Experimental constraints on the relative stabilities of the two systems monazite-(Ce) – allanite-(Ce) – fluorapatite and xenotime-(Y) – (Y,HREE)-rich epidote – (Y,HREE)-rich fluorapatite, in high Ca and Na-Ca environments under P-T conditions of 200–1000 MPa and 450–750 °C

Abstract

The relative stabilities of phases within the two systems monazite-(Ce)fluorapatiteallanite-(Ce) and xenotime-(Y)-(Y,HREE)-rich fluorapatite-(Y,HREE)-rich epidote have been tested experimentally as a function of pressure and temperature in systems roughly replicating granitic to pelitic composition with high and moderate bulk CaO/Na 2 O ratios over a wide range of P-T conditions from 200 to 1000 MPa and 450 to 750°C via four sets of experiments. These included (1) monazite-(Ce), labradorite, sanidine, biotite, muscovite, SiO 2 , CaF 2 , and 2 M Ca(OH) 2 ; (2) monazite-(Ce), albite, sanidine, biotite, muscovite, SiO 2 , CaF 2 , Na 2 Si 2 O 5 , and H 2 O; (3) xenotime-(Y), labradorite, sanidine, biotite, muscovite, garnet, SiO 2 , CaF 2 , and 2 M Ca(OH) 2 ; and (4) xenotime-(Y), albite, sanidine, biotite, muscovite, garnet, SiO 2 , CaF 2 , Na 2 Si 2 O 5 , and H 2 O. Monazite-(Ce) breakdown was documented in experimental sets (1) and (2). In experimental set (1), the Ca high activity (estimated bulk CaO/Na 2 O ratio of 13.3) promoted the formation of REE-rich epidote, allanite-(Ce), REE-rich fluorapatite, and fluorcalciobritholite at the expense of monazite-(Ce). In contrast, a bulk CaO/ Na 2 O ratio of~1.0 in runs in set (2) prevented the formation of REE-rich epidote and allanite-(Ce). The reacted monazite-(Ce) was partially replaced by REE-rich fluorapatite-fluorcalciobritholite in all runs, REE-rich steacyite in experiments at 450°C, 200-1000 MPa, and 550°C, 200-600 MPa, and minor cheralite in runs at 650-750°C, 200-1000 MPa. The experimental results support previous natural observations and thermodynamic modeling of phase equilibria, which demonstrate that an increased CaO bulk content expands the stability field of allanite-(Ce) relative to monazite-(Ce) at higher temperatures indicating that the relative stabilities of monazite-(Ce) and allanite-(Ce) depend on the bulk CaO/ Na 2 O ratio. The experiments also provide new insights into the re-equilibration of monazite-(Ce) via fluid-aided coupled dissolution-reprecipitation, which affects the Th-U-Pb system in runs at 450°C, 200-1000 MPa, and 550°C, 200-600 MPa. A lack of compositional alteration in the Th, U, and Pb in monazite-(Ce) at 550°C, 800-1000 MPa, and in experiments at 650-750°C, 200-1000 MPa indicates the limited influence of fluid-mediated alteration on volume diffusion under high P-T conditions. Experimental sets (3) and (4) resulted in xenotime-(Y) breakdown and partial replacement by (Y,REE)rich fluorapatite to Y-rich fluorcalciobritholite. Additionally, (Y,HREE)-rich epidote formed at the expense of xenotime-(Y) in three runs with 2 M Ca(OH) 2 fluid, at 550°C, 800 MPa; 650°C, 800 MPa; and 650°C, 1000 MPa similar to the experiments involving monazite-(Ce). These results confirm that replacement of xenotime-(Y) by (Y,HREE)-Editorial handling: L. Nasdala

Peak to post-peak thermal history of the Saglek Block of Labrador: A multiphase and multi-instrumental approach to geochronology

“…Harlov, 2011Harlov, , 2015Harlov et al, 2002Harlov et al, , 2005Harlov and Förster, 2003). Experimental results Budzyń et al, 2017) are consistent with natural observations that fluid activity is just as important as changes in P-T conditions for monazite breakdown reactions (Finger et al, 1998(Finger et al, , 2016Janots et al, 2008;Ondrejka et al, 2012Ondrejka et al, , 2016. The formation of monazite micro-inclusions in apatite may not be coeval with the breakdown of granoblastic monazite to apatite-allanite; the former may have been produced at different temperatures by fluids that were also responsible for monazite resetting at ca.…”

High-spatial resolution dating of monazite and zircon reveals the timing of subduction–exhumation of the Vaimok Lens in the Seve Nappe Complex (Scandinavian Caledonides)

“…Finger and Krenn 2007;Holder et al 2015;Kohn et al 2005;Williams et al 2011) and laboratory experiments (e.g. Budzyń et al 2011Budzyń et al , 2017Harlov et al 2011;Seydoux-Guillaume et al 2012;Shazia et al 2015). No relationship between Group 3 and the other monazite populations are observed.…”

REE Redistribution Textures in Altered Fluorapatite: Symplectites, Veins, and Phosphate-Silicate-Carbonate Assemblages from the Nolans Bore P-REE-Th Deposit, Northern Territory, Australia

“…The CaO contents of the britholite increased from~8.3 tõ 13.8% with temperature, indicating a higher fluo-rapatite component in britholite. Similarly, Budzyń et al (2017) reported increasing REE contents from~30 to 40% in fluorapatite with increasing temperature from 450 to 750 8C. Anenburg & Mavrogenes (2018) showed that an appreciable solid solution between fluorapatite and britholite is only observed at 700 8C and above, and two immiscible phases occur at lower temperatures.…”