10:45 〜 12:15
[SMP26-P06] Chemical composition of garnet including rutile exsolution lamellae in quartz-eclogite from the Sanbagawa metamorphic belt, Shikoku, Japan
キーワード：ザクロ石、三波川変成帯、石英エクロジャイト、ルチル、離溶ラメラ、チタンガーネット
Exsolution lamellae of Ti-mineals are caused by the stabilization of the Ti phase in a host mineral from one phase to two phases coexisting during depressurization (e.g., Zhang and Liou, 1999; Mposkos and Kostopoulos, 2001; Zhang et al., 2003). Generally, rutile exsolution lamellae in garnet are commonly found in rocks that have experienced ultra-high temperature (UHT) or ultra-high pressure (UHP) conditions (e.g., Kawasaki et al, 2011, Sato and Ogasawara, 2013, Keller and Ague, 2020). Some reports in synthesis experiments show a strong temperature dependence under UHT environment, and a strong pressure dependence in UHP to mantle environments (e.g., Zhang et al, 2003; Kawasaki and Motoyoshi, 2016). Here, we report the occurrence of rutile exsolution lamellae in garnet in high-pressure (HP) metamorphosed quartz-eclogites from the Mount Gongen, Sanbagawa metamorphic belt, in central Shikoku, and discuss the chemical composition characteristics and incorporation of Ti in the garnet.
Quartz-eclogite samples were collected from near the summit (near the stone shrine), the mountain path and the Tokonabe stream of Mt. Gongen. Quartz-eclogite is divided into quartz-poor/rich/mafic clots domains according to bulk chemical composition (Enami et al., 2018). The needle rutile exsolution lamellae occur only in the quartz-poor domain. The crystalline forms of rutile in garnet are needle, rod or spherical. Two types of rutile needles were observed: long type up to100 µm, and short type about 2–25 µm in length. Long type rutile needle lamellae are present in the core to mantle of the garnet, while short type rutile needles are present in the rim part of the garnet. The garnet is poor in Mn content and is of the almandine-pyrope series, with pyrope in the range of 28–36 mol%. The value of TiO2 is 0.06–0.19 wt% in the area with rutile exsolution lamellae. Na2O3 value is about 0.05 wt% in the rutile-rich part and about 0.02 wt% in the rutile-free part. For Ti incorporation into garnet crystal structure (X2Y3Z3O12; Y=octahedral site; Z=tetrahedral site) has been proposed in a theoretical framework that several substitution reactions enter octahedral or tetrahedral sites (Proyer et al., 2013), e.g. ⅥTi4++ⅧNa+↔ⅧX2++ⅣAl3+ in the octahedral site [experimental example: Ca2++Al3+ → Na1++Ti4+ (Zhang et al., 2003)]. Higher contents of Ti and Na in the regions containing rutile lamellae imply that this substitution reaction may occurred in the studied garnet.
Recently, cases of rutile needles formed under non-UHP conditions, such as a star-garnet in pelitic schists from Idaho have been reported, and the formation mechanism has been discussed through detailed analysis of crystallographic orientation relationship (e.g. Hwang et al. 2015). The rutile lamella in garnet identified in this study is also an example of formation under non-UHP/UHT environment, i.e., HP environment. Further investigation of the chemical composition of garnet grains will provide more details on the conditions under which the rutile lamella is formed.
Quartz-eclogite samples were collected from near the summit (near the stone shrine), the mountain path and the Tokonabe stream of Mt. Gongen. Quartz-eclogite is divided into quartz-poor/rich/mafic clots domains according to bulk chemical composition (Enami et al., 2018). The needle rutile exsolution lamellae occur only in the quartz-poor domain. The crystalline forms of rutile in garnet are needle, rod or spherical. Two types of rutile needles were observed: long type up to100 µm, and short type about 2–25 µm in length. Long type rutile needle lamellae are present in the core to mantle of the garnet, while short type rutile needles are present in the rim part of the garnet. The garnet is poor in Mn content and is of the almandine-pyrope series, with pyrope in the range of 28–36 mol%. The value of TiO2 is 0.06–0.19 wt% in the area with rutile exsolution lamellae. Na2O3 value is about 0.05 wt% in the rutile-rich part and about 0.02 wt% in the rutile-free part. For Ti incorporation into garnet crystal structure (X2Y3Z3O12; Y=octahedral site; Z=tetrahedral site) has been proposed in a theoretical framework that several substitution reactions enter octahedral or tetrahedral sites (Proyer et al., 2013), e.g. ⅥTi4++ⅧNa+↔ⅧX2++ⅣAl3+ in the octahedral site [experimental example: Ca2++Al3+ → Na1++Ti4+ (Zhang et al., 2003)]. Higher contents of Ti and Na in the regions containing rutile lamellae imply that this substitution reaction may occurred in the studied garnet.
Recently, cases of rutile needles formed under non-UHP conditions, such as a star-garnet in pelitic schists from Idaho have been reported, and the formation mechanism has been discussed through detailed analysis of crystallographic orientation relationship (e.g. Hwang et al. 2015). The rutile lamella in garnet identified in this study is also an example of formation under non-UHP/UHT environment, i.e., HP environment. Further investigation of the chemical composition of garnet grains will provide more details on the conditions under which the rutile lamella is formed.