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 the 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 between 3 and 4 wt% water at the stage of phenocryst precipitation. The partitioning 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 total) 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 annite and phlogopite in natural biotites. Based on the partitioning of BaO and TiO2 between biotite and liquid, we have formulated a thermometer and barometer. Over the range of 400 degrees C, TiO2 partitioning between phlogopite and liquid is a function of temperature (+/- 50 degrees C), and is insensitive to pressure and H2O and O-2 activities. BaO partitioning between phlogopite and liquid is a function of both temperature and pressure (+/- 4 kbar), the latter being most important. Applying the TiO2 and BaO partitioning expressions to lamprophyre and lamproite suites shows that Mexican minettes equilibrated at low pressures (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 degrees C). Experimental glass compositions and phenocryst fractionation calculations, together with the BaO- and TiO2- based pressure calculations indicate that felsic minettes from the Mexican suite of lavas can be generated by simple fractionation of a more mafic parent minette at mid to lower crustal pressures.