Geological Quarterly, 2015, 59 (2): 316-324 DOI: http://dx.doi.org/10.7306/gq.1223 The stability of xenotime in high Ca and Ca-Na systems, under experimental conditions of 250-350°C and 200-400 MPa: the implications for fluid-mediated low-temperature processes in granitic rocks Bartosz BUDZYŃ1' 2' * and Gabriela A. KOZUB-BUDZYŃ3 1 Polish Academy of Sciences, Institute of Geological Sciences, Research Centre in Kraków, Senacka 1,31-002 Kraków, Poland 2 Jagiellonian University, Institute of Geological Sciences, Oleandry 2a, 30-063 Kraków, Poland 3 AGH University of Science and Technology, Faculty of Geology, Geophysics and Environmental Protection, al. A. Mickie­ wicza 30, 30-059 Kraków, Po l and Budzyń, B., Kozub-Budzyń, G. A., 2015. The stability of xenotime in high Ca and Ca-Na systems, under experimental condi­ tions of 250-350°C and 200-400 MPa: the implications forfluid-mediated low-temperature processes in granitic rocks. Geo­ logical Quarterly, 59 (2): 316-324, doi: 10.7306/gq.1223 The stability of xenotime was tested by experiments in the presence of a silicate mineral assemblage and two different fluids, 2M Ca(OH)2 or Na2Si2O5 + H2O, under P-T conditions of 200-400 MPa and 250-350°C. The xenotime was stable in runs with 2M Ca(OH)2, replicating the low-temperature metasomatic alterations of granitic rocks, except in experiment at 350°C and 400 MPa, where some (Y,REE)-rich fluorapatite formed. Experi ments with Na2Si2O5 + H2O resulted in significant xenotime al teration and parti al replacement by an unknown (Y,HREE)-rich sili cate, and in the formati on of minor amounts of (Y,REE)-rich fluorapatite. The latter indicate preferential partitioning of Y and REE into silicates over phosphates during low-temperature, metasomatic processes in a high Na-Ca system, similar to peralkaline granitic rocks. Key words: xenotime, fluorapatite, yttrium silicate, rare earth elements, experimental petrology. INTRODUCTION Y-rich apatite and Y-rich epidote recognized in metamorphosed granitic rocks (Broska et al., 2005) or by fluorapatite and hingganite-(Y), documented in Skoddefjellet pegmatite from Xenotime, (Y,HREE)PO4, is an accessory mineral of gra­ Svalbard (Majka et al., 2011). The P-T conditions of xenotime nitic rocks, pegmatites, low- to high-grade metamorphic rocks, alterations are not tightly constrained, and experimental data and migmatites (Forster, 1998). Xeno-ime is also present as are necessary to fully understand the stabil i ty of xenotime as a detrital mineral in sedimentary rocks or as an authigenic phase, function of pressure-temperature conditions, accompanying commonly formed on zircon in siliciclastic rocks (Fletcher et al., mineral assemblage, and fluid composition. 2000; Rasmussen et al., 2004; Rasmussen, 2005). Because of Previous experimental works on xenotime focused on rela­ its U and Th con-ents, xeno-ime has applica-ions in iso-opic tively simple systems. The stabil ity of xenotime in the presence U-Pb dating using mass spectrometry (Rasmussen etal., 2004; of common metamorphic and igneous fluids (H2 O, NaCl and Rasmussen, 2005) and in chemical U-Th-total Pb dating using KCl brines, CaF2 + H2O, 1M and 2M HCl, 1M and 2M H2SO4, electron microprobe (Cocherie and Legendre, 2007; Hethering- 1M NaOH, and Na2Si2O5 + H2O) was tested at 500 MPa and ton et al., 2008; Suzuki and Kato, 2008). The application of 600°C, and 1000 MPa and 900°C, documenting the dissolution microanalytical methods, such as sensitive high-resolution ion and etchi ng of xenotime crystal faces, the formation of porous microprobe (SHRIMP) or elec-ron microprobe, to date xeno­ textures or the growth of small xenotime grains in some experi - time provides age data in textural context to constrain the age of ments - but no internal compositional alterations (Hetherington particular igneous, metamorphic or diagenetic processes. Al­ et al., 2010). The experiments at the same conditions of though xenotime is a relatively stable mineral, fluid-mediated al­ 500 MPa and 600°C, and 1000 MPa and 900°C, with start i ng teration may lead to xenotime breakdown and replacement by compositions of xenotime + SiO 2 + Al2O3 + ThSiO4 + Na2Si2 O5 + H2O, resulted in compositional alteration of xenotime and en­ richment in ThSiO4 along rims (Harlov and Wirth, 2012), par­ tially replicating the Th enrichment of xenotime in granitic pegmatites from the Hidra anorthosite, Norway (Hetherington * Corresponding author, e-mail: [email protected] and Harlov, 2008). Xenotime has also been tested for its solu­ Received: December 3, 2014; accepted: January 8, 2015; first bility in H2O and H2 O-NaCl fluids at 1 GPa and 800°C, showi ng published online: February 25, 2015 higher solubility of YPO 4 than CePO4 in pure H2 O to X NaCl = 0.27 The stability of xenotime in high Ca and Ca-Na systems, under experimental conditions of 250-350°C and 200-400 MPa: 317 and lower sol u bil ity with in creas ing NaCl con cen