Formation of Spinifex Texture in Komatiites: an Experimental Study

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

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

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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: http://dx.doi.org/10.1093/petrology/egl021

Publication date: 2006-08-01

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The Journal of Petrology provides an international forum for the publication of high quality research in the broad field of igneous and metamorphic petrology and petrogenesis. Papers published cover a vast range of topics in areas such as major element, trace element and isotope geochemistry and geochronology applied to petrogenesis; experimental petrology; processes of magma generation, differentiation and emplacement; quantitative studies of rock-forming minerals and their paragenesis; regional studies of igneous and meta morphic rocks which contribute to the solution of fundamental petrological problems; theoretical modelling of petrogenetic processes.

Formation of Spinifex Texture in Komatiites: an Experimental Study

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