A Complex Magma Mixing Origin for Rocks Erupted in 1915, Lassen Peak, California

The eruption of Lassen Peak in May 1915 produced four volcanic INTRODUCTION rock types within 3 days, and in the following order: (1) hybrid Magma mixing is an important process among interblack dacite lava containing (2) undercooled andesitic inclusions, mediate magmas in volcanic arcs (e.g. Turner & (3) compositionally banded pumice with dark andesite and light Campbell, 1986; Philpotts, 1990, and references therein). dacite bands, and (4) unbanded light dacite. All types represent Laboratory studies of synthetic analogs of silicate magma stages of a complex mixing process between basaltic andesite systems and mathematical modeling of mixing viscous and dacite that was interrupted by the eruption. They contain liquids have suggested a range of possible mixing mechdisequilibrium phenocryst assemblages characterized by the coanisms (e.g. Cashman & Bergantz, 1991). However, use existence of magnesian olivine and quartz and by reacted and of these results to interpret magmatic systems is limited unreacted phenocrysts derived from the dacite. The petrography and by the applicability of synthetic analogs to magmatic crystal chemistry of the phenocrysts and the variation in rock conditions and by the imprecise knowledge of the effective compositions indicate that basaltic andesite intruded dacite magma viscosity of silicate liquid–crystal mixtures at magmatic and partially hybridized with it. Phenocrysts from the dacite magma temperatures and pressures. If magmas mix incompletely, were reacted. Cooling, crystallization, and vesiculation of the hybrid i.e. mingle, features such as compositional banding or andesite magma converted it to a layer of mafic foam. The decreased undercooled inclusions are usually apparent, but if they density of the andesite magma destabilized and disrupted the foam. mix completely, mineralogical disequilibrium may be the Blobs of foam rose into and were further cooled by the overlying dacite magma, forming the andesitic inclusions. Disaggregation of only direct evidence for a mixing origin. andesitic inclusions in the host dacite produced the black dacite and If the thermal and compositional contrasts between light dacite magmas. Formation of foam was a dynamic process. two magmas are great and the ratio of silicic to mafic Removal of foam propagated the foam layer downward into the magma is large, there is little interaction between them, hybrid andesite magma. Eventually the thermal and compositional and the mafic magma is undercooled (e.g. Bacon, 1986). contrasts between the hybrid andesite and black dacite magmas were Many undercooled inclusions contain reacted phereduced. Then, they mixed directly, forming the dark andesite magma. nocrysts inherited from their host silicic magma (Heiken About 40–50% andesitic inclusions were disaggregated into the & Eichelberger, 1980), and Bacon (1986) pointed out host dacite to produce the hybrid black dacite. Thus, disaggregation that undercooled inclusions are typically formed from of inclusions into small fragments and individual crystals can be hybrid magmas. In general, the formation of undercooled an efficient magma-mixing process. Disaggregation of undercooled inclusions retards further mixing (Sakuyama, 1984; inclusions carrying reacted host-magma phenocrysts produces coThompson & Dungan, 1985; Sparks & Marshall, 1986; existing reacted and unreacted phenocryst populations. Koyaguchi & Blake, 1991). However, fragmentation and/ or disaggregation of undercooled inclusions plays an important role in hybridization in some magma systems (Thompson & Dungan, 1985; Coulon et al., 1986; Clynne & Christiansen, 1987; Clynne, 1989; Linneman & Myers,