The Journal of Petrology, Volume 38, Issue 11: November 1997.

The origin of Bl zones in komatiite flows

KE Silva^1,2, MJ Cheadle^2,* and EG Nisbet¹

¹Department of Geology, Royal Holloway College, University of London, Egham Hill, Egham TW20 0EX, UK, ²Department of Earth Sciences, University of Liverpool, Jane Herdman Laboratories, Brownlow Street, Liverpool L79 3BX, UK, ^*Corresponding author e-mail: mjc44@liv.ac.uk

B1 zones of differentiated komatiite lava flows are thin layers (less than a few tens of centimetres) composed predominantly of skeletal 'hopper' olivine crystals with hollow cores and irregular morphologies. Detailed textural and geochemical studies of two Archaean Komatiite flows from the Reliance Formation, Belingwe Greenstone Belt, Zimbabwe, reveal that the B1 olivines show reverse size grading and constant mg-number compositions (91.1-91.6%±1.5%). Olivines of this composition would have been in equilibrium with the estimated initial melt composition (20 wt % MgO), suggesting that the B1 crystals nucleated before or very soon after emplacement of the flow. Mass balance calculations indicate that the B1 zones are regions of net olivine accumulation. A model is presented for the formation of B1 zones, which is applicable to all komatiite flows that crystallized as closed magmatic systems. A convecting magma is erupted containing polyhedral phenocrysts and skeletal crystals subsequently grow during flowage. On emplacement, polyhedral phenocrysts settle rapidly to form the cumulate pile. Skeletal crystals with lower settling velocities become segregated from the polyhedral phenocrysts during settling. Some are maintained in convection, achieving large, elongate habits. These crystals eventually settle horizontally onto the cumulative pile, creating the reverse grading and typical fabric of B1 zones.

Key words: B1 zone; convecting magma; hopper olivines; komatiite; reverse grading

Pages 1565-1584