Devolatilization of metabasic rocks during greenschist–amphibolite facies metamorphism

The production of large volumes of fluid from metabasic rocks, particularly in greenstone terranes heated across the greenschist–amphibolite facies transition, is widely accepted yet poorly characterized. The presence of carbonate minerals in such rocks, commonly as a consequence of sea‐floor alteration, has a strong influence, via fluid‐rock buffering, on the mineral equilibria evolution and fluid composition. Mineral equilibria modelling of metabasic rocks in the system Na₂O‐CaO‐FeO‐MgO‐Al₂O₃‐SiO₂‐CO₂‐H₂O (NCaFMASCH) is used to constrain the stability of common metabasic assemblages. Calculated buffering paths on T–X_CO2 pseudosections, illustrate the evolution of greenstone terranes during heating across the greenschist‐amphibolite transition. The calculated paths constrain the volume and the composition of fluid produced by devolatilization and buffering. The calculated amount and composition of fluid produced are shown to vary depending on P–T conditions, the proportion of carbonate minerals and the X_CO2 of the rocks prior to prograde metamorphism. In rocks with an initially low proportion of carbonate minerals, the greenschist to amphibolite facies transition is the primary period of fluid production, producing fluid with a low X_CO2. Rocks with greater initial proportions of carbonate minerals experience a second fluid production event at temperatures above the greenschist to amphibolite facies transition, producing a more CO₂‐rich fluid (X_CO2 = 0.2–0.3). Rocks may achieve these higher proportions of carbonate minerals either via more extensive seafloor alteration or via infiltration of fluids. Fluid produced via devolatilization of rocks at deeper crustal levels may infiltrate and react with overlying lower temperature rocks, resulting in external buffering of those rocks to higher X_CO2 and proportions of carbonate minerals. Subsequent heating and devolatilization of these overlying rocks results in buffering paths that produce large proportions of fluid at X_CO2 = 0.2–0.3. The production of fluid of this composition is of importance to models of gold transport in Archean greenstone gold deposits occurring within extensive fluid alteration haloes, as these haloes represent the influx of fluid of X_CO2 = 0.2–0.3 into the upper crust.