Geochemical Modeling of Black Smoker Fluid Formation at the TAG Hydrothermal Vent Field

Abstract

Seafloor hydrothermal systems facilitate energy transport and chemical exchange between oceanic crust and seawater. The chemistry of hydrothermal fluids is attained through reactions with basaltic rock during fluid recharge, but the spatio‐temporal scales of fluid‐rock‐interaction at depth are difficult to constrain. We present a series of geochemical models of seawater‐basalt interaction within the recharge and upflow zones of a black smoker system, using data from the well‐studied TAG vent field for input compositions and model validation. Seawater‐basalt‐equilibrium is calculated using Gibbs Energy Minimization in a 16 element (Si, Ti, Al, Fe, Mg, Ca, Na, K, Cu, Zn, Pb, S, C, Cl, H, and O) system. Calculated fluid and rock chemistry reproduce observed hydrothermal alteration and element mobility. However, discrepancies with predicted stability of some minerals (amphibole, pumpellyite) need to be resolved in future thermodynamic database updates. A model describing the compositional evolution of seawater entering a hydrothermal system is developed, simulating sequential reactions with basalt at increasing temperature (25–430°C) and pressure (360–870 bar) over multiple cycles of seawater circulation. Models indicate that black‐smoker like fluids enriched in base metals, K, Ca, Si, and Fe and depleted of Mg are produced in hot (>400°C) and relatively shallow ( km bsf) recharge, with low integrated fluid‐rock mass ratios along the modeled path. Hydrothermal fluids generated from these models are used to simulate continuous reaction of basalt within the upflow zone of the hydrothermal system with passing hot fluids, successfully reproducing the mineralogical features of altered basalts beneath the TAG mound.