Three-component fluid mixing: Evidence from trace element and isotope systematics in vent fluids and sulphides from Maka volcano, North Eastern Lau Spreading Centre

The axial volcanic edifice of Maka at the North Eastern Lau Spreading Centre shows intense hydrothermal activity at two vent sites (Maka HF and Maka South) emitting fluids of distinct composition. We present trace element and isotope data for hydrothermal fluids and related sulphide precipitates that actively form on the seafloor at 1525 to 1543 m water depth. Hydrothermal activity at Maka HF is present as vigorously venting black smoker-type fluids reaching temperatures of ~330°C. High metal (e.g. Fe, Mn, Li) and REE contents in the vent fluids, are indicative for a rock-buffered hydrothermal system at low water/rock ratios. At Maka South venting of white smoke at up to 300°C occurs at several sites. Measured fluid pH (4.53-5.42) and Mg, SO4 and Cl concentrations are depleted compared to seawater, whereas Li, Mn and H2S are enriched, indicating a three-component mixing model between seawater, a boiling-induced low Cl vapor and a black smoker-type fluid at Maka South. Trace element systematics in hydrothermal pyrite also report on the contribution of these different fluid-types. Pyrite that precipitates from low Cl vapor-rich fluids at Maka South is characterized by high As/Co (>10) and Sb/Pb (>0.1) values that we relate to a boiling-induced element fractionation between the vapor (As, Sb) and liquid phase (Co, Pb). The Se/Ge ratio in pyrite may be used as a new tracer for fluid-seawater mixing. Sulfur and Pb isotopes in hydrothermal sulphides indicate a common metal(loid) source at the two vent sites by host rock leaching in the reaction zone.