MICROANALYTICAL EVIDENCE FOR MAGMA MINGLING AT MOUNT ST. HELENS

Mixing between basaltic and dacitic magmas has been invoked as an important process in the magmatic system of Mount St. Helens (MSH) (e.g. Gardner et al., 1995, Geology 23:523-526 and references therein). Bulk-rock major and trace element and isotopic studies of MSH andesites suggest they were generated by magma mixing; however, many MSH andesites do not exhibit mineralogical or textural features that clearly demonstrate mingling/mixing of different magma types. MSH tephra-fall layer Xb is an andesitic deposit erupted in the 16^th century, which has been noted to contain dacite as clots and bands in the scoria (Mullineaux, 1996, USGS Prof. Paper 1563 and references therein). We used electron microprobe (EMP) and scanning electron microscope methods to analyze abundances of major elements in Xb glass. In each individual lapillus analyzed so far, SiO₂ continuously varies from ~60 to 71 wt %, with concomitant variations in FeO, MgO, Al₂O₃, TiO₂, CaO, and K₂O. In one lapillus, glass with ~76 to 78 wt % SiO₂ has also been identified. In that sample, an analytical traverse reveals a break in silica (between ~67-71 and 76-78 wt %) that occurs at a boundary < 50 microns wide separating glasses with different colors and vesicle morphologies. In other lapilli, bifurcating trends in FeO, MgO and TiO₂ vs. SiO₂ suggest the involvement of three endmembers in magma mingling processes. Coexisting magnetite and ilmenite occurring as microphenocrysts (~10 to 50 microns) in Xb glass were also analyzed with the EMP in order to estimate temperature and oxygen fugacity via geothermobarometry. Within individual lapilli, temperatures and oxygen fugacities span a range of ~200°C and ~3 log units, respectively. In one lapillus, Fe-Ti oxides coexisting with the highest SiO₂ glass record the lowest temperatures, whereas Fe-Ti oxides in lower SiO₂ glass record systematically higher temperatures. However, other lapilli show no clear relationship between temperature and glass chemistry. The glass chemistry and T-ƒO₂ estimates for Xb samples suggest that micro-scale magma mingling processes were “caught in the act” via explosive eruption and rapid quenching, before blending or homogenization of hybrid magma was complete.