Experimental constraints on phlogopite chemistry in lamproites: 1. The effect of water activity and oxygen fugacity
Foley, Stephen F.
European Journal of Mineralogy Volume 1 Number 3 (1989), p. 411 - 426
published: Jul 27, 1989
manuscript accepted: Feb 15, 1989
manuscript received: Oct 27, 1988
ArtNo. ESP147050103011, Price: 29.00 €
Abstract 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. Lamproite chemistry and mineralogy requires a source containing F-rich perpotassic micas which corresponds to low fO2 and αH2O conditions in the experiments. However, these low fO2, low αH2O conditions are required only for the mantle enrichment event which produced the mica-bearing lamproite source rock. The later partial melting event which produced the lamproite magma may have taken place in more H2O-rich conditions, with high contents of Ba and high K/Al promoted by melting of accessory Ba-bearing, Al-free minerals such as priderite. MARID micas have Si + Al Al comparable to low fO2 experimental micas, possibly indicating a reduced source for MARID nodules. This is compatible with the occurrence of reduced volatile species in MARID micas, and supports suggestions that MARID nodules crystallised in more reducing conditions than previously estimated.