Evidence for amorphous calcium carbonate originated mid-lithospheric discontinuities

Abstract

Cratonic lithosphere is a vast host for deep recycled carbon, trapping up to several weight percent CO2 among its compositions1,2 at depths overlapping the seismic mid-lithospheric discontinuities (MLDs)3-5. However, the role of carbonates, especially for the latest discovered amorphous calcium carbonate (CaCO3)6, is underestimated in forming MLDs. Using the pulse-echo-overlap method in a Paris-Edinburgh press coupled with X-ray diffraction, we explored the acoustic velocities of CaCO3 under high pressure-temperature (P-T) conditions relevant to the cratonic lithosphere. Two anomalous velocity drops were observed associated with the phase transition from aragonite to amorphous phase as well as with pressure-induced velocity drop in amorphous phase around 3 GPa, respectively. Both drops are comparable with approximately 35% and 52% reductions for compressional (VP) and shear (VS) wave velocities, respectively. The VP and VS values of the amorphous CaCO3 above 3 GPa are about 1/2 and 1/3 of those of the major upper-mantle minerals, respectively, and they are the same with aragonite below 3 GPa. These velocity reduction by the presence of CaCO3 would readily cause MLDs at depths of 70–120 km dependent on the geotherm even if only 1-2 vol.% CaCO3 presents in the cratonic lithosphere. The CaCO3-originated MLDs is weak so as to be expected to influence the stability, rifting, and delamination of the craton7.

Lithospheric Structure of the Circum‐Pannonian Region Imaged by S‐To‐P Receiver Functions

“…The importance of MLD has been highlighted in relation with various tectonic processes such as the removal or destruction of cratonic roots (Wang & Kusky, 2019), displacement of shallow cratonic lithosphere (Wang et al., 2017), delamination of lower continental lithosphere or localized subsequent deformation events (Dunbar & Sawyer, 1988). The origin of the MLD is a timely and controversial topic which includes elastically accommodated grain boundary sliding (Karato & Park, 2018), channel flow (Yang et al., 2021), anisotropy (Selway et al., 2015; Wirth & Long, 2014); presence of amorphous carbonates (CaCO 3 ; Zhang et al., 2023), pyroxene and garnet crystallization (Liu et al., 2022), partial melting (Mierdel et al., 2007), fossilized melts and LAB (Rader et al., 2015) and break‐down of hydrous minerals (Green & Liebermann, 1976; Kovács et al., 2021).…”