IngentaConnect Formation of Spinifex Texture in Komatiites: an Experimental Stud...

Authors: FAURE, FRANÇOIS; ARNDT, NICHOLAS; LIBOUREL, GUY

Source: Journal of Petrology, Volume 47, Number 8, August 2006 , pp. 1591-1610(20)

Abstract:

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°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°C/h. We performed experiments that demonstrate that the platy habit of spinifex olivine or pyroxene is a consequence of slow cooling of ultramafic magma in a thermal gradient (7-35°C/cm). The charges were cooled at rates between 2 and 1428°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 platy spinifex-textured lavas. Under these conditions, olivine starts to crystallize at temperatures well below the equilibrium liquidus temperature (37°C < −ΔT< 56°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 upper parts of komatiite flows.

Keywords: komatiite; spinifex; cooling rate; experimental petrology; thermal gradient

Document Type: Research article

DOI: 10.1093/petrology/egl021

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Formation of Spinifex Texture in Komatiites: an Experimental Study