Apatite is a common accessory phase in igneous and metamorphic rocks. Its stability in magmatic-hydrothermal and hydrothermal systems is known to be a key control on the mobility of rare earth elements (REE). To better constrain how apatite is altered during fluid-rock interaction at comparably low temperatures, batch-type apatite dissolution experiments were conducted at 150 and 250 °C at saturated water vapor pressure in acidic to mildly acidic (pH of 2-4) aqueous fluids having variable salinities (0, 0.5, and 5 wt% NaCl). The study reveals the dominance of apatite dissolution textures with the formation of micron-scale etch pits and dissolution channels developing prominently along the c-axis of the apatite crystals.Backscattered electron imaging shows an increase in apatite dissolution with increased temperature and when reacting the crystals with more acidic and higher salinity starting fluids. This study also demonstrates an increase in dissolved REE in the experimental fluids corroborating with the observed apatite dissolution behavior. Backscattered electron imaging of secondary minerals formed during apatite dissolution and scanning electron microscopy-based energy dispersive spectrometer peaks for Ca, P, and REE support the formation of monazite-(Ce) and minor secondary apatite as deduced from fluid chemistry (i.e., dissolved P and REE concentrations). The studied apatite reaction textures and chemistry of the reacted fluids both indicate that the mobility of REE is controlled by the dissolution of apatite coupled with precipitation of monazite-(Ce), which are enhanced by the addition of NaCl in the starting fluids.This coupled process can be traced by comparing the REE to P ratios in the reacted fluids with the stoichiometry of the unreacted apatite crystals. Apatite metasomatized at temperatures <300 °C is therefore controlled by dissolution rather than dissolution-reprecipitation reactions commonly observed in previous experiments conducted above 300 °C. Further, this study This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.
The apatite crystal structure has been demonstrated to accommodate nearly half of the elements on the periodic table, including many of the harmful byproducts of nuclear fuel generation such as uranium (U) and thorium (Th). Therefore apatite has been proposed as a solid nuclear waste reservoir. This concept is largely predicated on the observation of naturally occurring apatite bearing elevated levels of U and Th. While numerous works suggest apatite can effectively uptake and retain U, Th, and other radionuclides over short timescales, it is still unclear if the apatite structure is stable while hosting high concentrations of these radionuclides over geologically relevant timescales. The current study has analyzed three suites of fluorapatites from the Mont Saint-Hilaire pluton, Quebec, Canada, via wavelength dispersive spectroscopy (WDS) and single crystal X-ray diffraction (SCXRD). WDS analyses revealed high Th contents with values reaching into the multiple weight percent, to date the highest recorded Th contents in natural fluorapatite. SCXRD results displayed a severely disordered structure, indicated by diffraction spots being streaked, displaying satellite spots, and a noted drop in diffraction at high two-theta values. Due to the significant disorder observed in the SCXRD data, and the high Th concentrations observed via WDS analyses, these fluorapatites were interpreted as being partially metamict and annealing experiments were performed in an attempt to heal the damaged structure. The results of these experiments, refinement of the crystal structure, and full results from WDS analyses will be presented.
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