A Comment on ‘Postcumulus Processes Recorded in Whole Rock Geochemistry: A Case Study from the Mirabela Layered Intrusion, Brazil’ by S.J. Barnes and M. Williams

Abstract

In their analysis of whole-rock data from the ultramafic section of the Mirabela Layered Intrusion, Barnes and Williams suggest that the rocks formed as a closed system of crystals + trapped liquid and that compaction played essentially no role in the formation of the rocks. Here it is shown that their stratigraphic trends are fully consistent with a system undergoing concurrent crystallization and compaction (CC&C). Specifically, phosphorus decreases upward owing to more time for compaction prior to apatite saturation away from the cooler margins. In contrast, sodium and strontium (mainly incorporated in interstitial plagioclase) increases upward owing to the initial interstitial liquid becoming more enriched in the plagioclase components over the crystallization of the ultramafic section, causing interstitial plagioclase to crystallize earlier in the compaction history up-section. Local coincident peaks in P, Na, and Sr, typically related to changes in rock type, can be ascribed to porosity variations that can develop during compaction of a layered, heterogenous mush. These and other observations lead to a conclusion contrary to that of Barnes and Williams: the Mirabela intrusion is instead an excellent case study for the consequence of CC&C in layered intrusions and potential role of non-conventional fractionation mechanism of the parent magma.

geochemistry, layered intrusions, differentiation, igneous compaction, Mirabela intrusion