tra tion The experiments were conducted at the Deutsche (Tropper et al., 2Q11). The experiments replicating systems of GeoForschungsZentrum (Potsdam, Germany) usi ng a stan­ natural rocks, involvi ng xenotime + albite + K-feldspar + biotite dard cold-seal, 6 mm bore, René metal au toclaves with H2O + muscovite + SiO2 + CaF2 with a variety of fluids, 2M KOH, 2M as the pressure medium. Four gently flattened capsules, two NaOH, 2M Ca(OH)2 or Na2Si2O5 + H2O, ran under conditions of for xenot ime experi ments with 2M Ca(OH)2 and Na2Si2O5 + 45Qoc and 590 MPa, decreasing to 54Q MPa over 16 days, re­ H2O fluids, and two for “twin” experiments on monazite sulted in xenotime alteration in all runs, with the formation of (Budzyń et al., 2013, 2015), were placed in each autoclave. Y-rich britholite and fluorapatite (Budzyń and Harlov, 2Q11). Three sets of experiments were run under P-T conditions and There is a lack of pubi ished experi menial works regard i ng the with a duration of: (1) 2QQ MPa, 25QoC, 4Q days; (2) 2QQ MPa, stability of xenotime in the presence of silicates and flu i ds repli­ 35QoC, 40 days; and (3) 4QQ MPa, 35QoC, 2Q days (Table 1). cating conditions of low-temperature metamorphism or hydro­ Durations of the experiments were chosen based on previous thermal, post-magmatic alterations in granitic rocks. experiments on the stabilities of monazite and xenotime This study experimentally explores the stability of xenotime (Hetheri ngton et al., 2Q1Q; Budzyń and Harlov, 2Q11; Budzyń under conditions of 200-400 MPa and 25Q-35QoC, in the pres­ et al., 2Q11; Harlov et al., 2Q11). Pressures and temperatures ence of silicate minerals assemblages and fluid. Experiments were stable during the runs. At the end of the experiments, the with 2M Ca(OH)2 al ka line fluid rep li cate con di tions of the autoclaves were cooled usi ng compressed air, reachi ng tem- low-temperature metamorphism of granitic rocks. The second peraiures of <1QQoc within <1 min. After the runs, the cap­ set of experi ments, with Na2Si2O5 + H2O alkali fluid, shows Y sules were weighed, opened, and dried in a 1Q5oc oven. The and REE partitioning between phosphates and silicates, during experi menial products were mounted in epoxy and pol ished the low-temperature metamorphic overprint of granites or dur­ for elec tron microprobe analyses. The second port ion was ing fluid-mediated post-magmatic processes in peralkaline gra­ sprinkled on the SEM mount with adhesive carbon tape for nitic rocks. back-scattered electron (BSE) imaging. ANALYTICAL METHODS EXPERIMENTAL AND ANALYTICAL METHODS The primary observations and analyses of the start i ng min­ EXPERIMENTAL METHODS erals and experimental products were performed using a Hitachi S-47QQ field emission scanning elect ron microscope The xenot ime used for the experi ments is a port ion of the (SEM) equipped with an energy-dispersive spectrometer (EDS) crysial from pegmatite from the North-West Frontier Provi nce at the Institute of Geological Sciences, Jagiellonian University (NWFP), Pakistan, also used in previous experimental works (Kraków, Poiand). by Hetherington et al. (2010), Budzyń and Harlov (2Q11), The chemical compositions of xenotime, (Y,REE)-rich Harlov and Wirth (2 Q1 2 ) and Budzyń et al. (2Q14). The other fluorapatite, and unnamed (Y,HREE)-rich sil icate were deter­ natural minerals used include gem-quality albite (Ab1QQ; mined us i ng a Cameca SX 100 electron microprobe equipped Roznava, Slovakia), labradorite (An6QAb27Kfs3; Chihuahua, with four-wavelength-dispersive spectrometers (WDS) at the Mexico), sanidine (Eifel region, Germany), muscovite (pegma­ Department of Special Laboratories, Laboratory of Electron tite, Siedlimowice, SW Poland), biotite (gneiss, Sikkim Microanalysis, Geological Institute of Dionÿz Stur (Bratislava, Himalaya, India), and garnet (Gore Mountain, NY, USA). The Slovak Republic). The xenotime was analyzed under conditions chosen mineral compositions and used weight proportions (Ta­ of 15 kV accelerating voltage, 18Q nA beam current, and 3 pm ble 1) roughly repl icate that of granitic rock. Relat ively high beam size, focused on the grain mount and coated with ca. 25 amounts of xenotime (ca. 5 mg of xenoii me in ca. 34.6 to nm carbon film. The natural and synihetic standards, and the 4Q.Q mg total charge of capsule) were added to guarantee ob­ corresponding spectral lines used for standardization were as servation of xenotime in specimens with experimental products, follows: apatite (P Ka ), PbCO3 (Pb Mß ), ThO2 (Th Ma ), UO2 because the material from each capsule was planned to be split (U Mß ), YPO4 (Y La ), LaPO4 (La La ), CePO4 (Ce La ), beiween 3-4 portions. CaF2 (Suprapure, Merck) was used in PrPO4(Pr Lß ), NdPO4 (Nd La ), SmPO4 (Sm La ), EuPO4 experi ments in excess as a source of Ca and F to increase re­ (Eu Lß ), GdPO4 (Gd La ), TbPO4 (Tb La ), DyPO4 (Dy Lß ), action rates, and to form fluorapatite.