Experimental phase-equilibrium study of Al- and Ti-contents of calcic amphibole in MORB; a semiquantitative thermobarometer

Given that Al tends to replace Si in tetrahedral coordination in Ca-amphibole with increasing T and that Al substitutes for Mg+Fe in the M2 octahedral site to progressively greater extents with increasing P, increasing Al2O3 contents of Ca-amphibole should reflect, in general, increasing P and T (i.e. calcic amphibole might serve as a crude thermobarometer in those rocks, such as metabasalts having MORB-like chemistry, in which they are a columetrically important phase). This study demonstrates that, provided that the host metabasalts are MORB-like in composition, the Al2O3 contents of Ca-amphiboles can serve as P-T indicators. P-T estimates provided by applying the new Ca-amphibole thermobarometer to natural assemblages from seven different plate-tectonic settings and P-T regimes were compared with the original P-T estimates for these same assemblages. In general, T for low-grade rocks show the best agreement; high-grade Ca-amphiboles exhibit low Ti contents for a given complement of Al and result in unrealistically high estimates using the proposed Ca-amphibole thermobarometer. Nonetheless, providing that these limitations are taken into account, the new Ca-amphibole thermobarometer can be used to estimate semiquantitatively the P-T conditions under which some metabasaltic assemblages formed.