Ca-phosphate grains (Ph; all white grains) in the trydimite (Si)-plagioclase (Pl)-pyroxene assemblage. Small irregular dark areas are holes in the section filled with mounting medium. purity elements such as K 2 O( wt.%), Al 2 O 3 ( wt.%.), and FeO ( wt.%) (Table 3). Ilmenite and spinel are the most common type of opaque minerals ( 1 vol.%, Table 1), and are often associated with each other. TiO 2 contents of the entire spinel population vary smoothly from Usp Chm (note Usp is mol. % ulvöspinel, Fe 2 TiO 4 ; Chm is mol. % chromite, FeCr 2 O 4 ), but the chemical variations of individual spinel grains are more limited (Fig. 5). One rectangular Cr-ulvöspinel ( m) in the finegrained lithology is rimmed by Fe-rich olivine (Fa 82.6 ). The other trace phases include troilite, Fe-metal and Fe-rich olivine, sometimes associated with oxides. Ca-phosphates occur as relatively large crystals, with rectangular to angular shapes. In the coarse-grained lithology (,18), Ca-phosphate grains are

7 Post-crystallization partial melting of eucrite 4.50 Ga ago 3583 Fig. 3. Compositions of pyroxenes in various lithologies of EET90020 in terms of mol. % endmembers ferrosilite (Fs; FeSiO 3 ), wollastonite (Wo; CaSiO 3 ) and enstatite (En; MgSiO 3 ; not indicated in Fig). The Mg/Fe ratios of each phase (pigeonite and augite) are completely homogenized, except for pyroxenes associated with the opaque assemblage. The intermediate compositions between low- and high-ca pyroxenes (pigeonite and augite) are artifacts due to EPMA beam overlap during analysis. only found in the large pyroxene-plagioclase-tridymite assemblage. Except for the largest one ( mm), all phosphates occur in a small area less than 0.6 mm across (Fig. 2b). In the fine-grained lithology (,28), on the other hand, we found only one slightly weathered Ca-phosphate grain ( m) and one blocky one of 250 m in size. This suggests that Ca-phosphate is distributed heterogeneously in the rock. A few Fig. 4. Compositions of plagioclases in various lithologies of EET90020 in terms of mol. % endmembers albite (Ab; NaAlSi 3 O 8 ), anorthite (An; CaAl 2 Si 2 O 8 ) and orthoclase (Or; KAlSi 3 O 8 ). zircon grains were found in the fine-grained lithology, one of which poikilitically encloses a tiny pyroxene grain. Zircon grains are m in size, and one which is large enough for analysis and consists of SiO , ZrO , and HfO wt.%. The unusually large opaque assemblage with poikiloblastic texture (Figs. 1c, 6) in the fine-grained lithology is composed of spinel (Ti-chromite) and ilmenite and contains subrounded inclusions (up to 200 m) of pyroxene, plagioclase, a minor silica mineral, and tiny ( m) Fe-metal grains. The spinel shows compositional zoning toward the boundary of ilmenite from Usp Chm (Fig. 6). Closer to the ilmenite grains, Al 2 O 3 and Cr 2 O 3 contents in the spinel increase and TiO 2 and FeO contents decrease, but minor MgO and MnO contents seem to be homogeneous. Ilmenite, on the other hand, contains minor amounts of MgO (0.62 wt.%) and MnO (0.65 wt.%) that are uniformly distributed, but Cr 2 O 3 contents are slightly zoned toward chromite crystals from 0 to 1.06 wt.%. This is not due to beam overlap during analysis, as in similarly sized ilmenite/spinel pairs, TiO 2 in spinel decreases towards ilmenite. Most pyroxene inclusions are rimmed by aggregates of small Fe-rich olivine crystals (20 30 m in thickness) (Fig. 6). In addition, a more complex rim texture is also observed: pyroxene inclusions are rimmed by thin augite and pigeonite, often associated with silica minerals, which are further surrounded by Fe-rich olivine (Fig. 7). However, a few pyroxene and plagioclase inclusions are observed that have no rims. Boundaries between the Fe-rich olivine rims and oxides and the pyroxene inclusions show zigzagged shape in which the pigeonite portion is often concave at the boundaries (Fig. 7). Compositions of the olivines in the opaque assemblage are homogeneous (Fa 78.2, Table 4), but are different from those in other portions of the rock. The Fe-rich olivines contain a significant amount of Cr 2 O 3 ( 0.4 wt.%) and TiO 2 (

