カンラン岩における単科輝石の再平衡と微細組織(<特集>造岩鉱物研究の現状と展望-地団研第33回総会鉱物学ミニシンポジウム-)

抄録

Clinopyroxene in garnet Iherzolite from Alpe Arami, Bellinzona, Switzerland has been re-equilibrated at two different stages during its emplacement. The solubility of Al, Cr, Na and Fe atoms in clinopyroxene decreases systematically throughout the re-equilibration, while that of Ca and Mg atoms increases. Primary crystals of clinopyroxene rarely remain, and are surrounded by recrystallized olivine, orthopyroxene, clinopyroxene and Mg-hornblende. The primary crystals have optically visible broad clinoamphibole lamellae parallel to (100). Electron microprobe analyses show that the marginal part of clinopyroxene is depleted in Al, Cr, Na and Fe content due to diffusion of atoms while attaining the re-equilibration. The core of the crystal, in most part, has undergone the same chemical depletions to form the broad clinoamphibole lamellae. Primary chemical composition only occurs in restricted areas sufficiently far away from the clinoamphibole lamellae to be free from detectable chemical depletion by electron microprobe analysis. Even in such areas, X-ray and electron microscopic observations reveal that clinopyroxene has re-equilibrated in sub-microscopic domain size. This re-equilibration texture forms intercalation of thin clinoamphibole lamellae (generally up to 100 A thick), having a coherent interface to the host, and related to planar defects parallel to (010). This suggests that the fluctuation of chemical composition is generally limited to a scale from the unit-cell size to several hundred angstroms.