Magmatic processes in mushy systems via coupled stable-radiogenic strontium isotopes

What happens to a magma from its formation deep in the mantle to its eruption at the surface of the Earth ? In undergraduate petrology and geochemistry courses we often model the process as simply : melt, evolve, erupt. Such models of melt generation and evolution usually create chemical variability by changing the degree of melt, the original (homogeneous) source composition, or by the degree of magmatic evolution via fractional crystalllisation.

However, the ‘plumbing system’ needed to get a melt through the lithosphere to the surface is complex. Crystal fractionation and the original degree of melt can go some ways to explaining the major element variations in erupted lavas, but chemical variation is ubiquitous in trace element and isotopic compositions. Commonly invoked processses to explain trace element and isotopic variation can be ‘exotic’. For example, additions directly to the magma source (e.g. subducted sediments or metasomatised mantle) or magmatic contamination and assimilation on the journey through the crust to eruption. A mushy magma system also may cannibalise itself through reactive flow processes (e.g., Jackson et al. 2018) and create chemical variations without the need for external inputs.

This project asks two broad question : (1) how much chemical variation can be generated within the lithosphere without the need for exotic inputs? and (2) can chemical variation in magmatic crustal rocks be used to determine the pathways of element cycling, enrichments and depletions and the physical parameters of crustal processes ?

To do this, we focus on the intermediate part of a magma’s journey through the lithosphere via plutonic xenoliths and cumulates thought to represent mushy magmatic systems. The dynamics mush system includes processes of reactive flow, re-melting, re-mobilisation of mushes and aging (e.g., Jackson et al., 2018), which all might create their own chemical variation without the need to invoke outside additives to a magma. Examining phases within plutonic and cumulate xenoliths for their trace element and isotopic compositions will allow us to assess the magnitude of variation generated through mushy lithospheric processes alone.

The backdrop for this project are recent geochemical studies on suites of cumulate xenoliths from Lesser Antilles arc: whole rock stable Fe and radiogenic Sr-Pb isotope data on one suite of plutonic xenoliths display a surprisingly large degree of chemical variability (Cooper and Inglis, 2021). In contrast, in the radiogenic Sr isotopic composition of a suite of cumulates from Martinique suggest a lack of exotic mantle inputs (Brown et al. 2021). It is difficult to disentangle chemical changes in mushy systems using a whole rock approach, which averages the chemical composition of the sample and looses the link to petrographic texture. It is also not possible to identify cannabalistic reactive flow processes via radiogenic isotopes, which are not fractionated by such processes.

The project initially focusses on cumulate and plutonic xenoliths from the Lesser Antilles, to determine mineral-scale trace element and isotopic heterogeneity. Specifically, both radiogenic and stable Sr isotope variations can be determined on the same feldspar crystals via micro-sampling. These initial xenoliths have a high degree of characterisation (e.g., Brown et al. 2021), including clear textural evidence for reactive flow. Conceptually, the coupled radiogenic-stable Sr approach should be able to distinguish three vastly different scenarios : (1) variation in radiogenic Sr indicates input from external sources, (2) lack of variation in radiogenic Sr, but variation in stable Sr indicates cannabalistic processes such as reactive flow (3) variation in both indicates a micture of reactive flow and external inputs. The novelty of this project is our ability to target in situ feldspars, via micro-milling, associated with reactive flow textures and compare them to feldspars lacking these textural clues.

The continental crust is formed in part by amalgamation of arc crust, therefore the results of the project have direct bearing on how the chemistry of the continental crust is determined and evolves. The complexity of mushy magmatic processes can yield new insight into the physical processes in the sub-arc lithosphere, quantify element transfer at subduction zones, and process of building the contintental crust.

Specifically, this project aims to:
• Determine coupled radiogenic and stable Sr isotopic composition of feldpsar in Lesser Antilles plutonic xenoliths
• Determine the extent to which exotic inputs or cannibalism dominants chemical variation
• Develop the coupled radiogenic-stable isotope approach as a fingerprint of mushy processes in the lithosphere

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Image Captions

Cumulate xenolith from the Lesser Antilles


The student will use methods including:
• Optical microscopy for textural evaluation of thin sections
• Laser ablation ICP-MS for trace element abundances of phases
• Stable and radiogenic Sr isotope analyses of micro-milled feldspar
• Integration of chemistry into existing numerical models

Project Timeline

Year 1

Literature review. Thin section characterisation via optical microscopy. Training in micro-milling, clean lab chemistry, column chromatography and mass spectrometry. Write/defend research proposal. Attend national Geochemistry Research in Progress and Volcanic Magmatic Studies Group conferences

Year 2

Measurement of isotopic ratio in mineral phases. Further develop writing skills and manuscript preparation for publication. Attend national Geochemistry Research in Progress and Volcanic Magmatic Studies Group conferences.

Year 3

Synthesise and model datasets; attend international conference(s); publication and thesis writing;

Year 3.5

Complete and submit thesis; finalise manuscripts for publication.

& Skills

This project would suit a student with a degree in Earth Sciences, Chemistry (or a related field) and strong interests in igneous process, petrology, trace element and isotope geochemistry.

Excellent time management skills coupled with strong numerical, verbal and written communication are important. Previous analytical experience would be an advantage but is not essential. Training will cover a wide range of geochemical methods, including: i) Laser abalation ICP-MS ii) Multi-collector Mass Spectrometry iii) micro-milling

The student will join both the Durham Volcanology Group and Durham Isotope Group. Both groups consist of postgrads, postdocs and members of staff, with weekly group meetings with a range of themes from scientific to ‘soft’ skills.

They will attend national and international conferences, networking events and outreach activities, developing an important network for feedback and future employment.

References & further reading

Brown, J.R., Cooper, G.F., Nowell, G.M., Macpherson, C.G., Neill, I., Prytulak, J. 2021. Isotopic compositions of plagioclase reveal crustal assimilation below Martinique, Lesser Antilles Arc. Frontiers in Earth Science, doi : 10.3389/feart.2021.682583.

Cooper, G.F., Inglis, E.C. 2022. A crustal control on the Fe isotope systematics of volcanic arcs revealed in plutonic xenolith from the lesser Antilles. Frontiers in Earth Science. 10.3389/feart.2021.795858.

Jackson, M.D., Blundy, J., Sparks, R.S.J. 2018. Chemical differentiation, cold storage and remobilisation of magma in the Earth’s crust. Nature, 564, 405.

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