Stable isotope geochemistry of carbonatites: New insights into the petrogenesis and evolution

Abstract

Carbonatites are carbonate-rich magmatic rocks that are commonly thought of having a mantle origin and are thus, of great interest to geologist for understanding the chemical evolution of the upper mantle. This thesis provides new perspectives on processes related to the origin, formation, and evolution of carbonatites using a multi-isotope approach. In the first of several studies in this thesis, variations in the stable carbon and oxygen isotope record of carbonatites through geologic time were assessed from newly acquired and the available literature data. Using the classic Amba Dongar carbonatite-alkaline complex of India as a case study, age constraints were placed on carbonatite magmatism which revealed its temporal relation to the peak Deccan volcanism. U-Pb dating of apatite in the carbonatites and associated nephelinites yielded 65.4 Ma as the emplacement age of the alkaline rocks, which reasserts the contemporaneity of alkaline magmatism and the Deccan volcanism. In a further study, an improved low-temperature acid digestion method was developed by employing a break seal vessel technique which yielded good precision and reproducibility in the long term for a suite of carbonate reference materials in clumped isotope measurements. On the application of the abovementioned technique that was developed, the abundance of multiply substituted “clumped” isotopologues in carbonate minerals (calcite, ankerite, dolomite) was used to derive the apparent equilibrium temperatures of carbonatites. These results reveal that the clumped-isotope temperatures derived for the carbonatites are much lower than typical magmatic temperatures, providing additional mechanistic insights into the formation and modification of the primary rocks due to fluid-rock interaction, recrystallisation, closed-system isotope exchange, and mineralisation of rare earth elements. The thesis further evaluates the effective use of triple oxygen isotope variations in carbonates (methods and applications) in understanding the isotopic fractionations in Earth surface processes and the interaction between different reservoirs. The findings clearly demonstrate the traceability of the isotopic ratios to the different origins and mechanisms of formation of the carbonates. Finally, the thesis work assesses the mantle origin of carbonatites using triple oxygen isotope compositions of several other global carbonatites. It was established that primary igneous carbonatites define a range of triple oxygen isotope values that are independent of mineralogy and overlaps with commonly investigated mantle rocks and minerals.