Experimental constraints on phlogopite chemistry in lamproites: 1. The effect of water activity and oxygen fugacity

Experiments on lamproite compositions with varying C-O-H fluid compositions delimit the effect of fluid speciation on mica composition at constant pressure and temperature. Fluid compositions were measured by a mass spectrometric monitoring technique and defined fO₂ conditions lower than those in previous experimental studies of ultrapotassic rocks, thus completing the fO₂ range of interest to mantle melting. Experiments with increasing H₂O/CH₄ and fO₂ show increasing Ba and Al and decreasing F, K and Si in mica, with relatively small variation of Ti. Mica in low fO₂, low αH₂O conditions has K/Al > 1 and Si + Al < 8. The remaining tetrahedral sites are unlikely to be occupied by Fe³⁺ at this low fO₂, and are probably occupied by M²⁺ cations. Experiments with increasing CO₂/H₂O show increasing Ti and decreasing Mg and cation totals in micas, indicating increasing octahedral vacancies in micas. Natural lamproite mica substitutions differ from those in the experiments due to the domination of crystal fractionation and P,T effects over those of fO₂ and αH₂O. -from Author