PHASE-EQUILIBRIA OF PHLOGOPITE LAMPROPHYRES FROM WESTERN MEXICO - BIOTITE-LIQUID EQUILIBRIA AND P-T ESTIMATES FOR BIOTITE-BEARING IGNEOUS ROCKS

Olivine and augite minette powders have been equilibrated from one bar to nearly 2.0 kbar (water-saturated), and from 900 to 1300 degrees C, and then quenched rapidly, at oxygen fugacities controlled between th e nickel-nickel oxide (NNO) and hematite-magnetite (HM) oxygen buffers . The liquidus of both samples is suppressed similar to 100 degrees C at water-saturated conditions and 1500 bar. Both lavas contained betwe en 3 and 4 wt% water at the stage of phenocryst precipitation. The par titioning of ferric and ferrous iron between phlogopite and liquid has been determined on eight samples across 3 log f(O2) units; when these determinations are combined with previous studies, Fe2O3/(Sigma FeO t otal) of Mg-rich biotite can be calculated knowing log f(O2), T, and X (Fe). Thermodynamic modelling of biotite-liquid equilibria results in two expressions for calculating activity coefficients (gamma) for anni te and phlogopite in natural biotites. Based on the partitioning of Ba O and TiO2 between biotite and liquid, we have formulated a thermomete r and barometer. Over the range of 400 degrees C, TiO2 partitioning be tween phlogopite and liquid is a function of temperature (+/- 50 degre es C), and is insensitive to pressure and H2O and O-2 activities. BaO partitioning between phlogopite and liquid is a function of both tempe rature and pressure (+/- 4 kbar), the latter being most important. App lying the TiO2 and BaO partitioning expressions to lamprophyre and lam proite suites shows that Mexican minettes equilibrated at low pressure s (5 to 15 kbar; +/- 4 bar) and temperatures (1090 to 1160 degrees C; +/- 50 degrees C), while Australian lamproites equilibrated at higher P (up to 30 kbar; +/- 4 kbar) and T (1125 to 1400 degrees C; +/- 50 de grees C). Experimental glass compositions and phenocryst fractionation calculations, together with the BaO- and TiO2- based pressure calcula tions indicate that felsic minettes from the Mexican suite of lavas ca n be generated by simple fractionation of a more mafic parent minette at mid to lower crustal pressures.