Application of Gibbs Method to Amphibole

Abstract

The differential thermodynamic method (Gibbs method) is applied to amphibole zonings in a metabasaltic system. A common mineral assemblage in metabasites (amphibole+epidote+ plagioclase+chlorite+quartz water) is analyzed in the system of SiO₂-Al₂O₃-Fe₂O₃-FeO-MgO-CaO-Na₂O-H₂O. Because the number of end-members of amphibole is larger than the degree of thermodynamic freedom, we can estimate pressure-temperature conditions solely from amphibole chemistries. Analyses for numerous amphiboles from Sanbagawa schists show stable P-T fields of calcic and subcalcic amphiboles, and reveal that typical zoning of the albitebiotite zone (barroisite-hornblende-actinolite) yields the decompression P-T path.
The gibbs method analysis minimizes systematic uncertainty when choosing the equilibrium set of mineral compositions, because the compositionset of coexisting minerals are not needed for the analysis. On the other hand, the propagated random uncertainty for calculated dT and dP becomes larger, as the change of monitor parameter becomes larger. To obtain accurate P-T paths, an appropriate way of minimizing total uncertainty is requied.