8 3584 A. Yamaguchi et al. Fig. 5. Compositions of spinels in various lithologies of EET90020 in terms of mol. % 2Ti, Cr, and Al (recalculated to 100%). The spinels are highly zoned. wt.%). Although we cannot completely rule out that these values are artifacts resulting from overlap of the EPMA beam near the chromite-olivine borders, the olivine areas are large (30 m), making this unlikely. The pyroxene rims in this assemblage are higher in Fe than those in the other portions of the rock (Fig. 3). Compositions of the plagioclase inclusions are very similar to those in other lithologies. Finally, a large, irregular tridymite ( mm) is found next to the large opaque, and the coexisting Fe-metal is nearly pure Fe (Ni 0.08 wt.%) Ion Probe Data We measured the abundances of the REE and selected other trace elements in plagioclase, pigeonite and Ca-phosphates from the coarse- and fine-grained lithologies (,18 and,28) (Table 5; Fig. 8; note that we did not discriminate between the portion with a granulitic texture and the coarse-grained area in,28 during ion microprobe analyses). The REE patterns for plagioclase are LREE-enriched, with large positive Eu anomalies. Abundances are uniform within each lithology, but the average abundances differ between them: they are almost a factor of three higher in the coarse-grained lithology than in the fine-grained lithology (Fig. 8a). Pigeonite REE patterns are HREE-enriched, with negative Eu anomalies. As noted for plagioclase, REE abundances are uniform within a given lithology, but they are significantly higher in pigeonites from the coarse-grained lithology than from the fine-grained lithology (Fig. 8b). Ca-phosphate grains large enough to analyze were found in both sections (Fig. 8c). Merrillite from the finegrained lithology has a LREE-enriched pattern with a negative Eu anomaly; abundances are similar to those observed in Ibitira and are two to four times lower than those of merrillite in other non-cumulate eucrites (Hsu and Crozaz, 1996). Apatite from the coarse-grained lithology has much lower REE abundances than merrillite, but a similarly LREE-enriched pattern (Fig. 8c). In addition to the REE, we measured other major, minor and trace element concentrations in plagioclase and pigeonite (Table 5). Sodium, K, Sr and Ba in plagioclase from both lithologies have narrow compositional ranges that fall within fields previously defined for non-cumulate eucrites (Hsu and Crozaz, 1996). In contrast, pigeonite compositions exhibit enrichments in several elements (Ti, Zr, and Y) similar to those previously observed for Ibitira (Hsu and Crozaz, 1996) Space Exposure Age Concentrations of He, Ne, and Ar were measured in 500 C and 1550 C extractions of an unirradiated sample of the finegrained lithology (,26) (Table 6). The 4 He and 40 Ar are radiogenic, and most of the other He, Ne, and Ar isotopes were produced by cosmic ray interactions. The cosmogenic 22 Ne/ 21 Ne ratio of 1.24 is larger than that of most eucrites (Eugster and Michel, 1995) and suggests that EET90020 was a relatively small object in space. We use cosmogenic production rates for eucrites given by Eugster and Michel (1995) to calculate the space exposure ages of 11 to 17 Ma (Table 6). The higher 21 Ne age compared to the 3 He and 38 Ar ages is partly due to a significant shielding correction to the 21 Ne production rate based on 22 Ne/ 21 Ne. For such relatively large shielding corrections, the production rates of Eugster and Michel (1995) become uncertain. If we had assumed average eucrite shielding, the 21 Ne age would be 15 Ma. Eugster and Michel (1995) report space exposure ages for 37 eucrites that lie between 5 and 50 Ma. The determined 81 Kr exposure age for Ibitira, another unbrecciated non-cumulate eucrite, is 13.4 Ma, although the 21 Ne age is 15.6 Ma and the ratio 22 Ne/ 21 Ne is (Eugster and Michel, 1995). Thus, space exposure for Ibitira and EET90020 may have been initiated by the same impact event, and these two eucrites may have been derived from the same parent body location.

