Abstract
Phase equilibrium experiments on two synthetic komatiite compositions have been used to develop a liquidus diagram that allows prediction of phase appearance sequences and crystallization paths for Munro Township komatiite lava flows. The experimentally determined phase relations and the observed mineral associations in thick, layered komatiite flows (e.g, Fred's Flow and the Alexo Flow) provide information on the bulk composition of these Munro komatiite lavas, the liquid line of descent and the magmatic processes that accompanied cooling and differentiation. Under equilibrium conditions a lava with bulk composition similar to that of Fred's Flow or the Alexo Flow should crystallize olivine as the liquidus phase, followed by spinel, pigeonite, plagioclase and finally augite. Komatiite lavas which have crystallized pigeonite as the first pyroxene are restricted to a limited range of bulk magma compositions. Cooling rate experiments reproduce successfully the mineral assemblages, the pyroxene zoning trends and the general textural characteristics of the olivine and pyroxene spinifex zones. In the upper part of the lava flow fractional crystallization of olivine moved the residual liquid composition into the pigeonite primary phase volume. Pigeonite crystallized, the liquid line of descent departed from equilibrium and augite and plagioclase crystallized under supercooled conditions. In the interior of these flows, within-flow magma mixing may have been important during the later stages of solidification, if the flow solidified under closed-system conditions.
