Earth and Planetary Sciences ETDs

Publication Date

Summer 7-29-2017

Abstract

Volatiles (N2, CO2, and He) are released by volcanism and hydrothermal activity during continental rifting and subduction processes. Analyses of volatile components have been conducted to obtain gas contents (CO2, SO2, H2S, N2, Ar, He, and so on), stable isotope compositions (δ13C, δ15N, and so on), and noble gas isotopes (3He/4He, 40Ar/36Ar, and so on). This dissertation includes four chapters to report new nitrogen isotope fractionation factors of bubbling gases during gas-water transfer at various water temperatures (Chapter 1), first measurements of massive amounts of CO2 released by incipient continental rifting in the Magadi and Natron Basin, East African Rift (Chapter 2), new results of gas chemistry, stable isotopes, and noble gas isotopes of hot springs in the same area as Chapter 2 (Chapter 3), and new nitrogen isotope compositions of springs at the Costa Rican subduction zone (Chapter 4). Chapter 1 (published in Geochemical Journal) is the first experimental work to acquire nitrogen isotope fractionation factors during N2 gas and water transfer at various temperature to examine nitrogen isotope shift for hydrothermal systems. This work reports measured δ15N values of dissolved N2 gas at 5 to 60°C. We obtained δ15N values of 0.91, 0.73, -0.04, and -0.42‰ at 5, 20, 40, and 60°C, respectively. Nitrogen isotope fractionation depending on temperature is more significant than previously published results, showing an isotopic 'crossover' at 40°C. A kinetic incorporation of 14N into water is enhanced by rising temperatures could explain the steep temperature dependence. In hydrothermal systems, small negative δ15N values could be attributed to kinetic fractionation between dissolved N2 and N2 in air. Chapter 2 (published in Nature Geoscience) is the first estimation of diffuse CO2 degassing (“Tectonic Degassing”) along faults which are away from active volcanic centers in the East African Rift. We used results of diffuse soil CO2 measurements combined with carbon isotopic compositions to quantify the flux of CO2 and constrain CO2 sources. This study reports that 4.05 mega tons per year of mantle-derived CO2 is released by faults penetrating the lower crust in the Magadi-Natron Basin. Extrapolated CO2 flux (71±33 mega tons per year) of the entire Eastern rift (~3,000 km long from Afar to Mozambique) is comparable to CO2 emission from the entire mid-ocean ridge system (53-97 mega tons per year). Therefore, widespread continental rifting and super-continent breakup could result in massive and long-term CO2 emissions, contributing prolonged greenhouse conditions likely during the Cretaceous. Chapter 3 (published in Journal of Volcanology and Geothermal Research) reports new results of gas compositions, stable isotopes (O, H, N, and C), and noble gas isotopes (He and Ar) of hot spring samples from the Magadi and Natron basin in the East African Rift (EAR). In dissolved gases, CO2 is the most abundant deep and shallow sources are mixed based on the N2-He-Ar abundances. δ18O and δD values of the springs waters indicates that the local meteoric water is dominant with minor evaporation. Most of δ15N and δ13C values and 3He/4He ratios suggest that Subcontinental Lithospheric Mantle (SCLM) is the major mantle source. The 4He flux values, significantly greater than the reported mean of global continental flux values, imply that elevated mantle 4He flux is due to magmatism and related heating, and crustal 4He is released by fracturing of old rocks in the Tanzanian craton and Mozambique belt. SCLM-derived volatiles can be ascribed to that a relatively small volume of lithospheric mantle has been replaced by asthenosphere during incipient rifting (Ma). Chapter 4 (in preparation) shows new results of gas compositions, nitrogen, and helium isotopes of springs at forearc and arc front areas in Costa Rica. Nitrogen isotope compositions (9-11N°) with less pelagic sediment contribution compared to further north (Guatemala and Nicaragua) result in insufficient nitrogen output to the atmosphere. This work supports the subduction erosion model in conjunction with seamount subduction. The overlying forearc crustal materials incorporated into the Costa Rican subduction zone dilute sediment-derived nitrogen signals. These results support the deep recycling of N into the deep mantle.

Degree Name

Earth and Planetary Sciences

Level of Degree

Doctoral

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Tobias P. Fischer

Second Committee Member

Zachary D. Sharp

Third Committee Member

Laura J. Crossey

Fourth Committee Member

Karl E Karlstrom

Fifth Committee Member

Jolante van Wijk

Project Sponsors

National Science Foundation

Language

English

Keywords

Volatiles, Hydrothermal systems, Volcanoes, Mantle, Stable isotopes, Noble gas geochemistry

Document Type

Dissertation

Available for download on Wednesday, July 31, 2019

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