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Rare earth elements (REEs) play a crucial role in technologies required for the transition to net zero, and their
occurrence is strongly linked to carbonatites. However, the mechanisms leading to REE enrichment in carbonatites
remain poorly understood. Economic REE mineralization is typically associated with phosphate minerals such as
monazite or xenotime. This research aims to understand the conditions leading to the crystallization of monazite
from carbonatite magma by examining the solubility of monazite in carbonate melts, as a function of pressure,
temperature, and melt composition. The study is structured into three main components:
1. Experimental determination of monazite solubility in carbonate melt.
2. Construction of a regression model to comprehend and forecast monazite solubility based on temperature,
pressure, and melt compositions.
3. Investigation of monazite crystallization in natural carbonatite through fractional crystallization.
Piston-cylinder experiments were conducted at upper mantle pressures of 1 and 2 GPa, and temperatures from
1000 to 1450°C. A synthetic sintered oxide mix, with the composition of natural monazite, and a sodic dolomitic
composition representing mantle-derived carbonatite melt were used. The experiments, conducted at a 1:1 ratio of
monazite to melt, resulted in monazite crystallization, confirming melt saturation in monazite. EPMA techniques
at the Centre of Advanced Microscopy were employed to analyze melt and monazite compositions, determining
percentages of phosphorous and REE oxides.
The effect of composition on monazite solubility was explored by systematically varying Ca# (from 0.2 to 1.0) and
introducing SiO2 (up to 15.3 wt%) and CaF2 (up to 10 wt%). This data was then used to develop a regression model,
providing insights into monazite solubility concerning temperature, pressure, and melt composition. The model
aims to predict conditions conducive to monazite formation in natural carbonatites. In the final phase of the project,
a primitive Na-calcio-carbonatite melt composition was used, derived during its separation from the associated
silicate liquid along with some rare earth oxides. The primary objective of this phase is to comprehend the processes
governing monazite crystallization within a natural carbonatite through fractionation, building upon insights gained
in earlier phases of the study. |