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Mechanical Interactions of Phenocrysts and Flow Differentiation of Igneous Dikes and Sills

1972; Geological Society of America; Volume: 83; Issue: 4 Linguagem: Inglês

10.1130/0016-7606(1972)83[973

1943-2674

Paul D. Komar,

Landslides and related hazards

Research Article| April 01, 1972 Mechanical Interactions of Phenocrysts and Flow Differentiation of Igneous Dikes and Sills PAUL D KOMAR PAUL D KOMAR Department of Oceanography, Oregon State University, Corvallis, Oregon 97331 Search for other works by this author on: GSW Google Scholar Author and Article Information PAUL D KOMAR Department of Oceanography, Oregon State University, Corvallis, Oregon 97331 Publisher: Geological Society of America Received: 01 Jun 1971 Revision Received: 22 Nov 1971 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1972, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1972) 83 (4): 973–988. https://doi.org/10.1130/0016-7606(1972)83[973:MIOPAF]2.0.CO;2 Article history Received: 01 Jun 1971 Revision Received: 22 Nov 1971 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation PAUL D KOMAR; Mechanical Interactions of Phenocrysts and Flow Differentiation of Igneous Dikes and Sills. GSA Bulletin 1972;; 83 (4): 973–988. doi: https://doi.org/10.1130/0016-7606(1972)83[973:MIOPAF]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract When a magma containing phenocrysts in concentrations greater than about 8 percent intrudes rock to form a dike or sill, mechanical interactions between the phenocrysts create a grain dispersive pressure. To maintain a balance in the dispersive pressure, the phenocryst concentration must decrease toward the walls of the intrusion because of the increase in the velocity gradient and the fluid viscosity. This mechanism offers an explanation for the observed rapid, but gradational, increases in content from phenocryst-poor margins to a phenocryst-rich center, especially in picritic dikes and sills. Using semi-empirical equations for the dispersive pressure, I found that a plug flow velocity profile yields a phenocryst distribution very similar to those observed in the field.If the phenocryst interactions are sufficiently intense, then the grain dispersive pressure would be also dependent upon the size of the phenocrysts. This dependence would cause the larger phenocrysts to migrate preferentially to the center of the intrusion and the smaller phenocrysts to the walls, giving a size variation as well as a concentration variation. It is uncertain whether the interactions are sufficiently intense to give size sorting by this mechanism.At the phenocryst concentrations normally encountered, a review of experimental studies indicates that the Magnus-effect and similar single particle forces are small in magnitude in comparison to the forces of grain interaction. They would have only a modifying effect on the phenocryst distribution produced by the grain dispersive pressure.Modifications of the phenocryst distribution produced by gravity during and after intrusion are examined. During intrusion, gravity settling of the phenocrysts would be balanced by a gradient of the grain dispersive pressure, causing the phenocryst distribution to be asymmetrical with respect to the center of the intrusion. Phenocryst interactions during gravity settling following intrusion, in which the faster settling larger grains capture smaller grains within their boundary layers, may be responsible for sharp discontinuities observed in the distributions of phenocrysts. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.