Lithium, boron and magnesium isotope compositions in high-temperature and acid-sulfate fluids from Brothers and Macauley volcano, Kermadec Arc, Western Pacific collected during SO-253 cruise. The acid-sulfate fluids exceed Mg concentrations of seawater and differ from previously known acid-sulfate fluids with seawater-like Mg concentrations. Based on our data, we explain the Mg enrichments with venting of a brine phase (Macauley volcano) and dissolution of caminite and Mg leaching from the ocean crust (Lower Cone, Brothers Volcano).