Volcanology: Analyzing Magmatic Evolution using SEM EDS

SEM EDS is a powerful tool for volcanologists who wish to understand more about the evolution of volcanic rock samples over tens of thousands of years. Compositional zoning of magmatic minerals is an outstanding method for the recording of geological processes and their conditions, such as the pressure, temperature and volatile content present in the magma chamber prior to eruption (see Ref. 1). Here we show spatially resolved EDS analyses of plagioclases with complex zoning in order to discover more about a sample's magmatic evolution.

EDS mapping is one of the fastest analytical techniques available for the collection of fundamental information to the nanometer scale on the quantitative element distribution in rocks and minerals. In just a few minutes EDS mapping can provide the modal analysis of a rock sample. Spatially-resolved analyses of the minerals’ composition down to the finest internal variations is also possible post-measurement using pixel-tagged spectrometry in the HyperMap feature of our ESPRIT analysis software.

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Element distribution map for Na, K and Ca in a volcanic rock with large plagioclases. Sample courtesy of Dr. Daniel J. Smith, Department of Geology, University of Leicester, UK.

The AutoPhase function in ESPRIT turns the element map into a phase map, the most effective and conclusive option to distinguish between compositionally different phases in a sample. Phase mapping using AutoPhase allows not only individual mineral types but even solid solution minerals such as plagioclases with different anorthite (An) content to be distinguished from one another. The distinction between phases when using multivariate compositional analysis is far more effective when compared to using intensity element maps (see images). Furthermore, AutoPhase calculates the area fraction of each phase thus making modal analyses considerably more efficient and accurate than point counting methods (see table).

Whereas the typical timescales for magma residence and differentiation are in the order of 10 - 100 thousand of years (see Ref. 2), the mapping and evaluation of the process-recording minerals take only a few minutes using XFlash^® 760 and ESPRIT.

Plagioclase phase map indicating the volcanic evolution with five cycles of magmatic rejuvenation, differentiation and resorption.

References:

[1] Ginibre, C., Wörner, G., Kronz, A. (2007): Crystal zoning as an archive for magma evolution. Elements, 3(4), 261-266.

[2] Turner, S., Costa, F. (2007): Measuring timescales of magmatic evolution. Elements, 3 (4), 267-272.