Magnesium isotopes of chlorite-rich hydrothermal sediments in the Okinawa Trough and indications for Mg cycle

Seafloor hydrothermal systems play a significant role in the oceanic Mg cycle due to ubiquitous deposits of secondary Mg-rich clays during the strong fluid-rock reactions. However, the magnitude of net Mg enrichment and Mg isotopic fractionation, particularly within the medium-high temperature hydrothermal systems in felsic-hosted settings, are not well studied yet. Here we report elemental and isotopic compositions of Mg in hydrothermal chlorite-rich sediments, volcanic materials, and terrigenous sediments collected during the IODP Expedition 331 drilled to the thick sediment-covered and felsic-hosted middle Okinawa Trough (Iheya North Knoll) in the West Pacific. We investigate the sources of Mg in chlorite and Mg isotopic behavior at medium-high temperature hydrothermal alteration. After 1 mol/L HCl leaching, Mg isotopic compositions of chlorite-rich sediments present overall similar values in the residual fractions and bulk samples albeit with slightly higher values in the leachates. Mineralogical differentiation primarily determines the Mg isotopic compositions, showing that siliciclastic residues have slightly higher delta Mg-26 values than the leachates dominated by carbonates and oxides/hydroxides. Significant Mg isotopic fractionation happened in the medium-high temperature (similar to 150 degrees C to 260 degrees C) felsic-hosted hydrothermal system, with Delta Mg-26(Chl-SW) ranging from 0.15 parts per thousand to 0.71 parts per thousand and yielding a negative correlation with temperature. This observation suggests the preferential incorporation of heavy Mg isotopes by the secondary chlorite precipitation. We infer that the medium-high temperature hydrothermal systems can take up about 8-14% of riverine input of Mg in the arc and back-arc regions. Incomplete removal of aqueous Mg in porewater and vent fluids by the medium-high temperature hydrothermal alterations in the arc and back-arc basins provides constraints on the Mg budget and isotopic composition of seawater.