Revisiting the Compositions and Volatile Contents of Olivine-Hosted Melt Inclusions From the Mount Shasta Region

Recent controversy over the origin of high-Mg andesites from the Mount Shasta region has prompted us to re-examine olivine-hosted melt inclusions from this area. We analyzed olivine-hosted (Fo88-94) melt inclusions from tephra of the S17 cinder cone (N flank Whaleback), north of Mount Shasta, using FTIR and electron microprobe. The melt inclusion compositions (uncorrected for post-entrapment crystallization) range from 54-63 wt% SiO2 and 4.0-6.7 wt% MgO, and they can be divided into high (>12 wt%) and low (<10 wt%) CaO groups with distinct P2O5/K2O ratios. High-CaO inclusions have Fo91-94 host olivines, whereas low-CaO inclusions have Fo87-88 host olivines. S, Cl, and F contents are 700-2300 ppm, 1300-2700 ppm and 0-600 ppm, respectively. H2O contents (uncorrected) vary from 0.5-3.8 wt% and agree with the range of H2O contents determined by Anderson (1974 and unpublished data) using the electron probe H2O-by- difference technique (0-6.4 wt%). CO2 contents range from below detection (~25 ppm) up to 800 ppm. S and Cl contents are generally higher for high-CaO inclusions than for low-CaO inclusions, whereas F, H2O, and CO2 contents are similar for both groups. All volatiles show trends consistent with degassing during crystallization and must be taken as minimum amounts dissolved in the melt. Comparison of melt inclusions with whole rock compositions (Baker et al., 1994) suggest significant Fe-loss from the glasses (~2-4 wt%). Total extents of post-entrapment crystallization were estimated by the addition of olivine to the melt composition (with 8 wt% FeOT) until equilibrium between the melt and host olivine was achieved (13-48 wt% crystallization). Corrected SiO2 and MgO contents of the high-CaO inclusions (Fo91-95 olivine) are 49-52 wt% and 17-19 wt%, respectively. The minimum initial H2O content for inclusions in high-Fo olivines is 2.5-4.7 wt% based on our new data and Anderson's (1974 and unpublished) data. Our data provide strong evidence for the existence of volatile-rich, high-Mg basaltic andesite melts beneath the Mount Shasta region.