Mantle-derived fluids in the basement of the Deccan Trap: evidence from stable carbon and oxygen isotopes of carbonates from the Killari borehole basement, Maharashtra, India
We report here for the first time geochemical, mineralogical and stable carbon and oxygen isotopic data on the crystalline basement rocks of the 1993 Killari earthquake region of Maharashtra (India), which is covered by a thick suite of Deccan volcanics. Our results revealed the hitherto unknown amphibolite–granulite nature of the 2.5 Ga basement, which contains 2.00–2.28 wt% of CO2. The stable carbon (δ13C) and oxygen (δ18O) isotopic measurements on carbonates separated from two basement samples KIL-13 (440.5 m depth) and KIL-20 (499.6 m depth) collected from the KLR-1 borehole drilled in the epicentral region showed the respective values of −6.23 and −6.22‰ versus PDB for δ13C and 7.94 and 8.11‰ versus SMOW for δ18O. The samples plot in the primary igneous carbonatite field, indicating the mantle origin of the carbonates, derived through the process of mantle metasomatism from the deep mantle carbon reservoir. This would suggest large-scale crust-mantle thermal fluid interaction beneath the Killari seismogenic region, which is characterized by massive upwarping of the high-velocity mafic crust and retrograde metamorphism.