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Ca Isotopes in Igneous and High-Temperature Metamorphic Systems and the Hydrothermal Chemistry of Paleoseawater

  • Author(s): Antonelli, Michael A
  • Advisor(s): DePaolo, Donald J
  • et al.
Abstract

In the last two decades it has been increasingly recognized that Ca isotope fractionation at high-temperatures can greatly exceed fractionation at surface conditions. These effects have been used to trace recycling of crustal materials into the mantle, to estimate equilibration temperatures in mantle rocks, and to understand kinetic isotope fractionation during diffusion in experimental melts. However, little is known about the competition between kinetic and equilibrium Ca isotope effects in natural samples, which suggests that the use of stable Ca isotope variations as a proxy for carbonate recycling (or for mantle temperature) is significantly underdetermined.

The Ca isotope evolution of continental crust, which is currently understudied, is intimately linked to the geochemical evolution of Earth’s mantle, oceans, and atmosphere. As such, this dissertation focuses dominantly on understanding radiogenic, kinetic, and equilibrium Ca isotope variations in igneous and metamorphic rocks from the continental crust, but also includes radiogenic Sr isotope models of hydrothermal circulation at mid-ocean ridges, which are critical to understanding continental weathering rates and the geochemical evolution of seawater over time.

Chapter 2 explores the potential feed-back between the chemical and isotopic composition of paleoseawater and high-temperature hydrothermal fluids, which has been implied by Sr-isotope measurements in hydrothermal minerals over time (Antonelli et al., 2017). Chapter 3 presents results from radiogenic Ca isotope measurements in lower-crustal rocks and minerals, confirming that K is effectively lost from the lower crust during high-T metamorphism (Antonelli et al., 2018). Chapter 4 focuses on stable Ca isotope measurements in lower-crustal rocks and minerals, demonstrating that kinetic Ca effects are abundant in nature, both at the whole-rock and inter-mineral scales, and that they can be used to understand paragenesis and Ca diffusion in lower-crustal rocks (Antonelli et al., 2019b). Chapter 5 presents stable Ca isotope results from volcanic and sub-volcanic rocks and minerals, which imply that Ca isotopes can be used to estimate crystal growth-rates in igneous crystals (e.g. phenocrysts, comb-layers, and orbicules) and that associated volcanic eruption/recharge events are generally short-lived (Antonelli et al., 2019a).

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