Melt extraction from crystal mushes: Numerical model of texture evolution and calibration of crystallinity-ordering relationships
Abstrakt
Mechanical crystal-melt interactions in magmatic systems by separation or accumulation of crystals or by extraction of interstitial melt are expected to modify the spatial distribution of crystals observed as phenocrysts in igneous rocks. Textural analysis of porphyritic products can thus provide a quantitative means of interpreting the magnitude of crystal accumulation or melt loss and reconstructing the initial crystal percentage, at which the process occurred.
We present a new three-dimensional numerical model that evaluates the effects of crystal accumulation (or interstitial melt removal) on the spatial distribution of crystals. Both processes lead to increasing apparent crystallinity but also to increasing spatial ordering expressed by the clustering index (R).
The trend of progressive crystal packing deviates from a random texture trend, produced by static crystal nucleation and growth, and it is universal for any texture with straight log-linear crystal size distribution. Illustrative quantitative evaluation of the crystallinity-clustering relationships to representative porphyritic granites from a single intrusive unit of the Krkonoge-Jizera pluton (central Europe) reveals a single crystal accumulation path starting at low initial crystallinity (5-7 vol.% K-feldspar phenocrysts), with 24-84% melt extracted leading to the observed crystallinity of 9-26 vol.%.
By contrast, a camptonite dyke from the Ceske stredohori volcanic province has experienced the onset of crystal accumulation later (18 vol.% amphibole crystals) and lost 23% interstitial melt only. The combination of modal and clustering analysis offers a sensitive tool for identifying differentiation processes in natural magma chambers, and here it illustrates examples of mechanically dominated open-system vs. in situ nearly closed-system crystallization from two contrasting magmatic settings.