Phase equilibria of phlogopite lamprophyres from western Mexico: biotite-liquid equilibria and P-T estimates for biotite-bearing igneous rocks

Abstract

 Olivine and augite minette powders have been equilibrated from one bar to nearly 2.0 kbar (water-saturated), and from 900 to 1300° 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 ∼100° 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/(Σ 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 (γ) 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° C, TiO₂ partitioning between phlogopite and liquid is a function of temperature (±50° C), and is insensitive to pressure and H₂O and O₂ 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 kbar) and temperatures (1090 to 1160° C; ±50° C), while Australian lamproites equilibrated at higher P (up to 30 kbar; ±4 kbar) and T (1125 to 1400° C; ±50° 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.