9 Post-crystallization partial melting of eucrite 4.50 Ga ago 3585 Fig. 6. X-ray scanning beam images of the large ( mm), poikiloblastic opaque assemblage in the fine-grained lithology (PTS,28) of EET It is mostly composed of Ti-chromite (grey in the CrK map) and ilmenite (white in the TiK map), containing pyroxene and plagioclase inclusions. Rimming of Fe-rich olivine around pyroxenes can be seen in the FeK map (light grey). Note that Ti and Al in the spinel are slightly zoned towards ilmenite Radiometric Ages Ar- 40 Ar age Calculated 39 Ar- 40 Ar ages and K/Ca ratios as a function of cumulative release of 39 Ar from the fine-grained (,26) and coarse-grained (,22) lithologies are given in Figures 9a and 9b, respectively. The uncertainty shown for the age of each individual temperature extraction is propagated from measurement uncertainty in the 40 Ar/ 39 Ar ratio and from uncertainties in the applied blank and reactor corrections, but does not reflect uncertainties in the irradiation constant. The plateau age we give below for each sample is the weighed average obtained from a series of individual extractions yielding similar ages. The uncertainty in these average ages is obtained by using the age equation to statistically combine the one sigma deviation about the mean of the average age with the uncertainty in the irradiation constant, J (Bogard et al., 2000). Not included in this average age uncertainty is a 0.5% uncertainty in the absolute age of the NL-25 hornblende or any uncertainty in 40 Ar decay constants (Bogard et al., 1995). The two EET90020 samples show similar 39 Ar- 40 Ar age spectra. For each, the K/Ca ratio is relatively constant at for the first 75% of the 39 Ar release, decreases to a value of 0.01 for two extractions, then rises again. The total K/Ca ratio of for both samples is equal to that for plagioclase (Table 5). Lower K/Ca ratios at 80% 39 Ar release probably include 37 Ar degassing from pyroxene. The slightly higher K/Ca ratio for the first 10% of the 39 Ar release may include Ar degassing from minor tridymite, which contains higher K concentrations. This tridymite phase may be the source of small diffusive losses of 40 Ar seen in the first 10% of the 39 Ar release. Alternatively, the first 10% of the Ar

10 3586 A. Yamaguchi et al. Fig. 7. BEI of a portion of the large opaque assemblage in the fine-grained lithology (PTS,28) of EET90020, showing complex rim textures. There are thin rims of Fe-rich olivine (Ol) between pyroxenes (Pig pigeonite; Aug augite) and oxides (Im ilmenite). Notice that the pigeonite portion is concave at the boundary with Fe-rich olivine. Pl plagioclase. release may be an Antarctica weathering effect. The finegrained sample (,26) may have experienced additional very small losses of 40 Ar over 10 to 32% 39 Ar release. The 39 Ar- 40 Ar age for both the fine- and coarse-grained lithologies summed overall extractions is the same at Ga and gives a minimum age for EET For the fine-grained lithology, the average age of eight extractions releasing 14 to 98% of the total 39 Ar, is Ga. If we omit lower temperature extractions, suggesting slight diffusive loss of 40 Ar, the average age of six extractions releasing 32 to 98% of the total 39 Ar, is Ga. We consider Ga to be the best 39 Ar- 40 Ar degassing age for the fine-grained lithology. For the coarse-grained lithology, the average age for nine extractions releasing 8 to 95% of the total 39 Ar, is Ga. This larger uncertainty compared to that for the fine-grained lithology arises from the lower age for one extraction at 82% 39 Ar release. Reactor-produced corrections to 39 Ar based on 37 Ar (and uncertainties in individual extraction ages) are largest for the two extractions at 80% 39 Ar release for each sample. We doubt, however, that this reactor correction has been improperly applied to the 1200 C extraction of the coarse-grained lithology, because the same correction to other extractions with low K/Ca from both lithologies gives ages consistent with the plateau ages. The amount of Ar released in the 1200 C extraction of the coarse-grained lithology is relatively small. If we omit the 1200 C extraction and consider only 8 extractions (over 8 95% 39 Ar release), the 39 Ar- 40 Ar plateau age for this lithology is Ga. As for the fine-grained lithology, the age over 0 to 31% of the 39 Ar release suggests slight diffusive loss of 40 Ar, so this age may be slightly low. Nevertheless, the 39 Ar- 40 Ar ages for both the fine- and coarse-grained lithologies have high relative precision and are identical at Ga. No evidence exists in the 39 Ar- 40 Ar age spectra for significant heating of EET90020 after this time. Table 5. Average major, minor and trace element concentrations of minerals in the coarse-grained (,18) and fine-grained (,28) lithologies of EET90020, as determined by ion microprobe techniques*,18,28 Plagioclase (9) Pigeonite (11) Apatite (4) Plagioclase (7) Pigeonite (6) Merrillite (2) Na Mg % % K Ca 11.4% 3.3% 11.5% 3.2% Sc Ti Fe % % Sr Y Zr Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu b.d b.d * Concentrations are in ppm, except as otherwise noted; errors are 1 standard deviation of the mean; the number of analyses are given in parentheses; b.d.: below detection.

