A LREE-, Si-, S-enriched fluorapatite with ~7.7 wt.% LREE2O3, 5.3 wt.% SiO2 and 3.5 wt.% SO3 from the Mushugai-Khuduk REE deposit, southern Mongolia, has been experimentally metasomatized in pure H2O at 900°C and 1000 MPa. Despite the high LREE content, inclusions or rim grains of monazite did not form during fluid-induced metasomatism. Under high-contrast BSE imaging, the reacted fluorapatite presents a mottled appearance with light, moderate and dark areas and hosts large fluid inclusions containing anhydrite as the solid phase. Dark areas show moderate depletion in all the LREE. Measurable HREE such as Gd and Y remain unchanged. In the areas of intermediate brightness, Ce and, to a lesser extent, La are moderately enriched relative to the original fluorapatite. In the light areas, Ce and La display strong enrichment, whereas the other LREE, Gd and Y remain little changed. In the dark areas, depletion in LREE is matched by depletion in Si and Na, satisfying the coupled substitution reactions Si4+ + (LREE)3+ = P5+ + Ca2+ and Na+ + (LREE)3+ = 2Ca2+. In the light areas, enrichment in LREE and P is accompanied by depletion in Si, Na and S, implying operation of two different coupled substitution reactions, namely: 2P5+ = Si4+ + S6+ and 2P5+ + LREE3+ = 2S6+ + Na+. In all areas, strong enrichment in F is accompanied by strong depletion in Cl and OH. The formation of large, fluid and anhydrite-filled inclusions associated with the LREE-enriched light areas is evidence of a negative volume change, combined with additional local dissolution of the fluorapatite. This occurred during the metasomatism of the original Cl- and OH-bearing fluorapatite to a nearly pure end-member fluorapatite. Local dissolution of fluorapatite in the space now occupied by the voids, combined with element remobilization from the body of the fluorapatite as a whole, provided the Ca and S needed for the formation of anhydrite as well as the P and LREE required for the enrichment of the bright areas.

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