Creating chemical heterogeneity in island arc crust

Overview

Subduction zones are the location of major mass transfer between the surface and the interior of the planet. They underpin all chemical cycling on Earth. Determining what happens to elements when they encounter a subduction zone therefore is a key piece of Earth’s geochemical puzzle. For example, does an element stay with the subducting slab and return to the deep mantle? Is it immediately liberated by fluids or melts from the slab to journey into the mantle wedge? Is it volatilised and immediately degassed in volcanic eruptions?

It is fairly intuitive to consider the end member scenarios: an element entering a subduction zone is either (a) returned to the mantle or (b) ejected in arc lavas. However, an element that is liberated from the slab and makes its way into a melt or fluid must still traverse the arc crust en route to its potential surface eruption. The arc crust can be >40 km thick and represents a complicated and poorly understood geologic setting – mostly due to its inaccessibility coupled with the challenges to high resolution shallow seismic imaging. The arc crust is, however, key to understanding diverse processes of magma recharge and storage, concentration of potentially economic deposits and via accretion, and the overall formation and evolution of the continental crust.

The current paradigm of crustal architecture envisions magma reservoirs connected through transcrustal mush system, comprised of crystals and melts in variable proportions (e.g., see review in Edmonds et al. 2019). The temperature, pressure, and rheological conditions at which this mush exist are key to understanding how ‘eruptible’ it is and how, in general, crustal assembly might take place.

This project uses novel geochemical approaches to examining and constraining crystal mush processes via a well-characterised suite of plutonic xenoliths from the Lesser Antilles arc that provide a snapshot of a mush system at different crustal depths.

Lesser Antilles island arc
The Lesser Antilles is a rare Atlantic subduction zone which has long been investigated due to the extremely radiogenic isotopic composition of some of the erupted lava, particularly on the island of Martinique. Two competing theories exist to explain the extremely radiogenic compositions of the arc lavas. The first invokes the incorporation of subducted sediments from the downgoing plate into the mantle source (e.g., Carpentier et al., 2008; Labanieh et al. 2012). The second proposes that the chemical heterogeneity and extreme signatures are inherited in the crust via assimilation (e.g., Davidson, 1985; Bezard et al. 2014). A suite of plutonic xenoliths from St. Vincent and Martinique have been collected (Cooper et al., 2016; Brown et al. 2021). Recent work by Brown et al. 2021 focussed on the question of whether the radiogenic Sr isotopic signature or Martinique lavas was obtained in the crust, or from subducted sediments. They found that the Sr isotopic composition of plagioclase from deep plutonic (ie crustal) xenoliths was not radiogenic, strongly suggesting that the enrichment process takes place in a heterogeneous arc crust, rather than as a primary mantle signature. The implication is that the suite of xenoliths from the Antilles, which are derived from different depths in the crust, may contain a record of increasing chemical heterogeneity and the favourable conditions under which heterogeneity is generated.

This project will use these same plutonic xenoliths to further investigate both source and process in the arc crust. Specifically, a novel combination of radiogenic isotopes to trace magmatic source(s) and stable isotope fractionation to constrain temperature and oxidation will be employed on mineral separates. Amphibole will be an initial target as it is an abundant, hydrous phase in the xenoliths, and is increasingly used to investigate crystal mush processes (e.g., Xu et al. 2021).

Specific questions include:
Is there evidence for multiple magmatic sources in the Lesser Antilles crust beneath Martinique and St. Vincent?
Does radiogenic isotope chemical heterogeneity decrease with increasing depth in the crust?
Is there a change in the relative oxidation state of fluids/melts with depth and or new magmatic sources?

The overall aim of the project is to deduce the conditions that promote chemical heterogeneity in the arc crust. The conditions must be better constrained to understand elemental cycling through subduction zones, the generation of ore deposits, and the long term evolution of the continental crust.

Methodology

The student will initially work on samples housed at Durham University. Depending on initial results, and if time permits, there is the potential to expand the sample suit to include field locations of a fossilised subduction zone in the Alps.

All Lesser Antilles samples have been characterised for major, trace and some radiogenic isotope systems. There is also preliminary laser ablation trace element data from amphibole. The student’s research will focus on mineral separation and investigation of mineral phases such as amphibole. They will determine both radiogenic (Pb-Nd) and stable (Fe-Zn) isotopic compositions on mineral phases throughout the xenolith suites.

Project Timeline

Year 1

Literature review, training in clean lab procedures, column chemistry and mass spectrometry. Write and defend research proposal at 9 month review. Attend national geochemistry research in progress and volcanic and magmatic studies conferences

Year 2

Sample and data processing, main geochemical approaches. Potential for time at St. Andrews should mineral compositions via electron microprobe be required. Further develop writing skills and manuscript preparation. Attend national geochemistry research in progress and volcanic and magmatic studies conferences

Year 3

Synthesize and model datasets, complete analytical work, attend and present work at international conference(s). Publication and thesis writing.

Year 3.5

Complete and submit thesis, finalise manuscripts for publication

Training
& Skills

This project would suit a student with a degree in Earth Sciences, Chemistry (or a related field) and strong interests in subduction zones, igneous process, trace metal 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 cutting-edge geochemical methods, including: i) ion chromatography; ii) isotope ratio measurement via MC-ICP-MS

The student will join the vibrant Durham Isotope Group, which includes research students and postdocs from the Earth Science, Geography and Archaeology departments. They will also benefit from interactions with the St. Andrews Isotope Geochemistry Group (StAIG) which will help develop complementary research skills and widen their network of collaborators.

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

References & further reading

Bezard, R., Davidson, J.P., Turner, S., Macpherson, C.G., Lindsay, J.M., Boyce, A.J. 2014. Assimilation of sediments embedded in the oceanic arc crust: myth or reality? Earth and Planetary Science Letters, 395, 51-60.

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

Carpentier, M., Chauvel, C., Mattielli, N. 2008. Pb-Nd isotopic constraints on sedimentary input to the Lesser Antilles arc system. Earth and Planetary Science Letters, 272, 199-211.

Cooper, G.F., Davidson, J.P., Blundy, J.D. 2016. Plutonic xenoliths from Martinique, Lesser Antilles: evidence for open system processes and reactive melt flow in island arc crust. Contributions to Mineralogy and Petrology, 171, 87.

Davidson, J.P. 1985. Mechanisms of contamination in Lesser Antilles island arc magmas from radiogenic and oxygen isotope relationships. Earth and Planetary Science Letters, 72, 163-174.

Edmonds, M., Cashman, K.V., Holness, M., Jackson, M. 2019. Architecture and dynamics of magma reservoirs. Philosophical Transactions of the Royal Society A. https://doi.org/10.1098/rsta.2018.0298

Labanieh, S., Chauvel, C., Germa, A., Quidelleur, X. 2012. Martinique: a clear case for sediment melting and slab dehydration as a function of distance to the trench. Journal of Petrology, 52, 2441-2464.

Xu, W., Zhu, D., Wang, Q., Weinberg, R., Wang, R., Li, S., Zhang, L., Zhao. 2021. Cumulate mush hybridization by melt invasion: evidence from compositionally diverse amphiboles in ultramafic arc cumulates within the eastern Gangdese Batholith, southern Tibet. Journal of Petrology, https://doi.org/10.1093/petrology/egab073

Further Information

For further information and informal enquiries, please contact Julie Prytulak, Durham University (julie.prytulak@durham.ac.uk)

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