METAMORPHISM AND FLUID-MEDIATED MASS TRANSFER IN SUBDUCTION ZONES AS RECORDED BY SORENA SORENSEN’S COOL MINERALS

Subduction zones are sites of intense geological activity, including explosive volcanism, earthquakes, and complex mass transfer between the Earth’s crust and mantle. Much of this activity takes place at depths of tens to hundreds of kilometers in the Earth and cannot be observed directly. While we can directly observe some of the inputs to subduction zones and some of the outputs, our understanding of the “subduction factory” in between remains incomplete. Sorena Sorensen’s detailed mineralogical and geochemical studies of high-pressure, low-temperature subduction-zone metamorphic rocks, beginning with the Catalina Schist in southern California, document an amazing array of complex fluid processes including fluid infiltration, metasomatism, veining. Our understanding of subduction-zone magmatism is based on inverting geochemical data, particularly trace element data, derived from primitive arc lavas. Focusing on subduction inputs, Sorena’s work demonstrates the importance of understanding the mineralogical hosts of minor and trace elements, the stability of these minerals during subduction, and their interaction with fluids. For example, Sorena’s work has demonstrated that phengite (Si-rich white mica) contains essentially all of the large-ion lithophile elements in meta-mafic rocks and most of the LILEs in meta-sedimentary rocks. Experiments demonstrate that phengite is stable to depths of ~300 km, so how is Li and other elements hosted by phengite in the subducting slab transferred to arc magmas? Accessory minerals, like titanite, epidote, and lawsonite, contain most of a rock’s rare earth elements. The stability of these minerals, as well as their element partitioning between and solubility in fluids and melts, are critical questions raised by Sorena’s research that must be answered if we are to invert arc geochemical data correctly. Sorena might be best described as a metamorphic mineralogist, with a passion for cool minerals, minerals that may well be the key to our unlocking the mysteries of subduction zones.