This presentation will explore the use of microscale oxygen isotope analysis using SHRIMP to investigate seafloor hydration and subduction dehydration of the altered oceanic crust. Hydration reactions in the oceanic crust incorporate large volumes of bound water. The most significant of these is the serpentinization reaction, forming serpentine minerals through the hydration of olivine and orthopyroxene. Mineral-bound water within the altered oceanic crust is subsequently released through metamorphic dehydration within subduction zones, carrying with it a suite of major, minor and trace elements. I will illustrate the utility of in-situ oxygen isotope analyses to track fluid-rock interaction in two studies of natural sample suites: investigating seafloor serpentinization conditions, and tracking subduction fluid transfer within Jurassic oceanic crust within in the NW Italian Alps.
The temperature dependence of oxygen isotope fractionation between serpentine and water can be utilized to provide constraints on serpentinization environment. Targeted in-situ oxygen isotope analysis of seafloor serpentinites from the Atlantis Massif (30°N, Mid-Atlantic Ridge) has revealed significant oxygen isotope heterogeneity between individual serpentinization stages (e.g. serpentine mesh and veins). Using existing fractionation models, compositions of successive generations of serpentine are consistent with serpentinization over a temperature range from 150 to >280°C, with increasing water/rock ratios.
Metamorphic dehydration fluids have geochemical signatures derived from their source lithologies, allowing fluid sources to be discriminated within heterogeneous reservoirs such as the subducted altered oceanic crust. An investigation of the metasedimentary cover within the subducted Jurassic upper oceanic crust uncovers previously undocumented metasomatism at peak metamorphic conditions. Strongly zoned metasedimentary garnets exhibit dramatic step-wise shifts in oxygen isotope composition. Garnets record at least two stages of external fluid infiltration at high pressure, likely sourced from nearby i) mafic and ii) ultramafic lithologies.