Titanium isotope and trace element insights into the redox evolution of the Oman ophiolite mantle sources and melts

Abstract:

The Oman ophiolite is the best preserved and largest ophiolite complex available for study. Long-standing debate surrounding its geodynamic context of formation hinges on two competing hypotheses: (1) the ophiolite formed at an open-ocean mid-ocean ridge, and thus a unique, invaluable direct analogue for fast-spreading oceanic crust, or; (2) it formed by spreading above a nascent subduction zone. Evidence supporting the latter is the presence of elevated water contents and arc-like geochemical signatures, but this has remained controversial. We here apply a novel combination of trace element ratios and Ti stable isotope compositions of sheeted dykes and lavas to differentiate between these hypotheses, with particular focus on the redox state of melts and mantle sources throughout the magmatic history of the ophiolite.

Primitive (Mg# >60) lavas and dykes from Oman, filtered from an overall database of >1200 samples, show large variations in concentrations and ratios of redox-sensitive elements. Importantly, the range of variation in these elements and ratios increases both spatially (from S to N) and temporally throughout the ophiolite. Mantle melting models taking into account varying source redox state indicate that the trace element systematics of the lavas and sheeted dykes can be replicated by melting of a progressively oxidised source. The trace element systematics are complemented by the first Ti isotope study of Oman lavas and dykes, a powerful new tracer of oxide-melt equilibrium in magmas that can be used to evaluate the redox state of mantle melts. Preliminary results reveal that Oman melts display Ti isotope compositions similar to that of arc magmas and distinct from tholeiitic suites typical of MOR settings, suggesting parental melts that are oxidised relative to N-MORB. Overall, the trace element and Ti isotope systematic of Oman ophiolite dykes and lavas suggest the progressive introduction of water in the active melting column under the Oman ophiolite and thus its likely formation in close proximity to a nascent subduction zone.