TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

defined as having crystallinities between 25% and 50%-55% [98] (Fig. 1.3). Pro-
cesses occurring within mush-layers and their interstitial spaces are thus best dis-
cussed in terms of solidification front processes. Crystallization within the mush
zone produces evolved interstitial melts [84, 98]. Continued crystallization traps
these evolved liquids within the solidification front. Dependent upon reservoir
geometry and magma supply rate, solidification fronts may propagate inward to
a point where the chamber is filled largely with a crystal mush that has a net-
work of interstitial spaces filled with variably evolved melts [98]. Flushing or filter
pressing of the crystal mush frees these evolved melts that can: 1) mix with more
primitive magmas; 2) be erupted; or 3) be intruded. A filter pressing mecha-
nism has been suggested for off-summit (axial) Hawaiian eruptions, where axial
tholeiitic Kilauea magmas are olivine-clinopyroxene-plagioclase-saturated versus
olivine saturated in summit magmas (e.g., [71, 98]). Interstitial liquids in the
crystal mush may be flushed out by less evolved magma rising from lower levels
of the magmatic system.
The crystal-free interior of the magma chamber is not affected by the solidi-
fication front differentiation processes, though evolved liquids may attain enough
buoyancy to rise and mix with undifferentiated magmas in the magma chamber
interior [84, 85, 101]. Scouring of magma chamber margins by vigorous input
of new magma may free evolved packets of magma and crystals by solidification
front erosion (e.g., [98]). Phases that are below the liquidus of the main body
of magma can grow in interstitial spaces of solidification fronts and mush piles
that are thermally and mechanically insulated from the hotter magma chamber
interior [84]. Crystal debris, including sub-liquidus phases, may be fully or par-
tially resorbed by the hotter less evolved magma in the chamber interior, which
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