12:00 〜 14:00
[R8P-05] コロラド台地産ローソン石エクロジャイト捕獲岩を構成するオンファス輝石の電子線散漫散乱：予察的報告
「発表賞エントリー」
キーワード：オンファス輝石、秩序無秩序転移、コロラド台地、ローソン石エクロジャイト、電子線散漫散乱
Ultrahigh-pressure lawsonite eclogite xenolith from the Navajo volcanic field, central Colorado Plateau, has attracted much attention because this xenolith should be a fragment of the Farallon plate that was rapidly brought to the surface by the diatreme formation. One can therefore expect that reconnaissance of its constituting minerals is useful to examine eclogitization processes without affected by retrogression. Notably, cation ordering processes in omphacite lead to various types of micrometer-scale textures such as antiphase domains, reflecting the thermal history of slab eclogitization.
Here, we report the crystallographic features of omphacite in the lawsonite eclogite xenolith. The omphacite grains are a little enriched in Fe3+(~aug35–45 jd40–50 aeg15–20) and characterized by local compositional heterogeneities. Despite the peak metamorphic temperature of ~620℃, below the critical temperature of its order-disorder transition (~865℃), our X-ray diffraction analysis of representative crystals demonstrated the absence of any super-lattice diffraction peaks belonging to the ordered phase. However, our observation of the omphacite from the same eclogite with transmission electron microscopy revealed the presence of diffuse scattering related to the ordered structure, although antiphase domains were not observed.
These omphacites should have undergone incomplete disordering after the prograde metamorphism. The most probable event that induced cation disordering was the Cenozoic thermal pulse before the diatreme emplacement. A previous study on a dunite xenolith suggested the presence of pre-emplacement heating (>~800℃) that lasted for <~1700 years (Smith, 2010). Therefore, we conducted several high-pressure annealing experiments of natural ordered omphacites by using a piston-cylinder apparatus; we obtained completely disordered omphacites after 24 h annealing at 1000℃, 3 GPa. This corroborates the hypothesis that cation disordering in omphacite can occur within such a short timescale. Although the heating might have overprinted the prograde omphacite microtexture, these peculiar omphacites could allow to discuss short timescale of the episodic heating, which cannot be evaluated only with conventional analyses of macroscopic diffusion profiles.
Here, we report the crystallographic features of omphacite in the lawsonite eclogite xenolith. The omphacite grains are a little enriched in Fe3+(~aug35–45 jd40–50 aeg15–20) and characterized by local compositional heterogeneities. Despite the peak metamorphic temperature of ~620℃, below the critical temperature of its order-disorder transition (~865℃), our X-ray diffraction analysis of representative crystals demonstrated the absence of any super-lattice diffraction peaks belonging to the ordered phase. However, our observation of the omphacite from the same eclogite with transmission electron microscopy revealed the presence of diffuse scattering related to the ordered structure, although antiphase domains were not observed.
These omphacites should have undergone incomplete disordering after the prograde metamorphism. The most probable event that induced cation disordering was the Cenozoic thermal pulse before the diatreme emplacement. A previous study on a dunite xenolith suggested the presence of pre-emplacement heating (>~800℃) that lasted for <~1700 years (Smith, 2010). Therefore, we conducted several high-pressure annealing experiments of natural ordered omphacites by using a piston-cylinder apparatus; we obtained completely disordered omphacites after 24 h annealing at 1000℃, 3 GPa. This corroborates the hypothesis that cation disordering in omphacite can occur within such a short timescale. Although the heating might have overprinted the prograde omphacite microtexture, these peculiar omphacites could allow to discuss short timescale of the episodic heating, which cannot be evaluated only with conventional analyses of macroscopic diffusion profiles.