OLIVINE CHEMISTRY OF INDIAN HEAVEN LAVAS, SOUTHERN WASHINGTON CASCADES

Indian Heaven is the most voluminous field of Quaternary basalt in the Cascades north of central Oregon. More than 50 vents have issued dominantly basalt; andesites are minor and dacites are absent. Basalts include low-K tholeiites (LKT) to intraplate basalts (IPB) and shoshonites (SHO), with MgO contents ranging from 5.8 to 8.7 wt %. We analyzed core compositions of olivine grains (~200~600 mm) occurring in 19 basalts and one andesite to assess whether the olivines were in equilibrium with their host liquids. The total range for olivine cores in the basalts is 66 to 89 mole % Fo. Most samples contain olivines that appear to be in equilibrium with their host liquid. Two samples contain olivines that are clearly out of equilibrium with the presumed host liquid, with cores of Fo = 66-70 and 71-75 vs. calculated Fo equilibrium values of ~88 and ~84, respectively. The andesite exhibits the largest range in Fo contents (59-82 mol %) and textural evidence for disequilibrium, including orthopyroxene rims and/or resorption textures. These characteristics are consistent with input from more silicic magmas and/or older crystal “mushes.”

Temperatures and pressures were calculated by the methods described by Leeman et al. (2005) for samples with >7.5 wt % MgO; the two enigmatic samples noted above were excluded from these calculations. Pre-eruptive conditions range from 1210 to 1248°C and from 0.69 to 1.02 GPa, with no apparent relationship to basalt type (e.g., LKTs vs. IPBs). In contrast, segregation T-P estimates range from 1258 to 1473°C and from 0.96 to 2.41 GPa (corresponding to depths of ~32-80 km). There is a clear relationship between basalt type and segregation T-depth estimates. The deepest, hottest conditions (1369-1473°C and 55-80 km) are observed for the LKTs, with progressively lower T-depth estimates for IPBs (1303-1380°C and 37-60 km) and the SHOs (1258-1263°C and 32-34 km). These high segregation temperatures imply melting was triggered by decompression, consistent with the lack of slab-fluid input (Leeman et al. 2005).

Leeman, W.P., Lewis, J.F, Evarts, R.C., Conrey, R.M., and Streck, M.J., 2005, J. Volcanol. Geotherm. Res.: 140:67-105