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

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  Å in the haplogranite melt, decreasing to CN Lu–O = 6 and r Lu–O = 2.29  Å 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 Å. 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.