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The fate of carbonate in oceanic crust subducted into earth's lower mantle

DOI: 10.1016/j.epsl.2019.01.041 DOI Help

Authors: James W. E. Drewitt (University of Bristol) , Michael J. Walter (University of Bristol; Carnegie Institution for Science) , Hongluo Zhang (University of Bristol; University of Science and Technology of China) , Sorcha C. Mcmahon (University of Bristol) , David Edwards (University of Bristol) , Benedict J. Heinen (University of Bristol) , Oliver T. Lord (University of Bristol) , Simone Anzellini (Diamond Light Source; CEA) , Annette K. Kleppe (Diamond Light Source)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Earth And Planetary Science Letters , VOL 511 , PAGES 213 - 222

State: Published (Approved)
Published: April 2019
Diamond Proposal Number(s): 10617 , 11896 , 15288 , 17994

Open Access Open Access

Abstract: We report on laser-heated diamond anvil cell (LHDAC) experiments in the FeO–MgO–SiO2–CO2 (FMSC) and CaO–MgO–SiO2–CO2 (CMSC) systems at lower mantle pressures designed to test for decarbonation and diamond forming reactions. Sub-solidus phase relations based on synthesis experiments are reported in the pressure range of ∼35 to 90 GPa at temperatures of ∼1600 to 2200 K. Ternary bulk compositions comprised of mixtures of carbonate and silica are constructed such that decarbonation reactions produce non-ternary phases (e.g. bridgmanite, Ca-perovskite, diamond, CO2–V), and synchrotron X-ray diffraction and micro-Raman spectroscopy are used to identify the appearance of reaction products. We find that carbonate phases in these two systems react with silica to form bridgmanite ±Ca-perovskite + CO2 at pressures in the range of ∼40 to 70 GPa and 1600 to 1900 K in decarbonation reactions with negative Clapeyron slopes. Our results show that decarbonation reactions form an impenetrable barrier to subduction of carbonate in oceanic crust to depths in the mantle greater than ∼1500 km. We also identify carbonate and CO2–V dissociation reactions that form diamond plus oxygen. On the basis of the observed decarbonation reactions we predict that the ultimate fate of carbonate in oceanic crust subducted into the deep lower mantle is in the form of refractory diamond in the deepest lower mantle along a slab geotherm and throughout the lower mantle along a mantle geotherm. Diamond produced in oceanic crust by subsolidus decarbonation is refractory and immobile and can be stored at the base of the mantle over long timescales, potentially returning to the surface in OIB magmas associated with deep mantle plumes.

Journal Keywords: carbonate; subduction; lower mantle; decarbonation; diamond

Subject Areas: Earth Science

Instruments: I15-Extreme Conditions

Other Facilities: ESRF