Constraining ascent rates of diamond-bearing kimberlite magmas using diffusion chronometry

Diamond crystal

Diamond crystal from the Colesberg Kopje mine (later the famous Kimberley mine) in South Africa © The Trustees of the Natural History Museum, London

Despite their economic importance and relevance to understanding the compositional evolution of the Earth’s upper mantle through geologic time, there are many aspects of kimberlite ascent and eruption that remain poorly constrained.

Project rationale

Kimberlite volcanism involves the rapid ascent of mantle-derived magmas that occasionally contain diamonds. Despite their economic importance and relevance to understanding the compositional evolution of the Earth’s upper mantle through geologic time, there are many aspects of kimberlite ascent and eruption that remain poorly constrained.

Arguably the most important of these is the volatile-driven (i.e., CO2, H2O) ascent velocities of primary kimberlite melts, which are enigmatic due to the intense alteration in most ancient kimberlitic rocks.

This project will investigate the geology of two contrasting, but relatively young and well-preserved diamond-bearing kimberlite deposits, in the USA (the Eocene Montana kimberlites) and in Tanzania (the Holocene Igwisi Hills; Brown et al., 2012). The latter is the youngest known kimberlite on Earth and remarkably fresh, containing primary (i.e. magmatic) olivine, calcite, perovskite and apatite.

The aim of the project is to determine kimberlite magma ascent velocities, capitalizing on recent advances in diffusion chronometry (Mutch et al., 2019; Costa et al., 2020) as applied to both zoned olivine and apatite phenocrysts, developed for other, more common magma types.

The study will provide a novel, quantitative understanding of kimberlite volcanism, and has potential to dramatically improve our understanding of kimberlite ascent and eruption rates.

Methodology

Samples from the two kimberlite deposits will be analyzed by X-Ray Fluorescence Spectroscopy and ionizing coupled plasma-mass spectrometry, to determine their bulk major and trace element compositions. They will then be thin sectioned for characterization of mineral abundances, compositions and zoning by optical and scanning electron microscopy. These analyses will all be undertaken in SOES, UoS.

As minerals precipitate in magmatic systems, changes in their storage conditions (e.g. pressure, temperature) or host magma composition can result in chemical zonation. As a chemical gradient exists between these zones, they diffusionally re-equilibrate at high temperatures; diffusion profiles can be compared with experimentally calibrated numerical models to calculate timescales of magmatic processes.

To mitigate diffusive anisotropy, crystals will be oriented using electron back-scatter diffraction (Trinity College Dublin). High-spatial resolution diffusion profiles will then be analyzed by Electron Probe Microanalysis (University of Bristol – Mg, Fe, Ni, Mn in olivine) and Secondary Ion Mass Spectrometry (NERC Ion Microprobe Facility – apatite volatiles).

Measuring fast diffusing species will elucidate pre- and syn-eruptive magmatic processes on timescale of days to weeks, which are believed to be characteristic of kimberlite ascent. Diffusion timescales will be calculated using new models (Mutch et al, 2019), which can be freely downloaded online.

Training

The INSPIRE DTP programme provides comprehensive personal and professional development training alongside extensive opportunities for students to expand their multi-disciplinary outlook through interactions with a wide network of academic, research and industrial/policy partners. The student will be registered at the University of Southampton and hosted in Ocean and Earth Science.

Specific training will include the use of optical and scanning electron microscopy to characterize the composition, texture and model abundance of different mineral phases.

Training will also be provided in performing trace element analysis using Inductively coupled plasma mass spectrometry (ICP-MS) at the University of Southampton. Novel approaches such as diffusion chronometry, previously applied to olivine and apatite, will be utilized to constrain magma ascent rates.

Training will be provided in writing proposals to use the high-resolution NERC ion microprobe at the University of Edinburgh. The student will gain experience both in national and international collaborations (e.g., with Trinity College Dublin, the University of Bristol and the Natural History Museum), which will prepare the student for a career path in academia or in industry.

Eligibility and how to apply

Read how to apply on the INSPIRE website.

The deadline for applications is 4 January 2021.

Apply for this project

Please check the INSPIRE website for details on how to apply.

Application deadline: 4 January 2021.

Supervisors

University of Southampton

Dr Thomas Gernon

Trinity College Dublin

Dr M Stock

University of Bristol

Dr Richard Brooker

The Natural History Museum, London

Dr Emma Humphreys-Williams

References

Brown, R. J., Manya, S., Buisman, I., Fontana, G., Field, M., Niocaill, C. M., Sparks, R. S. J., Stuart, F. M., 2012. Eruption of kimberlite magmas: physical volcanology, geomorphology and age of the youngest kimberlitic volcanoes known on earth (the Upper Pleistocene/Holocene Igwisi Hills volcanoes, Tanzania). Bulletin of Volcanology 74 (7), 1621–1643.

Costa, F., Shea, T., Ubide, T. 2020. Diffusion chronometry and the timescales of magmatic processes. Nature Reviews Earth & Environment 1, 201–214.

Mutch, E.J., Maclennan, J., Shorttle, O., Edmonds, M. and Rudge, J. F., 2019. Rapid transcrustal magma movement under Iceland. Nature Geoscience 12, 569–574.

Interdisciplinary Southampton Partnership for Investigators Researching the Environment

This a joint PhD training partnership between the Natural History Museum and INSPIRE a NERC Doctoral Training Partnership (DTP) creating an innovative multi-disciplinary experience for the effective training of future leaders in environmental science, engineering, technology development, business, and policy.

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