Insights into the controls on acid sulfate fluid geochemistry at arc and back arc systems from Brothers volcano
Abstract
Diversity in the composition of hydrothermal fluids at arc volcanoes and back-arc spreading centers is driven by variation in the conditions of fluid-rock reaction in the seafloor. The two styles of venting that exist at these settings include 1) convecting seawater-derived fluids that circulate through the crust in a similar manner as at mid-ocean ridges, forming 'black smokers,' and 2) the direct injection of magmatic fluids into unmodified seawater, forming 'acid sulfate' fluids, the focus of this study [1]. The low pH of the latter results from the addition of magmatic HCl, HF, and SO2, which disproportionates to H2SO4. As the dominant anion, the Cl content of an acid sulfate fluid exerts a key control on the aqueous transport of cations. Previous studies have increased our knowledge regarding the effects of these volatiles on fluid chemistry [1, 2, 3, 4]. However, the links between magmatic HCl-rich fluids and venting acid sulfate fluids can be further elucidated across arc and back arc systems.
During IODP Expedition 376, we intercepted upwelling acid sulfate fluids during coring of the Brothers volcano Upper Cone. Three high temperature (140-247°C) fluid samples were collected from within the drill string at depths of 160, 279, and 313 mbsf. Dissolved inorganic and volatile chemistry identifies a dominantly magmatic origin of these low pH (down to 1.8), high ΣSO4 (up to 89 mM) fluids. Fluid Na (305-337 mmol/L), Cl (480-500 mmol/L), Mg (39.6-40.6 mmol/L), and Br (0.652-0.680 mmol/L) contents lie along one mixing line. The abundances of these species in other natural acid sulfate springs also form their own mixing lines with seawater [1, 2, 3, 4]. Intriguingly, acid sulfate fluids from multiple arc and back arc systems, including Brothers Upper Cone, extrapolate to similar inferred source Cl compositions. These results contrast with mid-ocean ridge systems, where vent fluid Cl content varies over several orders of magnitude. The Cl content of the magmatic source fluids that feed seafloor acid sulfate springs may be set by a mechanism that is independent of different pressures in multiple magmatic systems. [1] de Ronde et al. (2011) Miner. Deposita, 541-584. [2] Gamo et al. (1997) Geology, pp. 139-142. [3] Butterfield et al. (2011) Geology, pp. 803-806. [4] Seewald et al. (2015) Geochim. Cosmochim. Acta, pp 179-199.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V32B..07M
- Keywords:
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- 0450 Hydrothermal systems;
- BIOGEOSCIENCES;
- 3616 Hydrothermal systems;
- MINERALOGY AND PETROLOGY;
- 8135 Hydrothermal systems;
- TECTONOPHYSICS;
- 8424 Hydrothermal systems;
- VOLCANOLOGY