Monazite saturation in carbonatite melts at crustal and upper mantle conditions
Year: 2025
Type: article
Abstract: Monazite (XPO 4 where X = light rare earth element) is one of the most important ore minerals for rare earth elements (REE) mined from carbonatites . This study explores the influence of pressure, temperature and melt composition on the solubility of monazite in carbonate melt using piston cylinder experiments. We aim to understand how such a highly soluble phosphate mineral may sometimes crystallise from carbonatite magma and form economic deposits of REE. Our findings reveal that the solubility of monazite in a sodic-dolomitic carbonate melt is high over the entire pressure (1.0–2.0 GPa), temperature (1000-1450˚C), and compositional space (Na 2 O-CaO-MgO-SiO 2 -F) of our study. It increases from ∼ 26 to ∼ 61 wt% CePO 4 equivalent over the temperature range of 1000–1450 °C at 2 GPa. However, the solubility product constant (lnK sp ) decreases by approximately ∼ 23 % and 21 % equivalent with increasing melt SiO 2 (from 0 to ∼ 18 wt% melt SiO 2 ) at 1275 °C and fluoride (from 0 to ∼ 5 wt% melt F - ) concentrations at 1100 °C respectively. Variations in melt Ca# (atomic Ca/(Mg + Ca) = 0.2–1.0) and pressure (1.0–2.0 GPa) have negligible impacts on monazite solubility. We use the experimental data to formulate an empirical model relating the solubility product constant (K sp ) of monazite in carbonate melt to temperature and melt compositional parameters: lnK sp = ln R E E 2 O 3 m e l t . P 2 O 5 m e l t = 1.31 0.50 - 8679 733 T - 3.35 ( 0.34 ) × SiO 2 - 1.68 ( 0.34 ) × F where [] represents the mole fractions of oxides in the melt of the rare earth elements, phosphorus and silicon, and fluoride, T is in Kelvin, and the parentheses indicate standard errors. R E E 2 O 3 m e l t represents the summation of the melt REE concentrations for those REE which are compatible in the experimentally synthesized monazite (La-Sm and Gd). These experimental results suggest that the formation of monazite requires relatively high concentrations of P 2 O 5 and Ce 2 O 3 under crustal pressure and temperature conditions, making it challenging to crystallise directly from carbonatite melts. However, monazite may instead form through reactions involving evolved carbonatite melts, potentially interacting with siliceous wall rocks or replacing pre-existing phosphates like apatite , as has been observed in natural carbonatites.
Source: Geochimica et Cosmochimica Acta
Institutions: Australian National University, Rice University
Language: English
Subfield: Geophysics
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: hybrid
Estimated APC paid: $4,090
OpenAlex ID: W4407133184