The Redox Budget of the Mariana Subduction System

Abstract:

Oceanic lithosphere is altered and oxidized as it spreads, until it subducts and fluids and melts from the subducting plate contribute to the composition of arc and back-arc basalts. The oxidized nature of Mariana arc magmas is likely acquired through the transfer of oxidized species from the slab to the mantle wedge via aqueous fluids [1]. Despite its critical role in the relationship between material recycling at subduction zones and oxidation, it is unclear what percentage of the oxidized material is transported into the mantle wedge and output during volcanism and what percentage is transported past subduction zones, into the deep mantle. We present Fe³⁺/ΣFe ratios determined on bulk sediments and altered oceanic crust recovered from ODP Site 801 in the western Pacific in order to constrain the bulk oxidation state of the Pacific plate prior to subduction. We performed micro-colorimetric determinations of the Fe²⁺O contents of 9 sediment samples, 8 variably altered MORBs, 5 alteration products, and 8 mixed composite powders from the sediment and upper 500 m of altered oceanic crust at ODP Site 801. Site 801 sediments have Fe³⁺/ΣFe ratios >0.69 and the altered oceanic crust (801 Super Composite) has Fe³⁺/ΣFe=0.51. Bulk Fe³⁺/ΣFe ratios of altered oceanic crust at Site 801 increase from 0.16 (pristine MORB glass measured by XANES, [2]) to 0.78 with increasing extent of alteration. Using bulk Fe³⁺/ΣFe ratios determined here, and Fe redox information from [1] and [2], we calculated a mass balance of Fe³⁺ and O₂ equivalent through the Mariana subduction zone. We subtracted the Fe³⁺ of pristine oceanic crust from that of altered oceanic crust to estimate the amount of Fe³⁺ taken up by oceanic crust during alteration on the seafloor. Comparing this value to the output of Fe³⁺ from Mariana arc and back-arc lavas, we find that 50-70% of the O₂ equivalent added to the oceanic crust by alteration on the seafloor is not output by arc or back-arc magmas. If this oxygen is retained in the slab that subducts into the deep mantle, it may contribute to mantle redox heterogeneity.

[1] Brounce, M.N., Kelley, K.A., and Cottrell, E. in revision. Variations in Fe³⁺/∑Fe of Mariana arc basalts and mantle wedge fO₂.

[2] Brounce, M.N., Kelley, K.A., and Cottrell, E. in prep. Temporal evolution of mantle wedge oxygen fugacity during subduction initiation.