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

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 com-positional 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 X1 S/X1 L (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(D ^ 1) to (b) (D ^ 1)/f, where f is the fraction of the active phaseçthe binary solution under consideration.The upper limiting curve, an extended fractiona-tion 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 his-tory, 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