Experimental study of monazite/melt partitioning with implications for the REE, Th and U geochemistry of crustal rocks

We report the results of monazite/melt partitioning experiments conducted in the piston-cylinder apparatus at 10-50 kbar and 750-1200 Celsius degree, using a synthetic granite mix with approximately 10 wt.% H₂O and doped with trace-elements in proportions corresponding to the composition of monazite. Monazite was produced in all experiments, generally in the form of small grains. Electron microprobe and laser ablation-ICP-MS analyses were carried out on the resulting "monazite-melt" mixes from these experiments, and the composition of the crystallized monazite calculated using regression analysis. The concentrations of LREE and Th in the melts coexisting with monazite increase sharply with increasing temperature. Monazite solubility decreases by 35-40% as pressure increase from 10 to 30 kbar. Monazite solubility in granitic melts with an Alumina Saturation Index above 0.85 and FeO+CaO+MgOb 3 wt.% can be described by the following equation:

ln?LREE = 16:16(+-0.3) + 0.23(+-0.07) ?H²O-11494(+-410)/T-19.4(+-4)P/T + lnX^LREE_mnz

Where H₂O is in weight percent, T is in Kelvin, P in kbar and ΣLREE is the sum of La-Sm in ppm; X^LREE_mnz is the molar ratio of LREE to the sum of all cations (REE, Th, U) in monazite. REE, Th, U, Y, V and As partition into monazite, whereas other trace elements (Li, Be, B, Sc, Ti, Mn, Sr, Zr, Nb, Ba, Hf, Ta and Pb) have monazite/melt partition coefficients less than unity. Monazite shows the greatest preference for LREE from La to Nd, with a progressive decrease in partition coefficients for Sm and the HREE. The partition coefficients for Th are 30% higher than those for the LREE, and Th/LREE ratios are independent of pressure and temperature. Partition coefficients for U are 4-23 times lower than for the LREE. The new experimental data provide a numerical basis for modeling the behavior of LREE, Th and U during fractional crystallization of granitic magmas, as well as the melting in the presence of monazite, both within the continental crust, and in subduction zones.