Mineralogical Controls on the Ti Isotope Composition of Subduction Zone Magmas

Abstract

The positive Ti isotope versus SiO2-content correlation in igneous rocks reflects the fractional crystallization of Ti-bearing oxide minerals. However, Ti isotope variations of subduction-related igneous rocks indicate that the Ti isotope compositions of their mantle sources are heterogeneous and additional mineral phases may promote Ti isotope fractionation. We have determined the Ti isotope composition of well-characterized subduction-related basalts, andesites and boninites. Samples from the Solomon Islands, the Troodos ophiolite in Cyprus, and Cape Vogel in Papua New Guinea show small but resolvable variations that may be related to differences in their mantle sources. Specifically, the δ49Ti of boninites (+0.109‰ to +0.168‰) is slightly higher than that of tholeiites (−0.027‰ to +0.111‰) from the same localities (Troodos in Cyprus and Cape Vogel in Papua New Guinea). Modeling suggests the partial melting of progressively depleted mantle sources where residual Cr-spinel plays a greater role in controlling the Ti budget during partial melting. More pronounced variations in δ49Ti are clearly linked to the fractional crystallization of Ti-oxides: Samples from Rabaul Volcanic Complex (New Britain, Papua New Guinea) show increasing δ49Ti (up to +0.373‰) with increasing Ti/V and decreasing Dy/Yb. Fractional crystallization models suggest that oxide minerals and amphibole are needed to sufficiently increase the δ49Ti of these magmas. Our study highlights that the combination of diagnostic trace element patterns and Ti isotope compositions in subduction-related igneous rocks can be a powerful tool to constrain petrogenetic processes and to discriminate between different crystallizing mineral phases.