A geochemical and Nd-Sr-Ca isotopic study of silicate weathering, crustal recycling and petrogenesis of carbonatites

Abstract

Weathering of silicate rocks releases cations and anions which are transported to the oceans by rivers where calcium carbonate precipitates from the seawater. This process results in the net consumption of atmospheric CO2 and has regulated the Earth’s surface temperature over geological timescales. These marine carbonates along with other sediments are eventually subducted resulting in mantle heterogeneity, which is subsequently sampled by mantle-derived magmas. This thesis investigates the geochemical and isotopic effects of weathering of silicate rocks on the surface of the Earth and the role of recycled crustal components in the petrogenesis of unique mantle-derived carbonate mineral-bearing igneous rocks called carbonatites. In the first part of the thesis, the geochemical and isotopic variability of a ~ 2.47 billion years old spheroidally weathered diabase has been studied. The variations in the major and trace element compositions of the weathered rindlets are explained by selective weathering of the rock-forming minerals plagioclase and clinopyroxene. Significant variations observed in the 87Sr/86Sr and 143Nd/144Nd ratios of the weathered rindlets have been used to estimate the timing of the peakweathering event which took place ~1.2-1.3 billion years ago, coinciding with the break-up of the supercontinent Columbia. It has also been shown that selective weathering of plagioclase and clinopyroxene can generate large variations in the stable Ca isotopic composition (δ44/40Ca) in a weathered diabase. These results have important implications for interpreting the δ44/40Ca variability observed in global rivers draining basalts. In the second part of the thesis, major, trace element and Nd, Sr and Ca isotopic compositions of globally distributed carbonatites (wholerock and acid-leached carbonate fractions) have been studied to investigate the role of recycled carbonates in the mantle-source of carbonatites. The δ44/40Ca values of carbonatites of widely varying eruption ages from different geographical locations show a large variability (~0.8 ‰). This variability is explained in terms of mantle mineralogy, recycling of carbonates into the mantle as well as contamination with the continental crust during magma emplacement. Additionally, the δ44/40Ca values of a wide-range of mantle-derived minerals like clinopyroxene, orthopyroxene, garnet, amphibole and phlogopite have been determined. The variation in δ44/40Ca of co-existing mantle minerals has implications for estimating the δ44/40Ca value of the bulk silicate earth (BSE). The acetic acid leached carbonate and non-carbonate fractions of carbonatites show different Nd, Sr and Ca isotopic compositions compared to the corresponding whole-rock samples which is interpreted in terms of isotopic disequilibrium between the coexiting carbonate and non-carbonate phases. Carbonatites and silicate rocks from the ~65 million years old Ambadongar carbonatite complex and the adjacent Phenai Mata region of western India were studied for their geochemical and isotopic compositions to understand the genetic link of these carbonatites with the spatially and temporally associated silicate rocks. The variability in the δ44/40Ca values of these rocks is interpreted in terms of continental crustal contamination, role of subducted carbonates and mantle mineralogy (phlogopite versus amphibole). The Ca isotopic data suggests that the Ambadongar carbonatites are derived from a deeper phlogopite-bearing mantle source whereas the silicate rocks are derived from a relatively shallow amphibole-bearing mantle source.