Tetrahedrally coordinated carbonates in Earth's lower mantle

TL;DR

This study provides experimental and theoretical evidence that carbonates in Earth's lower mantle transform into a tetrahedrally coordinated phase at high pressures, potentially affecting deep carbon cycling and geodynamics.

Contribution

It is the first to demonstrate the existence of tetrahedrally coordinated carbonates in Earth's lower mantle through combined experimental and computational methods.

Findings

Tetrahedral carbonates form above 80 GPa in the lower mantle.

Infrared spectroscopy identifies unique signatures of the high-pressure phase.

Tetrahedral coordination alters carbonate reactivity and chemical properties.

Abstract

Carbonates are the main species that bring carbon deep into our planet through subduction. They are an important rock-forming mineral group, fundamentally distinct from silicates in Earth's crust in that carbon binds to three oxygen atoms, while silicon is bonded to four oxygens. Here, we present experimental evidence that under the sufficiently high pressures and high temperatures existing in the lower mantle, ferromagnesian carbonates transform to a phase with tetrahedrally coordinated carbons. Above 80 GPa, in situ synchrotron infrared experiments show the unequivocal spectroscopic signature of the high-pressure phase of (Mg,Fe)CO$_3$. Using ab-initio calculations, we assign the new IR signature to C-O bands associated with tetrahedrally coordinated carbon with asymmetric C-O bonds. Tetrahedrally coordinated carbonates are expected to exhibit substantially different reactivity than low pressure three-fold coordinated carbonates, as well as different chemical properties in the liquid state. Hence this may have significant implications on carbon reservoirs and fluxes and the global geodynamic carbon cycle.