Determining the Effects of Cooling Rate on Magma Crystallisation Using a High Temperature Heating Stage

In order to understand igneous rock textures and the history of erupted products and the dynamics of lava flow emplacement, it is necessary to understand how crystals grow and what effects different cooling histories have on their growth. Previous studies on crystal growth have often assumed constant crystal growth rates and focused on crystallisation over long timescales more appropriate to igneous intrusions. This work aims to quantify and describe crystal growth rates, morphological variations and textural development of crystals growing from natural lava flow samples, primarily focussing on plagioclase feldspar. High Temperature Heating Stage experiments were carried out at temperatures and cooling rates appropriate to basaltic lava flows, in which wafers of the glassy rind from Blue Glassy Pahoehoe were melted and re-crystallised. It was possible to directly observe and record crystal growth over time at controlled cooling rates, and to extract information from the quenched products. The experiments in this study grew crystals at very low undercoolings, maintaining an interface controlled growth regime and facetted crystal morphologies. Bulk evolution of crystal growth indicated that growth rates were not constant over time, decaying as they grew. The morphology and aspect ratio of these crystals changed over time, with aspect ratio increasing as growth was significantly faster in the length direction during the observed period. The relationship between mean aspect ratio and crystallisation time proposed by Holness (2014) was experimentally verified. We also observed the ‘true’ crystallisation time of crystals, highlighting a need for better constraint on how crystal growth times are used to calculate growth rates. The results of this study will contribute to better future interpretations of magmatic histories and crystallisation conditions in natural basaltic lava flows, as well as refinement of Crystal Size Distribution studies

Melt diffusion-moderated crystal growth and its effect on euhedral crystal shapes

Crystal growth is often described as either interface-controlled or diffusion-controlled. Here, we study crystal growth in an intermediate scenario where reaction rates at the crystal-melt interface are similar to the rates of diffusive transport of ions through the melt to the advancing crystal surface. To this end, we experimentally investigated euhedral plagioclase crystal shapes in dry mafic (basaltic) and hydrous silicic (haplodacitic) melts. Aspect ratios and inferred relative growth rates of the 3D short (S) and intermediate (I) crystal dimensions vary significantly between mafic and silicic melts, with δS:δI = 1:6 – 1:20 in basalt and 1:2.5 – 1:8 in hydrous haplodacite. The lower aspect ratios of plagioclase grown in the silicic melt coincide with 10-100x lower melt diffusion rates than in the mafic melt. Using an anisotropic growth model, we show that such differences in melt diffusivity can explain the discrepancy in plagioclase aspect ratios: if interface reaction and melt diffusion rates are of similar magnitude, then the growth of a crystal facet with high interfacial reaction rates may be limited by melt diffusion while another facet of the same crystal with lower interfacial reaction rates may grow uninhibited by melt diffusivity. This selective control of melt diffusion on crystal growth rates results in progressively more equant crystal shapes as diffusivity decreases, consistent with our experimental observations. Importantly, crystals formed in this diffusion-moderated, intermediate growth regime may not show any classical diffusion-controlled growth features. The proposed model was developed for plagioclase microlites, but should be generalisable to all anisotropic microlite growth in volcanic rocks