Chromium isotope signature during continental crust subduction recorded in metamorphic rocks

摘要

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 delta Cr-53(NIST979) of greenschists, amphibolites, and eclogites ranging from -0.06 parts per thousand to -0.17 parts per thousand, -0.05 parts per thousand to -0.27 parts per thousand, and -0.01 parts per thousand to -0.24 parts per thousand, 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 delta C-53(Delta Cr-53(Cpx-Gt)) between coexisting clinopyroxene (Cpx) and garnet (Gt) from two garnet pyroxenites were -0.06 parts per thousand and -0.34 parts per thousand, 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 parts per thousand to -0.11 parts per thousand) < chromite-bearing peridotite (-0.07 parts per thousand) < chromite (-0.06 parts per thousand). These findings imply potential mantle heterogeneity as a result of partial melting or fractional crystallization associated with chromite.