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Reactive flow and its role in magmatic processes: from arc volcanism to layered intrusions
It is widely (albeit not universally) accepted that long-term magma storage in the continental crust occurs in low melt fraction (high crystallinity) ‘mush reservoirs’ rather than the high melt fraction (low crystallinity) ‘magma chambers’ that have dominated conceptual models of magma storage and differentiation for over a century. Recognition of the importance of mush reservoir processes has generated new questions around what those processes are, how they operate and what evidence they leave in the rock record.
Fractional crystallization is widely invoked to explain differentiation, assuming efficient crystal-melt separation in high melt fraction magma chambers driven by processes such as crystal settling. Yet differentiation in magma chambers is inconsistent with the evidence for low melt fraction magma storage. At low melt fraction, melt-crystal separation is typically assumed to occur by porous flow and compaction, yet microstructural evidence for compaction is scarce, at least in relatively shallow magmatic systems.
Magma reservoir processes are investigated here using numerical models that capture (i) buoyancy-driven separation of melt and crystals by crystal settling at high melt fraction and porous flow at low melt fraction; (ii) compaction and accompanying melt loss; (iii) transfer of heat by conduction and advection, and (iv) mass and component exchange between crystals and melt. A key aim is to identify the role of ‘reactive flow’ in driving melt fraction and compositional changes. Reactive flow occurs when melt-crystal separation causes changes in local bulk composition that require mass and/or component exchange to return melt fraction, and melt and crystal compositions, to their local equilibrium values.
Results show that compaction is not required to drive down melt fraction to small values to produce refractory crystalline residues, consistent with the relatively scarce microstructural evidence for compaction. Melt loss may instead leave evidence of mineral dissolution caused by reactive flow, which facilitates ongoing melt expulsion by preserving melt connectivity through the mush pore space. Where melt accumulates, interstitial mineral phases display textures that mimic those of interstitial melt.
Recent application of the model to layered intrusions suggests the combined effects of compaction and reactive flow can explain several observed features. They offer a mechanism to form igneous layering, including monomineralic layers such as anorthosite, which are not readily explained by fractional crystallization. Equilibrium and fractional crystallization do not represent ‘end-member’ behaviours, because they fail to capture the range of local bulk compositions created during melt-crystal separation.
We argue that reactive flow is a fundamentally important process in all magmatic systems in which there is relative flow of melt and crystals that can react with each other, yet its contribution is neglected in most conceptual models.
Date:
30 January 2026, 12:00
Venue:
Department of Earth Sciences, South Parks Road OX1 3AN
Venue Details:
Seminar rooms
Speaker:
Prof Matthew Jackson (Imperial College London)
Booking required?:
Not required
Audience:
Public
Editor:
Maria Petrunova
