IAP-25-001

Volatile happenings on the igneous side of metamorphism

Holistic Overview
Partial melting results in the production of two new rocks, a melt and a restite. This process redistributes material resulting in enrichment and depletion, respectively, and the movement of elements between systems is known as mass transfer. Ultimately, the process of mass transfer explains why Earth’s atmosphere contains +99% of Earth primordial noble gases, and why the crust is incompatible element enriched relative to the mantle. The focus of this PhD is to examine what happens to a key volatile element, nitrogen, when sediments melt. The melting of sediments produces granitic magmas, and fortunately, this process is sometimes preserved in migmatite complexes. This PhD project will focus aims to understand what happens to sedimentary-hosted nitrogen on the igneous side of metamorphism.

Why nitrogen?
The nitrogen cycle links all of Earth’s major reservoirs, and the rock record of the continental crust contains the oldest accessible archive of geological time. Traditionally, secular changes in Earth’s environmental history have been reconstructed using the sedimentary rock record. But this record includes fundamental biases. E.g., the sedimentary rock record shows a strong preservation bias towards shallow-water settings and the record for total organic carbon (TOC) and [N] overly relies on organic-rich shales; most importantly, the abundance of sedimentary rocks dramatically decreases with age. In fact, there is no sedimentary rock record for most of early Earth’s history because sedimentary rocks are metamorphosed and/or removed by the relentless churn of plate tectonics. However, during orogenic events, sediments are partially melted and produce a robust rock which dominates the oldest sections of Earth’s rock record – granite. We have shown that the geochemistry of granitic rocks of the continental crust is a novel archive for atmospheric and geobiological change (Mikhail et al., 2024).

Aims
The key known-unknown required to convert data from the granite record into the volume of biomass buried in sediments is what happens to sedimentary-hosted nitrogen during crustal melting? The answer contains the variables required to quantitively evaluate the effect changes in biomass burial have had on the N2/O2 and CO2/O2 ratios of Earth’s atmosphere. We have designed a project to answer this question which will enable us to quantitively assess how plate tectonics and changes in biomass burial have co-contributed to shaping Earth’s present-day environmental conditions, thus charting the co-evolution of Earth’s atmosphere, biosphere, and geosphere. We will quantify the behaviour of N (and its isotopes) during crustal metamorphism and partial melting by studying the N petrology and geochemistry of magmas and residual rocks at migmatite complexes. Ammonic Nitrogen (NH4+) substitutes for K+ and Rb+ in silicate minerals (mica, feldspar; Boocock et al., 2023a-b). Therefore, K-bearing phases are potential reservoirs for N storage in deep time, and SPGs host K-bearing phases (biotite mica and K-feldspar). Recent research has shown that granites produced via the melting of sediments show statistically significant enrichment in N in the Phanerozoic compared to the Proterozoic and Archean, mirroring what was found for P in SPGs (Mikhail et al., 2024). Because N and P are both enriched in biomass relative to abiogenic mineral phases, these data are straightforwardly explained by an absolute increase in biomass burial in the Phanerozoic by a relative factor of ~8 (Mikhail et al., 2024). However, converting the N or P composition of granites into volume of biomass in sedimentary protoliths requires specific data from key stages between the deposition of sedimentary rocks, metamorphism, melting, and the emplacement of strongly peraluminous granites (all of which are preserved in migmatite complexes).

The project involves fieldwork, petrological and geochemical lab work, and modelling to place these data into a geological framework. This PhD is part of a wider NERC-funded project entitled “Quantifying changes in biomass burial over geological time”.

Fieldwork. We will investigate the petrological behaviour of nitrogen during metamorphism and anatexis at Variscan-aged migmatite complexes exposed in Italy (Sardinia & Ivrea). These sites are selected because we can access mudstone and siltstone (pelitic) protoliths along prograde P-T pathways right up to the point of melting. We will sample [1] pelitic metasedimentary rocks along prograde metamorphic pathways to quantify the loss fraction for N during metamorphic devolatilization and [2] the zone of partial melting to quantify the behaviour of nitrogen during partial melting of metasedimentary rocks (anatexis).

Petrological Analysis. Migmatite complexes hosts a range of metamorphic rocks sharing a similar P-T history. They fall into two groups: [a] rocks which have experienced partial melting (neosome) and [b] those which have not (paleosome). We will sample both. The paleosome metasediments chart the behaviour of N during metamorphism up to the point of melting to chart the loss fraction of N (devolatilization) as a function of metamorphic grade. The neosome is comprised of two composite crystalline rocks, which represent; [i] melt (leucosome) and [ii] residuum (melanosome), and both have experience loss/enrichment by the process of incongruent partial melting. The leucosome is relatively felsic (light coloured) and the melanosome is mafic (dark coloured). Localised compositional variability for different leucosomes at a single neosome are known to be a factor which we must consider. Each neosome we sample needs to undergo a full petrographic and geochemical analysis to constrain the petrology of each specific leucosome-melanosome pair before we consider N geochemistry.

Geochemical Analysis. The reason little is known about the partitioning and stable isotope fractionation of N during partial melting and igneous differentiation is because these measurements are beyond the capabilities of most laboratories. At UStA we have established a custom-built vacuum line coupled with a Thermo MAT-253 IRMS optimised for the accurate determination of trace abundances and isotope ratios for N from silicate rocks and mineral separates (Boocock et al., 2020; 2023a,b; Mikhail et al., 2024). In addition, UStA hosts a JXA-iSP100 EPMA and laser ablation QQQ-ICP-MS for in situ major and trace element acquisition.

Modelling. Geochemical models will illuminate how biomass burial has changed through time and predict the corresponding effect this would have had on Earth’s atmospheric chemistry by fully utilising our novel archive of nitrogen in sediment-derived granitic rocks. We have found that the N geochemistry of SPGs preserve evidence for an increase in biomass burial in the Phanerozoic by a relative factor of ~8 (Mikhail et al., 2024). Using the data from this PhD we will construct a geochemical model to quantify the effect of changes in biomass burial on Earth’s atmosphere over the last 2.7 Ga and determine if the changes in δ15N values for SPGs over time reflects biological metabolisms or changes in P-T-X of the melting conditions. Armed with this new information, we can quantitively evaluate the effect changes in biomass burial have had on the N2/O2 and CO2/O2 ratios of Earth’s atmosphere.

Year 1

Fieldwork to Sardinia & Ivrea and petrological characterisation of samples brought back from the field.

Year 2

Full geochemical data acquisition period (major, trace, and volatile elements)
Drafting of first manuscript and presentation at national conference (the annual Metamorphic Studies Group or Geochemistry Group meeting).

Year 3

Construct geochemical model while finishing the acquisition of geochemical data.
Drafting of additional manuscripts and presentation at the international Goldschmidt

Year 3.5

Finalising manuscripts for submission alongside PhD thesis.

Training will be provided for all aspects of the scientific research (i.e., fieldwork, lab work, modelling). The PhD will gain experience in modelling, data management, fieldwork, and multi-disciplinary research, which are in the top 6 in the most-recent UKRI skills review list. They will work with leading experts in the field in the UK, Germany, USA, and Canada, present at several international conferences, and benefit from training and support offered by UStA’s Academic Skills Project led by the Centre for Educational Enhancement and Development team (alongside training provided by the Iapetus DTP).