Nitrogen characteristics of the lower continental crust and its geological significance
Creators: Du, Yifan
Publisher: name: University of Alberta Library
Year: 2023

<p>The geological nitrogen (N) cycle between Earth&#39;s deep interior and surface is a crucial aspect to understand the origin and fate of N in Earth&#39;s crust and atmosphere. One particular knowledge gap in the study of the geological N cycle is that the N budgets of most of the reservoirs in the solid Earth have not been well determined despite that speculative estimates suggest that over 70% of the Earth’s N is stored in the crust and mantle. In this thesis, I carried out a N stable isotope study on lower continental crustal rocks from around the globe aiming to adding in new understanding of the N signature (sources, gaining or losing mechanisms, isotopic composition) of the lower continental crust.</p>

<p>Chapter 1 is an introduction of the current state of research on the deep N cycle. </p>

<p>Chapter 2 is focused on a detailed investigation of the source and controlling mechanism on the N signature of the lower continental crust in the North China Craton (NCC). Two sets of petrologically and geochemically well studied lower crust samples in the NCC were used for this study. One set is composed of the Paleoproterozoic granulite-facies rocks exposed in the Huai’an terrane, and the other is a set of granulite-facies xenolith samples brought up by the Neogene Hannuoba intraplate basalts. These xenolith samples are composed of both Paleoproterozoic lower crust samples (minor) and their reworked equivalents (major) by mantle metasomatism during the cratonic destruction in 160-140 Ma. The comparison between these two sets of samples can provide valuable insights into the sources and affecting processes (e.g., crust-mantle interaction) of the N in the lower crust. The results show that, compared with the relatively low N concentrations (9.8 to 21.3 μg/g; average: 14.9±3.2 μg/g, 1sd, n=9) and mostly positive d15N values (-2.3‰ to +6.2‰; average: +2.2±2.4‰, 1sd, n=9) of the terrane samples, the xenolith samples exhibit systematically higher N concentrations (16.9 to 100.9 μg/g; average: 37.0±21.2 μg/g, 1sd, n=18) and much lower d15N values (-9.8‰ to +4.0‰; average: -1.9±3.7‰, 1sd, n=18). The data show a negative trend between N concentration and d15N value with high N concentrations coupled with low d15N values. This trend can be best explained by that the xenolith samples have assimilated variable amounts of ammonium produced by abiotic reduction of N2 released from the mantle following the mantle underplating during the destruction of the NCC. Modeling of these data requires an N2 outflux of 2.2E+9 to 3.6E+9 mol/yr (or 6.1E+10 to 1.0E+11 g N/ yr) from mantle degassing during this cratonic destruction period (160 to 140 Ma). Based on a C/N ratio of 295 of the mantle, I further estimated the CO2 released by the mantle degassing during this period to be 1.3E+12 to 2.1E+12 mol/ yr (or 1.5E+13 to 2.5E+13 g C /yr or 15 to 25 Mt C/ yr). This large flux could play an important role for the elevated atmospheric CO2 concentrations in the Early Cretaceous period.</p>

<p>Chapter 3 is focused on a better constraint on the N budget and d15N signature of the lower continental crust (LCC) at a global scale. In order to compile a representative dataset for the LCC, I analyzed 8 granulite-facies metasedimentary rocks from the Bohemian Massif (n=8) and 6 granulite-facies rocks form the Tongbai Orogen in China (3 felsic granulites and 3 mafic granulites). These data, together with the 27 data in Chapter 2, were integrated with previously published data of 80 LCC samples around the world, to gain insight into the N budget and d15N signature of the LCC. </p>

<p>Overall, these new data advanced our understanding of the deep N cycle involving granulite-facies metamorphism and crust-mantle interaction. </p>