Neodymium, strontium and chromium isotopic studies of the LEW86010 and Angra dos Reis meteorites and the chronology of the angrite parent body

Abstract

— Neodymium, strontium, and chromium isotopic studies of the LEW86010 angrite established its absolute age and the formation interval between its crystallization and condensation of Allende CAIs from the solar nebula. Pyroxene and phosphate were found to contain ∼98% of its Sm and Nd inventory. A conventional 147Sm‐143Nd isochron yielded an age of 4.53 ± 0.04 Ga (2 σ) and ∍143 Nd = 0.45 ± 1.1. An 146Sm‐142Nd isochron gives initial 146Sm/144Sm = 0.0076 ± 0.0009 and ∍143 Nd = −2.5 ± 0.4. The Rb‐Sr analyses give initial 87Sr/86Sr (I87Sr) = 0.698972 ± 8 and 0.698970 ± 18 for LEW and ADOR, respectively, relative to 87Sr/86Sr = 0.71025 for NBS987. The difference, ΔI87Sr, between I87Sr for the angrites and literature values for Allende CAIs, corresponds to ∼9 Ma of growth in a solar nebula with a CI chondrite value of 87Rb/86Sr = 0.91, or ∼5 Ma in a nebula with solar photospheric 87Rb/86Sr = 1.51. Excess 53Cr from extinct 53Mn (t1/2 = 3.7 Ma) in LEW86010 corresponds to initial 53Mn/55Mn = 1.44 ± 0.07 × 10−6 and closure to Cr isotopic homogenization 18.2 ± 1.7 Ma after formation of Allende inclusions, assuming initial 53Mn/55Mn = 4.4 ± 1.0 × 10−5 for the inclusions as previously reported by the Paris group (Birck and Allegre, 1988). The 146Sm/144Sm value found for LEW86010 corresponds to solar system initial (146Sm/144Sm)o = 0.0080 ± 0.0009 for crystallization 8 Ma after Allende, the difference between Pb‐Pb ages of angrites and Allende, or 0.0086 ± 0.0009 for crystallization 18 Ma after Allende, using the Mn‐Cr formation interval. The isotopic data are discussed in the context of a model in which an undifferentiated “chondritic” parent body formed from the solar nebula ∼2 Ma after Allende CAIs and subsequently underwent differentiation accompanied by loss of volatiles. Parent bodies with Rb/Sr similar to that of CI, CM, or CO chondrites could satisfy the Cr and Sr isotopic systematics. If the angrite parent body had Rb/Sr similar to that of CV meteorites, it would have to form slightly later, ∼2.6 Ma after the CAIs, to satisfy the Sr and Cr isotopic systematics.

Concordant Rb–Sr, Sm–Nd, and Ar–Ar ages for Northwest Africa 1460: A 346Ma old basaltic shergottite related to “lherzolitic” shergottites

“…11 shows very little difference between ( 87 Rb/ 86 Sr) S calculated for NWA 1460's mantle source, and measured bulk rock ( 87 Rb/ 86 Sr) B = 0.123-0.131 for NWA 1460 whole rock samples (Table 1). Although these present-day values are in excellent agreement with ( 87 Rb/ 86 Sr) BSM $0.13 as estimated by Shih et al (1999) for BSM, the Rb-Sr model ages of WRf1 and WR2 (Table 1) relative to ( 87 Sr/ 86 Sr) LEW = 0.698972 ± 8 for the LEW86010 (''LEW") angrite (Nyquist et al, 1994) are $5.9 Ga, and $5.6 Ga, respectively, reflecting some lowering of 87 Rb/ 86 Sr in the mafic magma from the time-averaged value in the mantle of $0.16. This probably represents some preferential extraction of Rb relative to Sr in earlier partial melting events, leaving the mantle sources depleted in Rb.…”

“…The typical average values of 87 Sr/ 86 Sr for NBS 987 and 143 Nd/ 144 Nd for Ames Nd metal standard during the course of the study are listed in Table 1. The isotopic data for samples analyzed contemporaneously with the standards were normalized to 87 Sr/ 86 Sr = 0.710250 for NBS 987 (Nyquist et al, 1994) and to 143 Nd/ 144 Nd = 0.511138 for Ames Nd (Wasserburg et al, 1981). The total procedural blanks for Rb ($5 pg), Sr ($20 pg), Sm ($5 pg), and Nd ($10 pg) are low and negligible.…”

“…The detailed sample dissolution techniques and chemical procedures of Rb, Sr, Sm, and Nd separations used in this study were reported previously (Nyquist et al, 1994;Shih et al, 1999). Rb-Sr and Sm-Nd analyses were performed on Finnigan-MAT 261 and 262 mass spectrometers in static-multicollection mode following the procedures of Nyquist et al (1994).…”

“…A small corner having exterior material was chipped off. The $731 mg interior sample was processed for this study following our standard procedures (Nyquist et al, 1994;Shih et al, 1999). The sample was first sonicated in ethanol to remove surface contaminants, and then crushed gently to pass a nylon sieve of opening size <149 lm.…”

“…Rb-Sr and Sm-Nd analyses were performed on Finnigan-MAT 261 and 262 mass spectrometers in static-multicollection mode following the procedures of Nyquist et al (1994). Tantalum oxide slurry was used as an emitter on Re filaments.…”