Stable and Radiogenic Isotope Studies of Iron-oxides as Paleoenvironmental and Tectonic Archives

Citation

Miller, Hayden Bryce Dutcher (2019) Stable and Radiogenic Isotope Studies of Iron-oxides as Paleoenvironmental and Tectonic Archives. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/945X-R138. https://resolver.caltech.edu/CaltechTHESIS:11072018-165610818

Abstract

Geochemical records of continental weathering environments are limited despite their critical value to understanding how past climates functioned. This thesis seeks to address this limitation by drawing together innovative lines of research in geochronology, stable isotope geochemistry, and chemical weathering. Two distinct projects are described; each project designed to provide new insight into the paleoenvironmental and tectonic history of continental weathering environments. These projects, though distinct in their methods and samples, are unified by their goal: to use the stable, radiogenic, and nucleogenic isotopic composition of iron oxides to provide new constraints on the geologic history of continental weathering environments.

The weathering of Fe-bearing rocks, coupled with the extreme insolubility of iron in moderately acidic to alkaline oxic waters, causes both goethite and hematite to be abundant chemical precipitates in near-surface environments. Goethite is favored in lower temperature and more acidic or alkaline conditions, while hematite precipitates more readily in near-neutral environments. These minerals are found in soils; spring, bog, and stream deposits; oxidized chemical sediments; and hydrothermal deposits. In many cases, substantial crystalline masses occur, which can take the form of nodules, pisoliths, botryoidal, stalactitic, and radiating masses, fibrous needles, pseudomorph, veneers, or as aggregates of flakes, tabular, or anhedral crystals. Time and temperature are arguably the two most fundamental variables we as geologists seek to constrain, and iron oxide deposits can provide a valuable archive of information on low-temperature, near-surface planetary processes.

The first project investigates how the stable oxygen isotopic composition of goethite, when combined with direct He dating on the same texturally resolved scales as stable isotope analyses, can be used to interpret water sources (Chapter 1) and formation temperatures (Chapter 2). The first chapter creates a record of the paleolatitudinal gradient in the oxygen isotope composition of meteoric water. The major finding of this study is the consistency in this gradient over geologic time. This second chapter proposes a new geothermometer using the intracrystalline oxygen isotopic composition of goethite. While stable isotopic compositions of goethite have long been utilized as a tool for reconstructing paleoenvironmental conditions, previous studies have focused on the bulk concentration of stable isotopes within this phase. Since goethite has two structurally non-equivalent oxygen sites, we show it is possible to extract two isotopically unique populations of oxygen, the composition of which we interpret to be dependent on temperature at time of mineral formation. In combination with the ability to directly date goethite by the (U-Th)/He method, we may utilize goethite to constrain both the temperature and timing of goethite formation, providing a valuable archive for information on continental paleoenvironments.

The second project utilized the paired He-Ne chronometer and ⁴He/³He method in hematite to produce thermal histories of the ancient Kaapvaal Craton over billion-year timescales. We applied these methods to hematite ore hosted within the Transvaal Supergroup in the Griqualand West (Chapter 3) and Transvaal Basin region (Chapter 4) of the ancient Kaapvaal Craton, South Africa. The application of hematite geo- and thermochronometry to these multi-billion year-old deposits represents the most challenging environments these methods have yet been applied to. We found, in some localities, hematite He-Ne ages provided further support of existing indirect age constraints on the timing of ore formation. In other localities, we found hematite He-Ne ages are uncorrelated with known tectono-thermal events. Modeled time-temperature histories indicate the Kaapvaal Craton has experienced exceptionally slow erosion rates over the last billion years, providing further evidence for the extreme tectonic stability of cratonic interiors over geologic timescales. This slow erosion took place over vast intervals of time, during which the craton was undergoing oxidative weathering, offering an additional constraint on understanding the history of atmospheric O₂ during Proterozoic time.

Thesis Files