The origin of spinifex texture in komatiites

Komatiites are high-temperature, fluid, magnesium-rich lavas typically of A rchaean age. A striking characteristic feature of such lavas is 'spinifex' texture-plate-like crystals of olivine ((Mg,Fe)(2)SiO4), millimetres to dec imetres long, in a fine-grained matrix of spherulitic clinopyroxene (Ca(Mg, Fe,Al)(Si,Al)(2)O-6), dendritic chromite ((Mg,Fe)(Cr,Al,Fe)(2)O-4) and alte red glass(1-4). Sheaves of olivine crystals can reach lengths exceeding one metre, even in komatiite flows less than 10 metres thick, in sharp contras t to the millimetre-scale post-eruption growth of crystals in more common v olcanic rocks. Crystal growth of this magnitude might be a consequence of t he high content of the constituent elements of olivine in komatiitic liquid , combined with the low viscosity and high chemical diffusivity of the lava s. But flows lacking spinifex texture are not uncommon, and those with such texture often contain substantial amounts of submillimetre olivine crystal s of unremarkable appearance, so chemical considerations alone do not appea r to provide a sufficient explanation. Here we present evidence that spinif ex texture develops as a result of large thermal gradients, coupled with co nductive and radiative heat transfer within olivine crystals lived in the c ool upper layers of the lava flows. This mode of growth has features in com mon with the high-temperature techniques used to grow large synthetic singl e crystals, but is rarely considered in geological contexts.