High-pressure Raman study of Na-Ca burbankite-A possible CO2 host in deep mantle

Na-Ca carbonates synthesized in high-pressure experiments and found in mineral inclusions inside diamonds, exhumed from mantle depths, attract a lot of attention as potential hosts of subducted CO2 in the deep mantle and sources of carbonate-rich magmas, involved in mantle metasomatism and diamond formation. However, the Na-Ca carbonates synthesized at pressures of mantle transition zone (13-24 GPa) were not characterized structurally or spectroscopically. At the same time, the stability of well-characterized Na-Ca carbonates synthesized at lower pressures (3-6 GPa) at higher pressures was never studied in in situ experiments. Here, we report an in situ Raman study of Na-Ca burbankite, Na2Ca4(CO3)(5), synthesized at 6 GPa using diamond anvil cell technique up to the pressures of 24 GPa, corresponding to the mantle transition zone. The obtained results link the observations from experiments corresponding both to the upper mantle and mantle transition zone by demonstration of structural stability of Na-Ca burbankite and the absence of phase transitions up to 24 GPa (at least at ambient temperature). The latter allows to preliminary assign the Na-Ca-carbonates synthesized in carbonated pelites and eclogites under pressures of mantle transition zone to the burbankite structural type.

Automated summary

This study characterizes the structure and stability of Na-Ca carbonates synthesized under high pressures, particularly in the mantle transition zone. The results demonstrate that Na-Ca burbankite remains structurally stable at pressures up to 24 GPa, suggesting its potential presence in the deep mantle.