Storage of Nitrogen in Silicate Minerals and Glasses

The processes by which organic nitrogen (N) is incorporated and redistributed within the geosphere are important for understanding modern volatile recycling and longer-term Earth degassing and atmosphere evolution. The N isotope system shows great potential for tracing the transfer of volatiles among Earth's major reservoirs, including the transfer of organic N into solid inorganic phases. In order to fully exploit the N isotope system as a tracer of pathways of organic components into the solid inorganic Earth, numerous high- and low-temperature processes must be examined closer. This research explores part of the biogeochemical cycle, focusing on three separate pathways for the transfer of organic N into solid inorganic phases in the shallow crust and on the sea floor.;Chapter 1 focuses on the potential for the storage of N (i.e., molecular N and possibly also as ammonium) within micropores or channels of the cyclosilicate minerals beryl and cordierite. Nitrogen in these systems is believed to be derived ultimately from the diagenesis of organic matter within low-high metamorphic rocks; however, the N concentrations and isotopic compositions of cyclosilicates in pegmatites obviously could reflect a wide range of metamorphic and igneous processes, including subsolidus devolatilization, partial melting, and later differentiation of melts. In metasedimentary rocks, organic N is transferred into clay minerals and later released into fluids during prograde dehydration reactions and can be incorporated into cyclosilicates during or after formation. For both the metamorphic and the igneous samples examined in this study, isotopic analyses of the molecular N residing in cyclosilicates could help elucidate fluid-rock interactions and could potentially contribute information regarding fluid-mineral fractionation useful in a wider range of studies employing the N isotope system.;Chapter 2 explores the possibility that measurable amounts of organic N, as molecular N, may be incorporated into the cages of low-temperature microporous silica phases. Melanophlogite, a silica clathrasil, was used in this study because it forms in hydrothermal settings and contains cages known to house molecular N. The hypothesis tested in this research was whether molecular N in the cages of melanophlogite would be retained at temperatures lower than those of its crystallization (<100°C), and if so, the extent to which the isotopic compositions of this molecular N could provide useful information regarding the source(s) of the N and the fluid-rock processes operating during their formation.;The work presented in Chapter 3 examined the ability of palagonized volcanic glasses to incorporate and retain N likely delivered from pore fluids in altered ocean sediments. The chemical exchange of hydrothermal fluids and/or seawater with oceanic crust leads to changes in the chemical compositions of oceanic crust and is viewed as likely impacting N crust-mantle cycling. Volcanic glasses are easily replaced by hydrous phases such as clays or zeolites and are more susceptible than silicates to microbial alteration. Because volcanic glass alters more readily than silicate minerals, the glass could, for some elements (such as N), contribute more significantly to the overall chemical mass-balance of seafloor alteration and could be an important pathway for geochemical cycling within the Earth, especially when it becomes subducted. This study focused on the N concentrations and isotopic compositions of suites of volcanic glasses showing a spectrum of alteration intensities that have been previously characterized for evidence of microbial alteration. Differences in the N contents and d15N of unaltered and altered glasses from the same section were compared, leading to some hypotheses regarding the sources of the N and the timing of incorporation relative to the cooling and degassing the lavas and later lower-temperature alteration.