The occurrence and mobility of carbonate-rich melts in Earth’s mantle are important factors that influence our understanding of the deep carbon cycle, as well as related geochemical and geophysical processes. Despite extensive studies on the occurrence and stability of carbonate-rich melts, physical properties (such as density, viscosity, and mobility) of carbonate melts are not well understood.
In a paper published in Nature Communications , Yoshio Kono and colleagues report viscosities of calcite and natural dolomite melts up to 6.2 GPa using an advanced technique of viscosity measurement with ultrafast synchrotron X-ray imaging. The imaging rate of 1,000 frames per second (fps), more than 15 times faster than that of conventional X-ray radiography (typically 30 to 60 fps) in large volume presses, enables precise determination of very low viscosity values. This study reveals that viscosities of calcite and dolomite melts are surprisingly low: in the range of 0.006-0.010 Pa s. These low viscosity values are more similar to those of water than to silicate melts.
Gravity-driven melt transportation is proportional to “hydrostatic melt mobility,” ∆r/h, where h is the viscosity of the melt and ∆r is density contrast between the melt and the surrounding solid rock. For rocks of similar permeability, the mobility of carbonate melts is approximately two to three orders of magnitude greater than that of basalt melts. Highly mobile carbonate melts therefore play an important role in several magmatic processes in Earth’s upper mantle, including fast melt migration and effective melt extraction underneath mid-ocean ridges. Considering the unexpectedly low viscosity of carbonate melts, these data have the potential to change how we view a number of mantle processes.
Image: Adapted from Figure 5a in reference . The image depicts ultralow viscosity of carbonate melts, which is approximately 90 to 720 times lower than that of the basalt melt along an adiabatic geotherm. View a larger version.