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An Origin for Harrisitic and Granular Olivine in the Rum Layered Suite, NW Scotland: a Crystal Size Distribution Study |
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| Authors: | O'Driscoll Brian; Donaldson Colin H; Troll Valentin R; Jerram Dougal A; Emeleus C Henry |
| Institution: | 1Department of Geology, Museum Building, Trinity College Dublin, Dublin 2, Ireland 2School of Geography and Geosciences, University of St. Andrews, St. Andrews KY16 9AL, UK 3Department of Earth Sciences, University of Durham, Science Laboratories, South Road, Durham DHI 3LE, UK |
| Abstract: | Dendritic crystal morphologies occur in a number of igneousrocks and are thought to originate from the rapid growth ofcrystals, yet many examples of dendritic morphologies are foundin plutonic igneous rocks where cooling rates should be low.Results from crystal size distribution (CSD) measurements onharrisitic olivines from Rum, Scotland, combined with estimatedolivine growth rates, suggest that the characteristic skeletalhopper and branching olivines of harrisitic cumulates that areup to centimetres long, may have exceptionally short crystalgrowth times (several hours to several hundreds of days). This,together with very low calculated nucleation densities for harrisiticolivine, supports the interpretation of harrisite being a disequilibriumtexture, developed in response to supersaturation of the magmain olivine. We propose that this supersaturation arose throughundercooling of thin picrite sheets emplaced along the Rum magmachamber floor, beneath cooler resident magma. It is envisagedthat the picrite sheets were largely free of suspended olivinecrystals. Coupled with the olivine-enriched composition of themelt and the increasing cooling rate, this allowed homogeneousnucleation of olivine to set in at deeper undercooling and greaterolivine supersaturation than if there had been plentiful suspendedolivines to act as heterogeneous nuclei. The enhanced supersaturationcaused rapid growth of olivine once nucleation began, with skeletaland dendritic shapes. It is suggested that the observed, interlayeredsequences of harrisite and cumulus peridotite found throughoutthe Rum Layered Suite are a result of multiple episodes of harrisitecrystallization resulting from picrite emplacement that alternatedwith periods of crystal growth and accumulation in the mainbody of magma at lesser degrees of undercooling. KEY WORDS: crystal size distribution; harrisite; crystal growth rates; Rum Layered Suite |
| Keywords: | : crystal size distribution harrisite crystal growth rates Rum Layered Suite |
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