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 δ¹¹B value (mostly lower than +10‰), whereas the non–eclogitic unit serpentinite from Omi–gawa is higher than +10‰. Although the δ¹¹B value of <0‰ was not measured from the eclogitic unit serpentinites, the relatively low δ¹¹B 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.