CHANGES IN HYDROTHERMAL FLUID ORIGIN CONDITIONS LEADING UP TO THE NEXT ERUPTION AT 9 °50´N EAST PACIFIC RISE (Invited Presentation)

The 9°50’N East Pacific Rise (EPR) hydrothermal field has been the focus of multidisciplinary studies during the past 30 years that span two periods of volcanic activity in 1991-1992 and 2005-2006. Shifts in the pressure and temperature of hydrothermal circulation induced by magmatic activity drive changes in the composition of venting fluid over the course of an eruptive cycle. Previous geothermobarometric model approaches used quartz solubility and vapor-phase Cl concentrations to estimate pressure and temperature conditions in the reaction zone where hydrothermal fluids originate, predict an imminent eruption, and investigate geochemical changes during post-eruption periods [1, 2]. A geothermometer based on dissolved Fe/Mn ratios [3] now provides additional insight on fluid temperature variations during non-eruptive years. Consequently, application of an updated geothermobarometric model is warranted.

This ongoing study focuses on the idea that a new magmatic event involving an eruption or shallow dike emplacement is likely to happen at the 9°50’N EPR axis in the next few years (~2021–2023), based on observed increases and variations in vent exit-fluid temperature captured by self-recording loggers installed in vent orifices. Our approach estimates the pressure and temperature conditions of fluid formation at historic high temperature vents in the 9°50’N EPR area between 2018 and 2021 in comparison with time series data since 1991. 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 quiet periods. As a result, phase separation, which is responsible for the production of vapor phase fluids, shifts from subcritical to supercritical conditions, with consequences for dissolved and volatile species partitioning. The chemical behavior and formation conditions of these hydrothermal fluids will be tracked for the next two years, with the intent 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.