Density of high SiO2 carbonatite liquids in the upper mantle.

Carbonatite melts, while representing a small fraction of all magmas on Earths, are important features of the deep carbon cycle [1]. These melts are very reactive and quickly evolve during their ascent from a CO2-rich (CO2 higher 40 %) and SiO2 poor (SiO2 around 0 % wt) to compositions including more SiO2 and relatively less CO2 [2,3]. The physical properties of what is known as transitional melts (high CO2 contents and SiO2 content below 15% wt) at high pressure are not well constrained, although they are important for understanding the dynamics of carbonatite melt migration and chemical evolution. In this study we determined one high magnesium carbonatite with a ratio Ca/(Mg+Ca) ≈ 0.2 and one dolomite carbonatite with a ratio Ca/(Mg+Ca) ≈ 0.5. Both compositions have a 12 % SiO2 content and 5 % wt H2O content. We applied the in situ X-ray absorption method in combination with XCT-tomography in a Paris -Edinburgh press at the Psiché beamline of Soleil synchrotron to determine the density of melts [4]. We measured for each composition the melt density between 2 -4 GPa and 1200 K -1900 K and  complemented the in situ data with sink/float experiments at 4 GPa and temperature of 1700 K, using B4C and forsterite markers. Both types of experiments showed that dolomite carbonatite melts (high Ca content) with 12 % SiO2 have densities in the range of 2.9 – 3.05 g.cm-3, closer to that of a forsterite than carbonatites with 0 % SiO2 (indicating that even small amounts of SiO2 tend to increase significantly the density compared to pure carbonatite melts [5]. The implications of this results for the mobility of transitional melts in the upper mantle will be discussed.[1] Jones et al. (2013) Rev. Min. Geochem. 75, 289.[2] Hammouda & Keshav (2015) Chem. Geol. 418, 171.[3] Moussallam, et al. (2015) Chem. Geol. 418, 198.[4] Ritter et al. (2020) EPSL 533, 116043.[5] Massuyeau et al. (2023) Chem. Geol. 622, 121275.