Phase relations in hydrous REE-bearing carbonatite at 1 GPa, 700-1000°C

Carbonatites are important sources of Rare Earth Elements (REEs). These rocks are the result of liquid immiscibility or fractional crystallization at low pressure; they are often extremely enriched in alkali and may contain up to 15.000 ppm of La (Cullers and Graf, 1984). REEs mainly reside in Ca-bearing phases (carbonates, apatites, Ca-Nb oxides, Ca-silicates) and in accessory phases such as monazite, bastnaesite (La, Nd, Ce) CO3F and hydroxyl-bastnaesite (La, Nd, Ce) CO3(OH, F). This study focuses on processes, precursor to the late differentiation, that lead to the formation of hydrous carbonatite melts at high pressure (>1 GPa). In particular we will investigate the stability of REE-bearing carbonates and silicates at near solidus conditions and the distribution of REEs among accessory phases and melt. Moreover, in alkali free systems, the complete miscibility between silicate and carbonate liquids is expected. However, it is widely known that, while silica glasses are experimentally recoverable, calcite is not quenchable. The compositional threshold at which carbonate-silicate liquids might form a glass is still unexplored. We performed single stage and end-loaded piston cylinder experiments in the model system CaO-SiO2-La2O3-H2O-CO2 in the range 700-1000°C, at 1 GPa. Starting materials were prepared as a powder mixture of La2(CO3)3, amorphous SiO2 and CaCO3. Three different bulk compositions at fixed La2(CO3)3 = 10 wt.% with SiO2:CaCO3 = 0.7, 1 and 1.4 have been considered. Gold and platinum capsules of 3 mm of diameter were loaded with starting mixtures, added with approximately 5-10 wt.% of H2O, and sealed while freezing the capsule in order to avoid the loss of volatile components. Run products, carefully prepared to avoid any contact with water and polished with diamond paste, are characterized by BSE images, X-ray diffractometry, Raman spectroscopy and chemically analyzed by electron microprobe. At subsolidus conditions all bulk compositions contain calcite and quartz coexisting with a Ca-Lanthanum silicate with up to 66 wt% of La2O3, 9.8 wt.% CaO and 23 .5 wt% SiO2. Preliminary X-ray diffraction data suggest that this phase has an apatite-type structure of general formula Ca2La8(SiO4)6O2. An additional Ca, La-bearing phase of few µm has been observed in BSE images. Although further investigation is required, microprobe analysis indicate a likely carbonate composition. Cullers R.L. & Graf J.L. 1984 Rare earth elements in igneous rocks of the continental crust: intermediate and silicic rocks–ore petrogenesis. Volume 2: Developments in geochemistry Elsevier