Geochemical insights into conditions of vent fluid origin and water-rock interaction over two eruptive cycles at 9° 50´N East Pacific Rise

Abstract

Multidisciplinary studies at the 9°50’N East Pacific Rise (EPR) hydrothermal field span three decades and encompass two periods of volcanic activity in 1991-1992 and 2005-2006. Shifts in the pressure and temperature of hydrothermal circulation induced by the magmatic cycle drive changes in the composition of venting fluids. Previous geothermobarometric model approaches used quartz solubility and fluid Cl concentrations to estimate pressure and temperature conditions in the zone where hydrothermal fluids originate [1, 2]. A geothermometer based on dissolved Fe/Mn [3] now provides additional insight on fluid origin temperatures. Consequently, application of an updated geothermobarometric model is possible. We estimate the pressure and temperature conditions of fluid formation at historic high temperature vents in the 9°50’N EPR area between 2018 and 2023 and compare them with time series data since 1991. These calculations focus on six vents that span 7 km north to south along the axial summit trough, including M, Bio9, P, V, L, and L-Hot8 vents. Immediately following eruptions, fluid origin pressures are considerably shallower at Bio9, M, and P vents (25-27 MPa) than during periods of lower magmatic activity (30-35 MPa). Additionally, fluid origin temperatures at the same vents rise from 390-400 °C for 1-2 years after eruptions to 410-430 °C during the periods between eruptions. Despite repeated observation of a continuous warming trend in vent fluid exit temperatures in the years preceding eruptions, fluid origin temperatures are relatively stable at a given vent location over the same time periods.  These results support previous assertions that the upflow zone may experience enhanced permeability immediately following an eruption, leading to seawater entrainment, and cooling prior to venting. These inferences are also supported by a significantly greater proportion of radiogenic, seawater-derived Sr isotope input to circulating fluids in the 1-2 years after eruptive events, followed by a shift toward less radiogenic, more basalt-derived Sr isotope signatures.  The vent fluids are presently circulating into the sheeted dikes and attaining maximum depths and temperatures similar to those previously observed leading up to the 2005/2006 eruption. The chemical behavior and formation conditions of these hydrothermal fluids will be tracked through 2025, with the goal to understand the hydrothermal response preceding the next magmatic event.  [1] Von Damm (2004) AGU Mono; [2] Fornari et al. (2012) Oceanography; [3] Pester et al. (2011) Geochim. Cosmochim. Acta.