EXPERIMENTAL DETERMINATION OF REE DISTRIBUTIONS BETWEEN MONAZITE AND XENOTIME: POTENTIAL FOR TEMPERATURE-CALIBRATED GEOCHRONOLOGY

Abstract

Solid solutions of the REE+Y orthophosphates monazite and xenotime were synthesized hydrothermally between 300°C and 800°C at 2 and 5 kbar using REE+Y-oxides and H3PO4 as starting materials. The REE bulk composition was similar to those found in metapelitic rocks. All experiments produced two solid phases, monoclinic monazite and tetragonal xenotime, in volumetric ratios similar to relative abundances in metapelites. REE concentrations of coexisting monazite-xenotime pairs were determined by EMP analysis, and REE distribution coefficients DREE=XREEm°nazite/XREExen°time were derived for the entire REE+Y suite. Values of DREE depend strongly on ionic radii of the REEs and on temperature. The log DREE vs. ionic radius curve shows a nearly linear decrease from La to Lu and comprises almost five orders of magnitude from DLa~102.5 to DLu~10-2. Between 450° and 800°C, most DREE values vary continuously by a factor of about 2 to 3. DNd decreases from 48 to 16 and DSm from 9 to 5; DTb increases from about 0.2 to about 0.4, DDy from 0.1 to 0.2 and DY from 0.06 to 0.12 along this temperature range. Experimentally determined values of DREE are in close agreement with those derived from natural monazite-xenotime pairs equilibrated between 400° and 700°C. REE partitioning between monazite and xenotime is appropriate for geothermometry. The most useful application is that equilibrium and disequilibrium partitioning of REE in metamorphic rocks are easily recognized. In the case of equilibrium distributions, U-Pb chronometry of single monazites or xenotimes provides a direct link to their growth conditions and information on temperature-time paths. Equilibrium distributions of Sm and Nd, expressed in molar fractions, result in KD=(XSmm°naziteXNdxen°time/(XSmxen°timeXNd m°nazite)=0.2 to 0.3 from 600° to 800°C, and Sm-Nd chronometry should provide reasonable two-mineral isochrons for such monazite-xenotime pairs.