Graphite–3R in a fault fracture zone associated with black jadeitite from Kanayamadani, Itoigawa, central Japan

Abstract

Graphite–3R was found at an outcrop in Kanayamadani, central Japan that had green–black jadeitite, altered jadeitite, and fracture zones in serpentine mélange. The graphite from the three zones was characterized using polarizing microscopy, XRD, Raman spectroscopy, SEM, HRTEM, and stable carbon–isotope mass spectrometry. Graphite in the black jadeitite zone is possibly crystallized simultaneously with prehnite in the stability field of prehnite (P <0.6 GPa and at T = 150–420 °C). The δ¹³C of graphite in the black jadeitite zone ranged from −8.570‰ to −7.870‰, suggesting that it does not have an organic origin. The graphite of the black jadeitite zone was possibly formed in CO₂–CH₄ rich fluid.
 XRD, Raman spectroscopy, and HRTEM observations suggested that the altered jadeitite zone had both low and high crystalline graphite. The altered jadeitite zone was located at the outer region of the green–black jadeitite zone and penetrated by many prehnite veins during a later stage. Amorphous carbon and graphite–3R found from the fracture zone may have formed as a result of graphite–2H crushing during fault movement between green–black and altered jadeitite zones. Changes in the crystal structure and detailed mineralogy of graphite can effectively be used to study tectonics and are key to understanding tectonic movement in geological processes.