Behavior of basalt-carbonate melts at high pressures

Properties of carbonate-rich silicate melt in the Earths upper mantle are important in our understanding of the deep carbon cycle, as well as related geochemical and geophysical processes. Behavior of carbonate-rich silicate melt may be complex, as demonstrated in the reported immiscible behavior in silicate-carbonate melts. However, most of these experiments are limited at low pressures of less than ~3 GPa. Here we investigate behavior of basalt-dolomite melts with 20-61 vol.% dolomite compositions, which corresponds to ~9-27 wt .% CO2, at high pressures to 7.3GPa by using newly developed X-ray phase contrast imaging technique in Paris-Edinburgh cell at beamline 16BMB, HPCAT, in the Advanced Photon Source. We find two phases in ~32-61 vol.% dolomite samples below ~5.5 GPa. Quench sample analysis shows that the two phases are carbonate-bearing silicate melt and a volatile phase, which imply that solubility of CO2 in basalt melt at this pressure condition is smaller than ~14wt.%. In contrast, the ~32-61 vol.% dolomite samples becomes a single liquid above ~5.5 GPa, which imply basalt melt can contain ~32-61 vol.% dolomite (~14-27 wt.% CO2). The abrupt change of CO2 solubility in basalt melt at ~5.5 GPa may play important role in understanding migration of carbonate-rich silicate melt in the upper mantle. For instance, Kono et al. (2014) (Nature Communications, 5, 5091, doi: 5010.1038/ncomms6091) reported significant difference in viscosity of basalt and carbonate melts. Viscosity of carbonate melts are ~2 to 3 orders of magnitude lower than those of basaltic melts in the upper mantle. Abrupt decrease of carbonate content in basalt melt may cause sudden increase of viscosity and the resultant speed-down of melt migration velocity below ~5.5 GPa.