Rock magnetic properties as a proxy for cooling rate in basaltic lavas: insights from Rajmahal Traps, Eastern India.

The present study aims to determine the relationship between cooling rate and rock magnetic parameters in the Rajmahal continental flood basalts of eastern India. Rock magnetic analyses are carried out which include magnetic susceptibility (χ) and its temperature (T) dependence (χ-T plots), Isothermal Remanent Magnetization (IRM) acquisition and backfield remanence coercivity of the IRM and Saturation IRM (SIRM). Also, some inter-parametric ratios like SIRM/χ and S-ratio are calculated. A thermo-magnetic deviation parameter (defined as the numerical difference between the Curie temperatures determined from the heating and cooling curves) is determined from the χ-T plots. Thermo-magnetic deviation varies with the distance from the eruptive centres, specifically, it decreases with increasing distance from the eruptive centre and cooling rate and is thus, considered a useful proxy for the cooling rate of lavas. It is shown that the concentration-dependent parameters (χ and SIRM) and grain-size related parameters (SIRM/χ) have direct or indirect proportionality with the cooling rates. SIRM and χ decrease with increasing cooling rate and mark a decrease in ferrimagnetic mineral concentration from the eruptive centre towards the periphery of the volcanic province. The SIRM/χ either decreases or shows an even pattern with increasing cooling rate, suggesting an abundance of Pseudo Single Domain (PSD)-sized magnetic grains. Despite this, the composition of the magnetic minerals (depicted by the S-ratio) is independent of the thermo-magnetic deviation and the cooling rate in the volcanic province. Research highlights: Study of rock magnetic properties with respect to cooling rate. Determination of the magnetic proxies to rate of cooling in a volcanic province. Introduction of a rock magnetic parameter called "thermo-magnetic deviation". [ABSTRACT FROM AUTHOR]