Sources of Mg enrichments in Vent Fluids from the Kermadec Arc recorded by Li, B and Mg Isotopes

Magnesium concentrations in acid-sulfate fluids from Brothers and Macauley volcano, Kermadec arc, Western Pacific, exceed those of seawater and differ from previously known acid-sulfate fluids with seawater-like Mg concentrations. Earlier studies explained these Mg enrichments as due to phase separation, caminite (Mg1.25SO4(OH)(0.5) x 0.5H(2)O) dissolution, water-rock interaction and/or "mining" of deep seated magmatic brines. Since these different and partly contradictory explanations could neither be confirmed nor refuted, we investigated B, Li, and Mg isotope compositions in acid-sulfate and black smoker fluids from both Brothers and Macauley to understand the origin of the high Mg concentrations and investigate a possible connection with the alteration of basement rocks. In addition, we performed seawater heating experiments to investigate Mg isotope fractionation during caminite precipitation. Lithium and B isotope signatures in all studied acid-sulfate fluids indicate water-rock interaction. Magnesium isotopes in fluids from Macauley correspond to seawater, implying Mg enrichment through phase separation in the fluids. In contrast, Mg-enriched acid-sulfate fluids from Brothers volcano have higher delta Mg-26 values (up to -0.62 parts per thousand) than seawater. Considering that our seawater heating experiment shows a preferential incorporation of Mg-24 into caminite, caminite dissolution would lead to lower delta Mg-26 values and thus cannot solely account for the enriched Mg concentrations seen at Brothers. Additionally, we thus suppose leaching of Mg from the basement during the interaction of acidic fluids with a relatively unaltered basement.