Reversal of carbonate-silicate cation exchange in cold slabs in Earth\u2019s lower mantle

Mingda Lv,Vitali B. Prakapenka,Stephan Borensztajn,Eran Greenberg,James Badro,Susannah M. Dorfman

doi:10.1038/s41467-021-21761-9

Abstract

The stable forms of carbon in Earth’s deep interior control storage and fluxes of carbon through the planet over geologic time, impacting the surface climate as well as carrying records of geologic processes in the form of diamond inclusions. However, current estimates of the distribution of carbon in Earth’s mantle are uncertain, due in part to limited understanding of the fate of carbonates through subduction, the main mechanism that transports carbon from Earth’s surface to its interior. Oxidized carbon carried by subduction has been found to reside in MgCO3 throughout much of the mantle. Experiments in this study demonstrate that at deep mantle conditions MgCO3 reacts with silicates to form CaCO3. In combination with previous work indicating that CaCO3 is more stable than MgCO3 under reducing conditions of Earth’s lowermost mantle, these observations allow us to predict that the signature of surface carbon reaching Earth’s lowermost mantle may include CaCO3.

Highlights

The stable forms of carbon in Earth’s deep interior control storage and fluxes of carbon through the planet over geologic time, impacting the surface climate as well as carrying records of geologic processes in the form of diamond inclusions
The pressure/temperature conditions of the reversal reaction as constrained by these experiments are similar to those of polymorphic phase transitions associated with sp2–sp[3] bonding changes in both MgCO3 and CaCO3, which suggests these transitions are related to the stabilization of a CaCO3 + MgSiO3 assemblage
The transition from sp2- to sp[3] bonds in MgCO3 has been identified at ~80 GPa with the stabilization of the C2/m structure[32,33,36], and the resulting densification of MgCO3 supports the forward reaction to MgCO3 + CaSiO3

Summary

Introduction

The stable forms of carbon in Earth’s deep interior control storage and fluxes of carbon through the planet over geologic time, impacting the surface climate as well as carrying records of geologic processes in the form of diamond inclusions. Carbon is transported from Earth’s surface to its interior mainly as carbonate minerals in subduction zones and is returned in carbon-bearing gas/fluid through volcanic degassing[2,3] These processes leave signatures in the mantle, including depletion of incompatible elements[4,5], diamond formation (and inclusions)[6,7], and isotopic abundances[8,9]. The temperature is not the only control on the fate of subducted carbonates: carbonates may interact chemically with the major phases of the ambient mantle or basalt-rich subducted crust In these compositions in the lower mantle, the silicates potentially reacting with carbonates are bridgmanite (bdg), post-perovskite (pPv), and Ca-perovskite (Ca-Pv)

Methods

Results

Conclusion