Preeruption conditions and timing of dacite‐andesite magma mixing in the 2.2 ka eruption at Mount Rainier

Abstract

Analytical, field, and experimental evidence demonstrate that the Mount Rainier tephra layer C (2.2 ka) preserves a magma mixing event between an andesitic magma (whole rock SiO2 content of 57–60 wt %) and a dacitic magma (whole rock SiO2 content of 65±1 wt %). The end‐member andesite (a mix of an injected and chamber andesite) and dacite can be characterized on the basis of the homogeneity of the matrix glass and phenocryst rim compositions. Many pumices, however, contain mixtures of the end‐members. The end‐member dacite contains a microlite‐free matrix glass with 74–77 wt % SiO2, orthopyroxene rims of Mg57–64, clinopyroxene rims of Mg66–74, and plagioclase rim anorthite contents of An45–65. The temperature and oxygen fugacity, from Fe‐Ti oxide compositions, are 930±10°C and 0.5–0.75 log units above NNO. The mixed andesite contains Mg73–84 orthopyroxene rims, Mg73–78 clinopyroxene rims, An78–84 plagioclase rims, and Mg67–74 amphibole rims. The temperature from Fe‐Ti oxides, hornblendeplagioclase, and two‐pyroxene geothermometry is 1060±15°C, and the oxygen fugacity is approximately one log unit above NNO for the injected andesite. The chamber andesite is estimated to be a magma with a ∼64–65 wt % SiO2 melt at 980°C and a NNO oxygen fugacity. We conclude that the andesitic and dacitic magmas are from separate magma storage regions (at >7 km and ∼2.4 km) due to differences in the bimodal whole rock, matrix glass, and phenocryst compositions and the presence or absence of stable hornblende. The time involved from the mixing event through the eruption is limited to a period of 4–5 days based on Fe‐Ti oxide reequilibration, phenocryst growth rates, and hornblende breakdown. The eruption sequence is interpreted as having been initiated by an injection of the 1060±15°C andesitic magma into the ∼980°C (>7 km) andesite storage region. The mixed andesitic magma then intersected a shallow, ∼2.4 km, dacitic storage system on its way toward the surface. The eruption became more dacitic over time, and the final products some show evidence of partial reequilibration between the andesite and dacite.