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Journal of Petrology Advance Access published online on November 27, 2008
Journal of Petrology, doi:10.1093/petrology/egn059
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© The Author 2008. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

The Internal Magma Reservoir of Large Intrusions Revealed by Multiphase Rayleigh Fractionation

S. A. Morse*

Department Of Geosciences, University Of Massachusetts, 611 North Pleasant Street, Amherst, Ma 01003-9297, USA

Received June 14, 2008; Revised typescript accepted October 16, 2008


  Abstract

The fractionation progress of a crystallizing basaltic magma is recorded in the changing mineral compositions of binary solutions that form a part of plagioclase, olivine, and augite. When such compositional data are plotted against an independent measure of progress such as volume, they can often be described by a Rayleigh equation that reveals details of the crystallization history. A binary solution crystallizing by itself follows a path determined by the Rayleigh exponent (D – 1) where D is the partition coefficient X1S/X1L (X is mole fraction, 1 is low-melting component, L is liquid, S is solid). In company with another phase, however, such a binary solution may follow a wide range of paths in an envelope about (D – 1), ranging from (a) f{alpha}(D – 1) to (b) (D – 1)/f{alpha}, where f{alpha} is the fraction of the active phase—the binary solution under consideration. The upper limiting curve, an extended fractionation path, is given by (a) above, whereas a full X2 depletion path (b) defines the lower limit. This range of alternatives is here codified and supplemented by considerations of residual porosity. The result is a Rayleigh equation in five variables that can illuminate magmatic history, including the probability of the existence of an internal reservoir that damps the depletion effect. The range of paths can help to identify evidence for, and causes of, leading-edge fractionation of ascending magma, small packet crystallization, pressure variations, off-cotectic excursions, and oscillating modal variation.

KEY WORDS: Magma reservoir; Rayleigh paths; binary solutions; Kiglapait; Skaergaard

*Corresponding author. Fax: 413-545-1200. E-mail: tm{at}geo.umass.edu


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