Search

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 Solitary Dykes, 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. [ABSTRACT FROM AUTHOR]

The melt–solid dihedral angle has been measured in a range of igneous rock types, ranging in composition from picrite, through basalt, phonolite, andesite and rhyolite, for the minerals quartz, leucite, plagioclase, olivine, amphibole and clinopyroxene. Populations of up to 104 true 3-D angles were measured in each sample using a universal stage mounted on an optical microscope. The median and standard deviation of the angle populations for each mineral are distinct (plagioclase ∼25°, with standard deviation (SD) 11°; clinopyroxene ∼38°, with SD 14°; olivine ∼29°, with SD 13°; quartz ∼18°, with SD 9°; leucite ∼20°, with SD 11°), with no control by either melt composition or degree of approach of the grains to their equilibrium shapes. [ABSTRACT FROM AUTHOR]