NITROGEN PARTITIONING AND ISOTOPIC FRACTIONATION IN GRANITIC ROCKS AND MINERALS: A CASE STUDY OF THE DEVONIAN LOCH DOON PLUTON, SW SCOTLAND

Nitrogen is an essential element for life, the 7^th most abundant element in the solar system and forms 78% of the modern atmosphere. However, many questions remain concerning the behaviour of nitrogen in geological systems. Nitrogen has long been considered an atmophile element. But it is now acknowledged that the geochemistry of nitrogen in geological systems played a key role in the development of Earth as a habitable planet. Current estimates of the nitrogen content of the continental crust are strongly skewed towards sedimentary facies, with limited existing granitic nitrogen studies. It is therefore imperative to develop a more quantitative understanding of nitrogen partitioning and isotopic fractionation during the differentiation of igneous systems. Here we present a detailed petrological and isotopic study of nitrogen geochemistry in a zoned pluton, Loch Doon, SW Scotland (outer diorite through to inner granite). The pluton is formed by nested incremental growth and accompanying fractional crystallisation in a calc-alkaline system (plagioclase-biotite-pyroxene). We focus on the partitioning and associated isotopic fractionation of nitrogen between biotite, orthoclase, plagioclase and bulk rock. Our findings indicate that assimilation of crustal material coupled with subducted slab fluids acted as the main source of nitrogen to these melts, supported by carbon and strontium isotopes, and trace metal geochemistry. We assess the degree to which oxidation state of the melt has influenced partitioning of nitrogen and fractionation of its isotope values and we evaluate the impacts of fractional crystallisation on nitrogen contents and isotope values. We find that nitrogen preferentially partitions into biotite and orthoclase with distinct isotopic compositions that differ by several permil, indicating that partitioning of nitrogen into different phases during crystallisation imparts a distinct isotopic fractionation. This dataset shows that relatively unaltered granites can potentially store more nitrogen than previously assumed, implying that the igneous portion of the continental crust is a significant nitrogen reservoir.