The question of geochemical flux in the mantle wedge during subduction is critical to our understanding of arc volcanism, and forms an important aspect of the global geochemical flux. This is one of the first order problems identified by the "Geochemical Earth Reference Model" (GERM) initiative, and by the "subduction factory" focus of the NSF "MARGINS" initiative. One way to study this problem is to examine outcrops of lithospheric mantle that underlie crust known to have formed by magmatism above a subduction zone. This mantle represents, in part, the source from which the overlying crust was extracted, and its mineralogy and composition reflect the processes that have affected it through time, including melt extraction, fluid phase enrichment, and subsequent interactions with melt derived from lower in the mantle. The primary advantage of this approach is that large tracts of supra-subduction lithosphere are commonly exposed at the base of many ophiolites, allowing us to examine their composition directly and on larger length scales than is currently possible in any active system.
This project focuses on the geochemical flux preserved in lithospheric mantle above the proto-Franciscan subduction zone, as represented by harzburgite and dunite tectonites that underlie the Coast Range ophiolite of California. The goal is constrain the nature and extent of these fluxes, as documented by whole rock major element and trace element analyses, by mineral analyses using electron and ion beam techniques, and by isotopic analyses of ultra-pure, hand-picked mineral separates. Major questions we will pose include the cumulative extent of melt extraction and the nature of the melt extracted, the nature, source, and extent of fluid flux to the mantle in the SSZ wedge, and the nature and extent of mantle-melt interactions subsequent to melt extraction (e.g., addition of melt from sub-lithospheric sources, or reaction of this melt with the previously depleted peridotites).
The geochemical evolution of the source region for island arc volcanics is an important problem with implications not only for the chemical budget of the Earth, but also for the origin of ore deposits that are commonly associated with island arc volcanism, the transfer of volatile elements from inside the Earth into the atmosphere, and the time scales over which these processes occur. While volcanoes are found on many terrestrial planets, plate tectonics and arc volcanism are unique to Earth and represent an important component of planetary evolution.
