Germanium in mid-ocean ridge flank hydrothermal fluids

We present concentrations of germanium and silicon in sediment pore waters, basaltic formation fluids, and bulk sediment from three ridge flank hydrothermal systems (RFHS). Basaltic formation fluids from warm (>30 degrees C) RFHS have much higher Ge concentrations and Ge: Si molar ratios than overlying sediment pore waters, requiring seawater-basalt reactions to dominate Ge concentrations in basaltic formation fluids. In contrast to warm RFHS, cool (similar to 20 degrees C) RFHS have similar Ge concentrations in basal sediment pore waters and underlying basaltic formation fluids, implying that there is little net exchange between these two fluid reservoirs. Despite this low net exchange, Ge: Si molar ratios in basaltic formation fluids are elevated compared to seawater and overlying sediment pore waters, implying that seawater-basalt reactions must influence Ge and Si cycling. Such seawater-basalt reactions are likely associated with secondary clay formation because increases in Ge concentration scale with Mg loss from basaltic formation fluids. Processes that control Ge cycling in cold (3-10 degrees C) RFHS are poorly constrained because our data are restricted to sediment pore waters that have been overprinted by diagenetic reactions and possibly sampling artifacts. Although net Ge fluxes from RFHS prevail over a wide temperature range, a refined estimate for the global RFHS Ge flux is currently not possible without data from cold RFHS springs or basaltic formation fluids because cold RFHS transport most of the convective heat and crustal fluid to the oceans.