Fluorapatite-monazite relationships in granulite-facies metapelites, Schwarzwald, southwest Germany

Abstract

Fluorapatite grains with monazite inclusions and/or rim grains are described in two of four samples from a set of granulite-facies metapelites collected from the Variscan Schwarzwald, southern Germany. Fluorapatite in all four samples appears to have experienced some dissolution in the partial granitic melt formed during granulite-facies metamorphism. Monazite inclusions and rim grains are highly deficient in Th and are presumed to have formed from fluorapatite in association with partial melting during granulite-facies metamorphism. Monazite inclusions range from very small (<1 mm) and very numerous to small (1À2 mm), sometimes elongated, and less numerous; both types are evenly distributed throughout the fluorapatite grain interior. Monazite rim grains tend to be 1À10 mm. The formation of monazite inclusions is proposed to be due to dissolution-reprecipitation of the fluorapatite by the aqueous fluids inherent in the granitic melt. We propose that an increase in inclusion size coupled with a decrease in inclusion number is due to Ostwald ripening (interfacial energy reduction), which is greatly facilitated by the presence of an interconnected, fluid-filled porosity in the metasomatized fluorapatite. We further propose that monazite rim grains formed principally during partial dissolution of the fluorapatite in the granitic melt and to a lesser extent by partial dissolutionreprecipitation of the fluorapatite grain rim area allowing for the partial removal of (Y+REE). We conclude that fluorapatite, with monazite inclusions and rim grains, experienced partial dissolution in a H 2 O-rich peraluminous granitic melt compared to fluorapatite with monazite rim grains and no inclusions which reacted with a similar, relatively less H 2 O-rich melt. In contrast, monazite-free fluorapatite experienced partial dissolution in a comparatively H 2 O-poor, subaluminous, possibly peralkaline melt.

Apatite: A Fingerprint for Metasomatic Processes

“…Hansen and Harlov 2007) and from experiments (e.g. Harlov et al 2005), the F, Cl, OH, and CO 3 content in apatite, coupled with the possible presence of monazite and xenotime inclusions, can serve as valuable "fi ngerprints" for recording metasomatic events in rocks.…”

“…Here voids, presumably once fl uid fi lled, are seen at specifi c points along the monaziteapatite interface. Monazite and xenotime inclusions can also form in fl uorapatite from H 2 O-rich metapelites that have experienced partial melting during granulite-facies metamorphism (Harlov et al 2007), apparently due to alkali-bearing fl uids expelled during crystallization of the alkali-SiO 2 -rich melt.…”

Ancient graphite in the Eoarchean quartz–pyroxene rocks from Akilia in southern West Greenland I: Petrographic and spectroscopic characterization

“…Depending on the composition of metamorphic fluids and, to a lesser extent, temperature and pressure, metasomatized apatites may contain monazite and xenotime inclusions (Harlov et al, 2002). For instance, monazite inclusions in fluorapatites from granulite-facies metapelites formed during the partial dissolution or dissolution-reprecipitation of fluorapatite when exposed to high-temperature metasomatism (Harlov et al, 2007). Since temperature estimates for graphite associated with apatite in the Akilia Qp rock are consistent with the granulite-facies metamorphism, the chemical composition of the apatite may have been altered during the high-grade metamorphic event at 3.65 Ga. Akilia apatites contain relatively high levels of rare Earth elements (up to 0.2 wt.%; Nishizawa et al, 2005), but they contain no REE-phosphate inclusions as may be expected from such metamorphic conditions.…”

Monazite-Fluorapatite Characteristics as Evidence for Interplay Between 2.04 Ga Fluid-Induced Dehydration and Melting of the Sand River Gneiss, Limpopo Complex, South Africa

“…Unlike the high-grade rocks investigated in these different studies, characterized by no obvious evidence of partial melting (see Harlov 2012 and references therein), other studies reported the occurrence of monazite inclusions and rim grains in fluorapatite from granulite-facies rocks that have been subjected to partial melting (e.g., Harlov et al, 2007). The closest correlative to the scenario at hand is the study by Harlov et al (2007), who related the occurrence of monazite inclusions and rim grains in fluorapatite grains from a granulite-facies metapelite to partial melting during granulite-facies metamorphism. Here, the suggestion of formation of monazite rim grains by partial dissolution of the fluorapatite in the granitic melt (Harlov et al, 2007) is similar to the results from experimental melts, which have so far shown monazite to form only as rim grains on fluorapatite (Wolf and London, 1995).…”