Advances in Igneous Petrology: Coupled chemical maps & thermodynamic models to tackle mushes crystallization dynamics

The assemblage and evolution in time through various differentiation processes of igneous reservoirs shapes magmatism expressions at depth and at surface. Our knowledge of those systems largely relies on theoretical or applied thermomechanical and kinetic models, on geophysical data from active systems, and on structural and petrological data from active and fossil systems. Petrological approaches commonly use textures, thermodynamic models, chemical compositions of major and trace elements to decipher on the dynamics of complex igneous processes. Although chemical maps are more and more common, the overwhelming of the published data reposes on punctual measurements.

Here by using the example of a basalt crystallizing from liquidus to solidus in closed system, we present a new petrological approach that couples chemical maps to thermodynamic models to provide the first maps of thermodynamic parameters with a value attributed to each pixel (e.g., temperature or melt fraction maps in plutonic rocks). If the cooling rate is quantified by other means (e.g., diffusion chronometry), then the first of its kind movies of the crystallization evolution can be built from the first crystal to form to the last melt drop crystallization. We will present such results, allowing the igneous petrologist to explore their data in a new perspective. As an example the studied samples could be observed at various stages of the crystallization path highlighting a heterogeneous interstitial melt spatial distribution during magma solidification. This result has potentially important implications on melt segregation at the scale of the igneous reservoir. The presented results highlight that such a heterogeneous melt distribution was already present at the magma/mush transition. Additional simplified numerical crystallization models suggest that this heterogeneous a melt distribution is related to an early heterogeneous nucleation process, and may be common in volcanic and igneous plumbing systems.

Overall, this new approach will eventually help to make progress in our understanding of igneous systems.