Abstract

A comprehensive survey of hydrothermal plumes and their geochemistry revealed 16 hydrothermally active volcanoes along 1,200 km of the Mariana arc from 13.5° N to 22.5° N. Of these 16, one volcano, NW Rota-1, is discharging sulfuric acid-rich fluids producing some of the largest chemical anomalies ever observed in non-buoyant hydrothermal plumes. Two types of venting were observed, one of focused flow rich in Al, S, Si, CO2, Fe, Mn, and 3He, and a second of diffuse flow rich in Fe, Mn, CO2, and 3He but without Al, S, and Si. The plume waters from the acid-rich focused flow showed decreases in pH up to 0.73 pH units compared to ambient seawater and an estimated 80 percent of this change was brought about by volcanic and/or hydrothermal SO2 (and possibly HCl) with the remaining 20 percent by CO2. Conservative estimates suggest that the pH of the venting fluids was <1.0. The chemistry of this plume was also greatly different from that observed in any other hydrothermal setting. An acid-sulfate or high-sulfidation fluid source was indicated from plume water samples rich in sulfuric acid, native S, Al-sulfate-phosphate minerals, and weathered silicate and amorphous silica phases. The alunite group minerals and acidic fluids indicate that the most important hydrothermal reactions taking place within the hydrothermal system were resulting in advanced argillic alteration of the host volcanic rocks. Gas chemistry of the fluids revealed an He isotope signature with R/Ra = 8.35 and 3He/CO2 = 3.25 × 109. These values are similar to those found along the global midocean ridge (MOR), suggesting that the source of the 3He and CO2 found here is predominantly from the upper mantle, consistent with the position of NW Rota-1 closest to the back arc in a chain of cross-arc volcanoes. The chemical signatures of the plumes arising from the diffuse flow remain greatly enriched in Fe, Mn, 3He, and CO2, but compared to the focused flow are low in mineral acidity (H+), Al, and S. The source of this diffuse flow is likely a fluid similar to that producing the acid-rich plume but with a lower water-to-rock ratio and a longer reaction time, resulting in the consumption of the mineral acidity. This results in an increase in pH and the deposition of the alunite-phosphate-sulfate and other minerals close to the seafloor. By comparison, it is likely that the low-pH focused flow ejects the vast majority of its mineral load into the water column which is not conducive to the formation of a mineral deposit at the sea floor, and potentially would produce a “barren prospect.”

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