The behaviour of nitrogen during differentiation of Earths igneous continental crust

Abstract

The geological and biological nitrogen cycles are intimately linked. Nitrogen is an essential element in the structure of amino acids, proteins, nucleic acids, and other molecules vital for life, as well as 78% of the Earths atmosphere and present in significant quantities in all of Earths geological reservoirs. Planetary scale processes such as degassing, subduction and differentiation play a key role in generating and supporting a nitrogen rich surface environment suitable for supporting life. However, the fluxes and stores of nitrogen between Earths mantle, crust and atmosphere remain poorly constrained. Constraining the behaviour of nitrogen during these processes is therefore essential. This thesis targets the processing and storage of nitrogen in the Earth’s crust, with a focus on identify fundamental partitioning and isotopic fractionation during magmatic differentiation and assessing what the crustal nitrogen record can inform us about changes in the biogeochemical nitrogen cycle. Firstly, I utilise two contrasting but well constrained magmatic systems, the aphyric lavas from Hekla volcano, Iceland, and the calc-alkaline Loch Doon zoned pluton, Scotland. These data are to the best of our knowledge the first combined abundance and isotopic ratio measurements for differentiating magmatic suites at a bulk and mineral scale. I show with the Hekla dataset that when undersaturated in a magma, nitrogen behaves more akin to lithophile rather than volatile elements and can be enriched significantly in Earths crust during differentiation. In Loch Doon, at a mineral scale, we find feldspars host 60-90% of the whole rock nitrogen contents in contrast with previous studies suggesting biotite as the main N host phase. Associated with this are significant feldspar-mica isotopic fractionation suggesting magmatic differentiation can impart significant measurable isotopic differences. Secondly, I assess the ability of the plutonic felsic crust to record changes in the terrestrial sedimentary nitrogen cycle through analysis of a suite of well characterised strongly peraluminous granitoids (SPGs). These data show a significant change in nitrogen biomass burial coincident with sedimentological, pedogenic and biological innovations coincident with the rise of a complex terrestrial biosphere at the onset of the Phanerozoic. Overall, this thesis improves our understanding of nitrogen in magmatic systems, shows that studying nitrogen in Earths igneous crust is essential for balancing the geobiological nitrogen cycle, and provides a strong basis for more work on igneous systems for future research.