Investigating the Deep Carbon Cycle through the Experimental Study of Simple Carbonate Systems

Abstract

Experiments in the simple binary carbonate systems of Na2CO3-MgCO3 and Na2CO3- CaCO3 between 10 and 55 GPa have shown that Na can dramatically reduce the solidus of pure carbonates. The addition of Na to pure magnesite will lower the solidus by almost 600 K at 11 GPa and 250 K at 50 GPa. Similarly the addition of Na to pure CaCO3 will lower the solidus by around 500 K between 15 and 40 GPa. This is in agreement with previous studies of complex carbonated MORB systems where the appearance of Na-carbonate in a magnesite assemblage will lower the solidus by 240 K while the solidus of an aragonite assemblage will decrease by 100 K with the appearance of Na-carbonate.

No changes in the carbonate phase assemblage are observed in either simple system between 21 and 50 GPa and there are no sudden changes in the solidi. This suggests that the assemblage in complex systems will not change between 21 and 50 GPa and that there will be no sudden changes in the solidi. As a result, cold subduction zone geotherms should not intersect the solidus of carbonated MORB as it increases with increasing pressure from 21 to 50 GPa. This means that carbonates will be subducted into the lower mantle along cold subduction zone geotherms.

Natrite and magnesite will react with SiO2 along a cold subduction zone geotherm at » 35 GPa to produce a Na-Mg-silicate phase and CO2-V. Along a cold geotherm CO2-V will break down to diamond + O2 at ~ 60 GPa. Therefore carbonates are unlikely to subduct beyond ~ 35 GPa and the stable carbon phase in subducted oceanic crust will be diamond, which could potentially reside in the deep lower mantle over long periods of geological time.

Date of Award	23 Jun 2020
Original language	English
Awarding Institution	University of Bristol
Supervisor	Oliver T Lord (Supervisor), Richard A Brooker (Supervisor) & Simon C Kohn (Supervisor)