Oxygen Isotope Studies of the Petrogenesis of Hawaiian Lavas and a Theoretical Study on Equilibrium Thermodynamics of Multiply-Substituted Isotopologues

Citation

Wang, Zhengrong (2005) Oxygen Isotope Studies of the Petrogenesis of Hawaiian Lavas and a Theoretical Study on Equilibrium Thermodynamics of Multiply-Substituted Isotopologues. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/zrza-wc92. https://resolver.caltech.edu/CaltechETD:etd-05192005-000025

Abstract

Temporal oxygen isotope variations have been discovered in both Mauna Kea and Koolau volcanoes after detailed studies of the Hawaiian Scientific Drilling Project (HSDP-phase II) and Koolau Scientific Drilling Project (KSDP). In both volcanoes, oxygen isotope compositions of olivine phenocrysts all converge to 'Mauna Loa-like' values from depleted or enriched composition, respectively and yet, the interpretations are totally different. In the studies of HSDP core, many lines of geochemical evidences strongly indicate that low-[delta18]O values characteristic of Kea trend Hawaiian lavas are the product of contamination by hydrothermally altered rocks in the volcanic edifice. In the studies of KSDP core and submarine Koolau landslides, a full diversity of [delta18]O values correlate with trace element and radiogenic isotope compositions of host lavas. These correlations are interpreted as reflecting mixing between partial melts of high-[delta18]O and normal-[delta18]O components. A magma mixing model has been constructed to simultaneously explain the oxygen isotope, major element, trace element and radiogenic isotope variations defined by these correlations. This model indicates that the Koolau shield-building lavas are mixtures of 75~100% melt of mantle peridotite with 0~25% eclogite melt, the protolith of which is Mid-Ocean Ridge Basalt (MORB) with Depleted Mantle Model (DMM) age of at least 1.8±0.3 billions years. [delta18]O value of this eclogite component is predicted to be 11.3±1.5‰, implying it is an upper crustal (layer 1 or 2) basalt or gabbro with a low-temperature alteration history; contains a small amount of sediment; and has experienced partial melting before forming eclogite.

I also derived a method for systematically evaluating clumped-isotope effects in the abundances of all isotopologues in thermodynamically equilibrated populations of O2, CO, N2, NO, CO2 and N2O between 1000 and 193 to 77 K. This method uses Urey-type algorithms to evaluate partition functions and equilibrium constants of isotope exchange reactions, and simultaneously solves for abundance of each isotopologue of a given molecule constrained by all independent equilibria. Calculation results show that, in most cases, multiply-substituted isotopologues of these mono-molecular gases are predicted to be enriched relative to stochastic (random) distributions by ca. 1 to 2 per mil at earth-surface temperatures.

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