The Petrological Consequences of the Estimated Oxidation State of Primitive MORB Glass

Abstract

Discrepancies in Fe²⁺/Fe^T in mid-ocean ridge basalt glasses may be an analytical problem with implications principally for the oxidation state of the source mantle, but the Fe²⁺/Fe^T of such glasses also influences their equilibrium with olivine and petrological models of magmatic temperature. Hence Fe²⁺/Fe^T affects the inference of potential temperature and extent of melting in the sub-ridge mantle and models of complementary formation of erupted basalts and cumulate lower crust. Conversely, evaluation of equilibria between olivine phenocrysts and host glasses may provide an independent constraint on oxidation state. This chapter reviews petrological methods for estimating primary basalt composition, potential temperature, and thickness of igneous crust, focusing on the influence of glass redox state. The 1990's consensus around mutually consistent average values of Fe²⁺/Fe^T of MORB glass, thickness of oceanic crust, and mantle potential temperature apparently needs revisiting on several fronts. Values of average MORB glass Fe²⁺/Fe^T < 0.88 are apparently inconsistent with olivine-glass data and require primary liquidus temperatures too low for traditional estimates of the average thickness of oceanic crust or of lower crustal cumulates. Critical assessments of the absolute accuracy of Fe²⁺/Fe^T measurements, of electron microprobe measurements of olivine composition, of olivine/liquid Fe/Mg partitioning models, and of thermodynamic models of peridotite melting are essential to resolving this inconsistency. New melting models, source compositions, crustal thickness estimates, or interpretations of the plutonic-volcanic relationship may be needed to reconcile the bulk properties of the oceanic crust and its layers with the measured Fe²⁺/Fe^T of MORB glass.