Constitutional Zone Refining of Magmatic Intrusions
If crystal settling is not the major cause of [layering andcrystal fractionation in magmatic intrusions, another mechanism is needed to explain the separation of interstitial liquid and the formation of monamineralic rocks that are characteristic of large, slowly cooled bodies. At least some of these features may be explained by recrystallization resulting from a process of re-equilibration of the intitial assemblage of crystals with a late-stage liquid moving through the zone of crystal mush.
A strongly excluded component, such as water in a magma crystallizing only anhydrous minerals, becomes enriched at an exponential rate in the diminishing vol urne of liauid in a thick zone of in-situ crystallization. The resulting steep chemical potential gradient and the relatively high diffusivity, which increases with water content, may cause water to move up the temperature gradient. Other factors, such as compaction of crystals and a density difference between a highly evolved water-rich liauid and a more iron-rich interstitial liquid above, could also contribute to the inward transfer of a late water-rich liquid.
When such a liquid enters hotter horizons, it will tend to act as a flux and cause selective remelting of the early-crystallized phases. Advancing through the crystallizing magma, this zone of remelting will have an effect similar to that of metallurgical zone refining, but in this case remelting results from nearly isothermal fluxing rather than reheating. Because the amount of melt tends to increase with the cancentraton of water and other excluded components, the liquid should not reacn saturation as quickly as it would if the amount of liquid were constant. As a result, the efficiency of segregation should be more effective than in conventional zone refining.
If the zone of remelting breaks through the inner boundary of the zone of crystallization, either because the amount of liquid increases at the expense of early-formed crystals or because it becomes lighter and gravitationally unstable, the remelted zone may mix with the main mass of interior magma contriouting excluded components scavenged from the crystal mush. The same event might also lead to modal layering when the svstem passes through repeated cycles.
KeywordsLayered Intrusion Lava Lake Magmatic Intrusion Bushveld Complex Crystal Mush
Unable to display preview. Download preview PDF.
- 1.Hess, G. B., 1972, Heat and mass transport during crystall ization of the Stillwater Complex. Geol. Soc. Amer. Mem. 132, 503–520.Google Scholar
- 3.Irvine, T. N., 1974, Petrology of the Duke Island ultramafic complex, southeastern Alaska. Geol. Soc. Amer. Mem. 138, 240p. Google Scholar
- 5.Butcher, A. R., Young, I. M., and Faithfull, J. W., 1985, Finger structures in the Rhum complex. Geol. Mac. 122, 491–502.Google Scholar
- 6.Shaw, H. R., 1974, Diffusion of H2O in granitic liquids: Part I Experimental data; Part II Mass transfer in magma chambers, in Geochemical Transport and Kinetics. A. W. Hoffmann et al., editors, Carnecie Inst. Washington Pub. 634, 131–170.Google Scholar
- 8.McBirney, A. R., in press, Constitutional zone refining in layered intrusions, in Origins of Igneous Layering. I. Parsons, editor.Google Scholar