Equilibrium partitioning and isotopic fractionation of nitrogen between biotite, plagioclase, and K-feldspar during magmatic differentiation

Abstract

A significant portion of the continental crust is composed of plutonic igneous rocks. However, little is known about the geochemical behaviour of N between the different minerals during magmatic differentiation. To provide new constraints for the behaviour of N during crust formation, we have characterised the geochemistry of nitrogen (N) in the compositionally zoned calc-alkaline pluton at Loch Doon, SW Scotland. We present N concentration and N isotope values for whole-rock data alongside biotite, plagioclase and K-feldspar mineral separates and assess the degree to which these data preserve equilibrium partitioning during magmatic differentiation. We show that whole rock likely inherited its N contents and δ15N signatures from the initial source composition and that this signature is homogenous at a pluton scale. Whilst the whole-rock data are best explained as crust-derived N in the source, the degree of homogenisation across a pluton scale is inconsistent with empirical N diffusivities, ruling out syn-emplacement crustal assimilation as the source of N. Instead, our data suggest a crustal signature inherited from depth associated with the Iapetus subduction zone. At a mineral scale, we find that N preferentially partitions into the feldspars over mica in this system in the order K-feldspar > plagioclase ≈ biotite > quartz, with average mineral–mineral distribution coefficients of DN plagioclase-biotite = 1.3 ± 0.6 and DN Kspar-biotite = 2.8 ± 0.6. Partitioning is accompanied by a large and near constant equilibrium isotope fractionation factor between biotite and both feldspars (averages are Δ15NPlag-Biotite = +7.8 ± 1.2‰ and Δ15NKspar-Biotite = +7.9 ± 1.0‰). In contrast, Δ15NKspar-Plagioclase closely approximates 0‰, where both minerals show δ15N values overlapping with the bulk rock δ15N values. These results show that mica crystallisation generates a 15N-depleted reservoir within plutonic rocks. Moreover, our dataset suggests that feldspars might be a more significant host of N in the igneous portion of Earth’s continental and oceanic crust than previously thought.