305-5 Strontium Isotope Ratios as a Proxy for Fluid-Rock Interactions in the Protolith of the Catalina Schist Metabasalts

Session: Subduction Zone Processes: Insights from Geology, Geochemistry, and Petrochronology (Posters)

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Chemical compositions of subducted rocks influence how they will deform at depth, making it crucial to understand the degree to which fluid-rock interactions alter basalts in the oceanic crust prior to subduction. We investigate the degree of seafloor alteration of metabasalts in the Catalina Schist (Pimu’nga/Santa Catalina Island, California), which represent an ancient exposure of underplated oceanic crust resulting from the subduction of the Farallon plate under the North American plate during the Cretaceous. We analyzed whole-rock ⁸⁷Sr/⁸⁶Sr ratios of samples from the Epidote Blueschist unit of the Catalina Schist, characterized by epidote porphyroblasts within a deformed fabric containing glaucophane and chlorite, minerals that are indicative of fluid flow and hydration. Initial Sr isotope ratios (calculated for 105-195 Ma) range from 0.702 to 0.705, with some samples shifted from a value indicative of unaltered mid-ocean ridge basalt (MORB; ⁸⁷Sr/⁸⁶Sr ~ 0.702) towards a value indicative of Jurassic seawater (⁸⁷Sr/⁸⁶Sr ~ 0.708). 

Using mass balance calculations for a single-pass, open-system equilibrium reaction, we estimate that minimum water/rock (W/R) ratios for the metabasaltic protoliths prior to subduction range from 0.4 to 8, comparable to those of basalts in modern hydrothermal systems. However, major-element compositions of our samples suggest little modification away from an average MORB composition towards typical MORB seafloor alteration products. Thin-section analysis reveals no significant variation in mineral assemblage or fabric development across samples with different estimated W/R ratios. 

The ⁸⁷Sr/⁸⁶Sr ratios and modelled W/R ratios indicate pre-subduction hydration of the basalts, likely serving as an important source water for later metamorphic dehydration reactions during subduction. The decoupling between Sr-isotopes, major-element compositions, and mineralogy suggests that either (1) seawater interactions were insufficient to alter major-element chemistry, or (2) these basalts started with similar Sr isotope ratios prior to subduction, and isotopic differences were introduced by variable amounts of sedimentary-derived fluid input during subduction and/or retrograde metamorphism. Our results suggest that the magnitude of W/R alteration of the epidote blueschist protoliths prior to subduction was not large enough to result in significant differences in deformation style. By connecting slab hydration with deformation textures, we provide insight into subduction zone processes that can be applied to similar rocks worldwide.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9460

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