11 Post-crystallization partial melting of eucrite 4.50 Ga ago 3587 Table 6. Noble gas concentrations (cm 3 STP/g) and space exposure ages for a sample of the fine-grained lithology (,26) of EET Cosmogenic production rates are from Eugster and Michel (1995). Species units 500 C 1550 C Total 3 He He Ne Ne Ne Ne cos Ar Ar Ar Ar cos He/ 21 Ne Ne/ 21 Ne Kr Xe Xe/ 132 Xe Production Rate Exposure Age, Ma 3 He Ne Ar Fig. 8. CI -normalized REE patterns for (a) plagioclase, (b) pigeonite, and (c) Ca-phosphates in the fine-grained (PTS,28) and coarsegrained (PTS,18) lithologies of EET All REE patterns shown are average abundances. The shaded region in (c) shows the range of REE abundances observed in merrillite from non-cumulate eucrites (NCE) and the solid line shows REE abundances in Ibitira merrillite (Hsu and Crozaz, 1996) Sm- 143 Nd and 146 Sm- 142 Nd Ages The Sm-Nd analytical results for the coarse-grained lithology are shown in Table 7, and the 143 Nd/ 144 Nd and 147 Sm/ 144 Nd data are plotted in an isochron diagram in Fig. 10. Least squares regression by the Williamson (1968) program yields an isochron age T SmNd Ga. The corresponding value of Nd This age is in good agreement with the 39 Ar- 40 Ar age of Ga for both lithologies. Furthermore, the (T SmNd, Nd )-values of EET90020 are nearly identical to those reported for the eucrite clast in Bholghati (Nyquist et al., 1990) and overlap values of T SmNd Ga and Nd reported by Prinzhofer et al. (1992) for Ibitira. Interpreted as primary igneous features of these eucrites, the positive Nd values imply their derivation from a LREE-depleted precursor. Laul and Gosselin (1990) considered this possibility for the Bholghati clast, as did Smith (1982) for clasts from Kapoeta. In the case of EET90020, however, it seems more likely that plagioclase incorporated more radiogenic 143 Nd * from adjacent phases during the last reheating event. We noted (Fig. 8) that LREE abundances in plagioclase and pyroxene are enriched relative to those of most non-cumulate eucrites. This is attributed to redistribution of REE from Caphosphates to the silicates during partial melting of the rock mesostasis. Plagioclase is more susceptible to REE exchange than pyroxene (Prinzhofer et al., 1992). As the REE redistribution appears to have been heterogeneous on a cm-scale, the variability in 143 Nd/ 144 Nd values of the two plagioclase analyses (Fig. 10) could be explained as due to the heterogeneity of sampling plagioclase. Two effects potentially accompanying the hypothesized LREE enrichment could increase Nd. First, LREE enrichment in plagioclase could lower the Sm/Nd ratio without significant change in 143 Nd/ 144 Nd, causing a portion of the radiogenic 143 Nd* in the plagioclase to appear unsupported by radioactive decay. This might happen if, for example, the reservoir from which Nd and Sm were partitioning relative to plagioclase had the Sm-Nd characteristics of Ca-phosphates. The 147 Sm/ 144 Nd ratio calculated for merrillite and apatite are 0.16 and 0.18, respectively (Table 5). These ratios are similar to that for plagioclase (0.11, Table 7), so that preferential partitioning of Nd relative to Sm from Ca-phosphate into plagioclase would cause 147 Sm/ 144 Nd for plagioclase to be lowered while 143 Nd/ 144 Nd increased slightly. This possibility is implied by the post-igneous redistribution of REE in EET90020 noted already by Phinney et al. (1993). Second, because plagioclase often shares long borders with pyroxene, some radiogenic 143 Nd * could preferentially migrate into plagioclase from

