Rare Time Series of Hydrothermal Fluids for a Submarine Volcano: 14 Years of Vent Fluid Compositions for Brothers Volcano, Kermadec Arc, New Zealand

Abstract

AbstractHydrothermal vent fluids from Brothers submarine arc volcano were previously collected in 2004, 2005, and 2017. We present new data from 2018 along with a unique time series of a submarine volcano hosting two distinct types of hydrothermal venting to better understand subsurface processes and how they evolve over time. Samples were collected from known venting locations along the NW Caldera, the Upper and Lower Cone sites, and the newly sampled Upper Caldera site. The NW Caldera wall and Upper Caldera vent fluid compositions are controlled by high-temperature water-rock interactions and phase separation between 2004 and 2018. Fe/Mn molar values suggest that the magmatic impact on all fields has been increasing over time. The Upper Cone has varying influences, including short-term pulses of magmatic degassing, as revealed in K-Mg-SO4 ternary diagrams, Fe/Mn values, and δD and δ18O data. The Lower Cone is dominated by low temperatures and CO2 degassing and shows a pulse in magmatic influence in 2017, followed by a decrease toward earlier conditions in 2018. The 2017 pulse was accompanied by the increase of an enigmatic, soluble MgSO4 phase, potentially due to the “mining” of a magmatic brine sequestered inside the cone. Stable isotope data across the sites indicate pulses of magmatic waters in 2004 and 2017, immediately followed by episodes of more seawater-dominated fluids in 2005 and 2018. Magmatic degassing, phase separation, permeability, and mineral solubility all strongly influence changes in vent fluid composition. This study highlights the dynamic nature of the Brothers volcano hydrothermal system and the changes in fluid chemistry that may affect resultant mineralization.