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

The relative stabilities of phases within the two systems monazite-(Ce) – fluorapatite – allanite-(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)-rich epidote is induced by a high Ca bulk content with a high CaO/Na 2 O ratio. These experiments demonstrate also that the relative stabilities of xenotime-(Y) and (Y,HREE)-rich epidote are strongly controlled by pressure.