Rock Magnetic Properties vis-à-vis Thermal Gradient in Volcanic Provinces: Studies from Rajmahal Traps, India

The study aims in developing a relationship between thermal gradient and rock magnetic parameters in igneous provinces. For the study, the Rajmahal Traps (~115 Ma) of eastern India are considered. Rock magnetic analyses including magnetic susceptibility including temperature dependence of magnetic susceptibility (χ-T plots), Isothermal Remanent Magnetization (IRM) acquisition and their backfield demagnetization of saturation IRM are carried out. Inter-parametric ratios like SIRM/χ, hard IRM, soft IRM and S-ratio are also calculated. A parameter, thermo-magnetic gradient (which equals the numerical difference between the heating and cooling Curie temperatures) is determined from the χ-T plots. The thermo-magnetic gradient is used to determine the variation of cooling rate from the eruptive centre towards the periphery of the lava flow. The thermo-magnetic gradient appears compatible with the distance from the eruptive centres and decreases with increasing distance i.e. decreases with increasing cooling rate. Thus, the thermo-magnetic gradient is considered a useful proxy for the cooling rate of lava flow. Once the relationship between the thermo-magnetic gradient and cooling rate is established, the variation in the values of the rock magnetic parameters and inter-parametric ratios are studied concerning cooling rate. From the results of the study, it is evident that the magnetic concentration (χ and SIRM) and grain-size (SIRM/χ) parameters have harmonic relations with the cooling rates of the lava flow. The SIRM and χ decreases with increasing cooling rate marking decreasing ferrimagnetism from the eruptive centre towards the periphery in an igneous province. The SIRM/χ also decreases with increasing cooling rate, depicting an abundance of Stable Single Domain (SSD) grains at the eruptive centre which gradually decreases towards the periphery. However, the magneto-mineralogical parameters (hard IRM, soft IRM and S-ratio) are independent of the thermo-magnetic gradient and cooling rate in igneous provinces.