17:15 〜 18:30
[SGC32-P02] Noble gas analysis and microstructure observation of the Finero phlogopite peridotite in Northern Italy to constrain the origin of deformation process of mantle wedge peridotite
キーワード：かんらん岩、マントル、希ガス、メタソマティズム
Since the rheology of mantle control the behavior of deep earthquakes and orogenic activity, it is important to understand the elementary processes of deformation of the mantle rocks. Alpine-type peridotites are rock bodies formed by the solid penetration of upper mantle fragments along large faults at convergent plate boundaries, and are likely to preserve deformation structures in the brittle-ductile regime during the ascending process. We focus on the effect of water in mantle region on the deformation characteristics preserved in Finero phlogopite peridotite in Northern Italy, which is an alpine-type peridotite and had been metasomatized by slab dehydration in the mantle wedge during the Paleozoic and Mesozoic (Selverstone & Sharp, EPSL 2011; Hartmann & Wedepohl, GCA 1993; [3] Zanetti et al., Contrib. Mineral. Petrol. 1999).
In order to clarify the water-rock interaction and deformation history of the peridotite, we performed: (1) microstructural observations using a scanning electron microscope and a transmission electron microscope, and (2) noble gases analysis of the fluid/melt inclusions in mineral grains, which provide information of the origin of fluids/melts. In the noble gas analysis, the crushing method was adopted to selectively extract noble gases from fluid/melt inclusions.
In the field survey, we found a strain localization zone about 80 m wide in the peridotite, and collected rocks in the strain localization zone and 750 m away from the strain localization zone, respectively. A comparison of the two rocks by thin-section observation shows that the rock of the strain localization zone may have been deformed with forming microcracks under hydrous condition and subsequently may have trapped the fluid/melt inclusions. On the other hand, no such characteristic was observed in the samples away from the strain localization zone. The results of the olivine slip system observation suggest that fluid may have penetrated the rock of the strain localization zone during brittle-ductile deformation.
Noble gas analyses of fluid/melt inclusions show that the He, Ne, and Ar isotope ratios of all samples can be accounted for by three-component mixing of atmosphere/seawater, radiogenic/nucleogenic and mantle components. Similar mixing relation has been observed in very fine-grained apatite and orthopyroxene separated from the Finero peridotite (Matsumoto et al., EPSL 2005). On the other hand, the Ne-Ar-Xe elemental ratios of the samples show that a seawater-derived component is dominant. In addition, while the 3He/4He ratios are uniform, the 40Ar/36Ar ratios become lower with increasing deformation of the sample rock. This suggests that the seawater-derived component may have added to the mantle + radiogenic/nucleogenic component, and that the metasomatism by slab-derived, water-rich fluids may have promoted plastic deformation of peridotite in the mantle region. The observation that the mantle + radiogenic/nucleogenic end members behave as a single component cannot be explained by an addition of fluid released from surrounding metamorphic rocks, in which abundant radiogenic/nucleogenic noble gases had accumulated, during exhumation of the Finero peridotite body. Alternatively, the radiogenic/nucleogenic component would result from accumulation of radiogenic/nucleogenic noble gases in the mantle wedge previously metasomatized by slab-derived fluids enriched in U, Th, and K.
In order to clarify the water-rock interaction and deformation history of the peridotite, we performed: (1) microstructural observations using a scanning electron microscope and a transmission electron microscope, and (2) noble gases analysis of the fluid/melt inclusions in mineral grains, which provide information of the origin of fluids/melts. In the noble gas analysis, the crushing method was adopted to selectively extract noble gases from fluid/melt inclusions.
In the field survey, we found a strain localization zone about 80 m wide in the peridotite, and collected rocks in the strain localization zone and 750 m away from the strain localization zone, respectively. A comparison of the two rocks by thin-section observation shows that the rock of the strain localization zone may have been deformed with forming microcracks under hydrous condition and subsequently may have trapped the fluid/melt inclusions. On the other hand, no such characteristic was observed in the samples away from the strain localization zone. The results of the olivine slip system observation suggest that fluid may have penetrated the rock of the strain localization zone during brittle-ductile deformation.
Noble gas analyses of fluid/melt inclusions show that the He, Ne, and Ar isotope ratios of all samples can be accounted for by three-component mixing of atmosphere/seawater, radiogenic/nucleogenic and mantle components. Similar mixing relation has been observed in very fine-grained apatite and orthopyroxene separated from the Finero peridotite (Matsumoto et al., EPSL 2005). On the other hand, the Ne-Ar-Xe elemental ratios of the samples show that a seawater-derived component is dominant. In addition, while the 3He/4He ratios are uniform, the 40Ar/36Ar ratios become lower with increasing deformation of the sample rock. This suggests that the seawater-derived component may have added to the mantle + radiogenic/nucleogenic component, and that the metasomatism by slab-derived, water-rich fluids may have promoted plastic deformation of peridotite in the mantle region. The observation that the mantle + radiogenic/nucleogenic end members behave as a single component cannot be explained by an addition of fluid released from surrounding metamorphic rocks, in which abundant radiogenic/nucleogenic noble gases had accumulated, during exhumation of the Finero peridotite body. Alternatively, the radiogenic/nucleogenic component would result from accumulation of radiogenic/nucleogenic noble gases in the mantle wedge previously metasomatized by slab-derived fluids enriched in U, Th, and K.