The Microstructural Record of Emplacement, Inflation and Convection of Sills, Lava Flows and Lakes

Abstract

The stratigraphic variation of plagioclase grain shape in the marginal regions of tabular bodies of magma provides a record of the balance of heat loss into the surroundings and heat brought into the body by magma flow during inflation and propagation. This record has the potential to constrain the amount of magma flow past any point in a sill, enabling mapping the construction of any extensive sill network. The variation of plagioclase grain shape in lava lakes may preserve a detailed history of lake filling, whereas the history of inflation of a thick ponded flood basalt flow is likely to be lost by melt-back of any early formed chill during prolonged flow at the base. Convection sufficiently vigorous to entrain crystals from the marginal solidification fronts is recorded by stratigraphic invariance of plagioclase grain shape and is not generally found in sills: instead, plagioclase grain shape varies with stratigraphic height, attesting to a predominance of in situ plagioclase nucleation and growth at the intrusion margins. While the stratigraphic variation of plagioclase grain shape in sills thinner than ~100 m is symmetrical, that in thicker sills varies in an asymmetric way with stratigraphic height, suggestive of a slower rate of inwards propagation of the roof solidification front compared to the floor, most likely indicative of moderately vigorous convection. Sills are generally finer-grained than dykes of the same thickness, regardless of whether the spatial variation of plagioclase shape is consistent with convection during solidification: this is likely to be a consequence of a greater extent of Ostwald ripening of suspended grains during long-lived convection in vertically extensive dykes.