Isotopic heterogeneity of the oceanic upper mantle: the crystal record - NERC GW4+ DTP project

This project is one of a number that are in competition for funding from the NERC GW4+ DTP. The GW4+ DTP consists of the Great Western Four alliance of the University of Bath, University of Bristol, Cardiff University and the University of Exeter plus six Research Organisation partners. The partnership aims to provide a broad training in earth and environmental sciences, designed to train tomorrow’s leaders in earth and environmental science. For further details about the programme, please see http://nercgw4plus.ac.uk/.

Background

The magnitude and scale of mantle heterogeneity reflects the time-integrated history of recycling of different rocks during the plate tectonic cycle. Reconstructing the magnitude and length scale of heterogeneities thus enable a robust reconstruction of the recycling process, and hence the geodynamic evolution of the planet.

Current constraints on the heterogeneity of the oceanic mantle come from isotopic analyses of mid-ocean ridge basalts (MORBs). However, MORBs are mixed in crustal magma chambers prior to eruption, and therefore no longer fully capture the heterogeneity of their mantle source.

This project will circumvent this problem by analysing the isotopic composition of phenocrysts in MORB and primitive cumulate crystals of the lower oceanic crust. These crystals formed prior to crustal mixing, thus retaining heterogeneity far exceeding that of MORB, enabling the true heterogeneity of mantle melts to be reconstructed.

Project aims and methods

This project aims to provide new constraints on the extent and length scale of heterogeneity of the mantle, using a novel approach of in-situ isotopic analysis of lower oceanic crustal cumulates and MORB phenocrysts.

There are very few isotopic data on MORB phenocrysts and their melt inclusions. However, all studies to date (see references) show that they retain significantly more heterogeneity than their host lavas. For example, individual Icelandic samples contain up to 90% of the Pb isotopic spread of the entire Atlantic Ocean (MacLennan, 2008). This is consistent with the petrogenetic history of these phenocrysts, which often record multiple stages of growth from different melts.

The scarce data on lower oceanic crustal rocks suggest that they, too, retain significant isotopic heterogeneity (Kempton et al., 1991). However, these data have been obtained using whole-rock or mineral separate analytical techniques, and hence would have integrated the multiple geochemical domains that are typically present in these rocks.

This project will capitalise on the heterogeneity preserved on the crystal scale by making in-situ isotopic analysis, focusing on Sr in plagioclase. This will involve extensive sample characterisation followed by microdrilling selected crystal domains for isotopic analysis using Cardiff’s recently installed multicollector ICP-MS and Bristol’s TIMS.

Candidate

This project would appeal to students interested in using geochemistry to improve understanding of the large-scale evolution of the Earth. An interest in igneous processes and an affinity for geochemical analytical techniques would be required.

Training

You will receive an extensive training programme in petrology/geochemistry. This comprises a wide range of techniques, with a particular focus on in-situ mineral analysis. Furthermore, you will join a research cruise to explore lower crustal exposures on the Pacific ocean floor, thus acquiring key field skills.

In addition to project-specific training and the DTP training courses, you will have access to the large range of Cardiff University Student Development courses, to maximise transferable skills.

Combined, the training package of the project will give you an excellent basis for a career in academia or industry.