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TitlePorous sidewall and sloping floor crystallization experiments using a reactive mush: Implications for the self-channelization of residual melts in cumulates
AuthorBédard, J H; Kerr, R C; Hallworth, M A
SourceEarth and Planetary Science Letters vol. 111, 1992 p. 319-329,
Alt SeriesGeological Survey of Canada, Contribution Series 51091
PublisherElsevier BV
Mediapaper; on-line; digital
File formatpdf
Subjectsigneous and metamorphic petrology; crystallization; porosity; fractional crystallization; cumulus processes
Illustrationsmodels; analyses
AbstractTank crystallization experiments with aqueous solutions were used to explore interactions between a buoyant residual fluid and a porous cumulate at a sloping floor and a vertical wall. The fluid flow path in a non-reactive mush (glass beads) was used as a control. For a vertical non-reactive mush the rise of the buoyant solute-depleted fluid was confined within it. Fluid flow paths were curved in the case of a sloping mushy floor, with the upper part of the mush being principally a plume-discharge zone and the lower part being a recharge zone. More realistic reactive mushes were created by supercooling a NH4Cl solution. Cooling against a vertical surface produced an initial mat of dendrites that were subsequently overgrown by a vertically grooved dendrite structure. The grooves contained upwardly flowing residual melt, while the ridges extended beyond the buoyant boundary layer into the thermally convecting, solute-rich part of the tank. With time, the grooves were covered over by a dendrite mat, but remained as open channels within the mush, focussing and accelerating the upflow of residual melt. A buoyant boundary layer on the outside of the mush was not observed. In the experiment with a sloping floor a thin mat of homogeneous mush was succeeded by a more organized deposit characterized by chimneys. Residual melt escaped as plumes through the chimneys. Chimneys formed because buoyant residual melt dissolved the mush as it rose and warmed. Chimney spacing increased, and the number of chimneys decreased as the mush thickened. Dissection of the mush showed that individual chimney channels were inclined upslope, so that melt residual from crystallization at the foot of the slope emerged from chimneys near the top of the slope. Formation of channels in cumulates would greatly increase fractionation efficiency, by increasing the effective volume from which residual melts could be extracted and by increasing the rate at which residual melts could be transferred to the upper part of the chamber.

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