Abstract
The average aspect ratio of plagioclase, measured in thin section, varies systematically through dolerite sills, with a symmetrical “M”-shaped profile observed in sills thinner than ~200 m. Thicker sills show the same marginal reversal at the base, but average aspect ratios appear to continue increasing towards the top, creating an “S”-shaped profile. A distorted “M”-shaped profile is visible in the stratigraphic variation of plagioclase average aspect ratios in the prehistoric Makaopuhi lava lake, with the centre of the “M” in the olivine-enriched horizon close to the base of the lake. Higher overall values of average aspect ratio are observed in thin sills compared to thicker sills, demonstrating that the plagioclase in more slowly crystallized bodies is more equant than that in more rapidly crystallized bodies. There is a strong correlation between the plagioclase average aspect ratio in the central parts of the sills and the crystallization time calculated using a simple one-dimensional thermal model assuming conductive cooling. The cause of the marginal reversals in average aspect ratio is not well understood, but may result from early grain impingement in the developing crystal mushy layer.
Highlights
Placing constraints on the time taken for cooling and crystallization of igneous rocks is a necessary first step in determining the timescales of processes such as mass transport and compaction, or the rheological properties of partially crystallized materials
Recent work has shown that the median value of the clinopyroxene–plagioclase–plagioclase dihedral angle in dolerites can be used as a direct proxy for crystallization times with no need for well-constrained growth rates (Holness et al 2012)
Two additional samples were chosen from the Makaopuhi lava lake with microstructures intermediate to these two extremes: MP151 contains the most equant plagioclase in the lake, whereas MP70 has the least equant plagioclase
Summary
Placing constraints on the time taken for cooling and crystallization of igneous rocks is a necessary first step in determining the timescales of processes such as mass transport and compaction, or the rheological properties of partially crystallized materials. Timescales of crystallization are commonly calculated using grain size distributions (e.g. Marsh 1988), but the results of such calculations depend on accurate knowledge of grain growth rates that are not sufficiently accurately known for the low undercoolings relevant to large igneous bodies (Cashman 1993; Brugger and Hammer 2010). Recent work has shown that the median value of the clinopyroxene–plagioclase–plagioclase dihedral angle in dolerites can be used as a direct proxy for crystallization times with no need for well-constrained growth rates (Holness et al 2012). In this contribution, the extent to which plagioclase grain shapes can be used to constrain crystallization times is explored. I present data obtained from a set of dolerite sills of varying thickness, together with a lava lake, and demonstrate a clear relationship between plagioclase
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