Subduction zones are regions where two tectonic plates converge, and one is forced underneath the other. They are the primary driver of plate tectonics, and a source of major earthquakes. These earthquakes occur at shallow depths where plates slide past one another quickly, but at greater depths, with higher pressure and temperature (PT) conditions, rocks behave more ductilely. The transition zone between brittle and ductile regions hosts slow slip events (SSEs), which accommodate motion between the plates during events that last months to years, as opposed to the seconds over which earthquakes occur. SSEs are an important mechanism for accommodating plate motion at depth, thereby affecting the occurrence of larger devastating earthquakes, but currently the processes which facilitate SSEs and the rocks that host SSEs are not well understood. By studying rocks formed in and ancient subduction zone from Santa Catalina Island in California, we can learn more about our own modern counterparts, as the rocks record the conditions where slow slip may have occurred. Using optical petrography and data from x-ray spectroscopy, I examine thin sections of epidote-rich blueschist from SSE PT conditions. Optical petrography allows me to characterize the mineralogy of this rock, and the x-ray data provide the chemical compositions of individual minerals. Using image-analysis software, I will pair these two datasets to estimate the bulk-rock chemical composition of my sample. These data will allow me to constrain the starting material (protolith) of this rock before it was metamorphosed in order to determine if it was originally a sedimentary or basaltic component of the subducting oceanic plate. Doing so will improve our understanding of the way in which rocks at those pressure-temperature conditions deform and chemically change to create the context in which modern SSEs occur.