12 3588 A. Yamaguchi et al. Fig Sm- 143 Nd isochron diagram for the coarse-grained lithology (,24) of EET Plag plagioclase, Px pyroxene, WR whole rock. Inset shows the deviation of the analytical data from the isochron in -units (parts in 10 4 ). Fig Ar- 40 Ar ages (rectangles) and K/Ca ratios (stepped line) as a function of cumulative release of 39 Ar for stepwise extractions of (a) the fine-grained (,26) and (b) the coarse-grained (,22) lithologies of EET Measured K and Ca concentrations are also given. Individual age uncertainties are indicated by the widths of the rectangles and include all analytical uncertainties, but not those associated with the irradiation constant and the absolute age of the hornblende standard. pyroxene if Nd is preferentially accepted by plagioclase relative to Sm. Thus, the apparently elevated value of Nd in EET90020 probably has no petrogenetic significance. Average Sm and Nd abundances in the two unleached whole rock samples of the coarse-grained lithology (WR1 and WR4) are 5.5 and 3.9 CI abundances (Anders and Grevesse, 1989). Not only are these values lower than in typical noncumulate eucrites, but the CI normalized Sm/Nd ratio is 1.4 compared to 1. This is most likely due to a deficit of Caphosphate in our sample of the coarse-grained lithology. Indeed, using the ion microprobe data for plagioclase, pyroxene, and merrillite (Table 5) and the modal data for plagioclase and pyroxene (Table 1), we calculate that the unleached whole rock samples contain less than 0.05 vol.% merrillite. The addition of only 0.1% merrillite to these whole rock samples would raise their Sm and Nd abundances to almost 10 CI and reduce their CI normalized Sm/Nd ratio to 1.1, i.e., values typical of noncumulate eucrites. Ca-phosphate, the main REE carrier in eucrites is very heterogeneously distributed in these meteorites and, indeed, is likely to be underrepresented in our samples. Sm and Nd abundances for the plagioclase mineral separates are 4.0 and 6.8 and those for pyroxene are 11.5 and 7.0 CI. These are about a factor of 2 higher than the corresponding ion probe values (Table 5). In addition, the mineral separate concentrations are too high to account for the Table 7. Sm-Nd analytical results for the coarse-grained lithology (,24) of EET90020 Sample a wt. (mg) Sm (ppm) Nd (ppm) 147 Sm/ 144 Nd b 143 Nd/ 144 Nd b,c 142 Nd/ 144 Nd b,c WR1 e WR2(r) WR Plag1 e Plag2(r) Px2(r) Px2a(r) e Ames Nd standard: Nd (15 analyses 7/97): d d NdO (10 analyses 3/97): d d a WR whole rock, Plag plagioclase, Px pyroxene, r residues. b Uncertainties correspond to last figures and represent 2 m error limits. c Normalized to 146 Nd/ 144 Nd and adjusted to 143 Nd/ 144 Nd of the Ames Nd standard (Wasserburg et al., 1981). d Uncertainties correspond to last figures and represent 2 p error limits. e Nd runs as NdO mode.

