Carbonate zonation to reveal P-T-t-fluid history of fossil slab and carbon cycle in subduction zones

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Two major questions surround the role of subducting oceanic slab in tectonic mass transfer: 1) how efficiently subduction sequesters carbon from surface reservoirs; and 2) how slabs return from depths >100 km and why such slabs are rare. To answer these questions, the primary objective of this project is to adopt carbonate as a new proxy of both slab decarbonation and pressure (P)-temperature (T)-time (t) evolution of subduction zone metamorphism. The timescales derived from carbonate zonation, in particular, are critical for the estimates of carbon flux as well as subduction channel dynamics. These new petrologic tools will be applied to the Western Tianshan ultra-high pressure (UHP) metamorphic belt that represents one of only two oceanic UHP slabs having exhumed in orogens. In this UHP belt, carbonate minerals and veins are abundant in both metabasite and metapelite. The matrix carbonates in the eclogite are predominantly dolomite-ankerite, with increasing Mg# from the core and sharp compositional zones.The project integrates experimental studies, field-based research, thermodynamic-kinetic simulations, and instrumental analyses, toward a general picture of the carbon cycle in the subduction zone and subduction-exhumation dynamics from a petrologic perspective. The proposed work will develop HQP because it will support 2 Ph.D. and 3 M.Sc. thesis projects as well as undergraduate involvement, with a commitment to engaging those from under-represented minority groups in sciences.The diffusive Fe-Mg exchange in dolomite-ankerite will be investigated experimentally using piston-cylinder apparatus in the high-pressure laboratory at the University of Toronto. The calibrated diffusion model will be applied to zoned carbonate in eclogite and high-pressure veins to derive the relative timescales, which are then compared with the estimates based on other minerals (e.g., Mn in garnet, Sr in Apatite) and radiometric dating of minerals that carry P-T information (e.g., rutile, titanite). The phase relations among silicate, carbonate, and electrolyte fluid will be analyzed to correlate the carbonate zones to metamorphic stages. In a similar scheme as commonly applied to zoned garnet, phase equilibria coupled with growth-diffusion simulation will reveal the overall P-T-t path of the UHP slab, placing petrologic constraints on the subduction-exhumation dynamics in subduction channel. Further analysis of open-system behaviors evaluates the extent of slab decarbonation by fluid-rock interaction, and its influence on carbonate zonation and the isotopic variations among carbonate zones. Finally, these estimates are combined with the mass balance analysis of macroscopic geochemical variations by metasomatism to evaluate the net carbon flux from the slab.