TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

studies of Kwaimbaita basalt by Sano and Yamashita [123] at the QFM buffer,
which is the approximate f O2 condition of OJP basalt crystallization suggested
by Roberge et al. [121]. This estimate of DFeOT is supported by the results of
Phinney [117] who reported little change in DFeOT at f O2 below the QFM oxy-
gen buffer, while DFeOT increased steadily at higher f O2, which was attributed to
greater amounts of Fe 3+ in the melt. If generally constant partitioning of Mg be-
tween plagioclase and melt is assumed (i.e., using the method of Bindeman et al.
[7]), we calculate an average DMg value of 0.024 ± 0.005 for the OJP plagioclase
crystals considered in this study. The minor variation of DMg, using the Bindeman
et al. [7] method, over the large compositional range of plagioclase examined in
this study (An65−86) is in accordance with Longhi [89] and Bindeman et al. [7]
who suggested An content has minimal effect on Mg partitioning. However, the
average DMg values calculated using the equations and constants of Bindeman et
al. [7] are inconsistent with the experimental results of Sano and Yamashita [123],
whose results for OJP basalts reveal slightly larger values for Kwaimbaita (0.1
MPa DMg = 0.043; 190 MPa DMg = 0.055) and Kroenke basalts (0.1 MPa DMg
= 0.04; 190 MPa DMg = 0.06) at elevated pressures. This suggests crystallization
pressure may have an effect on DMg [123]. I use DMg ≈ 0.047 to invert parent
magma compositions of OJP plagioclase crystals, which is an average of eight DMg
values from the experimental results of Sano and Yamashita [123] conducted at
0.1 and 190 MPa. Phinney [117] reported an average basaltic plagioclase DMg
value of 0.044, which is consistent with my selected DMg value.
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