Boron isotope compositions of antigorite–grade serpentinites in the Itoigawa–Omi area of the Hida–Gaien Belt, Japan

Abstract

The antigorite-grade serpentinite and Late Paleozoic high-pressure schists are main components of a serpentinite-matrix mélange in the Itoigawa-Omi area, Hida-Gaien Belt, Japan. Based on the petrologic characteristics of the high-pressure schists, the mélange is divided into two units, namely an 'eclogitic unit' and a 'non-eclogitic unit'. Our preliminary in-situ boron isotope analyses of five serpentinites using a laser-ablation multiple-collector inductively-coupled-plasma mass spectrometry (LA-MC-ICPMS) found a systematic difference of boron isotopic trends among the two units in the same mélange. The 'eclogitic unit' serpentinites from Yunotani and Kotagi-gawa are characterized by lower δ 11 B value (mostly lower than +10‰), whereas the non-eclogitic unit serpentinite from Omi-gawa is higher than +10‰. Although the δ 11 B value of <0‰ was not measured from the eclogitic unit serpentinites, the relatively low δ 11 B values of <+10‰ might have recorded the signature of fluids released from deep subducted dehydrating slab. In contrast, the non-eclogitic unit serpentinite might have been affected by fluids released from shallower portion. Our new data confirmed the potential sensitivity of the boron isotope signature of serpentinites reflecting variation of high-pressure metamorphism.

The versatility of petrological modeling: Thermobarometry of high‐pressure metabasites from the Renge and Sanbagawa belts and phase evolution during warm subduction at Nankai

“…The Renge belt consists of antigorite serpentinite-matrix mélange with HP rocks, mainly clastic sequences and metasedimentary schists, referred as the Renge Schists (Banno, 1958;Kunugiza et al, 2004;Tsujimori, 2002;Tsujimori & Harlow, 2017;Yamada et al, 2019;Yoshida et al, 2021). The Omi (Itoigawa-Omi) area has been well known as a locality of gem-quality jadeitite (Tsujimori & Harlow, 2017).…”

Revisiting Pb isotope signatures of Ni–Fe alloy hosted by antigorite serpentinite from the Josephine Ophiolite, USA

“…Geochemical studies of SSZ serpentinites thus seem to be necessary for understanding the dynamics of fluids at active margins. As part of our current geochemical reconnaissance study of serpentinites in various geologic settings (e.g., Yamada et al, 2019aYamada et al, , 2019b, this contribution focuses on exceptional aggregates of serpentinite-hosted Ni-Fe alloy, socalled josephinite, from the Josephine Ophiolite, Oregon, USA (e.g., Botto and Morrison, 1976;Göpel et al, 1990;Britten et al, 2017). The Josephine Ophiolite has been known as a volcanic arc-type SSZ ophiolite in the circum-Pacific ( Fig.…”

Two groups of fluid inclusions in the Yunotani eclogite from the Hida–Gaien Belt: Implications for changes of fluid salinity during exhumation

“…The 'Renge Schists' record mainly greenschist-to amphibolite-facies metamorphism and locally preserve blueschist-to eclogite-facies metamorphism. In the Omi area, the HP schists are divided into two distinct groups: an 'Eclogitic' (EC) unit and a 'Noneclogitic' (non-EC) unit (Tsujimori, 2002;Tsujimori and Matsumoto, 2006;Yamada et al, 2019) (Fig. 1).…”