Monazite-xenotime thermometry. III. Experimental calibration of the partitioning of gadolinium between monazite and xenotime
Gratz, René; Heinrich, Wilhelm
European Journal of Mineralogy Volume 10 Number 3 (1998), p. 579 - 588
published: Jun 22, 1998
manuscript accepted: Dec 19, 1997
manuscript received: Apr 30, 1997
ArtNo. ESP147051003016, Price: 29.00 €
Abstract Solid solutions of (Ce,Gd,Y)PO4 were synthesized hydrothermally between 500°C and 1000°C at 2 kbar. Run products were analyzed by electron microprobe and lattice parameters were refined by Rietveld analysis of XRD powder patterns. Over a wide range of bulk compositions, the two immiscible phases monazite and xenotime formed. Over the whole temperature range, a complete solid solution series with monazite structure along the Ce-Gd(PO4) binary exists. Similiarly, solid solutions with xenotime structure occur from pure YPO4 to Y0.04Gd0.96PO4- The temperature-dependent miscibility gap of the Ce-Y(PO4) binary is traced into the ternary up to the GdPO4 apex. At bulk compositions of (Ce0.45Gd0.10Y0.45)PO4 and low temperatures, Gd is preferentially incorporated in xenotime. The experimentally determined distribution coefficient DGd = XGdmonazite/XGdxenotime increases linearly from 0.25 at 500°C to 1.0 at 1000°C. Empirically derived Gd distribution coefficients of natural monazite-xenotime pairs from a metapelite suite are 0.25 in greenschist facies and about 0.5 in lower granulite facies rocks. This is an impressive agreement and it is clear that Gd distributions are appropriate for geothermometry and for testing equilibrium between the two orthophosphates. Simple Gd determinations by EMP may distinguish between equilibrium and disequilibrium distributions and provide a simple tool for the selection of “good” monazite-xenotime pairs for U-Pb and Sm-Nd chronometry. Combined with U-Pb and Sm-Nd age determination, the Gd geothermometer can be used to directly link the T-conditions and timing of growth of the REE-phosphates, and thus for deriving metamorphic P-T-t paths.