Experimental determination of REE distributions between monazite and xenotime: potential for temperature-calibrated geochronology

Solid solutions of the REE + Y orthophosphates monazite and xenotime were synthesized hydrothermally between 300 degrees C and 800 degrees 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 D-REE = X-REE(monazite)/X-REE(xenotime) were derived for the entire REE + Y suite. Values of D-REE depend strongly on ionic radii of the REEs and on temperature. The log D-REE vs, ionic radius curve shows a nearly linear decrease from La to Lu and comprises almost five orders of magnitude from D-La approximate to 10(2.5) to D-Lu = 10(-2). Between 450 degrees and 800 degrees C, most D-REE values vary continuously by a factor of about 2 to 3. D-Nd decreases from 48 to 16 and D-Sm from 9 to 5; D-Tb increases from about 0.2 to about 0.4, D-Dy from 0.1 to 0.2 and D-Y from 0.06 to 0.12 along this temperature range. Experimentally determined values of D-REE are in close agreement with those derived from natural monazite-xenotime pairs equilibrated between 400 degrees and 700 degrees 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 K-D = ( (XSmXNdxenotime)-X-monazite/( (XSmXNdmonazite)-X-xenotime) = 0.2 to 0.3 from 600 degrees to 800 degrees C, and Sm-Nd chronometry should provide reasonable two-mineral isochrons for such monazite-xenotime pairs. (C) 1998 Elsevier Science B.V. All rights reserved.