Carbonation of diverse ultramafic rocks: implications for Ca-rich protolith from the Setogawa Group, Central Japan

Abstract Ophicarbonate/listvenite and ultramafic rocks are distributed in various tectonic blocks in the western Izu Peninsula. These ultramafic rocks are likely of upper mantle origin, influenced by magmatic activity associated with the formation of the Izu-Bonin-Mariana intra-arc and back-arc basins within the Philippine Sea Plate. Carbonation of ultramafic rocks plays a fundamental role in fluid-rock interaction processes in subduction zone environments, yet the controls on carbonation pathways remain incompletely understood. In particular, the influence of protolith composition on reaction progress and mineral assemblage development has not been fully constrained. This study focuses on representative ophicarbonate/listvenite from the Setogawa Group, a Paleogene accretionary complex. The Setogawa Group contains diverse ultramafic rocks such as dunite, harzburgite, lherzolite and clinopyroxenite and is highly suitable for discussing carbonation of ultramafic rocks due to differences in original rock. Based on petrographic observations and whole-rock chemical compositions, the rocks are classified as Serpentine-Talc-Magnesite rock, Dolomite-Talc-Quartz rock, and listvenite. Thermodynamic modeling further reveals that clinopyroxene-rich ultramafic rocks undergo distinct reaction pathways compared to olivine-rich compositions, with earlier talc formation and different fluid-rock ratios required for listvenitization. These findings demonstrate that the bulk composition of the protolith, particularly Ca content, controls carbonation pathways, mineral assemblages, and the evolution of physical properties during fluid-rock interaction. Our results provide a unified framework for understanding variability in listvenite formation and highlight the importance of protolith heterogeneity in controlling carbonation processes in subduction zone environments.