Hf-W mineral isochron for Ca,Al-rich inclusions: Age of the solar system and the timing of core formation in planetesimals

作者：Burkhardt Christoph^*; Kleine Thorsten; Bourdon Bernard; Palme Herbert; Zipfel Jutta; Friedrich Jon M; Ebel Denton S

来源：Geochimica et Cosmochimica Acta, 2008, 72(24): 6177-6197.

DOI：10.1016/j.gca.2008.10.023

Application of (182)Hf-(182)W chronometry to constrain the duration of early solar system processes requires the precise knowledge of the initial Hf and W isotope compositions of the solar system. To determine these values, we investigated the Hf-W isotopic systematics of bulk samples and mineral separates from several Ca,Al-rich inclusions (CAIs) from the CV3 chondrites Allende and NWA 2364. Most of the investigated CAIs have relative proportions of (183)W, (184)W, and (186)W that are indistinguishable From those of bulk chondrites and the terrestrial standard. In contrast, one of the investigated Allende CAIs has a lower (184)W/(183)W ratio, most likely reflecting an overabundance of r-process relative to r-process isotopes of W. All other bulk CAIs have similar (180)Hf/(184)W and (182)W/(184)W ratios that are elevated relative to average carbonaceous chondrites, probably reflecting Hf-W fractionation in the solar nebula within the first similar to 3 Myr. The limited spread in (180)Hf/(184)W ratios among the bulk CAIs precludes determination of a CAI whole-rock isochron but the fassaites have high (180)Hf/(184)W and radiogenic (182)W/(184)W ratios up to similar to 14 epsilon units higher than the bulk rock. This makes it possible to obtain precise internal Hf-W isochrons for CAIs. There is evidence of disturbed Hf-W systematics in one of the CAIs but all other investigated CAIs show no detectable effects of parent body processes Such as alteration and thermal metamorphism. Except for two fractions from one Allende CAI, all fractions from the investigated CAIs plot on a single well-defined isochron, which defines the initial epsilon (182)W = -3.28 +/- 0.12 and (182)Hf/(180)Hf = (9.72 +/- 0.44) X 10(-5) at the time of CAI formation. The initial (182)Hf/(180)Hf and (26)Al/(27)Al ratios of the angrites D'Orbigny and Sahara 99555 are consistent with the decay from initial abundances of (182)Hf and (26)Al as measured in CAIs, suggesting that these two nuclides were homogeneously distributed throughout the solar system. However, the uncertainties oil the initial (182)Hf/(180)Hf, and (26)Al/(27)Al ratios are too large to exclude that sonic (26)Al in CAIs was produced locally by particle irradiation close to an early active Sun. The initial (182)Hf/(180)Hf of CAIs corresponds to an absolute age of 4568.3 +/- 0.7 Ma, which may be defined as the age of the solar system. This age is 0.5-2 Myr older than the most precise (207)Pb-(206)Pb age of Efremovka CAI 60, which does not seem to date CAI formation. Tungsten model ages for magmatic iron meteorites, calculated relative to the newly and more precisely defined initial epsilon(182)W of CAIs, indicate that core formation in their parent bodies occurred in less than similar to 1 Myr after CAI formation. This confirms earlier Conclusions that the accretion of the parent bodies of magmatic iron meteorites predated chondrule formation and that their differentiation was triggered by heating from decay of abundant (26)Al. A more precise dating of core formation in iron meteorite parent bodies requires precise quantification of cosmic-ray effects on W isotopes but this has not been established yet.

• 出版日期2008-12-15