Stability relationships of REE-bearing phosphates in an alkali-rich system (nepheline syenite from the Mariupol Massif, SE Ukraine)

Abstract

Primary REE-enriched fluorapatite and fluorbritholite-(ce) in nepheline syenite from the Mariupol Massif (SE Ukraine), contain textural and chemical evidence of late- to post-magmatic metasomatic alteration. REE mobilization and replacement of the primary phases by fluid-mediated coupled dissolution-reprecipitation strongly depended on the distance between the altered minerals in the host rock. fluorapatite and fluorbritholite-(ce) forming individual pristine grains were partially replaced by the same phase with a new composition, resulting in the presence of patchy zoning in altered grains. the increased REE contents in altered fluorapatite rim domains are related to REE mobilization from the altered REE-depleted rim domains of the fluorbritholite-(ce). the REEs were transported by a fluid with high f activity. the alteration of fluorapatite and fluorbritholite-(ce) grains in contact resulted in the partial replacement of the primary phases by the same phase with a new composition, but also in the partial replacement of the fluorapatite by secondary monazite and fluorite. the REE mobilized from the fluorbritholite-(ce) in the presence of a f-rich fluid in an alkali-rich system promoted formation of monazite as the new phosphate REE-host. the presence of secondary parisite in the altered domains of the fluorapatite and fluorbritholite-(ce) indicates a co2 component in the fluid during metasomatic alteration.