Limitations of chemical weathering indices and dating the peak weathering event using Nd, Sr isotope ratios in a hand-specimen sized spheroidally weathered diabase

Weathering of silicate rocks is a sink for atmospheric CO2 and plays an important role in long-term variability of terrestrial climate. The intensity of silicate weathering is geochemically quantified by considering the relative proportion of fluid-immobile to fluid-mobile elements in the weathered rock and several such chemical weathering indices (e.g., CIA) are widely used. We investigated the mobility of major and trace elements during spheroidal weathering of a hand-specimen sized 2.37 Ga old diabase, part of a major dyke complex in southern India. The values of the Chemical Index of Alteration (CIA) of the sub-samples show a wide range, some higher and some lower than the un-weathered dyke. Variations in the CIA values of the samples as well as the concentrations and ratios of major and trace elements including REEs like Eu are explained in terms of differential weathering of the major constituent minerals of the diabase, plagioclase and pyroxene. We conclude that at the hand-specimen scale, the variability in the weathering indices like CIA are controlled by differential weathering of minerals and might not accurately reflect the intensity of weathering. Considerable Nd and Sr isotopic variability at the hand-specimen scale is explained in terms of weathering-related fractionation of parent/daughter ratios, the fractionation having taken place long time ago to allow for radiogenic decay of the long-lived isotopes of 87Rb and 147Sm. The spread in the initial Sr and Nd isotope ratios of the weathered samples reach a minimum value around 1.2-1.3 Ga which we interpret as the timing of the peak weathering event which led to fractionation of the parent/daughter ratios. The timing of the weathering event coincides with the timing of the breakup of the Columbia supercontinent and follows wide-spread alkaline volcanism in the Indian subcontinent. This is the first such attempt to determine the timing of a weathering event in rocks using long-lived radioactive isotopes.