A new approach to estimating the evolution of nitrogen geochemistry in Earth’s crust and atmosphere


The mass and composition of Earth’s atmosphere are crucial variables for maintaining habitability, and yet one critical parameter remains poorly constrained: the evolution of atmospheric pressure over geologic time. The conservation of mass requires that changes in one reservoir (i.e., the atmosphere) must be balanced by reciprocal changes in other reservoirs (i.e., the crust). As such, exchange processes that cycle nitrogen between the mantle and atmosphere via the crust such as through weathering and the rock cycle need to be constrained if we are to better understand the processes that enrich this bio-essential element on planetary surfaces. Of Earth’s major reservoirs, the atmosphere and mantle are the largest and dynamically exchange via subduction zone plate tectonics and volcanism. Integral between these there is a third reservoir, Earth’s continental crust. The crust is important as it stores roughly half as much nitrogen as the present-day atmosphere. Although made of many lithologies, around 50% of the Earth’s upper crust is of felsic igneous composition and therefore magmatic rocks are a key building block for any telluric planetary crust. These felsic igneous rocks in the continental crust can contain up to ca. 250 μg/g nitrogen with a residence time of 1000+ Myr and therefore offer potential as long-term stores of nitrogen and as a key substrate for the development and evolution of extant life. The crustal reservoir can have two primary nitrogen sources: nitrogen-enriched sediments, and/or magmatic differentiation of mafic melts from the upper mantle. The former would mean that the felsic igneous nitrogen reservoir was derived from the atmosphere via biomass burial, implying that this amount of nitrogen was part of the atmospheric N2 budget. In contrast, magmatic enrichment would imply the exact opposite, meaning that crustal differentiation can act as a trap for mantle nitrogen and thereby limiting volcanically degassed atmospheric N2 accumulation.


This PhD will examine how nitrogen behaves during the differentiation of igneous rocks from mafic to felsic using suites of plutonic and extrusive igneous rocks. 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 (carbonatites + alkali intrusions) and silicic (basalt to rhyolite + gabbro to granite) extrusive and plutonic systems. The diversity of samples in this study enables the work to speak towards characterising a broad range of nitrogen reservoirs, from mantle sources (gabbro, carbonatite) to the continental crust (granite). In addition, this project will speak towards several processes including stable isotope fractionation (all samples), mineral-mineral partitioning (plutons), and the effects of volcanic degassing during volcanism and igneous differentiation (lavas). The project will determine several key variables required to develop a dynamic empirical model:[a] the partitioning behaviour of nitrogen during igneous differentiation of rocks[b] the controls on the behaviour of nitrogen (mineralogy, nitrogen speciation, redox, etc.)[c] the source of nitrogen in the systems investigated (isotope geochemistry)
This work builds on recent analytical developments enabling us to quantify nitrogen abundances and stable isotope values down to ca. 3 μg/g in silicate phases (Boocock et al., 2020). We have also 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).

Project Timeline

Year 1

Sample collection may require fieldwork which would be carried out 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 suites.

Year 2

Begin the task of quantifying the speciation, nitrogen abundance and isotopic values for samples from the three localities, sequentially.
Present results at national meeting (Volcanic and Magmatic Studies Group 2024, 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 the empirical 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.
Present results at international meeting (Goldschmidt 2025, USA)

Year 3.5

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

& 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

Hutchison, W., Mather, T.A., Pyle, D.M., Boyce, A.J., Gleeson, M.L.M., Yirgu, G., Blundy, J.D., Furgeson, D.J., Vye-Brown, C., Millar., Sims, K.W.W., 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., Barry, P.H., Sverjensky, D.A. 2017. The relationship between mantle pH and the deep nitrogen cycle. Geochimica Cosmochimica et Acta
Zerkle, A.L., Mikhail, S. 2017. The Geobiological Nitrogen Cycle: From Microbes to the Mantle. Geobiology
Mikhail, S., Sverjensky, D.A. 2014. Nitrogen speciation in upper mantle fluids and the origin of Earth’s nitrogen-rich atmosphere. Nature Geoscience
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

Further Information

For further information please contact Dr. Sami Mikhail (sm342@st-andrews.ac.uk).

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