The effect of fluorine, boron and phosphorus on the viscosity of pegmatite forming melts

The individual influences of F, B and P on viscosity of hydrous pegmatite forming melts have been determined experimentally. A starting glass composition (68.01 wt.% SiO2, 20.14 wt.% Al2O3, 7.73 wt.% Na2O and 4.26 wt.% K2O, Al / (Na + K) = 1.16) was doped with different amounts of F (up to 4.81 wt.%), B2O3 (0.93 wt.%) and P2O5 (up to 2.98 wt.%). The viscosity of melts containing 0.08 to 6.15 wt.% H2O was determined in the high and low viscosity range using the micropenetration technique and the falling sphere method, respectively. Falling sphere experiments were carried out at 200 to 650 MPa and 1173 to 1530 K. Micropenetration measurements were performed in the temperature range of 586 to 1124 K at ambient pressure.For all compositions a large decrease of viscosity upon hydration was observed, consistent with previous findings. The results also confirm that the viscosity decreases with the addition of F at all investigated temperatures. This decrease is more pronounced at low temperature and at low water content. According to our data, P and B do not play a major role on viscous flow in water-rich systems. However, the depolymerizing effect of H2O and F is not sufficient to explain very low viscosities of complex highly fractionated melts containing H2O, F, B, P and Li (Bartels et al., 2011). Thus, although we confirm that F is clearly a fluxing agent, Li must play a crucial role in lowering the viscosity of natural pegmatite forming melts and combined effects between different constituents need to be taken under consideration.

► The influence of F, B and P on the viscosity of pegmatitic liquids was determined.
► Our findings demonstrate that B and P do not play a major role in lowering the viscosity of pegmatitic melts.
► Recent multicompositional models were proven to be not well calibrated for the investigated melt compositions.