Melting of hydrous and hydrous-carbonated eclogite in the mantle

Water is a key component at Earth’s surface controlling geomorphological processes and establishing an environment capable of supporting life, but it is also present in the mantle as point defects in the lattice of nominally anhydrous minerals (NAMs). Even in trace amounts (i.e., ppm wt), H₂O exerts a profound influence on rock properties such as decreasing the melting temperature, which allows melting to occur at much greater depths with respect to volatile-free conditions, impacting the geochemical evolution of the deep mantle. Therefore, precisely modelling melting processes as a function of volatiles content is of great interest in petrology and requires a precise knowledge of the behavior of volatiles at mantle pressure and temperature.

A fundamental parameter used to investigate hydrous melting is the partition coefficient of H₂O (D_H2O^min/melt = C_H2O^min/C_H2O^melt), which can be studied at high-pressure high-temperature by performing laboratory experiments. A large number of studies have previously determined the D_H2O^min/melt for major mantle phases and mantle conditions in a hydrous system. However, a more comprehensive investigation of the distribution of H₂O in the mantle requires also the consideration of CO₂, which is the 2^nd most abundant volatile in the mantle and is expected to alter the activity of H₂O of the system and, consequently, the D_H2O^min/melt. Notably, the effect of CO₂ on D_H2O^min/melt at mantle conditions remains largely unconstrained today.

We present the results of a series of 2 GPa-1200°C piston cylinder experiments investigating an eclogitic system, where clinopyroxene (+garnet) crystals were equilibrated with melt. The experiments were conducted at hydrous and hydrous-carbonated conditions with different amounts of CO₂. The experimental charges were recovered, and minerals and melts were investigated by electron microprobe analyses, to determine chemical composition and Fourier Transform Infra-Red and Raman spectroscopy to determine the H₂O contents. The experiments highlight a strong effect of CO₂ content on decreasing D_H2O^min/melt. The new results are used to discuss melting processes interesting eclogitic lithologies in the deep mantle and at representative chemical compositions.