Be-daughter minerals in fluid and melt inclusions: implications for the enrichment of Be in granite-pegmatite systems
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Thomas, R and Webster, JD and Davidson, P, Be-daughter minerals in fluid and melt inclusions: implications for the enrichment of Be in granite-pegmatite systems, Contributions to Mineralogy and Petrology, 161, (3) pp. 483-495. ISSN 0010-7999 (2011) [Refereed Article]
The study of re-homogenized melt inclusions in the same growth planes of quartz of pegmatites genetically linked to the Variscan granite of the Ehrenfriedersdorf complex, Erzgebirge, Germany, by ion microprobe analyses has determined high concentrations of Be, up to 10,000 ppm, in one type of melt inclusion, as well as moderate concentrations in the 100 ppm range in a second type of melt inclusion. Generally, the high Be concentrations are associated with the H2O- and other volatile-rich type-B melt inclusions, and the lower Be concentration levels are connected to H2O-poor type-A melt inclusions. Both inclusion types, representing conjugate melt pairs, are formed by a liquid-liquid immiscibility separation process. This extremely strong and very systematic scattering in Be provides insights into the origin of Be concentration and transport mechanisms in pegmatite-forming melts. In this contribution, we present more than 250 new analytical data and show with ion microprobe and fs-LA-ICPMS studies on quenched glasses, as well as with confocal Raman spectroscopy of daughter minerals in unheated melt inclusions, that the concentrations of Be may achieve such extreme levels during melt-melt immiscibility of H2O-, B-, F-, P-, ± Li-enriched pegmatite-forming magmas. Starting from host granite with about 10 ppm Be, melt inclusions with 10,000 ppm Be correspond to enrichment by a factor of over 1,000. This strong enrichment of Be is the result of processes of fractional crystallization and further enrichment in melt patches of pegmatite bodies due to melt-melt immiscibility at fluid saturation. We also draw additional conclusions regarding the speciation of Be in pegmatite-forming melt systems from investigation of the Be-bearing daughter mineral phases in the most H2O-rich melt inclusions. In the case of evolved volatile and H2O-rich pegmatite systems, B, P, and carbonates are important for the enrichment and formation of stable Be complexes. © 2010 Springer-Verlag.
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