Solubility of manganotantalite and manganocolumbite in pegmatitic melts

Abstract

Solubility experiments of MnNb 2 O 6 and MnTa 2 O 6 were conducted in two nominally dry to watersaturated pegmatitic melts with different amounts of Li, F, P, and B at 700 to 1000 °C and 200 MPa to determine the maximum concentrations of Nb and Ta in pegmatitic melts. The Li 2 O, F, B 2 O 3 , and P 2 O 5 contents in the melts were 1.16, 2.99, 1.78, and 1.55 wt% for melt composition PEG1 and 1.68, 5.46, 2.75, and 2.75 wt% for melt composition PEG2 and the resulting Al/(Na+K+Li) ratio for both melts is 0.92. The experimental data show that the solubility product of manganocolumbite increases by a factor of three upon increasing the water concentration from 0 to 4 wt%. Considering that pegmatitic melts at pressures above 50 to 100 MPa are hydrous (>4 wt% H 2 O), the increase in solubility by this magnitude, over the stated range of water concentration, is not significant for pegmatites. The data also point out that the solubility of MnNb 2 O 6 and MnTa 2 O 6 is strongly temperature dependent, increasing by a factor of 50 for manganocolumbite and 15-20 for manganotantalite from 700 to 1000 °C under water-saturated conditions. The solubility also increases with increasing content of fluxing elements like Li, F, B, and P. In the pegmatite melt containing the highest amount of fluxing elements, the maximum concentrations of Ta and Nb are higher by nearly one order of magnitude when compared to a subaluminous rhyolitic melt.

Evidence for lithium-aluminosilicate supersaturation of pegmatite-forming melts

“…4) shows a weak dependence on temperature with lower values measured at lower temperatures. This observation is consistent with the results from previous studies, which demonstrated that the solubility of other elements in silicate melts saturated with crystalline phases containing large concentrations of those elements showed a positive correlation with temperature (e.g., Harrison and Watson 1984;Pichavant et al 1992;Linnen 1998;London et al 1998;Evensen et al 1999;Bartels et al 2010;Van Lichtervelde et al 2010;Che et al 2013). …”

GEOCHEMICAL FRACTIONATION OF Nb-Ta OXIDES IN Li-BEARING PEGMATITES FROM THE BARROSO-ALVAO PEGMATITE FIELD, NORTHERN PORTUGAL

“…Recent work by Bartels et al (2010) on the solubility of tantalite-(Mn) and columbite-(Mn) in felsic melts confirm that the solubility of tantalite-(Mn) is higher, and the concentrations of Li, F, P, and B may increase the solubility of tantalite-(Mn) and columbite-(Mn) by a factor of 10.…”

The Origin of Niobium and Tantalum Mineralization in the Nechalacho REE Deposit, NWT, Canada

“…12). If manganese was retained in the melt due, in part, to the presence of phosphorus, then this association may represent a magmatic signature (Bartels et al, 2010).…”

“…This research has shown that the Mn/(Mn + Fe) and Ta/(Ta + Nb) ratios of the columbite group minerals increase during progressive crystal fractionation, a behavior that can be attributed to differences in the solubility of the Nb and Ta end members of the columbite group minerals in the magma and the concentration of Mn in the residual melt through the crystallization of Fe-rich phases or high fluorine abundances in the melt (Linnen and Keppler, 1997;Linnen, 1998;Bartels et al, 2010;Wise et al, 2012). Little consideration, however, has been given to the effect of hydrothermal fluids on the composition of the columbite group minerals or the possibility that these fluids may deposit the minerals directly.…”