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

摘要

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.