TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

tion driven plumes, or stirring by new magma input [83, 98]. In the case of the
OJP magma chamber system, magma in the chamber interior(s) was the most
eruptible, had Kwaimbaita or Kroenke basalt-like composition(s), and made up
the most significant compositional and textural fractions of basalts erupted to
the surface. Disruption of solidification fronts released evolved material (liquid +
crystals) to mix with the main body of magma e.g., [98]. Mixing small fractions of
evolved material from disrupted solidification fronts into the main magma body
can lead to changes in basalt chemistry (Marsh, 1996). However, if the geometry
of the OJP magma chamber system was such that inputs from disrupted solid-
ification fronts were small relative to the size of the magma body, the input of
more evolved material would have had little affect on the bulk magma chemistry.
Evidence for this process is thus best preserved by allochthonous crystals e.g.,
[37, 98], much like the An65−73 phenocrysts with evolved parent magma composi-
tions and the An80−86 xenolith crystals with primitive magma compositions, which
are all out of equilibrium with the host Kwaimbaita basalt. Parent magma trace
element compositions of An65−73 phenocryst zones, which trend to much more
evolved compositions than whole-rock data, suggest some of these crystals grew
deep within solidification fronts that were eroded and mixed into the main magma
body. Crystals with similarly evolved compositions may also form in dikes and
conduits within the magma chamber system that become readily congested. In
these regions geometry and magma supply rate allow solidification fronts to prop-
agate inward and meet filling the space with a crystal mush that has a network
of interstitial spaces filled with variably evolved melts [98]. Within the interstices
of the crystal mush significant melt differentiation can occur where the mush is
infrequently flushed with new (primitive) melt. I suggest the xenolith crystals and
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