An experimental study of B-, P- and F-rich synthetic granite pegmatite at 0.1 and 0.2 GPa

Abstract

Extreme enrichment in H2O, B, P and F is characteristic of many evolved granites and pegmatites. We report experimental phase relations of a synthetic peraluminous pegmatite spiked with P2O5 ,B 2O3 and F (5 wt% of each), Rb2O, Cs2O (1 wt% of each) and Li2O (0.5 wt%). Experiments were carried out in H2O-satu- rated conditions in cold-seal rapid-quench pressure vessels at 0.1-0.2 GPa. Crystallisation starts at about 820 C with berlinite and topaz. Quartz appears at 700- 750 C. Topaz is replaced by muscovite at about 600 C. At near-solidus temperatures (450-500 C) amblygonite, lacroixite and a Cs-bearing aluminosilicate crystallise. In all charges aluminosilicate melt coexists with low-density hydrous fluid and hydrosaline melt. The latter is strongly enriched in Na3AlF6 and H3BO3 components. Experi- mental evidence of the liquid immiscibility and mineral reactions documented in our study offers new explana- tions of many enigmatic features of natural pegmatites.