Date of Award
Doctor of Philosophy in Oceanography
Katherine A. Kelley
Oxygen fugacity is a fundamental thermodynamic property that describes reduction-oxidation (redox) equilibria in the solid Earth. It controls material transfer from the interior to the exterior of the planet by dictating the speciation of multi-valent elements (e.g., Fe, V, S). Oceanic crust ages and oxidizes as it moves from spreading centers to subduction zones, where it returns to the mantle in modified form. Subducting slabs release H2O-rich fluids and SiO2-rich melts to the mantle wedge in subduction zones, contributing significantly to the isotopic and major, trace, and volatile element composition of arc and back-arc magmas, however the effect that the oxidized nature of subducting slabs on arc basalts remains unclear. Arc basalts have a higher proportion of oxidized (Fe3+) relative to reduced (Fe2+) iron, expressed as the Fe3+/ΣFe ratio, than do mid-ocean ridge basalts (MORB) but there is disagreement as to whether this arises due to shallow level differentiation processes (e.g., crystal fractionation, crustal assimilation, degassing) in the arc crust or to differences in the fO2 of the mantle source. This thesis addresses this problem by examining the oxidation state of Fe and other transition element proxies for fO2 in (1) altered oceanic crust prior to subduction, (2) modern eruptive products from the active Mariana arc and back-arc, and (3) eruptive products representative of subduction initiation and margin evolution in the Marianas. Melt inclusions and submarine glasses record variable magmatic compositions that have the potential to record changes in magma chemistry during crystal fractionation and volcanic degassing. Recent innovations in synchrotron technologies have made studies of Fe redox possible in situ, on small scales (>10 microns), allowing direct observation of changes in Fe redox during shallow level differentiation processes in arc and back-arc magmas. This study reports observations of Fe redox variation from several Mariana arc volcanic centers as well as from the Mariana trough, demonstrating that shallow level differentiation processes are not responsible for generating the oxidized nature of arc basalts. Constraints for mantle source fO2 show that the mantle wedge is more oxidized than MORB source mantle and link this oxidation to influence from recycling slab fluids. Additionally, we explore other transition row element proxies for fO2 and show that these proxies need not preclude oxidized mantle wedge conditions in the Marianas. We examine changes in Fe redox in samples that record subduction initiation and margin evolution, demonstrating that in zones of melt generation within the mantle wedge, oxidation occurs immediately upon subduction initiation and that the mantle wedge remains oxidized for the majority of a subduction zone’s lifetime. We further constrain the fluxes of Fe3+ in to and out of the Mariana convergent margin, demonstrating that the Pacific plate is very oxidized prior to subduction and that 50-70% of this oxidized signature survives the recycling process to be subducted into the deep upper mantle.
Brounce, Maryjo N., "A Geochemical Investigation of Oxygen Fugacity in the Marianas Subduction Factory" (2014). Open Access Dissertations. Paper 249.