Chromite in komatiites 3D morphologies with impli

Authors: Bélinda Godel Stephen J Barnes Derya Gürer Peter Austin Marco L Fiorentini

Publish Date: 2012/10/04

Volume: 165, Issue: 1, Pages: 173-189

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Abstract

Highresolution Xray computed tomography has been carried out on a suite of komatiite samples representing a range of volcanic facies chromite contents and degrees of alteration and metamorphism to reveal the wide range of sizes shapes and degrees of clustering that chromite grains display as a function of cooling history Dendrites are spectacularly skeletal chromite grains formed during very rapid crystallization of supercooled melt in spinifex zones close to flow tops At slower cooling rates in the interiors of thick flows chromite forms predominantly euhedral grains Large clusters up to a dozen of grains are characteristic of liquidus chromite whereas fine dustings of mostly individual ~20μm grains form by in situ crystallization from trapped intercumulus liquid Chromite in coarsegrained olivine cumulates from komatiitic dunite bodies occurs in two forms as clusters or chains of euhedral crystals developing into “chickenwire” texture where chromite is present in supracotectic proportions and as strongly dendritic semipoikilitic grains These dendritic grains are likely to have formed by rapid crescumulate growth from magma that was close to its liquidus temperature but supersaturated with chromite In some cases this process seems to have been favoured by nucleation of chromite on the margins of sulphide liquid blebs This texture is a good evidence for the predominantly cumulus origin of oikocrysts and in situ origin of heteradcumulate textures Our 3D textural analysis confirms that the morphology of chromite crystals is a distinctive indicator of crystallization environment even in highly altered rocksBélinda Godel is funded by the CSIRO Office of the Chief Executive PostDoctoral Fellowship scheme Analytical costs were partially funded by a “Pump Priming Grant” from the Faculty of Natural and Agricultural Sciences at The University of Western Australia Accessibility to supercomputing facilities was provided by iVEC at the Australian Resources Research Centre Perth Dr Greg Hitchen provided assistance with electron microprobe analyses This paper is an output from the CSIRO Minerals Down Under National Research Flagship Associate editor Chris Ballhaus and two anonymous referees are acknowledged for their comments on the manuscript

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