Structure and Viscosity of Carbonate-Silicate Melts Using in situ Techniques

Abstract:

The chemical properties of carbon-rich magmas at high pressures and temperatures are a major factor controlling the deep-Earth carbon cycle, and may play a crucial role in global magmatism at depth. We have measured the structure of carbonate-silicate liquids as a function of carbon content along the calcite-wollastonite (CaCO₃-CaSiO₃) and calcite-forsterite (CaCO₃-Mg₂SiO₃) compositional joins at ~1800 ^oC, and 3 or 6 GPa in a Paris-Edinburgh press using X-ray diffuse scattering. Pair distribution functions calculated from the scattering data confirm that CaCO₃ behaves as an ionic liquid, while compositions with 40-100 wt% wollastonite contain polymerized silicate with an average connectivity (Q_n) of at least two bridging O per Si (Q₂). Even liquid compositions with as little as 35 wt% forsterite component contain silicate polymers, despite the absence of bridging O in the crystalline orthosilicate. As the carbonate content of the melt is increased, the average connectivity of SiO₄ units linearly increases from ~Q₂ for pure wollastonite liquid to >Q₃ for 40 wt% wollastonite, even though less silicate is present. Analysis of Ca-Ca and Ca-Si pair correlations, as well as Ca-O bond distances, indicate that Ca²⁺ bonds more strongly to the silicate framework as carbonate content increases. Increasing pressure from 3 to 6 GPa at constant composition causes Q_n to fall near Q₀. The trends in local atomic structure with composition and pressure explain falling sphere viscometry measurements in carbonate-silicate liquids at upper mantle conditions. These viscosities are as low as 6x10^-3 Pa-s for pure CaCO₃, only half a log unit higher than that of water, and span ~1.6 log units between the carbonate and silicate end-members. Structural and viscosity trends taken together indicate carbonate-rich, depolymerized and highly mobile liquid at depth which transitions into silicate-rich, polymerized and much less mobile liquid as the melt ascends into the crust.