REE-BEARING CARBONATE SYSTEMS AT HIGH PRESSURE:AN EXPERIMENTAL STUDY

Abstract REE bearing carbonate systems at high pressure: an experimental study Carbonatites are important sources of Rare Earth Elements (REE). REE mainly reside in Ca-bearing phases carbonates, apatites, Ca-Nb oxides, Ca-silicates and in accessory phases such as monazite, burbankite, bastnäsite. At liquid phases, carbonate melts display remarkable physical properties. In particular, carbon-rich and silica-poor melts, i.e. transitional melts, are efficient metasomatizing agents of carbon between the mantle and the crust. This study focuses on two main issues: I) the effect of REE such as La and Y on the structure and melting behavior of transitional carbonate-silicate melts, which has been investigated in simple model with variable CaCO3:SiO2 ratio at 1 GPa; and II) the stability and mineral physics of REE-bearing carbonates at high pressure, in particular of La-burbankite. Few experimental works have explored phase relations, structure and the role of REE in carbonate-rich melts. The typical unquenchable nature of carbon-rich melts makes for a difficult, determination of the structure of carbon-rich glasses. Wyllie and Jones (1986) synthesized a REE-bearing carbonatite glass in a series of experiments performed in the system CaO–CaF2–BaSO4–CO2–H2O–La(OH)3 using a composition similar to carbonatites ore deposits in Mountain Pass (California). At 0.1 GPa the investigated carbonatite composition starts to melt at temperatures as low as 550°C. As today, there are no data available regarding melting behavior, structural properties of melts, and the stability of REE-bearing phases in model system of hydrous carbon-rich low silica. Furthermore, although in alkali free systems a complete miscibility between silicate and carbonate liquids is expected, the compositional threshold at which carbonate-silicate liquids might form a glass, i.e. are quenchable, is still scarcely explored in simple model systems. In this study, I investigated i) how viable is the quenching of a carbon-rich melt as a function of the bulk SiO2:CaCO3 ratio, ii) the structure of these glasses, iii) and the role of REE in the molecular structure(La, Y). I also determined the CO2 solubility in transitional melts by micro-Raman spectroscopy. Single stage and end-loaded piston cylinder experiments have been performed at 1 GPa in two model systems: CaO–SiO2–La2O3–H2O–CO2 in the range 700–1250°C, and CaO–SiO2–Y2O3–H2O–CO2 in the range 1200–1250°C. The starting materials were prepared as a powder mixture of La2(CO3)3 or La2O3 or Y2O3 and amorphous SiO2 and CaCO3 with approximately 5-10 wt.% of H2O. Different bulk compositions with different SiO2:CaCO3 ratio have been considered. Run products were characterized by backscattered electrons images (BSE), X-ray diffractometry, micro-Raman spectroscopy, nuclear magnetic resonance (NMR) of selected experiments, and chemically analyzed by electron microprobe. At subsolidus conditions (T < 1000°C), all bulk compositions in CaO–SiO2–La2O3–H2O–CO2 system contain calcite and quartz coexisting with a Ca-La silicate phase with an apatite-type structure of general formula between La3Ca2(Si3O12)OH and La4Ca(Si3O12)O. Liquidus conditions have been observed in runs on bulk compositions with SiO2:CaCO3 = 0.28 1.4 at T >1150°C for both investigated systems. Homogeneous quenched glasses have been retrieved for composition with up to SiO2:CaCO3=0.28 ratio, whereas at lower ratio (0.12) dendritic textures are visible. Deconvolution of Raman spectra of glasses reveal a CO2 content up to 20.40% in the CaO–SiO2–La2O3–CO2–H2O system and up to 10.80% in the CaO–SiO2–Y2O3–CO2–H2O system classifying the melts as carbonate-silicate transitional melt. While studiyng the behavior of Ca-rich carbonatitic hydrous glasses with REE, we also investigated the influence on the REE distribution of an alkaline carbonate, burbankite [(Na,Ca)3(Sr,Ba,Ce,REE)3(CO3)5], at upper mantle conditions. Recently, the alkaline-carbonate system has been observed at High Pressure and High Temperature (HP-HT) by Shatzky et al., (2016); they found a new class of Ca-rich alkaline-alkaline earth carbonates and burbankite with the latter being abundantely presents in carbonatites that constitutes important ore concentrations of strategic metals, including Nb and REE elements on Earth’s surface. A La rich burbankite [Na3Ca2La(CO3)5] was synthesized at 5 GPa and 1000°C, to scope out the possibility that REE can enter the structure of this carbonate also at upper mantle conditions. Furthermore, the elastic properties of the synthesized La-burbankite have been determined by in-situ HP and HT single crystal X-ray diffraction measurements at synchrotron facilities using a diamond-anvil cell for HP experiments and a quartz-glass capillary for the HT experiments. The determination of these thermoelastic parameters allowed to calculate the possible density of this phase at upper mantle conditions, that is ca. 3.2 g/cm3 at 5.5–6.0 GPa and 900–1000°C. Results suggest that potentially the La-burbankite could fractionate at HP and HT from a carbonatitic melt, because the latter has a lower density (ca. 2.1-3.1 g/cm3) and may constitute a REEs reservoir at upper mantle conditions.