IAP-24-114

How important is mantle carbonation in Earth’s long-term carbon cycle?

The carbon cycle involves each and every one of Earth’s layers (core, mantle, crust, oceans, atmosphere). This project is concerned with what happens in the mantle wedge during subduction, thus linking the surface to the interior. Geological processes at subduction zone margins govern seismicity, plutonism and associated volcanism and geochemical cycling between the ocean, crust, and mantle. However, the influence of these processes on global geochemical budgets remains poorly constrained, with uncertainties spanning several orders of magnitude. This project will define the geochemical processes and refine the quantities of carbon emplacement into the shallow mantle at subduction zones by assessing the conditions that facilitate carbonation of tectonically disrupted mantle rocks, leading to the formation of fully carbonated peridotites, known as listvenites. Currently, natural carbonation of ultramafic rocks is of great interest owing to their potential to aid efforts in geoengineering to limit rising atmospheric CO2 levels.

Sampling of the mantle wedge beneath active subduction zones is not possible because these processes occur below the maximum sampling depth afforded by drilling. More specifically, these processes occur at, cf. >7 km depth. Therefore, to investigate solid reaction products in the leading edge of the mantle wedge, we rely on ophiolites–ancient sections of ocean crust preserved on land. Fully carbonated peridotites, termed listvenites, consist of magnesite + quartz (+ dolomite) + chromite and form between ~50 and 250 °C during fluid ingress into ultra mafic mantle rocks. For example, in Oman, the thrusting of the Samail ophiolite over metasediments of the Arabian margin was initiated ~96 Ma, lasting ~15-20 Myr, with carbonation of partially serpentinitised peridotite occurring at depths between 7 and 20 km within ~500 m of the basal thrust (Falk and Kelemen, 2015). These listvenites illustrate the complex carbonation processes of the mantle wedge in the shallow subduction zones, characterized by complex cross-cutting relationships between carbonate minerals. The Oman Drilling Project cored through the listvenite section and identified graphite throughout the cores. Under reducing conditions, the reduction of carbonate to graphite is possible, as observed in subduction zone settings. This reaction is important as it suggests that the carbonate destabilisation does not necessarily lead to decarbonation (CO2-loss) and may act as a mechanism to stabilise carbon in the subducting slab (Galvez et al, 2013) or forearc mantle (resulting in billion-year-scale carbon storage). However, the volumetric importance of this process remain to be quantified. Thus, assessing the fluid flow and redox conditions that drive mantle rock carbonation is crucial in order to gain a predictive understanding of how different carbon species behave at these zones, and what governs which carbon species predominates in nature.

A suite of samples from the Oman Drilling Project listvenite cores (BT1b) will be the focus of this study. The main objectives are to:
1. Identify the processes and conditions required for the deposition of graphite.
2. Determine the source(s) and probable volumes of fluid that have driven carbonation.
3. Characterize fluid-rock interactions that control the chemistry of fluids responsible for carbonation.
4. Elucidate redox conditions responsible for the formation of both oxidised (haematite, carbonate) and reduced (graphite) phases within listvenites.
5. Identify the conditions that favour mantle rock carbonation with the aim of assessing where such conditions may persist in the modern tectonic settings to explore the feasibility of engineering similar carbonation for enhanced CO2 drawdown.

Petrological, geochemical, and isotopic characterisation of listvenite samples will be undertaken:
1) Detailed petrography and mineralogy of listvenite samples (including SEM) to characterise differences between different vein generations and types to enable selection of samples for isotopic scrutiny.
2) Differing generations of carbonate veins will be analysed for major, trace and REE, carbon, oxygen, and strontium isotopes. A subset will undergo Li isotopes to complement the O isotope work and build a tracer transport model of fluid flow to estimate fluid fluxes.
3) Graphite will undergo Raman spectroscopic characterisation. Following characterisation, different types and generations that have been identified will be separated for carbon and nitrogen isotope analyses. If graphite is deposited during mixing of CH4-rich fluids generated during serpentinisation and CO2-rich fluids from decarbonation reactions, this would have a distinct carbon isotope signature from graphite formed through the direct carbonate destabilisation.
4) Transition metal isotopes (e.g. Cr, Fe) of different vein generations will enable assessment of redox conditions during different reactions.
5) A geochemical thermodynamic model will be constructed to investigate controls on peridotite carbonation and metasomatism, to define conditions that favour carbonation and graphitisation of mantle rocks.

Year 1

Thorough literature review; training in elemental and isotopic methods (ICP-MS, ICP-AES, SEM, 87Sr/86Sr, C and O isotopes of carbonates), sample petrography and elemental and isotope measurements on carbonates.
Training and initial characterisation of graphite by Raman.

Year 2

Selection and characterisation of key carbonate vein samples for detailed isotope work; further isotopic measurements (inc. Li and transition metal isotopes, training in MC-ICP-MS).
Method development of transition metal isotopes (e.g. Cr, Fe, V) and testing utility of each isotope for tracing redox conditions in the paragenesis of interest.
Finish Raman characterisation of graphite and separate and analyse samples for C and N isotopes. Interrogation of carbonate dataset and writing initial manuscript draft on fluid sources and fluxes driving carbonation.
Presentation of initial carbonate vein results at national conference (Geochem. Group Research in Progress meeting).

Year 3

Complete isotope analyses of carbonate and graphite.
Submit manuscript on fluid sources and fluxes.
Data interrogation, draft manuscript on graphite spectroscopic characterisation and C and N isotopes.
Start to build thermodynamic model to investigate conditions favouring carbonation and graphitisation.
Presentation of results at national and international meetings (Geochem. Group Research in Progress meeting & and The Goldschmidt conference).

Year 3.5

Finalise thermodynamic modelling.
Complete any remaining final isotope analyses.
Submit graphite manuscript.
Write up final thesis chapters and start to draft manuscript on conditions favouring carbonation using redox isotope proxies and thermodynamic modelling data.

Training in the preparation of geological materials for geochemical analyses.
2. Measurement of trace and rare earth element abundances (ICP-MS).
3. Training in measurement of novel stable and radiogenic isotopes using high precision MC-ICP-MS and TIMS.
4. Training in Raman spectroscopic characterisation.
5. Training in analyses of C and O stable isotopes in carbonates and C and N stable isotopes in carbonaceous material.
6. Training in thermodynamic modelling using Thermocalc and/ or Geochemist’s Workbench modelling software.
7. Presentation of research at national and international conferences.
8. Training in writing skills through detailed feedback on manuscripts and thesis drafts.