Structure and density of molten fayalite at high pressure

Abstract

The structure of molten fayalite was studied up to 7.5GPa by means of in situ energy-dispersive X-ray diffraction. The pressure-range studied covers the fayalite–spinel–liquid triple point at 6.2GPa. For pure molten fayalite, Fe–O coordination increases gradually from 4.8(2) at ambient pressure (P) to 7.2(3) at 7.5GPa. Compressibility of the melt is derived from the extrapolation of the structure factor to q=0Å−1, enabling the determination of density as a function of P with an unprecedented P-resolution. This is a promising method to extract the equation of state of non-crystalline materials at moderate P. The link between observed structural changes and density increase and the fact that structural changes occur over a broad but limited P-range in silicate melts implies that the equation of state should not be extrapolated at P-values higher than obtained in measurements, and that a single equation of state cannot accurately describe the density evolution over the whole terrestrial mantle P–temperature (T) range. Fe-rich melts are expected to have a higher densification rate than their Mg counterparts in the 0–10GPa range due to the increase of Fe–O coordination number. As a consequence, Fe-rich melts are more likely to be trapped at depth.