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Rates and Mechanisms of Magmatic Processes: Isotopic and Geochronological Evidence

  • Author(s): Peterson, Brook Thomas
  • Advisor(s): DePaolo, Donald J
  • et al.
Abstract

Magmatism has played an integral part in the evolution of the Earth through geologic time. Mantle-derived mafic magmatism is a primary mechanism by which the bulk silicate Earth segregated into mantle and crust, and the continental crust has further differentiated through metamorphism, anatexis, and assimilation. The study of magmatic processes has given rise to our understanding of how the Earth has evolved.

Central to this understanding are chemical, isotopic, and geochronological studies. Chemical and isotopic studies provide unparalleled richness of information; this is especially true now, as technological advances extend our analytical and computational capabilities. The difficulties presented in geochemical field studies of magmatic processes are primarily twofold: we are mostly limited to surface samples at the end of their evolutionary story (while the processes of interest are those occurring from source to surface), and it can be difficult to disentangle the myriad causes that produce the chemical and isotopic effects observed. These difficulties can be addressed by combined geochemical-geophysical-geospatial studies, by computational modeling based on an extensive (and growing) body of thermodynamic data, and by application of novel geochemical techniques.

As with chemical data, geochronological data are fundamental descriptors in their own right, but they are also crucial because they anchor rates. Almost all geologic processes or features are rate-dependent, from growth of crystals on an atomic scale, to morphologies of volcanoes, to the evolution of the crust. An understanding of chronologies and rates is therefore fundamental to understanding the processes of magmatic evolution.

The diverse chapters in this thesis reflect my diverse interests in the Earth sciences, but all are motivated by desire to understand both fundamental aspects of the chemical evolution of magmas, and how that evolution informs our broader understanding of volcanoes, volcanic fields, and continental crust evolution.

The first chapter is a geochemical study of the San Francisco Volcanic Field in central Arizona. In this study, I combine geochemical, geochronological, geospatial, and isotopic data with the computational thermodynamic modeling package alphaMELTS to understand the chemical evolution of the volcanic magmas from source to surface. I show that the isotopic and chemical characteristics of the SFVF can be largely explained by assimilation of the deep crust by parent magma derived from fertilized asthenospheric mantle. The computational treatment with alphaMELTS allows for quantitative limits to be placed on the mineralogy, chemistry, and dynamics of the source of the volcanic field.

The second chapter is a geochronological study of magmas from the Hawaii Scientific Drilling Project phase 2 core. This study works to extend the U--Th/He dating technique to olivine phenocrysts in relatively undegassed submarine basalts, rocks which are normally difficult to date by other chronometers.

The third chapter investigates the evolution of felsic magma chemistries and the changing nature of continental crust source rocks during the Himalayan collision. In this study, I apply relatively new (Mg, Ca) and established (Sr, Nd) isotopic techniques to a sample suite of diverse crustal lithologies (primarily granitoids and felsic volcanics) from the Lhasa, Tibet region. I show that Ca and Mg isotopes are sensitive to specific mineralogies and processes, including carbonate-bearing shale assimilation, and that the Ca and Mg isotopic systems are promising tools in studies of granite petrogenesis, metamorphism, assimilation, and subduction recycling.

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