They also give us deeper insights into the mechanism of mineral replacement reactions and porosity development.ABSTRACT: Alkali-rich F-Cl-CO2-bearing aqueous fluids are thought to be common ore-forming fluids of carbonatite-associated rare earth element (REE) deposits.Thus interpretation of results from these geochronometers requires a greater understanding of their structure, stability, and kinetic properties than does zircon.
Solubilities of synthetic monazite-(La) and xenotime-(Y) end-members in such fluids were determined at 800 °C and 0.5 GPa in a series of piston-cylinder experiments.
At run conditions, solutions of Na Cl/KCl (1.9 mol/kg) ± CO2 (5% mole), Na Cl/KCl (1.7 mol/kg) ± Na F/KF (0.2 mol/kg) were trapped as synthetic fluid inclusions in quartz, and subsequently analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS).
These results indicate that during the evolution of carbonatitic fluids, enrichment of Na and F can significantly increase solubilities of all REE, whilst enrichment of K may promote the fractionation of HREE from LREE in the presence of F or CO2.
ABSTRACT: Large quantities of monazite from different pegmatite bodies of the Itambe pegmatite district were investigated to assess their suitability as U–Pb and Sm–Nd isotope reference materials for LA-ICP-MS and to track the origin of a piece of the Moacyr monazite (termed here Itambé), a widely used reference material for LA-ICP-MS U–Pb geochronology.
The dominant occurrence of phosphate minerals in crystalline rocks is as accessory phases, most notably apatite, monazite, and xenotime.
Because these minerals tend, to varying degrees, to partition U and Th into their structures they can often contain the majority of those elements in a rock.
Exclusively at 600 °C, monazite alteration takes the form of symplectite development at the reaction front as vermicular intergrowths of vitusite and britholite.
The higher temperature experiments also show substantially more reactivity by fluorapatite, which is partially pseudomorphically altered into britholite.
In fact, the silicate zircon comes reasonably close to meeting these criteria.
Zircon has a broad stability field, is refractory under a wide variety of geologic environments (e.g., weathering, sedimentary transport, anatexis, and metamorphism), and can be highly retentive of daughter products in the U-Th-Pb decay system.
The fluorapatite replacement is enhanced by monazite alteration through a self-perpetuating, positive feedback mechanism between these two reactions, which enhance the REE mobility in alkali-bearing fluids and further drives bulk re-equilibration.