Evolution in Fluid Composition During Subduction

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Abstract Fluid‐rock reactions play important roles in seismicity, arc‐magma chemistries, and volatile cycling at convergent plate boundaries. In subduction environments, metamorphic reactions involving hydrous fluids facilitate metasomatic alteration and mass transport. Fingerprinting the source of fluids that move across subducting lithologies is, however, challenging. We predict evolving mineralogy and fluid compositions during subduction along representative PT ‐paths, assessing closed and open system assumptions using phase assemblage modeling. Our calculations consider simplified hydrated harzburgite, hydrated mid‐ocean ridge basalt, average subducted sediments, and subducted carbonates. Predict equilibrium mineralogy, fluid composition, and this allows us to identify fluid solute ratios that might be unique to a given source lithology or mineral breakdown reaction. In general, fluids derived from average subducting sediments are predicted to have a higher proportion of Na to Mg and Si than those derived from basalts at the same subduction PT condition. Fluids produced during deserpentinization of hydrated harzburgite contain a higher proportion of Mg and Ca. Together, these results help to constrain the potential sources of fluids that metasomatize subducting rocks and the overlying mantle wedge. Comparing results to examples of metasomatism and fluid inclusions from the rock record indicates that although imperfect, modeled major element fluid signatures can be an effective tool to assess the source lithology of altering fluids.

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