Experimental Melting of Phlogopite Websterite in the Upper Mantle between 1.5 and 4.5 GPa

Abstract Reaction experiments have confirmed that phlogopite websterite can be formed by the interaction of peridotite with hydrous alkaline- or silica-rich melts. Phlogopite websterites commonly occur as xenoliths in orogenic and intraplate volcanism but do not receive much attention. We have experimentally investigated the melting behaviour of a phlogopite websterite at 1.5 GPa (1050–1300 °C), 3.0 GPa (1100–1500 °C), and 4.5 GPa (1200–1500 °C) to contribute to understanding the sources of ultrapotassic rocks that occur in different settings. The solidus temperature of the investigated phlogopite websterite rises with increasing pressure, bracketed between 1050 and 1100 °C at 1.5 GPa, 1100 and 1150 °C at 3.0 GPa, and between 1200 and 1250 °C at 4.5 GPa. At 1.5 GPa, phlogopite websterite melts incongruently to form olivine and melt, whereas orthopyroxene, garnet, and melt are formed at 3.0 and 4.5 GPa. The transition of orthopyroxene from reactant to product with increasing pressure results in changes in the SiO2 content of melts. The experimental melts reach a maximum K2O content when phlogopite is consumed completely at temperatures ~150 °C above the solidus. The melting reactions are similar to those of phlogopite lherzolite, but the low Al2O3 starting materials result in lower Al2O3 in the melt than in melts of phlogopite lherzolite. Comparison with natural ultrapotassic rock compositions reveals that the sources of ultrapotassic rocks in convergent settings may be dominated by phlogopite websterite, phlogopite lherzolite, and phlogopite harzburgite. Sources of ultrapotassic rocks in intraplate settings are more likely to include phlogopite clinopyroxenite ± CO2 and K-richterite. In all melting experiments on phlogopite-bearing rocks, K2O from phlogopite passes into the melt, and hence the highest K2O contents in ultrapotassic rocks must be an indication of the minimum stoichiometric coefficient of phlogopite in the melting reaction. In cases where phlogopite websterite or phlogopite lherzolite is identified as the source, the minimum modal percentage of phlogopite in the source can be inferred from the highest K2O content. When applied to the Milk River minettes and New South Wales leucitites, the estimated modal proportion of phlogopite in the sources is greater than 20 wt %. Phlogopite can survive the subduction process and melt later in the post-collisional environment, whereas thermal perturbations are necessary to trigger the melting of phlogopite-bearing assemblages at the base of the lithosphere in intraplate settings.