The origin of spinifex texture in komatiites

Mark Shore and Anthony D. Fowler ()
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Mark Shore: University of Ottawa
Anthony D. Fowler: University of Ottawa

Nature, 1999, vol. 397, issue 6721, 691-694

Abstract: Abstract Komatiites are high-temperature, fluid, magnesium-rich lavas typically of Archaean age. A striking characteristic feature of such lavas is ‘spinifex’ texture—plate-like crystals of olivine ((Mg,Fe)2SiO4), millimetres to decimetres long, in a fine-grained matrix of spherulitic clinopyroxene (Ca(Mg,Fe,Al)(Si,Al)2O6), dendritic chromite ((Mg,Fe)(Cr,Al,Fe)2O4) and altered glass1,2,3,4. Sheaves of olivine crystals can reach lengths exceeding one metre, even in komatiite flows less than 10 metres thick, in sharp contrast to the millimetre-scale post-eruption growth of crystals in more common volcanic rocks. Crystal growth of this magnitude might be a consequence of the high content of the constituent elements of olivine in komatiitic liquid, combined with the low viscosity and high chemical diffusivity of the lavas. But flows lacking spinifex texture are not uncommon, and those with such texture often contain substantial amounts of submillimetre olivine crystals of unremarkable appearance, so chemical considerations alone do not appear to provide a sufficient explanation. Here we present evidence that spinifex texture develops as a result of large thermal gradients, coupled with conductive and radiative heat transfer within olivine crystals fixed in the cool upper layers of the lava flows. This mode of growth has features in common with the high-temperature techniques used to grow large synthetic single crystals, but is rarely considered in geological contexts.

Date: 1999
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DOI: 10.1038/17794

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