Computer Science
Scientific paper
Sep 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007gecoa..71.4609h&link_type=abstract
Geochimica et Cosmochimica Acta, Volume 71, Issue 18, p. 4609-4627.
Computer Science
12
Scientific paper
Recent 182Hf 182W age determinations on Allende Ca-, Al-rich refractory inclusions (CAIs) and on iron meteorites indicate that CAIs have initial ɛ182W (-3.47 ± 0.20, 2σ) identical to that of magmatic iron meteorites after correction of cosmogenic 182W burn-out (-3.47 ± 0.35, 2σ). Either the Allende CAIs were isotopically disturbed or the differentiation of magmatic irons (groups IIAB, IID, IIIAB, and IVB) all occurred <1 m.y. after CAI formation. To assess the extent of isotopic disturbance, we have analyzed the elemental distribution of Hf and W in two CAIs, Ef2 from Efremovka (CV3 reduced), and Golfball from Allende (CV3 oxidized). Fassaite is the sole host of Hf (10 25 ppm) and, therefore, of radiogenic W in CAIs, with 180Hf/184W > 103, which is lowered by the ubiquitous presence of metal inclusions to 180Hf/184W > 10 in bulk fassaite. Metal alloy (Ni ˜ 50%) is the sole host of W (˜500 ppm) in Ef2, while opaque assemblages (OAs) and secondary veins are the hosts of W in Golfball. A large metal alloy grain from Ef2, EM2, has 180Hf/184W < 0.006. Melilite has both Hf and W below detection limits (<0.01 ppm), but the presence of numerous metallic inclusions or OAs makes melilite a carrier for W, with 180Hf/184W < 1 in bulk melilite. Secondary processes had little impact on the 182Hf 182W systematics of Ef2, but a vein cross-cutting fassaite in Golfball has >100 ppm W with no detectable Pt or S. This vein provides evidence for transport of oxidized W in the CAI. Because of the ubiquitous distribution of OAs, interpretations of the 182Hf 182W isochron reported for Allende CAIs include: (i) all W in the OAs was derived by alteration of CAI metal, or (ii) at least some of the W in OAs may have been equilibrated with radiogenic W during metamorphism of Allende. Since (ii) cannot be ruled out, new 182Hf 182W determinations on CAIs from reduced CV3 chondrites are needed to firmly establish the initial W isotopic composition of the solar system.
Grossman Lawrence
Humayun Munir
Simon Steven B.
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