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, B₂O₃ and F (5 wt% of each), Rb₂O, Cs₂O (1 wt% of each) and Li₂O (0.5 wt%). Experiments were carried out in H₂O-saturated 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. Experimental evidence of the liquid immiscibility and mineral reactions documented in our study offers new explanations of many enigmatic features of natural pegmatites.