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E402: Early Earth evidence for a magnetic field (Lead Supervisor: Helen Williams, Earth Sciences)

Supervisors: Helen Williams (Earth Sciences) and Richard Harrison (Earth Sciences

Importance of the area of research:

The Earth possesses an active geomagnetic field, which shields the atmosphere from erosion by the solar wind. The significance of the Earth's geomagnetic field can be illustrated by comparison with the Moon and Mars, which initially possessed magnetic fields that shut down between 3 and 4 Ga when their cores cooled to a point where vigorous convection could not be sustained, resulting in the loss of their initial atmospheres. The earliest evidence for a magnetic field on Earth arises from studies of ancient rocks where data has been obtained on magnetic minerals trapped in silicate minerals such as zircon, quartz and feldspar. However, interpreting palaeomagnetic data in mineral inclusions is challenging due to the lack of geological contextual information and it has been proposed that the samples were remagnetisated by later events (e.g. see http://www.sciencemag.org/news/2015/07/oldest-rock-crystals-point-ancient-magnetic-shield-earth). New approaches are required to resolve this controversy.  This project will take a multidisciplinary approach and will apply a series of geological, petrological, (nano)palaeomagnetic and geochemical tools to this problem.

Project summary:

This project will use a range of techniques to verify the primary nature and temperature of magnetic mineral growth in early Earth rocks and will use this information to test the concept of an early magnetic field.  This project will look at iron isotopes, which are partitioned between oxide and silicate minerals in a systematic, temperature-dependent way (Shahar et al., 2008) that should allow primary and secondary magnetic minerals to be distinguished. Primary high-temperature phases are expected to display a restricted range of Fe-isotope compositions, whereas secondary magnetic minerals formed at lower temperatures or via fluid-rock interaction will have variable Fe-isotope signatures. It will therefore be possible to determine whether the magnetic signals recorded by ancient rocks, mineral inclusions or even meteorites relate to primary or secondary magnetic mineral growth.

What the student will do:

The student will focus on the distribution of Fe isotopes in bulk samples and mineral separates from previously collected samples from the 2 Ga Bushveld Complex in South Africa, where there is an uncontested primary record of the Earth's magnetic field, and abundant oxide and silicate minerals with which to calibrate Fe isotope behaviour. The student will use these sample and synthetic analogues to develop chemical and mechanical methods for analysing magnetic minerals included in feldspars and other silicate minerals for their Fe isotope compositions.  This work will be carried out in conjunction with palaeomagnetic analyses at a range of lengthscales, from the bulk rock to the submicron, using the state-of-the-art facilities (https://wserv4.esc.cam.ac.uk/nanopaleomag/). The student will then apply these techniques to 3.5 Ga rocks collected from the Barberton Belt in South Africa and other localities and will combine this information with field, petrographic, mineralogical information to verify whether these samples truly provide a rock record of the Earth's ancient magnetic field.

Please contact the lead supervisor directly for further information relating to what the successful applicant will be expected to do, training to be provided, and any specific educational background requirements.

References:

Tarduno, J. A., Cottrell, R. D., Davis, W. J., Nimmo, F. and Bono, R. K. (2015) A Hadean to Paleoarchean geodynamo recorded by single zircon crystals. Science, 349(6247) pp. 521-524.

Weiss, B. et al., (2015) Pervasive Remagnetization of Detrital Zircon Host Rocks in the Jack Hills, Western Australia and Implications for Records of the Early Geodynamo, EPSL, 430 pp. 115-128

Williams, H.M. et al., (2004) Iron isotope fractionation and the oxygen fugacity of the mantle. Science 304 (5677), pp. 1656-1659

Follow this link to find out about applying for this project.

Other projects available from the Lead Supervisor can be viewed here.

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