Abstract
The microstructure in basaltic dykes is significantly different from that in sills and lava lakes of the same bulk composition. For a given width of intrusion (or depth of lava lake), vertical tabular bodies are coarser grained than horizontal bodies, with an invariant plagioclase shape across the intrusion. When comparing samples from sills and dykes for which the average grain size is the same, the dyke samples contain fewer small grains and fewer large grains than the sill samples. In contrast, the variation of median clinopyroxene-plagioclase-plagioclase dihedral angles in dykes correlates precisely with that observed in sills and is a function of the rate of diffusive heat loss. These patterns can be accounted for if the early stages of crystallization in dykes primarily involve the growth of isolated grains suspended in a well-mixed convecting magma, with the final stage (during which dihedral angles form) occurring in a crystal-rich static magma in which heat loss is primarily diffusive. In contrast, crystallization in sills occurs predominantly in marginal solidification fronts, suggesting that any convective motions are insufficient to entrain crystals from the marginal mushy layers and to keep them suspended while they grow. An exception to this general pattern is provided by members of the Mull SolitDykes, which propagated 100-1000 km SE from the Mull Palaeogene Igneous Centre, Scotland, through the shallow crust. These dykes, where sampled >100 km from Mull, have a microstructure indistinguishable from that of a sill of comparable thickness. We suggest that sufficient nucleation and crystallization occurred in these dykes to increase the viscosity sufficiently to damp convection once unidirectional flow had ceased.
Highlights
IntroductionThe extent and vigour of convection during the solidification of mafic intrusions (sills and larger, more equant, layered intrusions), flows and lava lakes remain controversial (Brandeis & Marsh, 1989; Marsh, 1989, 2015; Worster et al, 1990; Huppert & Turner, 1991; Latypov, 2009)
The extent and vigour of convection during the solidification of mafic intrusions, flows and lava lakes remain controversial (Brandeis & Marsh, 1989; Marsh, 1989, 2015; Worster et al, 1990; Huppert & Turner, 1991; Latypov, 2009)
Geological observations used to argue for convection include: evidence for effective magma mixing on a chamber-wide scale, demonstrated by the progressive development of concentric zones containing compositionally identical minerals on the walls, roof and floor of intrusions (e.g. Salmonsen & Tegner, 2013); modal layering attributed to scouring and resedimentation of crystals (e.g. McBirney & Nicolas, 1997; Irvine et al, 1998); and the increasing dominance of floor crystallization as the intrusion size increases (Worster et al, 1990), this has been used as an argument to support the creation of intrusions by the injection of a succession of crystal-rich slurries (Marsh, 2013)
Summary
The extent and vigour of convection during the solidification of mafic intrusions (sills and larger, more equant, layered intrusions), flows and lava lakes remain controversial (Brandeis & Marsh, 1989; Marsh, 1989, 2015; Worster et al, 1990; Huppert & Turner, 1991; Latypov, 2009). Salmonsen & Tegner, 2013); modal layering attributed to scouring and resedimentation of crystals
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