UC San Diego

Titanium (Ti) isotopes are a well-established tracer of magmatic differentiation capable of providing useful insight into the magma series (e.g., calc-alkaline versus tholeiitic) and geodynamic setting of rock formation. Titanium isotope fractionation is sensitive to oxygen fugacity, water content, and the initial geochemical composition of a melt, however the impacts of partial melting during plate subduction upon the Ti isotopic composition of a rock are still not well constrained. In this study, we use a suite of samples from two relatively modern, well-characterized sites from the Aleutian arc thought to have little to no influence from subducted sediment in the source of the arc magmas to probe how formation processes (e.g., partial melting versus fractional crystallization/magma mixing) influences the measured Ti isotope compositions. We present Ti isotopic compositions for 18 volcanic samples from two Aleutian arc sites; Western Aleutian seafloor lavas from the Ingenstrem Depression, thought to have formed through partial melting of mid-ocean ridge basalt (MORB) in eclogite facies, and lavas from Korovin Volcano on Atka Island, which formed from fractional crystallization and/or magma mixing. Samples dredged from the Ingenstrem Depression span a relatively narrow range in Ti isotopic composition (δ49Ti = +0.031 to +0.147‰) with a relatively muted trend in Ti isotopic composition with respect to increasing silica content (50.41 to 66.30 wt.%). The samples from Korovin Volcano span a larger range (δ49Ti = -0.021 to +0.355‰) and more pronounced Ti isotopic compositions with increasing silica content (50.38 to 64.09 wt.%). We use thermodynamic isotopic modeling (using THERMOCALC software 3.53c) to demonstrate that the rocks from Ingenstrem Depression were formed through partial melting of MORB in eclogite facies and rocks from Korovin Volcano are likely formed by mixing of a mafic and dacitic magmas.