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

James W.E Drewitt,Michael J Walter,Hongluo Zhang,Sorcha C Mcmahon,David Edwards,Benedict J Heinen,Oliver T Lord,Simone Anzellini,Annette K Kleppe

doi:10.1016/j.epsl.2019.01.041

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.

Highlights

Carbon is essential for habitability at earth’s surface and understanding how it cycles through exterior and interior reservoirs is fundamental for modelling the evolution of carbon through geological time (Sleep and Zahnle, 2001)
We report on laser-heated diamond anvil cell (LHDAC) experiments and subsequent analysis of the recovered run products by synchrotron X-ray diffraction and micro-Raman spectroscopy, and use these data to constrain the stability of carbonate in deeply subducted oceanic crust
Identification of non-ternary silicate phases by X-ray diffraction Fig. 3 shows diffraction patterns from the FMSC and CMSC systems that illustrate the detection of decarbonation reactions in our synthesis experiments

Summary

Introduction

Carbon is essential for habitability at earth’s surface and understanding how it cycles through exterior and interior reservoirs is fundamental for modelling the evolution of carbon through geological time (Sleep and Zahnle, 2001). The abundance of carbon in earth’s interior is highly uncertain with estimates for carbon in bulk silicate earth varying by more than an order of magnitude from ∼20 to 800 ppm (Dasgupta and Hirschmann, 2010; Marty, 2012; Sleep and Zahnle, 2001), and estimates for the core ranging from very little to several weight% (Chen et al, 2014; Dasgupta and Walker, 2008; Wood et al, 2013). While absolute carbon abundances are challenging to constrain, estimates of the H/C ratio in bulk silicate earth indicate that this ratio is greater than the primitive chondritic ratio (Halliday, 2013; Hirschmann and Dasgupta, 2009; Marty, 2012), a feature which can ostensibly be explained by solution of carbon into ironrich liquid metal during high pressure–temperature core segregation. Drewitt et al / Earth and Planetary Science Letters 511 (2019) 213–222

Methods

Results

Conclusion