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The role of fluorine and oxygen fugacity in the genesis of the ultrapotassic rocks

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Abstract

The effects of H2O, CO2, CH4 and HF on partial melting of a model phlogopite harzburgite mantle are considered with regard to the production of ultrapotassic magmas. Fluorine has a polymerising effect in H2O-poor conditions, but in the presence of abundant H2O where HF rather than F is dominant, the overall effect is depolymerisation. Methane also dissolves by forming (OH)− groups, and so has a depolymerising effect. Group I ultrapotassic rocks (lamproites) probably originate from primary magmas with SiO2 contents ranging from around 40 wt% to at least 52 wt%. This range can be explained by differing depths of origin from a similar source with a similar reduced H2O-CH4-HF volatile mixture. The formation of silica-rich initial melts from a model phlogopite harzburgite is assisted by the presence of CH4 and HF. Dissociation of less than 0.1 wt% H2O, driven by H2 loss, is sufficient to cause oxidation during emplacement to observed oxidation states. Silica-poor ultrapotassic rocks could be produced at higher pressures in a reduced environment, or in an oxidised environment with high CO2/(CO2 + H2O) ratios.

Group II (African Rift) potassic rocks may originate in H2O-poor conditions in which fluorine will maintain a large phlogopite phase field, so that initial melts will be magnesian and silica-undersaturated.

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  1. Stephen F. Foley

    Present address: Abteilung Kosmochemie, Max-Planck-Institut für Chemie, Saarstraße 23, Postfach 3060, D-6500, Mainz, FRG

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  1. Geology Department, University of Tasmania, Box 252 C, 7001, Hobart, Tasmania, Australia

    Stephen F. Foley, Wayne R. Taylor & David H. Green

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Foley, S.F., Taylor, W.R. & Green, D.H. The role of fluorine and oxygen fugacity in the genesis of the ultrapotassic rocks. Contr. Mineral. and Petrol. 94, 183–192 (1986). https://doi.org/10.1007/BF00592935

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