Compaction of Density-Stratified Cumulates: Effect on Trapped-Liquid Distribution

Igneous rocks formed by the accumulation of crystals from a cooling magma body compose a significant fraction of both the terrestrial oceanic crust and the lunar crust and mantle. Here we report the results of numerical simulations of compaction of accumulating crystals for which the bulk density is not constant. The simulations are designed to provide insight into the behavior of interstitial liquid during compaction when a sharp drop in the density of the cumulus minerals is encountered (e.g., at the ultramafic-mafic boundary in a layered intrusion). The simulations provide an explanation for trapped-liquid distributions associated with sharp mineral density transitions. Density transition in the mineral assemblage is a feature common to many layered intrusions and the oceanic crust. This process does not require external mechanisms, such as injections of new and different magmas, and has important implications for the formation of Pt-ores found at ultramafic-mafic transitions. The model also has implications for the redistribution of liquid in the intrusive portions of the oceanic crust.