Experimental constraints on trace element partitioning between coesite and hydrous silicate melt at 5 GPa and 1500-1750°C

Trace element partitioning between coesite and hydrous silicate melt has been investigated at 5 GPa and 1500-1750 degrees C. High-P experiments successfully produced large coesite crystals in equilibrium with large silicate melt pools (plus kyanite and corundum crystals in some cases). Scanning electron microscopy and micro-Raman spectroscopy were employed to characterize the phases and the textures. Wavelength-dispersive electron microprobe analyses were performed to quantify conventional major elements, and laser ablation-inductively coupled plasma-mass spectrometry analyses were successfully conducted to quantify trace elements. Eventually, high-P partition coefficients were obtained for 33 elements. In general coesite is a very pure phase. With a few possible exceptions like Sc, Ti, and V, nearly all other trace elements are incompatible in coesite. Moreover, the partitioning behaviors of nearly all trace elements except some 4+ cations cannot be readily described by the lattice strain model, presumably implying a minor role for the cation size in the trace-element partitioning. Combining our experimental results with the results in the literature, some T and P effects on the element partitioning behavior have been observed: T seemingly has different effects on different trace elements, but P might negatively correlate with the partition coefficients in all cases. Due to its large modal fraction in some subducted materials such as the continental crustal material, coesite might play an important role in the distributions of some trace elements, Ti for example.