| dc.description.abstract | Earth is the only rocky planet in the Solar System with a compositionally evolved (felsic) continental crust. This incompatible element enriched layer is dominantly formed by extraction of melt from the upper mantle leading to the formation of a complimentary depleted mantle reservoir. Although volumetrically insignificant (~0.6% of mantle mass), the continental crust contains a significant proportion (up to 60%) of incompatible trace elements. The emergence of a felsic crust paved the way for complex nutrient cycling and diversification of life. The exposed continental crust is further chemically weathered to release cations and anions, which are eventually transported to the oceans. Silicate weathering has implications for oxygenation of the atmosphere and acts as a thermostat by modulating atmospheric greenhouse CO2 concentrations over time. This thesis utilizes geochemical, radiogenic and metal stable isotopes to study the continental crust and can be divided into two parts.
The first part of the thesis attempts to constrain the evolving chemical composition of the continental crust by studying geological archives such as glacial diamictites and loess. Glacial sediments form by mechanical abrasion of the bedrock in cold and dry climate, which limits chemical weathering. Thus, they are considered as robust compositional archives of the upper continental crust (UCC). The glacial diamictites studied in this thesis (n = 24) are from four continents (North and South America, Africa and Asia) and varying geological ages (Mesoarchean, Paleoproterozoic, Neoproterozoic and Paleozoic). Investigation of source signatures using radiogenic Sr isotopic composition (87Sr/86Sr) of the diamictites reveal variable post-depositional weathering changes which perturbed the Rb/Sr (up to 90% Sr loss) ratio and led to erroneous initial 87Sr/86Sr calculated at sedimentation and mantle separation ages. The measured Rb/Sr are corrected by combining 87Sr/86Sr and 143Nd/144Nd ratios to estimate the time-integrated Rb/Sr (Rb/SrTI) and subsequently, calculate the initial 87Sr/86Sr (87Sr/86SrTI) at sedimentation age. The Rb/SrTI displays secular variation, which coincides with major episodes of crustal growth. Further, the Rb/SrTI and 87Sr/86SrTI of the Paleozoic diamictites agree well with modern UCC estimates, making it a robust archive of crustal growth reflecting differentiation. Multiple crustal growth models (exponential, delayed and punctuated) have been proposed; in this study the crustal growth curves were numerically modelled to constrain the geochemical evolution of the continental crust using a multi-reservoir open system model with mass exchange along open boundaries. Results show that the exponential growth model best approximates the 87Sr/86Sr ratios of the UCC as constrained from the Paleozoic diamictites. The diamictites also have very high K/Ca ratios (up to 8) and high radiogenic Ca (d40/44Ca up to 22.6) which is reflected in their d44/40Ca compositions due to ingrowth of radiogenic 40Ca. Another archive used to estimate present-day UCC composition is wind-blown glacial sediments or loess. The loess samples of this study (n = 9) were deposited after the recent Pleistocene glaciation and sampled across three different continents (Australia, North America and Europe). A multi-isotopic investigation of stable and radiogenic Ca and Sr isotopes was conducted for these loess samples; the study reveals that these sediments reflect homogenized provenance signatures rather than the UCC composition and highlights the limitations of loess as archives of UCC isotopic composition, especially for soluble elements.
The second part of this thesis aims to understand the geochemical and isotopic modifications of crustal rocks during weathering. A multi-poxy study was performed on saprolites (n = 16) developed on a metadiabase dike (parent rock) from South Carolina, USA. The geochemical and isotopic (87Sr/86Sr, 143Nd/144Nd, d44/40Ca and d88/86Sr) compositions of these saprolites display significant variation across the weathering profile. Allochthonous dust inputs or selective mineral dissolution cannot explain the observed geochemical and isotopic variabilities. The d44/40Ca of the saprolites are heavier than the parent rock and increase with progressive weathering as indicated by increasing CIA values and decreasing [Ca]Ti and bulk density. The d44/40Ca variability is best explained by secondary clay formation, which preferentially incorporates the heavier Ca isotopes. These saprolites have extremely high rare earth element (REE) concentrations (up to 2633 ppm) at shallower levels (0-6 m) compared to the parent rock sample (55 ppm, 30 m depth) and display highly fractionated, LREE enriched patterns. Additionally, the saprolites display more radiogenic 87Sr/86Sr and less radiogenic 143Nd/144Nd compared to the parent rock. The REE-enriched and radiogenic 87Sr/86Sr signatures of the saprolites are derived from the weathering of the surrounding Liberty Hill granite which has radiogenic 87Sr/86Sr and contains LREE rich accessory minerals such as titanite. The REE patterns of the saprolite are further affected by changes in chemical conditions (e.g., redox and pH), ligand complexations, and mineralogy of secondary minerals (kaolinite versus smectite) with kaolinites preferentially adsorbing the LREEs. The relative proportion of parent rock and granite is used to estimate the d88Sr composition of the weathering fluid. Kaolinite rich saprolites also display high d88Sr while the smectite rich samples show lower d88Sr indicating that mineralogy of secondary clays exerts a strong control on d88Sr compositions in weathered profiles | en_US |
