Metasomatism and Fluid Infiltration of Actinolitite, Talc-, and Chlorite-Actinolite Schist during Subduction on Santa Catalina Island, California: An Isocon Analysis

Episodic tremor and slip(ETS) remains poorly understood, particularly the potential role that processes such as metamorphism, metasomatism (chemical alteration of a rock caused by hydrothermal fluids), and mechanical mixing play in producing them. The Epidote-Amphibolite(EA) unit on Santa Catalina Island, CA, is a paleo subduction terrane containing exhumed hybrid rocks from 450-580℃ and 7-12 kbar, representing typical P-T conditions for ETS. Previous studies have examined areas of the Catalina Schist of lower- and higher temperatures, but little comparable work has been done to understand the geochemical evolution of rocks from the intermediate P-T (450-580℃ and 7-12 kbar) conditions. Here, we fill this gap by determining major and trace element changes to improve our understanding of the geochemical evolution of subduction zone interfaces and how this produces weak minerals that may host ETS.

We measured whole-rock major and trace element geochemistry from eight samples from three different lithologies: actinolitite, talc-actinolite schist, and chlorite-actinolite schist grouped by metasedimentary or meta-ultramafic protoliths. We used isocon analysis to solve Gresens' equation for change in mass and concentration in metasomatic alteration. Results indicated a total mass increase of all samples ranging from ~47% to 480% except a chlorite-actinolite schist sample, which had a ~25% mass decrease. Sr was gained by the meta-ultramafic samples and lost by the metasedimentary, indicating some local exchange. However, losses and gains of the other major and trace elements did not balance, ruling out metasomatism by solely mechanical mixing or local exchange by diffusion. The metasedimentary samples all had significant increases in CaO, MgO, and MnO, and the meta-ultramafic samples had an increase in SiO2, CaO, Na2O, MgO, and TiO2. These increases can be explained by interaction with fluids sourced from subducted carbonate-rich sediments and/or metabasalts. Decreases in large-ion lithophile elements and rare earth elements indicate there was leaching by fluids in the reaction zone. These results suggest that both local exchange and external fluid infiltration are required to create these chemical changes and produce weak rocks that may have hosted slow slip events in the EA unit on Santa Catalina Island.