Temperature dependence of yttrium partitioning between garnet and xenotime: an experimental study

Yttrium is a notable trace element particularly compatible with garnet. Lanzirotti (1995) provided evidences that, among major metapelitic minerals, yttrium preferentially partitions into garnet and that the mode of major metapelite phases besides garnet have little effect on its fractionation. On the contrary trace elements are extremely sensitive to changes in accessory mineral assemblage (e.g. Ganguly, 2010). Xenotime (YPO4) is a common accessory mineral in metapelites and arguments for garnet growth in equilibrium with xenotime are presented in several papers (e.g. Martin, 2009). Pyle & Spear (1999) described a relevant temperature control on the solubility of yttrium in garnet in xenotime-bearing metapelites from New England (USA). On the basis of a strong negative correlation between Y concentration and temperature they derived an empirical calibration to be used as geothermometer. However, no experimental studies do exist to date on the temperature dependence of Y partitioning between garnet and xenotime. In order to unravel this relation, high pressure (up to 2.0 GPa) xenotime – saturated synthesis of garnet have been performed in an end-loaded piston cilynder. The simple model system MgO-FeO-Al2O3-SiO2 has been investigated at temperature between 800 and 1000°C running compositions falling along the join almandine-pyrope + 5 wt% YPO4. Gels have been prepared as starting materials using tetraethylorthosilicate (TEOS) as silica source, pure Mg-, Al-, Ca-, Y-nitric solutions, ferric benzoate and ammonium dihydrogen phosphate (NH4H2PO4) digested in deionised water. Gels were fired in a gas-mixing furnace at fO2 conditions approaching the IW (iron-wustite) buffer at 1 atm for 3 hours. The powder was tightly packed into a gold capsule with an internal graphite sleeve to keep the oxygen fugacity low. Run products were preliminary identified by X-ray powder diffraction, carefully inspected on back-scattered electron images and by X ray element maps, and analysed by electron microprobe and particle-induced X-ray emission (micro-PIXE). The use of the proton microprobe stems from its higher spatial resolution and lower X-ray background with respect to electron microprobe. This allows to measure trace element concentrations down to levels of ~ 1 ppm on a 1 m beam spot (Fraser, 1990). Preliminary results will be discussed. References Biggar, G.M. & O’Hara, M.J. (1969). A comparison of gel and glass starting materials for phase equilibrium studies. Mineralogical Magazine 37, 198-205. Fraser, D.G. (1990). Applications of the high-resolution scanning proton microprobe in the Earth Sience: an overview. Chemical Geology 83, 27-37. Ganguly, J (2010). Cation diffusion kinetics in aluminosilicate garnets and geological applications. Reviews in Mineralogy and Geochemistry 72, 559-601. Heald, E.F., Reeher, J.R. & Herrington, D.R. (1969). Gel preparation of starting materials in iron-containing silicate systems. American Mineralogist 54, 317-320. Lanzirotti, A. (1995). Yttrium zoning in metamorphic garnet. Geochimica et Cosmochimica Acta 59, 4105-4110. Martin, A.J. (2009). Sub-millimeter Heterogeneity of Yttrium and Chromium during Growth of Semi-pelitic Garnet Journal of Petrology 50, 1713-1727 Pyle, J.M., & Spear, F.S. (1999). An empirical garnet (YAG)-xenotime thermometer. Contributions to Mineralogy and Petrology 138, 51-58.