EVOLUTION OF PHENOCRYSTS AS A RESULT OF MAGMA MIXING AT MOUNT ST. HELENS VOLCANO, WASHINGTON

At Mount St. Helens, a series of eruptions led to a remarkable dacite-andesite-dacite stratigraphy in less than 165 years during the Kalama eruptive period (1480- ~1770s A.D.). This rapid and strong change in composition is attributed to mixing of basaltic magma with dacitic magma to make andesite (Pallister et al., 1992; Smith & Leeman, 1993). We are interested in how phenocrysts that originated from dacitic and basaltic magmas responded to being suddenly submerged in andesitic liquid. This has implications for studying mixed andesites in other locations where the temporal record is not as well constrained. We have investigated the phenocrystic assemblages of the mixed andesite magma(s) (X-set tephra and lava flows) and of likely dacitic and basaltic endmembers (W-set tephra & lava and Z-set lava as dacitic endmembers; lavas of Castle Creek period, 1600-2200 yrs before present, as basaltic endmembers). Phenocrysts of ‘dacitic' origin found in andesitic magma are amphibole and orthopyroxene, while olivine and clinopyroxene are clearly derived from basaltic magma. Compositional ranges in the cores of olivine (~Fo_68-85, n.d.-0.34 wt.% NiO) in andesite and Castle Creek basalts are comparable. The range of olivine compositions in andesite may suggest mixing with variably fractionated basalt magmas. Phenocrystic amphibole of andesitic magma is compositionally similar to those found in the dacitic magmas, but occurs as distinctly smaller crystals with strong textural evidence of resorption. In contrast, basaltic clinopyroxene and dacitic orthopyroxene continued to crystallize, however both phases indicate pronounced compositional shifts. Clinopyroxene phenocrysts of the mixed andesite have high Mg # (81), high Cr (up to 0.56 wt.% Cr2O3) cores that are generally overgrown by lower Mg # (75) rims. On the contrary, orthopyroxenes have low Mg # (52-60) cores overgrown by higher Mg # (60-71) rims. Our findings indicate that pyroxenes preserve the clearest petrographic evidence for magma mixing compared to amphibole which is readily resorbed, and thus is least likely to survive as a record of magma mixing events similar to the one described here.

Pallister, J.S., Hoblitt, R.P., Crandell, D.R., Mullineaux, D.R., 1992, Bull. Volcanol 54: 126-146.

Smith, D.R., Leeman, W.P., 1993, J. Volcanol. Geotherm. Res. 55: 271-303.