MSA Roebling Medal Lecture: Two examples of element-exchange equilibria in petrology

Bishop Tuff rhyolites contain quartz, sanidine, plagioclase, biotite, magnetite, ilmenite, and in some cases two pyroxenes. FeTi-oxide temperatures range from 700°C in high-Si rhyolites to 820°C in less evolved tuffs from deeper in the magma chamber. The FeTi-oxides show excellent element-exchange equilibrium (Mg/Fe2+, Mg/Mn, Mn/Fe2+) with each other and with biotite, but not with pyroxene. Extracted temperatures correlate with ilm% in the rhombohedral phase, whereas spinel is constant at 26% usp. Rather than non-reactivity of the spinel, or sub-solidus re-equilibration of ilmenite, this behavior represents the T-fO2 path of magma evolution driven by Fe/Mg enrichment resulting from fractional crystallization, involving reactions universally consuming hematite but not the magnetite component. Unlike the usp content, Mg/Fe2+, Mg/Mn, and Mn/Fe2+ ratios of magnetite all correlate with T. FeTi-oxides register eruption temperatures, whereas the 824±15°C pyroxenes record prior magma conditions. Orthopyroxene is the bad actor in QUILOp thermobarometry. The pyroxenes are compositionally antecrystic, but this designation defies their euhedral habit and evidence from their inclusion population.

The typically Mg-rich nature of the serpentine in serpentinized mantle peridotite can be ascribed to Mg/Fe2+ equilibration with an environmental exchange potential supplied by the abundant olivine and orthopyroxene in the protolith. Mass balance then requires precipitation of magnetite and evolution of hydrogen. KDs match those of prograde metamorphosed olivine-antigorite meta-serpentinites. In massif serpentinites XΣFe values (1σ) are Liz: 0.058±0.030(n=460) and Chr: 0.034±0.018(n=198). The variation in XΣFe of mesh and bastite Liz correlates with uptake of the cronstedite component Fe2+2Fe3+2SiO5(OH)4. This substitution is favored at low T and low fH2, and may be associated with decreasing a(SiO2). Chr has less Fe3+ than Liz, and so its lower XΣFe more accurately reflects the environmental Mg/Fe2+ potential.