Lydéric France , Valentine Charvet , Aurore Toussaint
Journal of Petrology, Volume 66, Issue 5, May 2025, egaf044
Voir en ligne : https://doi.org/10.1093/petrology/egaf044
Abstract: Chemical maps of igneous samples contain prodigious information constraining the evolution and solidification of magmas and mushy systems. Nevertheless, their full potential remains unrealized; they are mostly used only to highlight phase relations or mineral zonations. Here, based on developments in metamorphic petrology over the last decade, we demonstrate that quantified parameters describing igneous systems (e.g. crystallization temperature, melt fraction or composition, or other thermodynamic parameters) can be coupled with quantified chemical maps to map such system parameters at the scale of a thin section. This approach has great potential to improve our understanding of the dynamics of magma and mushy igneous systems. To highlight the potential of our approach, we apply it to a gabbro dike from a slow-spreading oceanic center, a rare sample representing a most studied igneous system (MORB) melt crystallized in situ in a closed system (i.e. its bulk composition corresponds to that of the initial melt). This peculiar sample provides the unique opportunity to study mush processes that are usually overprinted by more classical open-system processes during oceanic plutonic accretion (crystal settling, reactive porous flow, magma/mush deformation, including compaction and related melt extraction…). Our results highlight that local crystal–melt segregation occurred during progressive solidification, creating melt-rich and melt-poor domains. A simplified numerical model testing the effects of homogeneous versus heterogeneous crystal nucleation indicates that the latter has the potential to trigger such heterogeneities with melt-rich and melt-poor domains. More generally, we show that heterogeneous nucleation can likely not produce melt lenses of several meters, but that it could produce widespread melt pockets >25 000 times larger than those produced by homogeneous nucleation, and thus has a strong potential to facilitate melt mobilization during any subsequent rejuvenation event.