TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

should, however, be noted that in their experimental studies Sano and Yamashita
[123] observed crystallization of An82 plagioclase from Kroenke basalts (parental
to Kwaimbaita basalts) at higher temperatures (∼1200 ◦ C) and lower pressure (0.1
MPa). Although the Mg# of a residual magma decreases with increased olivine
fractionation, I see no evidence of olivine-dominated fractionation in xenolith or
phenocryst parent magmas (i.e., no systematic difference of parent magma Mg#
between the xenolith crystals and phenocrysts). The ranges of Mg# for parent
magmas of both xenolith crystals and phenocrysts overlap the range observed for
Kwaimbaita basalts (Fig. 2.9a; Table 2.3). Parent magmas of An65−79 phenocryst
zones from the three Site 1183 basalt units are compositionally similar with regard
to Mg number and Ti abundance (e.g., Fig. 2.9a). This first order observation
suggests they share a common parent magma, although some lower An phenocryst
zones (i.e., 63R2P4, 59R2P1, and 59R2P2) appear to have more evolved parent
magmas (Fig. 2.9). However, when individual units from Site 1183 are considered,
the Mg# of the parent magmas changes little with quite a large range in An
content (Fig. 2.10b-d), suggesting that the Mg# is buffered.
Parent magmas of the An-rich xenolith crystals and phenocrysts extend to
more primitive compositions than the Kwaimbaita whole-rock composition (Ta-
ble 2.2; Fig. 2.9a). This is contrary to the interpretation of Sano and Yamashita
[123] that An-rich zones grew in a relatively evolved and H2O-rich boundary layer.
It is thus necessary to elucidate other factors, in addition to growth in a water-rich
boundary layer, which favor crystallization of An-rich plagioclase.
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