Magnesium isotopic composition of submarine vent fluids from arc and back-arc hydrothermal systems in the western Pacific

Abstract

Seafloor hydrothermal systems are important sinks for Mg as their concentration in high-temperature hydrothermal vent fluids decreases to near zero through the formation of secondary minerals. However, the problem of mass balance still persists regarding the global sink for Mg via sub-seafloor high-temperature hydrothermal reactions. In this study, we measured isotopic compositions of Mg in high-temperature hydrothermal vent fluids from 5 arc and back-arc systems in the western Pacific. This was done to better understand Mg behavior during hydrothermal circulation and to quantify the oceanic Mg cycle. The samples with significantly low Mg concentrations showed 26Mg depletion due to Mg fractionation and Mg fixation during hydrothermal circulation. Addition of Mg to the permeable sub-seafloor during venting is the probable reason for the isotopic variation of Mg. The vent fluids (return flux) with lighter Mg isotopic compositions contain much smaller amounts of Mg than seawater. This confirms that the high-temperature hydrothermal sinks have a less significant effect on the oceanic Mg isotopic composition. Based on the simple steady state equations and low-temperature hydrothermal sinks of 26Mg (−0.25‰ to 0.00‰ of δ26Mg), 7–26% of the riverine Mg input is presumed to be removed through the high-temperature sink. This estimate is lower than that proposed by Mottl and Wheat (1994), which was 10–40%. Thus, other Mg sinks such as the low-temperature hydrothermal sink and/or the dolomite sink need to be considered.