Experimental constraints on phlogopite chemistry in lamproites

1. The effect of water activity and oxygen fugacity

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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

Original languageEnglish
Pages (from-to)411-426
Number of pages16
JournalEuropean Journal of Mineralogy
Issue number3
Publication statusPublished - 1989
Externally publishedYes

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