Ultra-hydrous conditions and local readjustment of magnetite in evolving carbonatitic magma at Sung Valley, Shillong Plateau: Evidence from fluid inclusions in calcite and presence of manasseite, ferrohogbomite and amesite within magnetite

Mineral and fluid inclusions in carbonatites play a vital role in understanding the nature and evolution of primary carbonatitic magma. In this study, aqueous-carbonic bi-phase, aqueous saline bi-phase and poly-phase fluid inclusions were examined in calcite; and mineral inclusions were observed in accessory magnetite in the Sung Valley carbonatites from NE India. These carbonatites are part of an Ultramafic-Alkaline-Carbonatite Complex (UACC), related to the Kerguelen mantle plume. All the studied inclusions are primary in nature. Raman spectroscopy and microthermometry of these inclusions suggest that the bi-phase and poly-phase fluid inclusions hosted by calcite are dominantly composed of water, whereas the mineral inclusions hosted by magnetite in the Sung Valley carbonatites are manasseite, ferrohogbomite, and amesite. These extremely rare hydrous mineral inclusions occurring in the magnetite are not formed through hydrothermal alteration but represent primary magmatic characteristics of the crystallizing carbonatitic magma. This study suggests that ultra-hydrous conditions have prevailed during the crystallization of the Sung Valley carbonatitic magma. Our study also suggests that magnetite is quite sensitive to these ultra-hydrous conditions prevailing during the crystallization of carbonatitic magma, which results in the local readjustment of this magnetite into these hydrous phases and preserving them as inclusions during the crystal growth.