TRACING THE FORMATION OF TALC RICH MÉLANGE ROCKS WITH MAGNESIUM ISOTOPES

Subduction zones host abundant seismic activity along the subduction interface, an area that undergoes metasomatic reactions due to interactions between rock and fluids. With a vast quantity of reactions occurring within the interface region of subduction zones it is unknown how these reactions or their products could influence deformation. Talc, in particular, is a weak mineral produced by metasomatism that can change the deformation behavior of the subduction interface. Therefore, understanding the drivers of talc formation could reveal why these regions of subduction zones are so seismically active. The reactions forming talc remain controversial, with its formation commonly attributed to both the addition of carbonate and silica to ultramafic rocks. The magnesium isotope system is sensitive to the presence of carbonate, so can be particularly useful in determining the reaction(s) forming talc. By applying Mg isotopes tracing to mélange rocks on Santa Catalina Island’s exhumed ancient subduction zone, we aim to investigate the reactions that form talc in hopes to understand where, when and how the necessary conditions for slow slip might occur in modern subduction zones. Samples obtained from Santa Catalina Island reached peak conditions of epidote-amphibolite facies, comparable to the P-T condition for episodic tremor and slow slip in modern subduction zones. Samples consist of subduction interface mélange matrix rich in talc, amphibole, and chlorite, and Mg isotopes were measured in whole rock powders by multi-collector ICPMS. Magnesium isotopes reveal the source of fluid-mobilized elements responsible for the formation of talc in mantle-wedge-derived ultramafic rocks and their relationship with adjacent carbon-bearing metasediments and infiltrating fluids from underlying metabasalts. This provides a clearer picture of the rock types, tectonic setting and depths of talc formation and its potential distribution in modern subduction zones. This is particularly relevant given recent work that identified evidence of episodic slow slip events hosted in talc-rich samples from this site.