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.