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

Boocock, T. J., Stüeken, E. E., Bybee, G. M., König, R., Boyce, A. J. , Prytulak, J., Buisman, I. and Mikhail, S. (2023) Equilibrium partitioning and isotopic fractionation of nitrogen between biotite, plagioclase, and K-feldspar during magmatic differentiation. Geochimica et Cosmochimica Acta, 356, pp. 116-128. (doi: 10.1016/j.gca.2023.07.010)

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.

• Item Type: Articles
• Additional Information: We acknowledge funding from a NERC studentship (to TJB; NE/R012253/1), a National Environmental Isotope Facilities grant (to EES, SM, and TJB; NEIF – 2313.0920), a NERC Frontiers grant (to EES; NE/V010824/1) and a NERC standard grant (to SM; NE/PO12167/1).
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Boyce, Professor Adrian
• Authors: Boocock, T. J., Stüeken, E. E., Bybee, G. M., König, R., Boyce, A. J., Prytulak, J., Buisman, I., and Mikhail, S.
• College/School: College of Science and Engineering > Scottish Universities Environmental Research Centre
• Journal Name: Geochimica et Cosmochimica Acta
• Publisher: Elsevier
• ISSN: 0016-7037
• ISSN (Online): 1872-9533
• Published Online: 17 July 2023
• Copyright Holders: Copyright © 2023 The Author(s)
• First Published: First published in Geochimica et Cosmochimica Acta 356:116-128
• Publisher Policy: Reproduced under a Creative Commons license