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

Boocock, Toby J.; Stüeken, Eva E.; Bybee, Grant M.; König, Ramona; Boyce, Adrian J.; Prytulak, Julie; Buisman, Iris; Mikhail, Sami

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.

Citation

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

• Journal Article Type: Article
• Acceptance Date: Jul 11, 2023
• Online Publication Date: Jul 17, 2023
• Publication Date: 2023
• Deposit Date: Oct 24, 2023
• Publicly Available Date: Oct 24, 2023
• Journal: Geochimica et Cosmochimica Acta
• Print ISSN: 0016-7037
• Publisher: Meteoritical Society
• Peer Reviewed: Peer Reviewed
• Volume: 356
• Pages: 116-128
• DOI: https://doi.org/10.1016/j.gca.2023.07.010
• Public URL: https://durham-repository.worktribe.com/output/1816570