Petrology of Medicine Lake Highland volcanics: Characterization of endmembers of magma mixing |
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Authors: | David C Gerlach Timothy L Grove |
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Institution: | (1) Dept. of Earth and Planetary Sci., Massachusetts Institute of Technology, 02139 Cambridge, MA, USA |
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Abstract: | Lavas from Medicine Lake volcano, Northern California have been examined for evidence of magma mixing. Mixing of magmas has
produced basaltic andesite, andesite, dacite and rhyolite lavas at the volcano. We are able to identify the compositional
characteristics of the components that were mixed and to estimate the time lag between the mixing event and eruption of the
mixed magma. Compositional data from pairs of phenocrysts identify a high alumina basalt (HAB) and a silicic rhyolite as endmembers
of mixing. Mg-rich olivine or augite and Ca-rich plagioclase are associated with the HAB component, and Fe-rich orthopyroxene
and Na-rich plagioclase are associated with the rhyolitic component. Some lavas contain multiple phenocryst assemblages suggesting
the incorporation of several magmas intermediate between the HAB and silicic components. Glass inclusions trapped in Mg-rich
olivine and Na-rich plagioclase are similar in composition to the proposed HAB and rhyolite end members and provide supportive
evidence for mixing. Textural criteria are also consistent with magma mixing. Thermal curvature of the liquidus surfaces in
the basalt-andesite-rhyolite system allows magmas produced by mixing to be either supercooled or superheated. Intergranular
textures of basaltic andesites and andesites result from cooling initiated below the liquidus. The trachytic textures of silicic
andesites form from cooling initiated above the liquidus. Reversed compositional zoning profiles in olivine crystals were
produced by the mixing event, and the homogenization of the compositional zoning has been used to estimate the time interval
between magma mixing and eruption. Time estimates are on the order of 80 to 90 h, suggesting that the mixing event triggered
eruption. |
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