Melting phlogopite-rich MARID: Lamproites and the role of alkalis in olivine-liquid Ni-partitioning

In this study, we show how veined lithospheric mantle is involved in the genesis of ultrapotassic magmatism in cratonic settings. We conducted high pressure experiments to simulate vein + wall rock melting within the Earth's lithospheric mantle by reacting assemblages of harzburgite and phlogopite-rich hydrous mantle xenoliths. These comprised a mica-, amphibole-, rutile-, ilmenite-, diopside (MARID) assemblage at 3–5 GPa and 1325–1450 °C. Melting of the MARID assemblages results in infiltration of melt through the harzburgite, leading to its chemical alteration. At 3 and 4 GPa, melts are high in K₂O (> 9 wt%) with K₂O/Na₂O > > 2 comparable to anorogenic lamproites. Higher pressures and temperatures (5 GPa/1450 °C) lead to increasing MgO contents of the melt and to some extent lower K₂O contents (5–7 wt%) at equally high K₂O/Na₂O ratios. Our experiments provide insights into the role of alkalis in nickel-partitioning (D_Ni) between olivine and ultrapotassic melt. We observe that the high contents of Na, K, and Al are indicative of high D_Ni values, implying that the melt polymerization is the dominant factor influencing the olivine/melt nickel partitioning. The change of D_Ni as a function of melt composition results in a pressure independent, empirical geothermometer: [Formula presented].  Element oxides represent the composition of the glass (in wt%), and D_Ni is the liquid/olivine Ni-partitioning coefficient. We propose that this geothermometer is applicable to all natural silicate melts that crystallized olivine in a temperature interval between 1000 and 1600 °C. Application to glass-olivine pairs from calc-alkaline settings (Mexico), MORB (East Pacific Rise), and OIB (Hawaii) yielded reasonable values of 996–1199 °C, 1265 °C, and 1330 °C, respectively.