SLAB VS CRUSTAL MELTING: IMPLICATIONS FOR THE SR/Y SIGNATURE IN ARC MAGMAS

Arc magmas originate in subduction zones where the subducting oceanic lithosphere dehydrates beneath the overriding oceanic or continental lithosphere. In this tectonic scenario, melts can derive from different sources including the mantle wedge, subducting slab, and the crust of the upper plate. Arc magmas may ultimately represent mixtures of melts from these different sources. While arc magmas are often relatively similar in major-element composition, trace-element ratios, such as the Sr/Y ratio, can be used to distinguish possible melt sources. However, the interpretation of the Sr/Y ratio remains controversial

In this work, we explore the effects of source and intracrustal processes on the Sr/Y signature in arc magmas by applying forward petrological modeling. We model different geodynamic scenarios for arc magmatism including slab melting and melting of the lower continental crust and show how different the Sr/Y signature is as a function of melt source. In both scenarios, we consider the effects different melt mechanisms (fluid-flux melting vs dehydration melting) and melt migration. We further evaluate the effects of partial melting of different lower continental crust compositions as well as different crustal thicknesses (pressure). Our preliminary results show that slab melts display considerably higher Sr/Y compared to melts derived from the anatexis of the lower crust; this is also true for other geochemical proxies such as La/Yb and Nb/Ta. The Mg concentrations [and Mg#?] from slab and lower crustal melts are similar, and depleted in MgO in both cases relative basaltic and andesitic magmas. For crustal melting, different estimates for the compositions of the lower continental crust also have an impact on the melt’s Sr/Y ratio. Bulk compositions that preferentially stabilize garnet over plagioclase yield higher Sr/Y ratios; bulk compositions with low Sr/Y ratios result in melts with corresponding low Sr/Y signatures. Overall, our preliminary results show that depth and source processes play a more important role on controlling the Sr/Y signatures in arc magmas. When melting continental crust, thickness of the crust the pressure of melting controls the Sr/Y ratio more than the effects of crustal compositional heterogeneities or melting mechanism (i.e., flux melting vs dehydration melting).