Date

2013

Document Type

Thesis

Degree

Master of Science

Department

Earth and Environmental Sciences

First Adviser

Bebout, Gray E.

Abstract

Metamorphism of subducting oceanic crust and carbonate-rich seafloor sediments likely plays an important regulatory role in the long-term global C cycle by controlling the fraction of subducting C emitted into the atmosphere via arc volcanic gases as compared with the fraction of C continuing into the deep mantle. Metasedimentary suites representing high-pressure (HP)- and ultrahigh-pressure (UHP)-metamorphosed Jurassic oceanic sediment cover were sampled from locations across the Italian Alps (the Schistes Lustres, Zermatt-Saas ophiolite, and at Lago di Cignana). These samples were selected for the broad range of peak P-T (1.5-3.0 GPa; 330-550°C) conditions that they experienced during subduction (Agard et al., 2001a; Bebout et al., 2013), corresponding to depths approaching those beneath volcanic fronts. Reported in this thesis are field, petrographic, and geochemical evidence for the efficient retention of C in subducting shale-carbonate sequences through forearcs to depths approaching those beneath volcanic fronts. This information can inform models of the global C budget and long-term evolution of the atmosphere. Calc-silicate minerals occur only at the higher grades, in minor abundance, indicating minor decarbonation in this suite. Carbonate δ13C values retain the signatures typical of marine carbonates and are relatively uniform across grade, with some shifts to lower values in low-calcite samples containing abundant reduced C. Carbonaceous matter shows some increase in δ13C with increasing grade that could reflect decarbonation, but more likely varying degrees of exchange with carbonate in the same samples. All carbonate analyzed has δ18O significantly lower than is typical for marine carbonates, partly caused by exchange with silicate phases but, for more carbonate-rich samples, seemingly requiring some infiltration by externally-derived fluids. These results indicate that relatively little decarbonation occurred in carbonate-bearing sediments subducted to depths of up to 90 km, arguing against extensive decarbonation driven by massive infiltration of the sediments by H2O-rich fluids released from mafic and ultramafic parts of the underlying slab (as was modeled in some theoretical studies). Metapelitic rocks intercalated with the carbonate-rich rocks released some H2O-rich fluid during prograde metamorphism, providing a more proximal source for fluid to shift the δ18O values of the carbonates.

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