Melt migration in basalt columns driven by crystallization-induced pressure gradients

Abstract

The structure of columnar-jointed lava flows and intrusions has fascinated people for centuries and numerous hypotheses on the mechanisms of formation of columnar jointing have been proposed. In cross-section, weakly developed semicircular internal structures are a near ubiquitous feature of basalt columns. Here we propose a melt-migration model, driven by crystallization and a coeval specific volume decrease inside cooling and solidifying columns, which can explain the observed macroscopic features in columnar-jointed basalts. We study basalts from Hrepphólar (Iceland), combining macroscopic observations, detailed petrography, thermodynamic and rheological modelling of crystallization sequences, and Anisotropy of magnetic susceptibility (Ams) of late crystallizing phases (that is, titanomagnetite). These are all consistent with our proposed model, which also suggests that melt-migration features are more likely to develop in certain evolved basaltic lava flows (with early saturation of titanomagnetite), and that the redistribution of melt within individual columns can modify cooling processes.

Ardnamurchan 3D cone-sheet architecture explained by a single elongate magma chamber

“…(3) The emplacement-related fabrics in sheet intrusions can be modified by e.g. post-emplacement crystal growth, melt migration, and dyke closure (cf404142.). Hence, the locally observed sub-horizontal flow fabrics in the Ardnamurchan cone-sheets may not be exclusively primary in nature and, even if they were, do not rule out semi-vertical emplacement.…”

Anatomy of an extinct magmatic system along a divergent plate boundary: Alftafjordur, Iceland

“…In the inner part of the dike, there is a reorganization of the phenocrysts and opaque minerals along subhorizontal orientations orthogonal to the dike plane, which does not reflect the original magma emplacement. This fabric could reflect secondary processes producing horizontal readjustment of grains due to lithostatic pressure or during magma cooling stage [ Mattsson et al , ; Almqvist et al , ]. The origin of this geometrically inverse fabric will be discussed elsewhere.…”

Magma flow within dykes in submarine hyaloclastite environments: an AMS study of the Miocene Cabo de Gata volcanic units

“…This mineral is typically recognized as a primary Fe -Ti oxide, formed during the last stage of crystallization (Mattsson et al 2011), whereas the presence of small amounts of magnetite can be related to exsolution or metasomatic processes (Rochette et al 1999). The rock magnetic results definitively allow us to exclude the formation of a new magnetic phase or alteration of a pre-existent magnetic mineral in our samples.…”