Monazite-xenotime thermometry; III, Experimental calibration of the partitioning of gadolinium between monazite and xenotime

Solid solutions of (Ce,Gd,Y)PO (sub 4) were synthesized hydrothermally between 500 and 1000 degrees C at 2 kbar, and the products determined by EPMA and the cell parameters refined by Rietveld analysis of XRD patterns. Over the whole T range, a complete solid solution series with the monazite structure exists along the CePO (sub 4) -GdPO (sub 4) binary; similarly, solid solutions with xenotime structure occur from pure YPO (sub 4) to Y (sub 0.04) Gd (sub 0.96) PO (sub 4) . The T-dependent miscibility gap in the CePO (sub 4) -YPO (sub 4) binary is traced into the ternary system up to the GdPO (sub 4) apex. At bulk compositions of (Ce (sub 0.45) Gd (sub 0.10) Y (sub 0.45) )PO (sub 4) and low T, Gd is preferentially incorporated in xenotime; the experimentally determined distribution coefficient D (sub Gd) = X (super mon) (sub Gd) /X (super xen) (sub Gd) increases lineraly from 0.25 at 500 to 1.0 at 1000 degrees C. Emperically derived Gd distribution coefficients of natural monazite-xenotime pairs from a metapelite suite are 0.25 in the greenschist facies and approximately 0.5 in lower granulite-facies rocks. If combined with U-Pb and Sm-Nd age determinations, the Gd geothermometer could be used to link the T conditions and timing of the REE phosphates and hence for deriving metamorphic P-T-t paths.