High-pressure elastic properties of dolomite melt supporting carbonate-induced melting in deep upper mantle

Abstract

Deeply subducted carbonates likely cause low-degree melting of the upper mantle and thus play an important role in the deep carbon cycle. However, direct seismic detection of carbonate-induced partial melts in the Earth’s interior is hindered by our poor knowledge on the elastic properties of carbonate melts. Here we report the first experimentally determined sound velocity and density data on dolomite melt up to 5.9 GPa and 2046 K by in-situ ultrasonic and sink-float techniques, respectively, as well as first-principles molecular dynamics simulations of dolomite melt up to 16 GPa and 3000 K. Using our new elasticity data, the calculated VP/VS ratio of the deep upper mantle (∼180–330 km) with a small amount of carbonate-rich melt provides a natural explanation for the elevated VP/VS ratio of the upper mantle from global seismic observations, supporting the pervasive presence of a low-degree carbonate-rich partial melt (∼0.05%) that is consistent with the volatile-induced or redox-regulated initial melting in the upper mantle as argued by petrologic studies. This carbonate-rich partial melt region implies a global average carbon (C) concentration of 80–140 ppm. by weight in the deep upper mantle source region, consistent with the mantle carbon content determined from geochemical studies.

Revision of the CaMgSi2O6-CO2P-T phase diagram at 3–6 GPa

“…10). A recent experimental study (Xu et al 2020) shows that the presence of carbonate-rich melts in the deep upper mantle is consistent with seismic observations.…”

Experimental Evidence Supporting an Overturned Iron‐Titanium‐Rich Melt Layer in the Deep Lunar Interior

“…The sound velocity of the melt increases with pressure from 3,204 m/s at 1.41 GPa and 1,919K to 3,764 m/s at 4.25 GPa and 1,919K, and shows a weak temperature dependence (Figure 1a). Combined with the room-pressure density calculated from the partial molar volumes of oxide components for silicate melts (Lange & Carmichael, 1987), we fit our P-T-Vp data to the third-order Birch-Murnaghan EOS (Birch, 1952) using a Monte-Carlo approach (Xu et al, 2020(Xu et al, , 2022 (Text S4 in Supporting Information S1), with the best-fit results listed in Table S1 in Supporting Information S1. The calculated velocity profiles using these fitting parameters recover the experimental data well (Figure 1a).…”

The Speciation and Coordination of a Deep Earth Carbonate‐Silicate‐Metal Melt

“…We examine the speciation and the coordination of a carbonate‐silicate‐metal melt at conditions relevant to a magma ocean and to the present‐day Earth’s lower mantle. Previous studies examine carbonate melts (Koura et al., 1996 ; Li et al., 2017 ; Xu et al., 2020 ), carbon‐bearing silicate melts (Bajgain & Mookherjee, 2021 ; Ghosh & Karki, 2017 ; Ghosh et al., 2017 ; Karki et al., 2020 ), carbon and iron‐bearing silicate melts (Karki et al., 2020 ; Solomatova & Caracas, 2021 ; Solomatova et al., 2019 ), and carbon partitioning between silicate and iron melts (Zhang & Yin, 2012 ). The melt composition of this study differs from that of previous studies in that our melt has subequal amounts of carbonate, silicate, and metal, and acts as a representative composition of the types of mixed melts that could be present in the deep Earth.…”