CONSTRAINING THE THICKNESS OF THE CRYSTAL MUSH IN LAYERED MAFIC INTRUSIONS

When gabbroic magma stalls in the crust, cooling leads to growth of a marginal mushy layer. The thickness of this crystal mush can be constrained using the step-changes in textural maturity (quantified by the median augite-plag-plag dihedral angle, Θ_cpp) caused by the change in fractional latent heat accompanying the arrival of a new liquidus phase.

At the instant of its saturation in the bulk magma, the top of the mush is marked by the first appearance of the new primocryst phase. The high porosity upper zone of the mush comprises poorly consolidated material, with no cpx-plag-plag junctions: at deeper levels porosity decreases by primocryst overgrowth, growth of interstitial augite, and compaction. The mush zone can be divided into 3: the upper zone, where melt is adjacent to all plag-plag junctions; the middle zone where augite fills some of the pore corners; and the lower zone where augite fills all pore corners. The base of this lower zone corresponds to the point at which diffusive change of grain boundary orientations has effectively ceased.

The change in fractional latent heat accompanying the addition to the liquidus assemblage will be fully recorded within the upper zone since, once they have formed, all cpx-plag-plag junctions will experience the new thermal regime: Θ_cpp will be high. The middle zone will record a mixture of the new and old thermal regime: junctions which were melt-filled at the moment of arrival of the new phase will receive the full impact of the new regime, while those which were filled by augite will receive a reduced impact. Θ_cpp will therefore range from the new higher value at the top of middle zone, to some lower value at the base. Cumulates in the lower zone had no melt-filled junctions: Θ_cpp will increase from a low value corresponding to the old thermal regime recorded at the base of the lower zone to meet the lowest value recorded at the base of the middle zone. The width and shape of the step-change in Θ_cpp thus tells us about mush thickness and structure.

This method, applied to apatite-in in the Skaergaard Layered Series, reveals that the mush was of the order 4m thick in the centre of the intrusion, but 150m thick near the walls. The difference reflects variable contributions of eroded material from the vertical chamber walls.