Liping Qin

Tungsten Nuclear Anomalies in Planetesimal Cores

Use of the extinct 182 Hf- 182 W chronometer to constrain the timing of planetary accretion and differentiation rests ontheassumptionthatthesolarnebulahadhomogeneoustungstenisotopiccomposition.Here,wereportdeficiencies of � 0.1 part in 10,000 in the abundance of 184 Win group IVB iron meteorites relative to the silicate Earth. These are mostlikelyduetoincompletemixingattheplanetesimalscale(2Y4kmradiusbodies)oftheproductsof slow(s-)and rapid (r-) neutron-capture nucleosynthesis in the solar nebula. The correction that must be applied to the 182 Hf- 182 W model age of core formation in IVB irons due to the presence of these nuclear anomalies is � 0.5 Myr. Subject headingg minor planets, asteroids — nuclear reactions, nucleosynthesis, abundances — solar system: formation — stars: abundances

Chromium isotope signature during continental crust subduction recorded in metamorphic rocks

The chromium isotope compositions of 27 metamorphic mafic rocks with varying metamorphic degrees from eastern China were systematically measured to investigate the Cr isotope behavior during continental crust subduction. The Cr isotope compositions of all samples studied were Bulk Silicate Earth (BSE) like, with δ53CrNIST979 of greenschists, amphibolites, and eclogites ranging from −0.06‰ to −0.17‰, −0.05‰ to −0.27‰, and −0.01‰ to −0.24‰, respectively. The lack of resolvable isotopic variability among the metamorphic rocks from different metamorphic zones indicated that no systematic Cr isotope fractionation was associated with the degree of metamorphism. However, the Cr isotopic variability among homologous samples may have reflected effects induced by metamorphic dehydration with a change of redox state, rather than protolith heterogeneity (i.e., magma differentiation). In addition, the differences in δ53Cr (Δ53CrCpx-Gt) between coexisting clinopyroxene (Cpx) and garnet (Gt) from two garnet pyroxenites were 0.06‰ and 0.34‰, respectively, indicating that significant inter-mineral Cr isotope disequilibria could occur during metamorphism. To provide a basis for comparison with metamorphic rocks and to provide further constraints on the potential Cr isotope heterogeneity in the mantle and in the protolith of some metamorphic rocks, we analyzed mantle-derived chromites and the associated peridotites from Luobusa, and we obtained the following general order: chromite-free peridotites (−0.21‰ to −0.11‰) < chromite-bearing peridotite (−0.07‰) < chromite (−0.06‰). These findings imply potential mantle heterogeneity as a result of partial melting or fractional crystallization associated with chromite.