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      Crystal settling and convection in the Shiant Isles Main Sill


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          The 168 m-thick Shiant Isles Main Sill is a composite body, dominated by an early, 24 m-thick, picrite sill formed by the intrusion of a highly olivine-phyric magma, and a later 135 m-thick intrusion of olivine-phyric magma that split the earlier picrite into a 22 m-thick lower part and a 2 m-thick upper part, forming the picrodolerite/crinanite unit (PCU). The high crystal load in the early picrite prevented effective settling of the olivine crystals, which retain their initial stratigraphic distribution. In contrast, the position of the most evolved rocks of the PCU at a level ~80% of its total height point to significant accumulation of crystals on the floor, as evident by the high olivine mode at the base of the PCU. Crystal accumulation on the PCU floor occurred in two stages. During the first, most of the crystal load settled to the floor to form a modally and size-sorted accumulation dominated by olivine, leaving only the very smallest olivine grains still in suspension. The second stage is recorded by the coarsening-upwards of individual olivine grains in the picrodolerite, and their amalgamation into clusters which become both larger and better sintered with increasing stratigraphic height. Large clusters of olivine are present at the roof, forming a foreshortened mirror image of the coarsening-upwards component of the floor accumulation. The coarsening-upwards sequence records the growth of olivine crystals while in suspension in a convecting magma, and their aggregation into clusters, followed by settling over a prolonged period (with limited trapping at the roof). As olivine was progressively lost from the convecting magma, crystal accumulation on the (contemporaneous) floor of the PCU was increasingly dominated by plagioclase, most likely forming clusters and aggregates with augite and olivine, both of which form large poikilitic grains in the crinanite. While the PCU is unusual in being underlain by an earlier, still hot, intrusion that would have enhanced any driving force for convection, we conclude from comparison with microstructures in other sills that convection is likely in tabular bodies >100 m thickness.

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                Author and article information

                Contrib Mineral Petrol
                Contrib Mineral Petrol
                Contributions to Mineralogy and Petrology. Beitrage Zur Mineralogie Und Petrologie
                Springer Berlin Heidelberg (Berlin/Heidelberg )
                19 January 2017
                19 January 2017
                : 172
                : 1
                : 7
                [1 ]GRID grid.5335.0, ISNI 0000000121885934, Department of Earth Sciences, , University of Cambridge, ; Downing Street, Cambridge, CB2 3EQ UK
                [2 ]Unilever Research and Development, Colworth Science Park, Bedford, MK44 1LQ UK
                [3 ]GRID grid.5335.0, ISNI 0000000121885934, BP Institute for Multiphase Flow, , University of Cambridge, ; Madingley Road, Cambridge, CB3 0EZ UK
                [4 ]GRID grid.5335.0, ISNI 0000000121885934, Department of Applied Mathematics and Theoretical Physics, Centre for Mathematical Sciences, , University of Cambridge, ; Wilberforce Road, Cambridge, CB3 0WA UK
                Author notes

                Communicated by Timothy L. Grove.

                Author information
                © The Author(s) 2017

                Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License ( http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

                : 30 June 2016
                : 22 December 2016
                Funded by: FundRef http://dx.doi.org/10.13039/501100000270, Natural Environment Research Council;
                Award ID: NE/J021520/1
                Award Recipient :
                Funded by: FundRef http://dx.doi.org/10.13039/501100000288, Royal Society;
                Award ID: University Research Fellowship
                Award Recipient :
                Original Paper
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                © Springer-Verlag Berlin Heidelberg 2017

                convection,crystallisation,microstructure,grain size,composite intrusion


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