The formation of plagioclase chains during convective transfer in basaltic magma

The basaltic rock in the lower part of the thick Holyoke lava flow in Connecticut and Massachusetts has been shown to have a remarkable texture, with crystals of feldspar linked together in a continuous three-dimensional network of chains. Heating experiments have revealed that this network persists to temperatures where the rock is 75% liquid, and therefore the network was interpreted to have formed at an early stage of crystallization and to have played an important role in the compaction of crystal mush in the lower part of the flow. Despite the texture's importance to our understanding of how such basalt flows form, the origin of the texture has remained uncertain. Here we show that, although the network is present in the lower third of the flow, it was actually formed in the upper solidification front and was transported down in plumes of dense crystal mush. Convection of this type has been postulated for intrusive magma chambers, but corroborative field evidence has been equivocal, especially in lava lakes and flows. Preservation of the roof-generated texture in the lower part of a thick flood-basalt flow therefore constitutes important evidence for the role of convection in the solidification and differentiation of a simple magma sheet.