17:15 〜 18:45
[SCG45-P10] Combined XnCT-TEM analysis of micro-inclusions in the “black-colored” olivine from SDW in the Horoman peridotite complex, Hokkaido
キーワード：幌満かんらん岩体、黒色かんらん石、包有物、SDW、X線ナノトモグラフィー、透過型電子顕微鏡
The Horoman peridotite complex is composed of mantle-derived rocks and has two characteristic suites: MHL (Main Harzburgite-Lherzolite suite, the main part) and SDW (Spinel-rich Dunite-Wehrlite suite). Although SDW is considered a cumulate crystallized from the magmas originated from MHL [1], the origin of the magma that formed SDW is controversial. Based on isotopic compositions, it is believed that SDW was formed through partial melting and melt transport in a mid-ocean ridge [2]. In contrast, arc-related origin of SDW was proposed in terms of “black-colored” olivine composing SDW [3]. The “black-colored” olivine has magnetite and diopside inclusions (typically ~20 µmΦ), exhibiting particular crystal orientation relationship on the host olivine. Based on this result, [3] hypothesized that the inclusions were precipitated from solid solution components of olivine (fayalite, monticellite, hydrous olivine), accompanied by the release of hydrogen. Combining the mineralogical result with rock magnetic analysis, they estimated a required water amount is 306 (range: 77 - 430) ppm, which is higher than those estimated from the sub-cratonic mantle lithosphere (up to 300 ppm, mostly <100 ppm) [4].
In this study, we revisit these micro-inclusions using synchrotron radiation X-ray nano-tomography (SR-XnCT) and transmission electron microscope (TEM). We found various types of inclusions in the “black-colored” olivine and can provide a revised estimation of required water amount that formed the “black-colored” olivine based on our new results. SR-XnCT was performed at BL47XU in synchrotron facility SPring-8 (Hyogo, Japan). The phases and crystal orientation were clarified using a TEM at Kyoto University.
The samples are A5-511-C1 (C1, black) and A5-511-D (D, gray green), divided from A5-511 rock from SDW. TEM analysis revealed that their inclusions can be classified into three types and have the following mineral assemblages and crystal relationships with the host olivine.
Type 1 inclusions in C1 consist of magnetite (Mag), diopside (Di), and chlorite (Chl). Mag and Di have a particular orientation relationship with the host, but orientation of Chl has not been determined clearly. The area ratio of magnetite in olivine is 0.16 (range: 0.08 - 0.20) % by backscatter electron (BSE) observation of 15 olivine grains. Type 2 inclusion in D consists of monticellite which has the same crystal orientations as the host. Type 3 inclusions in D consist of chromite (Chr), Mag, and unknown mineral with NaAlSiO4 composition. Chr and Mag have a particular orientation relationship with the host.
The results indicate that various types of inclusions can be recognized in the “black-colored” olivine and the inclusions described in [3] correspond to Type 1 inclusion. Due to the ubiquitous existence of chlorite, the reaction proposed by [3] needs to be revised as follows. According to 3D observation of 20 Type 1 inclusions, the volume ratio of each phase is almost constant (Mag: Di: Chl = 8: 16: 1), and the composition of the entire inclusions is close to olivine stoichiometry. It is suggested that the reaction forming Type 1 inclusions is a redox reaction involving water in olivine because of the occurrence of magnetite and chlorite. We propose a scenario that some water in olivine partitioned into chlorite when SDW was formed, while others escape because the amount of hydrogen equivalent to Fe3+ in magnetite is higher than in chlorite. Based on this assumption, we estimated required amount of water by the volume ratio of magnetite determined by the careful BSE observation: 208 (range: 104 - 265) ppm H2O. This estimation falls in the error range of [3] but is slightly lower. This result does not strongly deny the mid-ocean ridge origin of the magma that formed the “black-colored” olivine in SDW.
[1] Takahashi, (1991) J. Mineral. Petrol. Econ. Geol [2] Takazawa et al. (1999) J. Petrol.
[3] Arai et al. (2021) Lithos [4] Peslier et al. (2017) Space Sci. Rev.
In this study, we revisit these micro-inclusions using synchrotron radiation X-ray nano-tomography (SR-XnCT) and transmission electron microscope (TEM). We found various types of inclusions in the “black-colored” olivine and can provide a revised estimation of required water amount that formed the “black-colored” olivine based on our new results. SR-XnCT was performed at BL47XU in synchrotron facility SPring-8 (Hyogo, Japan). The phases and crystal orientation were clarified using a TEM at Kyoto University.
The samples are A5-511-C1 (C1, black) and A5-511-D (D, gray green), divided from A5-511 rock from SDW. TEM analysis revealed that their inclusions can be classified into three types and have the following mineral assemblages and crystal relationships with the host olivine.
Type 1 inclusions in C1 consist of magnetite (Mag), diopside (Di), and chlorite (Chl). Mag and Di have a particular orientation relationship with the host, but orientation of Chl has not been determined clearly. The area ratio of magnetite in olivine is 0.16 (range: 0.08 - 0.20) % by backscatter electron (BSE) observation of 15 olivine grains. Type 2 inclusion in D consists of monticellite which has the same crystal orientations as the host. Type 3 inclusions in D consist of chromite (Chr), Mag, and unknown mineral with NaAlSiO4 composition. Chr and Mag have a particular orientation relationship with the host.
The results indicate that various types of inclusions can be recognized in the “black-colored” olivine and the inclusions described in [3] correspond to Type 1 inclusion. Due to the ubiquitous existence of chlorite, the reaction proposed by [3] needs to be revised as follows. According to 3D observation of 20 Type 1 inclusions, the volume ratio of each phase is almost constant (Mag: Di: Chl = 8: 16: 1), and the composition of the entire inclusions is close to olivine stoichiometry. It is suggested that the reaction forming Type 1 inclusions is a redox reaction involving water in olivine because of the occurrence of magnetite and chlorite. We propose a scenario that some water in olivine partitioned into chlorite when SDW was formed, while others escape because the amount of hydrogen equivalent to Fe3+ in magnetite is higher than in chlorite. Based on this assumption, we estimated required amount of water by the volume ratio of magnetite determined by the careful BSE observation: 208 (range: 104 - 265) ppm H2O. This estimation falls in the error range of [3] but is slightly lower. This result does not strongly deny the mid-ocean ridge origin of the magma that formed the “black-colored” olivine in SDW.
[1] Takahashi, (1991) J. Mineral. Petrol. Econ. Geol [2] Takazawa et al. (1999) J. Petrol.
[3] Arai et al. (2021) Lithos [4] Peslier et al. (2017) Space Sci. Rev.