Partitioning of trace elements between Na-bearing majoritic garnet and melt at 8.5 GPa and 1500-1900 °C

摘要

New experimental data on trace element partitioning between Na-bearing majoritic garnet and melt at P = 8.5 GPa and T = 1500-1900 °C applicable to partial melting of Na-rich eclogite are presented. We have found that Na-bearing garnet is a liquidus phase of the system at 1850-1650 °C being accompanied by enstatitic pyroxene at lower temperatures. With decreasing temperature, Na concentration in garnet increases up to >1 wt.% Na2O due to progressive incorporation of a Na majorite component (Na2MgSi5O_(12)). Most of the studied trace elements are incompatible, except for Er, Tm, Yb (in some runs), Lu, and Sc (in all runs), which are distributed into garnet. The main characteristic of the trace-element partitioning in our experiments is a different behaviour of the LREE (La, Ce, Pr) in comparison with MREE and HREE (Nd, Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, and Lu). In particular, a significant increase of D values for LREE with the increase of Na2O concentration in garnet is observed. As predicted from lattice strain, partitioning coefficients for REEs entering the X site of garnet exhibit a near-parabolic dependence on ionic radius. The results of the study are applied to the formation of inclusions of Na-bearing majoritic garnets in diamonds, and equilibrium melts significantly enriched in LREEs.