Plagioclase compositions
An example elemental plagioclase map is shown in Fig. 6a. The molar
anorthite number (An#; mol CaO/[CaO + NaO0.5 +
K0.5]) is calculated for each plagioclase pixel of the
scan and shown in the distributions of Fig. 6b. The statistical mode for
distributions in six of the seven samples are relatively consistent
(centered at around An # 0.15 – 0.25). This is especially surprising
considering the range in bulk wt % SiO2 (62 to 74 wt
%) of these six samples (Fig. 6c). One sample, SMLG, however, has a
distinctly lower modal An # (about 0.05) and a considerably higher wt%
SiO2 value (78 wt%). Finally, for comparison, the
expected equilibrium range of molar anorthite numbers in plagioclase as
a function of bulk rock molar anorthite number is shown in Fig. 6d. The
range in equilibrium plagioclase compositions derives from the
equilibrim constant, Keq, from the exchange reaction
between plagioclase and melt (Drake, 1976, Dungan et al. , 1978,
Putirka, 2008, Rhodes et al. , 1979). Here, Keq =
0.10 ± 0.05 (Putirka, 2008), and depends on the molar ratio of
AlO1.5/ SiO2. The upper and lower bounds
for the equilibrium range plotted in Fig. 6c are obtained by taking the
lowest and highest AlO1.5/ SiO2 ratios
amongst the yellow samples in Fig. 6c. The grey circles in Fig. 6c are
calculated using the bulk molar anorthite number and
AlO1.5/ SiO2 ratio for each sample to
predict the equilibrium molar anorthite number in plagioclase. Fig. 6d
highlights that the analyzed samples are shifted off the equilibrium
line and have anorthite contents lower than would expect given that of
the bulk rock. SMLG, however, approaches equilibrium.