incorporation in magmas at depth: changes in melt local environment and the influence partitioning

The structure of two Lu doped (4000 ppm) model end member silicate liquids, a highly polymerised haplogranite (Si-Al-Na-K-O) and a less polymerised anorthite-diopside (Si-Al-Mg-Ca-O), have been studied up to 8 GPa using in situ x-ray diffraction techniques. The results are the first to identify trace rare Earth element incorporation in silicate melts at high pressure. At pressures below 5 GPa, the bonding environment of Lu-O was found to be dependent on composition with coordination number CN Lu − O = 8 and bond distance r Lu − O = 2 . 36 ˚A in the haplogranite melt, decreasing to CN Lu − O = 6 and r Lu − O = 2 . 29 ˚A in the anorthite-diopside melt. This compositional variance in coordination number at low pressure is consistent with observations made for Y-O in glasses at ambient conditions and is coincident with a dramatic increase in the partition coefficients previously observed for rare Earth elements with increasing melt polymerisation. With increasing pressure we find that CN Lu − O and r Lu − O remain constant in the haplogranite melt. However, an abrupt change in both Lu-O coordination and bond distance is observed at 5 GPa in the anorthite-diopside melt, with CN Lu − O increasing from 6 to 8-fold and r Lu − O from 2.29 to 2.39 ˚A. This occurs over a similar pressure range where a change in the P-dependence in the reported rare Earth element partition coefficients is observed for garnet-, clinopyroxene-, and olivine-melt systems. This work shows that standard models for predicting trace elements at depth must incorporate the effect of pressure-induced structural transformations in the melt in order to realistically predict partitioning behaviour. element studies due to the low concentrations of trace elements ( < 0.1 wt%) required to represent natural systems. X-ray diffraction has the benefit of providing information on both the short and medium range structure of the melt, includ- ing absolute distance and average coordination numbers, without relying on structural models. In this study we employ two end member melt compositions, silicic haplogran- ite and a ‘model basalt’ iron-free anorthite-diopside to monitor structural changes at high pressure and temperature that may affect the incorporation of Lu at 4000 ppm concentration into the