TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

water altered basalt or materials with a seawater brine component as the source
of widespread Cl contamination of OJP basaltic glasses. Additionally, Neal and
Davidson [108] invoked seawater altered basalt as the contaminant to the parental
magma that produced megacrysts entrained in post-OJP alnite pipes on Malaita,
Solomon Islands. However, I note that the assimilation of such a seawater-altered
component did not radically elevate the water content of the magma within the
different chambers. Magma chambers are more sheet-like near the Earths surface
[97]. The maximum vertical extent of interconnected sheet-like chambers would
have been where the supply of intruding magma was the greatest (i.e., toward the
center of the plateau; Fig. 2.11). The total integrated thickness of the dike and
sill system was thus less along the margins of the OJP, which allowed magmas
distinct from Kwaimbaita-type to ascend through the mush column and erupt
(see Fig. 2.11). Indeed, the greatest compositional diversity is observed along the
plateau margins e.g., [46, 91, 93, 94]. Along the thinner margins of the plateau
the magma chamber system and overlying rock would have acted as less of a
density filter than the thicker central portion of the plateau (Fig. 2.11). I cal-
culated average OJP magma densities using the KWare Magma program written
by Dr. Ken Wohletz using the glass compositions reported by Michael [103] and
Roberge et al. [121] at a temperature of 1180 ◦ C and a pressure of 1 kbar (KWare
Magma program is available at http://geont1.lanl.gov/Wohletz/Magma.htm). It
would have been possible for more (e.g., Kroenke-type: 2869 ± 3 kg/m 3 ) and
less dense (Singgalo-type: 2860 kg/m 3 and Kwaimbaita-type: 2861± 3 kg/m 3 )
magmas to ascend the mush pile and reach the surface along the margins of the
plateau where there was less overlying volume of intruded and extruded magma.
Indeed, Kroenke basalt flows are found to be stratigraphically above Kwaimbaita
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