Tungsten solubility in evolved granitic melts: An evaluation of magmatic wolframite

A variety of parameters that potentially control the solubilities of synthetic hubnerite (MnWO4) and ferberite (FeWO4) in haplogranitic melts rich in fluxing compounds have been determined at 200 MPa in order to test the hypothesis that wolframite can occur as a magmatic mineral. The melts, considered representative of highly evolved pegmatites, contain 1.1, 1.7 and 2.02 wt.% of Li2O, P2O5, and B2O3, respectively. Although the molar Al/(Na + K) ratio is one, if Li is considered to be an alkali element the molar Al/(Na + K + Li) (ASILi) of the melt is 0.88, and the melt is peralkaline. Hubnerite and ferberite solubilities at 800 °C are strongly controlled by melt composition. They are much lower in subaluminous melt, (ASILi = 1.05), than in peralkaline melt. By contrast, hubnerite and ferberite solubilities are nearly independent of the fluorine content of the melt, for up to 8 wt.% F. Hubnerite and ferberite solubilities at 800 °C are also nearly independent of oxygen fugacity over a logfO2 range of approximately Ni–NiO − 3 to Ni–NiO + 2, which implies that the predominant oxidation state of tungsten in the melts is +6, even at moderately reduced conditions. A series of experiments on haplogranitic melt, rich in fluxing compounds, with 0 and 6 wt.% F at 850–650 °C shows that temperature strongly influences hubnerite solubility, e.g., the solubility product of hubnerite (Ksphub) in the melts with 6 wt.% F decreases from 71.0 ± 7.9 × 10−4 mol2/kg2 at 850 °C to 4.4 ± 2.3 × 10−4 mol2/kg2 at 650 °C.Our experimental results show that ferberite solubilities are much higher than those of hubnerite with the same composition of melt, but the Mn, Fe and W concentrations in natural melt inclusions indicate that these melts were undersaturated with wolframite at 800 °C. However, flux-rich melts crystallize at lower temperatures in nature and the occurrence of natural, magmatic wolframite was evaluated by comparing experimental solubility product values extrapolated to 500 °C with wolframite activities at these temperatures. Magmatic wolframite is unlikely in flux-rich peralkaline melts but by contrast, flux-rich subaluminous to peraluminous melt inclusions appear to have been saturated with wolframite at approximately 500 °C. Because of the strong temperature dependence of wolframite solubility the dominant effect of fluxing compounds is to lower the solidus temperature of the melt.