変成P-T経路の見積りにおける相平衡熱力学的フオワードモデリングの適用

抄録

One of the important aims of metamorphic petrology is to unveil the physicochemical conditions of the Earth's interior. Decoding the record of metamorphism is a case of the inverse problem based on observations of stable equilibrium mineral assemblages and mineral compositions. Mineralogical forward-modeling is an alternative approach to estimate the metamorphic conditions of a rock. It enables a prediction of equilibrium composition of zoned minerals or mineral inclusions with the matrix phase assemblage. Thus, the forwardapproach tests the assumption of an equilibrium mineral assemblage, which is critical for inversion analysis.
In this paper, we introduce thermodynamic forward-modeling in the field of metamorphic petrology. Then, we show an example of its application in estimating a prograde metamorphic P-T path of a whiteschist from the Kokchetav ultrahigh-pressure terrane in Kazakhstan. Garnet in the whiteschist shows prograde compositional zonation and contains mineral inclusions. We carried out geothermobarometric estimates using ilmenite-rutile composite inclusions in garnet, combined with an evaluation of the equilibrium mineral assemblage in a P-T pseudosection of the whiteschist in the K₂O-CaO-MgO-FeO-Al₂O₃-SiO₂-H₂O system. The result yielded a counter-clockwise prograde P-T path for the rock. The amount and equilibrium composition of garnet were sequentially calculated for model P-T conditions along the P-T path, and the results were compared with a line-profile observation of the garnet. A consistency between the model and the observations was confirmed for X_Ca in garnet, however X_Fe and X_Mg had a large inconsistency. Both uncertainties in the equilibrium model and nonequilibrium effects during the crystal growth are possible reasons for such a discrepancy. Hence, it must be noticed that there are complexities in estimating the equilibrium composition of a zoned garnet and matrix minerals.