The Randolph Bromery Fall 2018 Seminar Series

Description

Kayla Iacovino, a post-doctoral fellow at the School of Earth and Space Exploration at Arizona State University, will present a talk entitled "Tracking Redox Reactions from the Surface to the Mantle and Back: Can Subduction Help Oxidize Our Atmosphere?" as part of the Bromery Lectures.

Abstract:

Subduction-related volcanism is responsible for the largest global flux of volatiles from the deep Earth to the atmosphere today, and subduction zones are the principle conduits through which materials are returned to the mantle. This exchange controls the balance of life-sustaining elements, including oxygen via redox reactions, on the Earth's surface. It is widely accepted that fluids shed from the slab during subduction are the most efficient carriers of many elements into the mantle and mantle melts, however their capacity as oxidation agents remains hotly debated. Thus the efficiency of fluid-mediated redox transfer as a mechanism to deliver oxidized material to the surface is largely unknown. Here we report the first measurements of the composition and oxidation state of experimentally produced slab-derived fluids and model the interaction of these fluids with the arc mantle wedge. Our results show that the dehydration of natural serpentinite at subduction zone conditions produces moderately oxidizing fluids (oxygen fugacity 1-2 log units above the quartz-fayalite-magnetite buffer) with elevated concentrations of Ca, Mg, K, and Na. Equilibrium modelling of fluid-rock reaction between our experimental fluids and theoretical mantle lithologies indicates that 10-100 kg of serpentinite-derived fluid is required to increase the Fe3+/ΣFe in 1 kg of sub-arc mantle from MORB-like values (0.15) to values measured in primitive arc basalts (0.2 - 0.3). Given this range of fluid/rock ratios, we calculate that a slab fluid flux of 5x109 - 5x1013 kg-yr-1 is required to oxidize enough sub-arc mantle to produce the global average volcanic output at arcs. This flux range is within the range of estimates for H2O flux in subduction zones up to 100 km depth, or beneath the arc front. Our results suggest that the oxidation of the sub-arc mantle due to the flux of slab fluids is therefore expected and requires slab fluids with only slightly elevated oxygen fugacities.