Silicon isotope compositions of metaperidotites from the Franciscan Complex of California- implications for Si isotope fractionation during subduction dehydration

Abstract

Slab-derived fluids have a great significance in material transfer in subduction zones and exert a great influence on element and isotope characteristics of the overlying mantle wedge. To investigate Si isotope fractionations during fluid-rock reactions in subduction zones, and to constrain Si isotope compositions of the residual slab after metamorphic dehydration, we studied a set of metaperidotite samples (four groups) with good characteristics from the Franciscan Complex of California, USA. Due to multistage fluid metasomatism, a series of fluid-mineral reactions including serpentinization, talcification, and tremolitization, occurred in the subduction channel. Mantle wedge peridotites successively reacted with slab-derived fluids to produce partially and completely serpentinized peridotites (Group 1 & 2), talc-dominated metaperidotites (Group 3) and tremolite-dominated ones (Group 4). All of these four groups of samples exhibit relatively homogeneous Si isotope compositions, and δ30Si values vary from −0.39‰ to −0.26‰, with an average value of −0.32 ± 0.08‰ (2SD, n = 15). This observation demonstrates that the Si isotope compositions of mantle peridotites may be not modified significantly during metasomatism by slab-derived fluids. Mass balance calculations indicate that the slab-derived fluids in the Franciscan Complex have Si isotope compositions close to those of mantle peridotites. The Si isotope compositions of the residual subducting slab may also not be changed significantly during metamorphic dehydration processes.