ADVANCES IN USING CALCIC AMPHIBOLES IN GEOTHERMOBAROMETRY

Experimental determination of mineral stability boundaries and compositional exchange reactions in simple chemical systems remains the "ground truth" by which petrologists correlate the pressures and temperatures of formation with the composition of a mineral that exhibits extensive solid solution. Research conducted over the past 15 years at Binghamton University has helped to define this chemistry-to-formation condition relationship for calcic amphiboles. In particular, experimental investigation of (1) the location of the upper-thermal stability of tremolite, (2) the Al content of tremolite in the presence of clinopyroxene, anorthite, and quartz, (3) the temperatue-composition relations of Mg-rich actinolite, (4) the location of the upper-thermal stability of ferro-actinolite, and (5) the site partitioning of Fe and Mg in synthetic actinolites can now be combined to obtain a relatively complete understanding of the variation in amphibole composition in the system CaO-FeO-MgO-Al₂O₃-SiO₂-H₂O for the mineral assemblage amphibole-clinopyroxene-plagioclase-quartz. Thermodynamic modeling has been used to predict the exchange of Fe, Mg, and Al between these phases based on the water-conserving reactions: tschermakite + 2 diopside + 2 quartz = tremolite + 2 anorthite; tremolite + 5 hedenbergite = ferro-actinolite + 5 diopside, and Ca-tschermak + quartz = anorthite. This model also includes intracrystal site-partitioning of Fe and Mg and non-ideality of solid solution using regular solution theory. The model predicts (i) that the total Al content in the amphibole remains sensitive to pressure in the Fe-bearing system, as it does in the Fe-free system, (ii) that there is an increase in the ferro-actinolite and diopside components in amphibole and pyroxene, respectively, with increasing temperature, and (iii) that the positive correlation between Fe and Al observed in natural calcic amphiboles is replicated in this model. Plots of the equilibrium constants for the first two reactions provide a suitable geothermobarometer for amphibolite assemblages closely modeled by this chemical system. Future experimental work will need to focus on the substitution of Al into Fe-Mg actinolite in this model chemical system to better understand the interaction energies of Fe, Mg, and Al on the amphibole M2 site.