A New Source of Basaltic Meteorites Inferred from Northwest Africa 011

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Only three groups of basaltic meteorites with asteroidal origin (eucrites, angrites, and basaltic inclusions of mesosiderites) have been sampled to date. The basaltic inclusions in mesosiderites are mineralogically similar to eucrites, but the genetic relationships between the two basalts have been yet controversial. Angrites are a small group of basaltic meteorites, mineralogically different from eucrites. The oxygen isotopic compositions of these basaltic meteorites indicate derivation from a common isotopic reservoir (23). In contrast to the strong link between eucrite and 4 Vesta, the parent bodies of angrites and mesosiderites remain unidentified. See (2) for detail.

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NWA011 was found in the Sahara desert with total known mass of 40 g, and was initially classified as a eucrite (5). A broken surface of NWA011 shows a rusty appearance due to the presence of weathering products. Polished thin and thick sections of NWA011 were examined by optical and scanning electron microscopy and by electron microprobe. A portion of the sample weighing ∼1.5 g was powdered for standard wet chemical analysis, instrumental neutron activation analysis (INAA) including prompt gamma-ray analysis (PGA), and inductively coupled plasma–mass spectrometry (ICP-MS). Other portions were used for noble gas and oxygen isotopic analysis. The analytical procedures were described in (27–32). Note that two oxygen isotopic analyses were performed with and without HCl wash to remove weathering products, and the two results are the same. This indicates that the oxygen isotopic composition of NWA011 was not changed during the weathering processes in the desert. This result is consistent with the very low Fe₂O₃ contents determined by wet chemical analysis (7).

Supplementary material is available on Science Online at www.sciencemag.org/cgi/content/full/296/5566/334/DC1

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The isotopic compositions of He, Ne and Ar represent the predominance of cosmogenic components and radiogenic ⁴⁰Ar (7). Heavy noble gases, Kr and Xe, are composed of trapped (probably adsorbed terrestrial atmospheric gas), cosmogenic and fission components. The cosmic-ray exposure ages, determined from cosmogenic ³He, ²¹Ne and ³⁸Ar, are 11, 30, and 23 Ma, respectively. The shorter ages from ³He and ³⁸Ar are probably due to partial loss of these gases by terrestrial weathering (33). The ⁸¹Kr-Kr age for NWA011 is 39 ± 5 Ma, which can be interpreted as an upper limit of the cosmic-ray exposure age (7). Abundance of radiogenic ⁴He is very low (<1 × 10⁻⁷ cm³/g), and the measured ³He/⁴He ratio is purely cosmogenic. From this amount and the bulk U content for NWA011 (7), the U-⁴He age can be constrained as <0.03 billion years (Ga). Radiogenic ⁴He may have been lost during atmospheric entry to the earth or by terrestrial weathering.

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Oxygen isotopic compositions are expressed as delta-values, the deviation in per mil relative to a standard composition (standard mean ocean water, or SMOW) δ¹⁸O = {[(¹⁸O /¹⁶O)_sample/(¹⁸O/¹⁶O)_SMOW] – 1} × 1000 (‰) and similarly for δ¹⁷O. The deviation of the oxygen isotopic composition from the terrestrial fractionation line is expressed by Δ¹⁷O, where Δ¹⁷O = δ¹⁷O – 0.52δ¹⁸O.

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We thank K. Shiraishi, H. Takeda, T. Hiroi, K. Keil, and T. Mikouchi for discussion, K. Hayano for technical assistance, and the Musée National D'Histoire Naturelle (Paris) for providing the sample of Juvinas. We acknowledge support by Grants-in-Aid for Scientific Research from the Ministry of Education, Science, Culture, Sports, Science and Technology, Japan, Nos. 12740300 (A.Y.), 1144017 (M.E.), and 11640487 (K.N.), and NSF grant EAR9815338 (R.N.C.).