FLUID PULSES DURING SUBDUCTION AND THEIR IMPACTS ON FLUID COMPOSITION, ROCK VOLUME, AND METASOMATISM

Fluid-rock reactions impact seismicity, arc-magma chemistries, and volatile cycling at convergent plate boundaries. In subduction environments, metamorphic reactions involving hydrous fluids lead to volume changes and drive metasomatic alteration. We use phase assemblage modeling to predict evolving mineralogy and fluid compositions during subduction along representative PT-paths, assessing both closed-system and fluid fractionating assumptions. Our calculations consider simplified hydrated mid-ocean ridge basalt, hydrated harzburgite, and average subduction sediments. Thermodynamic predictions identify specific phase changesand allow the calculation of their impacts on the volume and composition of fluids along different subduction geotherms. For example, in a hydrated harzburgite composition, three fluid pulses are predicted to occur during subduction, each with a subtly different composition and resulting in different ∆V. The first reflects brucite and antigorite break down to olivine, the second is antigorite breakdown to olivine, chlorite, and orthopyroxene, and the third involves chlorite and orthopyroxene forming olivine plus garnet. These reactions are expected to occur in hot or cold subduction geotherms, but at temperatures 40° C warmer and significantly higher pressures along a colder geotherm such as Honshu. The breakdown of brucite during the first pulse of fluid corresponds with a spike in the molality of Mg and Ca in the hydrous fluids. Mg and Ca content of the fluid then declines as antigorite breaks down, until antigorite out. Additionally, the volume of the rock changes after each reaction. The rock volume without fluid is predicted to decrease by 8% after the breakdown of brucite along a Honshu geotherm. Along the same geotherm for a MORB composition, the fluid pulse released at the start of lawsonite breakdown is associated with a 0.5% ∆V. Each lithology is associated with a range of fluid abundance and composition dependent on subduction path.