Compaction of Igneous Cumulates

When initially deposited, mafic igneous cumulates consist of a solid matrix of mineral phases plus 50-60% interstitial liquid. Due to its greater density the matrix compacts, squeezing the interstitial liquid upward. A numerical fluid mechanical model of this process indicates that if the cumulate deposition rate is constant, compaction occurs in one of two distinct models. For rapid deposition, the system is permeability controlled with compaction occurring only at the base of the cumulate pile. For slowly deposited cumulates an additional compaction zone that is partially viscosity-controlled develops at the top of the pile. If the extent of compaction and thickness of this zone are known for a real system, the model can be used to estimate the permeability and effective viscosity of the compacting cumulates. Such calculations yield a permeability of 1.5 × 10⁻³a²f³/(1 - f)² for olivine cumulates in the Muskox Intrusion. Reliable viscosity estimates await an empirical law relating matrix viscosity to melt fraction. This work places only a lower limit of 5 × 10¹⁴ Pa s on the matrix viscosity. The actual value is probably 2-3 orders of magnitude higher. Future studies on intrusions where compaction occurred to differing degrees should lead to an empirical law relating matrix viscosity and melt fraction in igneous cumulates.