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Origin of finger structures in the Rhum Complex: phase equilibrium and heat effects

Published online by Cambridge University Press:  01 May 2009

S. A. Morse ,
Brent E. Owens  and
Alan R. Butcher
S. A. Morse
Affiliation:
Department of Geology and Geography, University of Massachusetts, Amherst, MA 01003, USA
Brent E. Owens
Affiliation:
Department of Earth and Planetary Sciences, Washington University, St Louis, MO 63130, USA
Alan R. Butcher
Affiliation:
Geochronology Division, National Physical Research Laboratory, CSIR, P.O. Box 395, Pretoria 0001, South Africa
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Abstract

The finger structures described earlier by Brown and later by Butcher, Young & Faithfull involve dissolution of troctolite during crystallization of olivine, followed by crystallization of pyroxene around olivine grains in the fingers. However, the ingestion of troctolite takes the liquid away from pyroxene saturation rather than toward it. The pyroxene field can be encountered metastably, and pyroxene caused to crystallize, by supercooling the olivine-rich liquid against the troctolite. The melt corrosion represented by the fingers, and other field relations, suggest that the mafic layers were emplaced as sills of mafic magma into nearly solid troctolites. Melting at the base of mafic liquid layers was impeded by a bed of olivine crystals releasing light solute upward, causing compositional convention and rapid heat transfer to the top of the layer, where melting demonstrably occurred. Recognition of this process introduces the novel concept of a magmatic heat pump driven by compositional convection. The crystallization path ol–px–pl(–sp) is also found next to xenoliths in the Kiglapait Intrusion where the magma was normally saturated only in ol+pl, directly demonstrating the effect of supercooling on the crystallization sequence.

Type
Articles
Information
Geological Magazine , Volume 124 , Issue 3 , May 1987 , pp. 205 - 210
Copyright
Copyright © Cambridge University Press 1987

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