The effect of B2O3 on the viscosity of a haplogranitic liquid (K2O-Na2O-Al2O3-SiO2) has been determined at 1 atm pressure in the temperature interval of 600–1600 °C. Viscosity measurements of a haplogranite, haplogranite + 4.35 wt% B2O3, and haplogranite + 8.92 wt% B2O3 have been performed using the concentric cylinder and micropenetration methods. The viscosity of a B-enriched natural rhyolite obsidian, macusanite from Macusani, Peru, has also been determined.

The viscosity of haplogranite liquid decreases with the addition of B2O3 at all temperatures investigated. The viscosity decrease is nonlinear, with the strongest decrease exhibited at low B2O3 concentration. The temperature dependence of the viscosity of all the investigated liquids is Arrhenian, in strong contrast to the case for B2O3 liquid. The Arrhenian activation energy is much lower in the B2O3-bearing liquids than in the B2O3-free haplogranite, with the result that the effect of B2O3 on viscosity is a strong function of temperature. At temperatures corresponding to the crystallization of B-rich granitic and pegmatitic systems the addition of I wt% of B2O3 decreases the viscosity 2 orders of magnitude. The macusanite liquid exhibits a reduced viscosity compared with B-free rhyolite that is consistent with the synthetic liquid systematics. B must be considered as a fluxing agent in B-rich granitic and pegmatitlc systems.

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