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

Abstract

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 δ30Si of these samples range from −0.38‰ to −0.27‰, with an average of δ30Si=−0.32±0.06‰ (2SD, N=50), regardless of the depth and extent of alteration. The δ30Si 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.Because the altered oceanic crust could be partially melted during subduction or in the convective mantle, we estimated the Si isotope fractionation between the melt and residue minerals of the altered oceanic crust. Our calculations show that Si isotopes can be fractionated between the melt and residues enriched in garnet and clinopyroxene. Although the bulk altered oceanic crust has a mantle-like Si isotopic signature, the residual solids of the recycled altered oceanic crust could be enriched in light Si isotopes and the melt could be enriched in heavy Si isotopes. This indicates that melting and recycling of the subducted oceanic crust can produce heterogeneous Si isotope compositions in the convective mantle, which could contribute to the source of ocean island basalts with low δ30Si.