Date of Award

Summer 8-21-2020

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Geological Sciences

Advisor

Alicia M. Cruz-Uribe

Second Committee Member

Horst R. Marschall

Third Committee Member

Edward S. Grew

Additional Committee Members

Amanda Olsen

Martin Yates

Abstract

Subduction zones are the site of long-term chemical exchange between Earth’s surface and interior geochemical reservoirs. Subducting slabs are progressively depleted in volatiles and other mobile elements through dehydration and melting reactions. These elements are then introduced to the mantle wedge and volcanic arc. One consequence of this cycle is that volcanic arcs produce the most oxidized magmas on Earth. Sulfur, which exhibits a range in valence states from S2- to S6+, is one of the few elements in the subducting slab capable of oxidizing the arc and mantle wedge. Sulfides in the slab may also act as an important host of chalcophile and siderophile minor and trace elements (CSEs), such as Co, Ni, Cu, and As. The extraction of sulfur from the slab may also liberate and transfer these elements to the arc, where they may contribute to the formation of ore deposits. Such chemical and advective processes, which occur at depths in excess of 50 km, are not directly observable. Rocks and minerals in exhumed high-pressure terranes provide a chemical archive of sulfur and CSE extraction during subduction. In this dissertation, I examine the sulfur isotopic composition of sulfides in high-pressure and slab fluids, predict the concentration and speciation of sulfur in slab fluids, and characterize the CSE budget of the subducted mafic crust.

Sulfur isotopes are a sensitive indicator of redox conditions and sulfur mobility. In situ sulfur isotope measurements were conducted using secondary ion mass spectrometry (SIMS) on sulfides in high-pressure metamorphic rocks. Sulfides are classified into two categories: 1. Metamorphic sulfides, which preserve a record of prograde subduction metamorphism, and 2. Metasomatic sulfides precipitated from slab-derived fluids. The isotopic composition of metamorphic sulfides reflects their protolith compositions: −4.3 to +13.5 ‰ for metabasic rocks, and −32.4 to −11.0 ‰ for metasediments. From these data, I estimate that prograde sulfide breakdown will release slab fluids with an isotopic composition of −11 to +8 ‰. Metasomatic sulfides exhibit a range in δ34S values from −21.7 to +13.9 ‰, significantly larger than the range of predicted slab fluid compositions. The observed range of metasomatic sulfide compositions likely indicates large isotopic fractions between and oxidized slab fluid and the precipitating sulfide grains.

In support of the sulfur isotope observations, I used equilibrium thermodynamic forward modeling software to predict the speciation and concentration of sulfur in slab fluids. Sulfur loss from the slab is sensitive to protolith Fe3+/SFe ratio and subduction zone thermal structure. Subduction of altered oceanic crust in cold subduction zones was found to produce significant fluxes of sulfate and sulfite species. In contrast, subduction along a warm subduction geothermal gradient produces a bi-modal release of sulfur-bearing fluids, with a low-volume shallow flux of reduced sulfur, followed by a high-volume deep flux of sulfate and sulfite species. Sulfur oxidation in these systems is balanced by ferric iron reduction. Matrix sulfides in high-pressure metamorphic rocks are rarely associated with peak metamorphic conditions, suggesting that sulfur is almost completely extracted from the slab across the blueschist to eclogite transition. The thermodynamic models predict that sulfur oxidation is balanced by iron reduction in the eclogitic slab residuum. This prediction is supported by the observation that oceanic eclogites display lower bulk rock Fe3+/SFe when compared to blueschist and altered oceanic crust.

Prograde sulfur-loss is anticipated to redistribute and/or release sulfide-hosted minor and trace CSEs. In situsulfide and silicate compositions were determined by laser ablation inductively coupled mass spectrometry (LA-ICP-MS) for the elements Cr, Co, Ni, Cu, Zn, Ga, Ge, As, Mo, Ag, Cd, In, Sn, Sb, Te, Tl, and Bi. Sulfides host nearly the entire Cu, As, Ag, Cd, and Te budget of subducted rocks. These elements are incompatible in garnet, omphacite, phengite, and rutile, and are almost completely lost from the slab during prograde sulfide breakdown. Cobalt, Ni, Zn, Ga, Ge, Mo, Sn, and Tl are hosted in silicate and oxide phases. While Co, Ni, Zn, Ga, and Pb may be partially lost during lawsonite, epidote, and amphibole dehydration reactions, Ge, Tl, Sn, and Mo are largely retained in garnet, phengite, and rutile. Mobilization of the latter elements may require high fO2 conditions or melting. Consistent with estimated CSE budgets, minor and trace element zoning in metasomatic sulfide and silicate minerals suggest that Co, Ni, and As are mobile in slab fluids. Sulfur lost from the slab is expected to oxidize the subarc mantle and arc magmas, whereas mobile CSE elements may contribute to the formation of arc-related ore deposits.

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