TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

kbar), high temperature (>1200 ◦ C) crystallization environment favors An-rich
plagioclase growth from known compositions of magmas parental to Kwaimbaita
basalt.
2.6.1.3 An-rich OJP Plagioclase: Influence of Water
Sano and Yamashita [123] invoked a role for water in the genesis of low-MgO,
An-rich OJP plagioclase crystals, which they envisaged to form in a water-rich
(and presumably incompatible trace element-rich) crystal mush layer. Their in-
terpretation is consistent with the hydrous experimental data of Sisson and Grove
[126]. However, while the plagioclase observed by Sisson and Grove [126] grown
in H2O saturated parent magmas were low-MgO and An-rich, the crystals they
observed were more An-rich (generally An90) and lower MgO (< 0.2 wt.% MgO)
than OJP plagioclase crystals (< An90, and > 0.2 wt.% MgO) reported in this
study or by Sano and Yamashita [123] (Table 2.1). Plagioclase data from the
anhydrous experiments of Grove et al. [61] and Bartels et al. [3] produced a neg-
ative correlation of MgO with An content, which was also documented for OJP
plagioclase xenolith crystals and phenocrysts ( [123]; this study; Fig. 2.8). The
experiments of Bartels et al. (1991) were run at 10-20 kbar, and the experiments
of Grove et al. (1982) run at 10-3 kbar. The lower pressure experiments of Grove
et al. [61] produced crystals with MgO and An contents similar to natural OJP
plagioclase observed in this study and by Sano and Yamashita [123]. While I can-
not rule out the influence of H2O, MELTS modeling and previous experimental
studies provide evidence that crystallization of low MgO, An-rich OJP plagioclase
from low pressure, high temperature magmas can account for my observations
without a significant role for H2O. It is, however, impossible to determine un-
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