IAP2-22-356

The elemental and isotopic partitioning behaviour of nitrogen during magmatic differentiation

The geochemistry of nitrogen in high temperature systems is a topic rife with intrigue. For example, debate is ongoing regarding the crust-mantle tectonic flux(es) and the storage capacity of Earth’s silicate reservoirs. This is in part because the behaviour of nitrogen is complex, given that nitrogen is redox sensitive where the main species of nitrogen in the Earth system – nitride, oxide, N2, and ammonic nitrogen – share no overlapping behavioural traits and the redox conditions required to convert between these species overlaps with key redox gradients of the bulk silicate Earth. We have designed a PhD project which capitalises on recent results and developments, led by the supervisors, which will speak to the geochemistry of nitrogen in igneous systems. The big picture goal is to generate an empirical model to quantify the role of the continental crust in the modulation of Earth’s atmospheric mass over billion-year timescales.

The PhD student will examine a combination of volcanic and plutonic samples from different tectonic settings. The samples cover both alkali and subalkaline intrusive and extrusive igneous rocks. The diversity of samples in this study have been selected to constrain the role and effect of specific variables (P-T-X-fO2). For example, the work to speak towards characterising a broad range of nitrogen reservoirs, from mantle sources (gabbro, carbonatite) to the continental crust (granite, rhyolite). In addition, this project will speak towards key fundamental processes including:

[1] stable isotope fractionation during magma evolution (all samples)

[2] mineral-mineral partitioning between key igneous mineral phases (clinopyroxene, biotite, plagioclase, and orthoclase) using samples from the Boggy Plain zoned pluton

[3] the effect of igneous differentiation on volcanic degassing using samples from alkali stratovolcanoes in the Afar and main Ethiopian rifts (e.g., Dabbahu and Aluto)

Methodology

This PhD project applies the fundamentals of igneous petrology to the geochemistry of nitrogen. The student will examine the elemental and isotopic partitioning behaviour of nitrogen between key igneous mineral phases (clinopyroxene, biotite, plagioclase, and orthoclase) alongside whole rock data from a zoned pluton and suites of differentiated volcanic units.

This work builds on recent analytical developments at St Andrews which enable us to quantify nitrogen abundances and stable isotope values down to ca. 3 μg/g in silicate phases (Boocock et al., 2020). In addition, we have recently installed a Soft-X Ray Spectrometer and acquired a new Electron Microprobe which will be employed to quantify nitrogen speciation and mineral chemistries with high-spatial resolution (on the micron scale). The project will determine several key variables required to develop dynamic empirical models:

[a] the partitioning behaviour of nitrogen during igneous differentiation

[b] the controls on the behaviour of nitrogen partitioning (mineralogy, nitrogen speciation, redox, etc.)

[c] the source of nitrogen in the systems investigated (isotope geochemistry)

Project Timeline

Year 1

Sample collection may require fieldwork in the first year of the PhD.

Initial training in isotope ratio mass spectrometry, SEM-SXES, EPMA, and LA-ICP-MS (St Andrews & Durham).

Sample characterisation (sample prep and major & trace element data acquisition) to constrain the petrology (P-T-fO2) of the sample suite(s).

Begin initial training in geochemical modelling to express the partitioning behaviour and controls on the partitioning behaviour of nitrogen in igneous systems.

Year 2

Begin the task of quantifying the speciation, nitrogen abundance and isotopic values for samples from the three localities, sequentially.

Refine geochemical as data are generated.

Begin first manuscript for submission with input and assistance from supervisory team.

Present results at national meeting (Volcanic and Magmatic Studies Group 2025, UK).

Year 3

Finalise the task of analysing the speciation, nitrogen abundance and isotopic values for samples from the remaining cache.

Use data to develop final geochemical model to predict the behaviour of nitrogen during the differentiation of igneous rocks and to quantify the controls on the behaviour of nitrogen in igneous systems.

Ideally, work on second and third manuscripts for submission with input and assistance from supervisory team.

Present results at international meeting (Goldschmidt 2026, Europe)

Year 3.5

Write, submit, and defend Ph.D. thesis.

Training
& Skills

Training will be provided throughout the project on all techniques.

This IAPETUS2 DTP project will provide training in petrology, stable isotope geochemistry, and geochemical modelling. The focus on petrological characterization of minerals, advanced analytical isotope ratio geochemistry will provide the student a skill set to competitively acquire postdoctoral research positions, or to transition from an academic to industrial, economic, and databased careers upon completion of their Ph.D. degree.

References & further reading

Boocock. T.J., Mikhail, S., Prytulak, J., Di Rocco, T., Stüeken., E.E. 2020. Nitrogen mass fraction and stable isotope ratios for fourteen geological reference materials: Evaluating the applicability of Elemental Analyser versus Sealed Tube Combustion methods. Geostandards and Geoanalytical Research
Hutchison, W., Mather, T.A., Pyle, D.M., Boyce, A., Gleeson, M.L., Yirgu, G., Blundy, J., Ferguson, D.J., Vye-Brown, C., Millar, I.L., Sims K.W.W. and Finch, A.A. 2018. The evolution of magma during continental rifting: new constraints from the isotopic and trace element signatures of silicic magmas from Ethiopian volcanoes. Earth and Planetary Science Letters
Mikhail, S., Sverjensky, D.A. 2014. Nitrogen speciation in upper mantle fluids and the origin of Earth’s nitrogen-rich atmosphere. Nature Geoscience
Mikhail, S., Barry, P.H., Sverjensky, D.A. 2017. The relationship between mantle pH and the deep nitrogen cycle. Geochimica Cosmochimica et Acta
Stow, Madeleine A., Prytulak, Julie, Humphreys, Madeleine C. S. & Nowell, Geoffrey M. 2022. Integrated Petrological and Fe-Zn Isotopic Modelling of Plutonic Differentiation. Geochimica et Cosmochimica Acta
Stüeken, E.E., Boocock, T.J., Robinson, A., Mikhail, S., Johnson, B.W. 2021. Hydrothermal recycling of sedimentary ammonium into oceanic crust and the Archean ocean at 3.24 Ga. Geology
Zerkle, A.L., Mikhail, S. 2017. The Geobiological Nitrogen Cycle: From Microbes to the Mantle. Geobiology

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