Solubility and stability of beryl in granitic melts

Beryllium contents of granitic melts at saturation in beryl (+/- chrysobery l, phenakite, or quartz) have been determined by forward- and reverse-direc tion experiments at 650-850 degrees C and 200 MPa using natural beryl or th e compositionally equivalent mixture of phenakite + alumina + quartz added to metaluminous haplogranite, peraluminous haplogranite, and macusanite (a peraluminous Li, B, F, and P enriched rhyolite obsidian from Macusani, Peru ). Chrysoberyl coexists with beryl +/- quartz in moderately to strongly per aluminous systems (ASI approximately greater than or equal to 1.05); the as semblage phenakite + beryl + chrysoberyl is stable in silica-undersaturated bulk compositions. The BeO content of the melts, which ranged between 0.03 -0.40 wt%, varies principally with temperature (the solubility of beryl fal ls with decreasing T) and with the activity product of beryl, (a(BeO))(3)(a (Al2O3))(a(SiO2))(6). Beryllia contents are lowest in strongly peraluminous and quartz-saturated haplogranite melts; however, the highest BeO content measured in glass products occurs for the strongly peraluminous macusanite at 850 degrees C (4016 ppm), we infer that the greater solubility of beryl in macusanite melt results from melt speciation reactions involving Li, F, B, and P. For all bulk compositions studied, the BeO content of melt projec ts to a narrow range of similar values at low T, near the solidus of haplog ranite. Thus, metaluminous to peraluminous granitic magmas with and without common volatile and fluxing components will, if cooled to subsolidus tempe ratures, have similarly low BeO requirements for beryl saturation. The smal l BeO content of melt near the solidus of haplogranite results in a minor ( 10 degrees C) depression of the freezing point. Compared to Be-free haplogr anite melt, the mean melt composition at the beryl-saturated granite minimu m shifts slightly toward quartz. Beryl is common in peraluminous granitic r ocks in part because lower BeO contents are required to saturate these melt s in beryl, and possibly because these melts acquire higher BeO contents by mica melting reactions at their sources, Beryl is also a common constituen t of border-facies assemblages in granitic pegmatite dikes, regardless of t heir ASI values. This early crystallization of beryl results from the low B eO content required re saturate any granitic melt in beryl at the low-T env ironments in which pegmatite dikes solidify.