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Pyroxene accumulation in spinifex-textured rocks

Published online by Cambridge University Press:  01 May 2009

I. H. Campbell  and
N. T. Arndt
I. H. Campbell
Affiliation:
J. Tuzo Wilson Research Laboratories, Earth and Planetary Sciences, Room 3032, Erindale Campus, University of TorontoMississauga, Ontario L5L 1C6, Canada
N. T. Arndt
Affiliation:
Max-Planck-Institut für Chemie, Saarstrasse 23Postfache 3060, D-6500 Mainz, West Germany
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Summary

Natural pyroxene spinifex-textured rocks, free of olivine, may have MgO contents as high as 15%. If these rocks are allowed to crystallize in the laboratory under equilibrium conditions, olivine is the liquidus phase at 1350°C and pyroxene does not appear until the temperature has fallen to 1180°C. A similar problem is encountered in high MgO komatiites. If spinifex-textured and aphyric komatiites from Munro Township are plotted on an MgO-CaO-Al2O3 diagram, olivine control is evident until the MgO content of the melt falls to 15%. At this point the liquid trend moves away from the CaO-MgO boundary, indicating the crystallization of pyroxene. If a high MgO komatiite is crystallized in the laboratory under equilibrium conditions, pyroxene does not appear until the MgO content of the liquid reaches 9%. It is suggested that experimental and natural systems behave differently because pyroxene has crystallized metastably in the natural system under conditions of extreme supercooling. The early metastable crystallization of pyroxene can only affect the liquid trend if the spinifex zone contains a cumulate component.

Type
Articles
Information
Geological Magazine , Volume 119 , Issue 6 , November 1982 , pp. 605 - 610
Copyright
Copyright © Cambridge University Press 1982

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