Quantifying crystal-melt segregation in dykes
Résumé
The dynamics of magma flow is highly affected by the presence of a crystalline load. During magma ascent, it
has been demonstrated that crystal-melt segregation constitutes a viable mechanism for magmatic differentiation.
However, the influences of crystal volume fraction, geometry, size and density on crystal melt segregation are still
not well constrained. In order to address these issues, we performed a parametric study using 2D direct numerical
simulations, which model the ascension of crystal-bearing magma in a vertical dyke. Using these models, we have
characterised the amount of segregation as a function of different quantities including: the crystal fraction (), the
density contrast between crystals and melt (), the size of the crystals (Ac) and their aspect ratio (R). Results
show that crystal aspect ratio does not affect the segregation if R is small enough (long axis smaller than 1/6 of
the dyke width,Wd). Inertia within the system was also found not to influence crystal-melt segregation. The degree
of segregation was however found to be highly dependent upon other parameters. Segregation is highest when
and Ac are large, and lowest for large pressure gradient (Pd) and/or large values ofWd. These four parameters can
be combined into a single one, the Snumber, which can be used to quantify the segregation. Based on systematic
numerical modelling and dimensional analysis, we provide a first order scaling law which allows quantification of
the segregation for an arbitrary Snumber and , encompassing a wide range of typical parameters encountered in
terrestrial magmatic systems.