Formation of spinifex texture in komatiites: an experimental study

The formation of platy olivine spinifex, the texture that characterizes komatiite lavas. has long been enigmatic. A major problem is that the dendritic morphology, of the olivine resembles that of crystals grown in laboratory experiments at high cooling rates (> 50 degrees C/h), but at the position where these textures form, up to several meters below the komatiite flow top, the cooling rate cannot have been greater than 1-5 degrees C/h. We performed experiments that demonstrate that the play habit of spinifex olivine or pyroxene is a consequence of slow cooling of ultramafic magma in a thermal gradient (7-35 degrees C/cm). The charges were cooled at rates between 2 and 1428 degrees C/h and, even at the low cooling rates, the thermal gradient led to constrained growth and the development of preferentially oriented dendritic crystals with morphologies like those in natural play spinifex-textured lavas. Under these conditions, olivine starts to crystallize at temperatures well below the equilibrium liquidus temperature (37 degrees C < -Delta T < 56 degrees C) depending on the composition of the starting material. When the cooling rate is high, the thermal gradient has a negligible effect on the texture and the crystals have a random orientation. like that in the zipper parts of komatiite flows.