An experimental study of the effect of boron on a water saturated haplogranite at 1 Kbar vapour pressure

An experimental study of the effect of boron in the water saturated Q-Or-Ab-B₂O₃-H₂O system has been performed at P=1 Kbar to provide experimental data and explain the role of boron in some late magmatic and early hydrothermal events. Experiments were conducted between 500° C and 800° starting from a gel, or a previously crystallized gel, and variable amounts of boron (0 to 18% B₂O₃) added to water. The phases obtained were: quartz, sanidine, albite, silicate liquid quenched to glass, and aqueous vapour phase. Boric acids, borates and isotropic low index materials were found in the quenched vapour phase. An aluminium silicate-like mineral, not yet fully identified, is also present.The solidus temperature of the Q-Or-Ab composition is lowered by 60° C when 5 wt. % B₂O₃ is added and by more than 130° C when 17wt. % B₂O₃ is added. Compositions of equilibrated silicate melts and vapours were obtained between 780° C and 750° C for various B₂O₃ concentrations. The vapour phase is B and Si rich. It is also enriched in Na with respect to K, and in alkalis with respect to Al. Its silicate solute content is higher than in experiments with pure water. The solubility of water is increased by the addition of boron in Q-Or-Ab melts. Microprobe data show that the melts equilibrated with vapour phases become hyperaluminous and more potassic than sodic. The partition coefficient of boron is in favour of the vapour (k _D=B₂O₃% in melt/B₂O₃% in vapour=0.33±0.02). The effect of the interaction between the silicate phases and the vapour is discussed. Comparison is made between the behaviour of boron and that of chlorine and fluorine. Geological applications are also provided, which concern the influence of boron on minimum melting, on muscovite stability and on the hypersolvus-subsolvus transition.