Silicon isotopic compositions of altered oceanic crust: Implications for Si isotope heterogeneity in the mantle

摘要

This study reports Si isotope compositions of the altered oceanic crust recovered from IODP Site 1256, East Pacific Rise to investigate the behavior of Si isotopes during low-temperature seawater and/or high-temperature hydrothermal alteration processes in the oceanic crust. These samples, including basalts and gabbros, were altered by seawater and hydrothermal fluids at various temperatures and water/rock ratios. The delta Si-30 of these samples range from -0.38% to -0.27%, with an average of delta Si-30 = -0.32 +/- 0.06% (2SD, N = 50), regardless of the depth and extent of alteration. The delta Si-30 values of the altered oceanic crust are in general agreement with the previous estimate of the global fresh MORBs (-0.27 +/- 0.06%, 2SD; Savage et al., 2014), suggesting that either low-temperature seawater alteration or high-temperature hydrothermal alteration does not cause resolvable Si isotopic fractionation at the bulk-rock scale in the altered oceanic crust.