Journal of Petrology Advance Access originally published online on April 27, 2006
Journal of Petrology 2006 47(8):1591-1610; doi:10.1093/petrology/egl021
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Formation of Spinifex Texture in Komatiites: an Experimental Study
1 CENTRE DE RECHERCHE PÉTROGRAPHIQUE ET GÉOCHIMIQUES CNRS-UPR2300, BP 20, 54501 VANDOEUVRE LES NANCY, FRANCE
2 LABORATOIRE DE GÉODYNAMIQUE DES CHAÎNES ALPINES, UMR 5025, UNIVERSITÉ JOSEPH FOURIER BP 53, 38041 ST MARTIN D'HÈRES, FRANCE
3 COLE NATIONALE SUPÉRIEURE DE GÉOLOGIE-INPL BP 40, 54501 VANDOEUVRE LES NANCY, FRANCE
RECEIVED OCTOBER 14, 2004; ACCEPTED MARCH 24, 2006
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 < –
T< 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.
KEY WORDS: komatiite; spinifex; cooling rate; experimental petrology; thermal gradient
*Corresponding author. Telephone: +33-383-594226. Fax. +33-383-511798. E-mail: ffaure{at}crpg.cnrs-nancy.fr
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