13 Post-crystallization partial melting of eucrite 4.50 Ga ago 3589 Fig Sm- 142 Nd isochron diagram for the coarse-grained lithology (,24) of EET Mineral identifications are as in Fig Nd/ 144 Nd values are normalized to 146 Nd/ 144 Nd precisely defined than that of EET90020, but is consistent with it within error limits. The value of ( 146 Sm/ 144 Sm) I found for EET90020 implies closure of the Sm-Nd isotopic system 75 ( 28/ 24) Ma after ( 146 Sm/ 144 Sm) in the angrite LEW Referenced to an angrite age of Ga (Lugmair and Galer, 1992), this implies Nd-isotopic closure at Ga, in good agreement with the conventional 147 Sm- 143 Nd age. The apparent coincidence in the ages of EET90020 and the cumulate eucrites is discussed later in the paper. The Sm-Nd ages are in excellent agreement with the 39 Ar- 40 Ar age of this lithology, and it appears that the Ar and Nd isotopic systems of this eucrite closed simultaneously, at the end of the last major heating event, 4.50 Ga ago. The value of ( 146 Sm/ 144 Sm) I for EET90020 is significantly less than that for Ibitira (Prinzhofer et al., 1992; Nyquist et al., 1999), suggesting that although the two eucrites may have simultaneously closed to Ar-outgassing 4.50 Ga ago, they experienced differing intensities of reheating. reported whole rock analyses, even if one makes the unreasonable assumption that all the REE in the whole rock samples are located in plagioclase and pyroxene only. This indicates the presence in the mineral separates of some phosphate grains that were not removed by the leaching procedure. Similarly, leaching of a whole rock sample does not seem to have been as efficient at removing Ca phosphate as anticipated. The short-lived 146 Sm- 142 Nd chronometer is less sensitive to factors affecting the Sm-Nd ratio of mineral phases. Fig. 11 shows the 142 Nd/ 144 Nd data plotted vs. the 147 Sm/ 144 Nd ratio. The slope of the isochron gives initial ( 146 Sm/ 144 Sm) I , a value similar to those obtained previously for the cumulate eucrites Moama ( ; Jacobsen and Wasserburg (1984) and Moore County ( ; Tera et al. (1997)). The 147 Sm- 143 Nd age of EET90020 is identical to the Ga age of the large non-cumulate eucrite clast in the Bholghati howardite (Nyquist et al., 1990, see also Reid et al., 1990). Ages of 4.51 Ga also have been found for two other non-cumulate eucrites. Tera et al., (1997) determined precise Pb-Pb mineral isochron ages of Ga and Ga for Nuevo Laredo and Bouvante, respectively. Another non-cumulate eucrite, Bereba, was tentatively assigned an age of 4.52 Ga by these authors, based on its mineral Pb-Pb isotopic systematics. Initial ( 146 Sm/ 144 Sm) I for the Bholghati clast is less Rb- 87 Sr data The Rb-Sr analytical data for the coarse-grained lithology (,24) are given in Table 8. In spite of large variations in Sr concentrations among plagioclase, pyroxene and whole rock, these three samples have very similar 87 Rb/ 86 Sr and 87 Sr/ 86 Sr ratios. Tridymite has surprisingly high Rb and Sr concentrations of 2.76 and 35.8 ppm, respectively. The Rb concentration, combined with the K 2 O concentration in tridymite of 0.26% (Table 3) suggests K/Rb 780 in this phase. This seems a plausible value, compared to K/Rb 300 in lunar KREEP basalts, for example. The high abundance of alkali and alkaline earth elements in tridymite seems generally consistent with an origin via melting and recrystallization of mesostasis originally present in this basalt, as suggested above. However, the high Sr abundance in tridymite is somewhat surprising in view of its close association with plagioclase in tridymite-plagioclase-pyroxene assemblages (Fig. 2.) Tridymite has by far the most radiogenic Sr isotopic composition of any analyzed phase. However, the Rb-Sr data for tridymite are displaced from a 4.51 Ga reference isochron through data for the other mineral phases (Fig. 12). Initial 87 Sr/ 86 Sr (I Sr ) for age T 4.51 Ga and for T 4.56 Ga. Both values are within error limits of 87 Sr/ 86 Sr for the LEW86010 angrite (Nyquist et al., 1994). Thus, initial 87 Sr/ 86 Sr Table 8. Rb-Sr analytical results for the coarse-grained lithology (,24) of EET90020 Sample a wt. (mg) Rb (ppm) Sr (ppm) 87 Rb/ 86 Sr b 87 Sr/ 86 Sr b,c WR2(r) Plag2(r) Px2 (r) Tridymite NBS 987 Sr standard: Sr (8 analyses 6/97): d (7 analyses 8/97): d a WR whole rock, Plag plagioclase, Px pyroxene, r residues. b Uncertainties correspond to last figures and represent 2 m error limits. c Normalized to 88 Sr/ 86 Sr and adjusted to 87 Sr/ 86 Sr of the NBS 987 Sr standard (Nyquist et al., 1990). d Uncertainties correspond to last figures and represent 2 p error limits.