Phase relations in hydrous ree-bearing carbonatite AT 1 GPA, 700-1250°C

REE are known to be concentrated in carbonatites. Due to the difficulties in obtaining quenched glasses, the structure and the role of REE in carbonate-rich melts is currently poorly known (Jones and Wyllie, 1983). The aim of this study is to experimentally investigate the distribution of La in low-silica hydrous carbonate system at lithospheric conditions in order to define the role of La in the melt structure, establish phase relations and the distribution of REE (La). Additionally, experiments will allow to study the glass transition as a function of SiO2:CaCO3 ratio. Single stage and end-loaded piston cylinder experiments have been performed at 1 GPa in the model system CaO-SiO2-La2O3-H2O-CO2 at temperature between 700-1250°C. Starting materials were prepared as powder mixtures of La2(CO3)3 or La2O3, amorphous SiO2 and CaCO3. Different bulk compositions at fixed La2O3 ~ 10 wt.% with SiO2:CaCO3 = 0.12, 0.2, 0.28, 0.5, 0.7, 1, 1.4 have been considered. Au and AuPd capsules were loaded with starting mixtures, adding ~10-15 wt.% of H2O, and sealed while freezing in order to avoid the loss of volatile component. Run products, carefully prepared to avoid any contact with water and polished with diamond paste, have been characterized by BSE images, X-ray diffractometry, Raman spectroscopy and chemically analyzed by electron microprobe. At subsolidus conditions 700-900°C all bulk compositions contain calcite and quartz coexisting with a Ca,La-silicate (~5-40 μm) with an apatite structure and pseudo-hexagonal prismatic shape. At 1000°C wollastonite forms and coexists with Ca,La-silicate and calcite or quartz depending on the starting bulk composition. The presence of silica spherules testifies to the coexistence of a high pressure fluid. Melting has been observed in bulk compositions with SiO2:CaCO3 = 1.4, 0.5, 0.28 at 1250°C, 1150°C and 1200°C respectively. Run products quenched to glasses and show evidence of fluid present conditions. In particular, fluid inclusions under the surface of the glass have been identified as molecular H2O by Raman spectroscopy. The bulk composition with the lowest SiO2:CaCO3 ratio (0.14), run at 1200°C, does not quench to glass, but show calcite with typical dendritic textures. Glasses compositions mostly overlap the starting bulks, in agreement with a high degree of melting. They are rather homogeneous and microprobe analyses are in agreement with a significant amount of dissolved volatiles that is proportional to the initial CaCO3 wt.% of the starting material. Raman spectroscopy on glasses suggests a low degree of polymerization, and significant solubility of H2O and CO2. The latter is present mainly as carbonate ion (CO3 2-) with a peak at 1080 cm-1 although molecular CO2 is also observed. H2O Raman signal presents the typical asymmetric band between 2800-3750 cm-1. A peak at ~ 850 cm-1 is preliminary assigned to LaQ0 species, in agreement with its role as structure modifier in low viscosity carbonate rich melts. References: Jones AP, Wyllie PJ (1983) Econ Geol., 78:1721-1723 Melt physics