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Centre for Earth and Environmental Science Research

The use of granitic intrusions to determine the Earth's magnetic field strength through geological time

Executive Summary
ResearchersJennifer Halliday, CEESR
Dr Neil Thomas, CEESR
Prof. Nicholas Petford, CEESR & Pro-VC Research, Bournemouth University
Dr Kenneth McCaffrey, University of Durham
Funding Body/SourceKingston University
DurationOctober 2004 - ongoing
Project SummaryThis project aims to evaluate whether materials found within granitic plutons are suitable for palaeointenity studies.

Background

Palaeomagnetism is the study of the Earth's magnetic field as it was in the past and utilises the observable fact that certain minerals retain a 'memory' of the past orientation and intensity of the field. Magnetic and palaeomagnetic data show that the geomagnetic field varies on timescales from seconds to millions of years (Bloxham and Gubbins, 1985). Long term variations, known as secular variation are predominantly of internal origin. Therefore the variation of the Earth's magnetic field strength (palaeointensity), recorded in surface rocks through geological time, is a primary quantifiable effect of processes occurring in the deep interior of our planet.

Traditionally mafic, extrusive rocks (e.g. basaltic lava flows) are used to determine palaeointensities. Lava flows typically represent an "instantaneous" measurement of the magnetic field because they tend to cool quickly relative to changes of the internal magnetic field (Merrill et al., 1996). The primary factors controlling the quality of palaeomagnetic data are the chemistry, mass-fraction and grain-size of the constituent magnetic minerals. Basic rocks are characterised by the presence of olivine, pyroxene, calcium-rich feldspars and Fe-Ti oxide solid solutions (Best, 2002), this mineralogy makes them ideal for palaeomagnetic studies. The fine grained nature of extrusives means that mafic materials possess greater palaeomagnetic stability. The principal reasons for the paucity of data in the current palaeointensity record can be attributed to a lack of such 'suitable' material in critical time windows, or poor magnetic behaviour of these rocks during experiments (Biggin & Thomas, 2003). Therefore it would be useful to have other potential sources of palaeointensity data in the surface rock record.

Granitic intrusions occur frequently throughout the geological record, are often well exposed and in general well dated. Therefore, such rocks are potentially ideal for palaeointensity studies aimed at significantly enhancing our understanding of the temporal behaviour of the Earth's magnetic field. However, granites per se are discriminated against for palaeointensity studies, and have not been previously been exploited. Granites have been neglected due to their perceived lower primary magnetic content and problems associated with the stability of their magnetic signatures. However, granitic plutons show an internal grain-size variation controlled by the differential cooling rates within the body. Additionally, they contain mafic enclaves, xenoliths and sheets derived from the primary melt. Therefore a range of magnetic behaviour, and hence a range of potential suitability for palaeointensity work can be expected within a single pluton.

Fig. 1 Coastal Batholith, Peru | Examples of enclaves, sheets & textures seen in the Pluton

This project aims to address the question of whether granites are generically unsuitable for palaeointensity work or whether certain types of granite, or zones and facies within granitic bodies, are more suitable than others. This hypothesis will be tested experimentally using detailed rock magnetic and palaeomagnetic characterisation of specifically targeted I-type granites. The anticipated outcomes of the project will be to improve our understanding of the magnetic behaviour and characterisation of granitic intrusions, and to evaluate their potential for use in palaeointensity work. This will add essential evidence to increase our understanding of the long-term evolution of the geomagnetic field (e.g. Goguitchaichvilli et al, 2002a,b; Thomas & Biggin, 2003; Heller et al, 2002). The results of this project are expected to significantly change the traditional approach to palaeointensity work. This study will recommend a new strategy for future sampling and experimentation.

Fig. 2 Isle of Mull, Scotland | Dr Neil Thomas and Myself performing an emergency procedure on the drill with parcel tape... now that is resourceful!

REFERENCES

Best, M.G., 2002. Igneous and Metamorphic Petrology. Blackwell publishing. 2nd Edition

Biggin, A.J. & Thomas, D.N., 2003. Analysis of long-term variations in geomagnetic poloidal field intensity and evaluation of their relationship with global geodynamics, Geophys. J. Int., 152, 392-415.

Bloxham, J. & Gubbins, D., 1985. The secular variation of Earth's magnetic field. Nature, Vol. 317, 31, October 1985, pp777-781

Goguitchaichvilli, A., Alva-Valdiva, L., Urrutia-Fucugauchi, J., Morales, J. & Lopes, O.F., 2002a. On the reliability of the Mesozoic Dipole Low: New absolute palaeointensity results from the Parana Flood Basalts (Brazil), Geophys. Res. Lett., 29, 13, doi: 10.1029/2002GL015242.

Goguitchaichvilli, A., Urrutia-Fucugauchi, J. & Alva-Valdiva, L., 2002b. Mesozoic dipole low: Myth or reality? Eos, Trans. AGU, 83, 41, 457-461.

Heller, R., Merrill, R.T., & McFadden, P.L., 2002. The variation in intensity of Earth's magnetic field with time. Phys. Earth Planet. Inter., 131 (3-4), 237-249.

Merrill, R.T., McElhinny, M.W. & McFadden, P.L., 1996. The Magnetic Field of the Earth: Paleomagnetism, the Core and the Deep Mantle. International Geophysics SS. Academic Press.

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