Petrogenesis of Mount St. Helens Dacitic Magmas

The most frequent and voluminous eruptive Kurasawa, 1983], (4) partial melting of the lower crust products at Mount St. Helens are dacitic in composition, [e.g., Grant et al., 1984; Reid and Cole, 1983], and (5) although a wide variety of magma types (basalt to formation of compositional gradients in response to rhyodacite) is represented. To address the petrogenesis of convective and diffusive processes in evolving magma the dacites, we present major and trace element analyses chambers [e.g., Smith, 1979; Hildreth, 1981]. of samples of pumice clasts and dome or flow lavas In the present study, we describe the petrology and erupted during the past -40,000 years. The dacites have geochemistry of representative dacitic eruptive products similar (in some cases even lower) contents of many and evaluate the above models for their origin at Mount incompatible lements (e.g., Zr, Hf, REE, U, Be, Ta, Nb) St. Helens. In particular, we address possible genetic compared with those in associated basalts and andesites, relationships among dacites, the role of basaltic and whereas Ba, Rb, K, Cs, and Sr are relatively enriched. andesitic magmas as parental liquids, the involvement of The unusual depleted nature of the dacites and generally crustal material in the origin of the silicic magmas, and low bulk distribution coefficients (estimated from temporal patterns in the compositions of the dacites. glass/whole rock pairs) for numerous trace elements preclude an origin of these magmas principally by crystal Analytical Methods fractionation of associated mafic magmas. A more plausible model for their origin involves melting of The eruptive history of Mount St. Helens has been metabasaltic crustal rocks that have been enriched in Ba, investigated through studies of detailed stratigraphic Rb, Cs, and Sr by either intercalation of sediments with correlations, geologic mapping, carbon14 and tree ring depleted basalt or selective metasomatic enrichment of the chronology, historical accounts, and remanent magnetism source region. Melting at crustal levels presumably is [Verhoogen, 1937; Lawrence, 1939, 1954; Hopson, 1971, related to intrusion of mantle derived basaltic magmas. 1,972; Hyde, 1975; Crandell et al., 1975; Mullineaux et al., Compositional diversity among the erupted dacites can be 1975; Crandell and Mullineaux, 1973, 1978; Hoblitt eta!., attributed to spatial or temporal heterogeneity of the 1980; Mullineaux and Crandell, 1981; Mullineaux, 1986]. magma sources or, in some specific cases, to such Results of previous studies are summarized in Table 1. processes as crystal fractionation, assimilation, and Eruptive activity during the last 40,000 years was magma mixing. subdivided by Mullineaux and Crandell [1981] into several eruptive periods, each lasting tens to thousands of years, Introduction separated by dormant intervals of comparable duration. Andesites were erupted during only the Castle Creek, During the last 40,000 years Mount St. Helens volcano Kalama and Goat Rocks periods, and basalts were erupted (Washington state) erupted a wide range of magma types only during the Castle Creek period. including basaltic to andesitic lavas, andesitic scoria, The detailed stratigraphy established by previous dacitic pumiceous tephra and pyroclastic flows, and workers and the long-lived, though intermittent, activity dacitic domes and flows [e.g., Hoblitt et al., 1980; at Mount St. Helens provide a unique opportunity to Mullineaux and Crandell, 1981]. Dacite is apparently the investigate the petrology of the eruptive units as a most frequently erupted magma type and represents the function of time. To this end, we sampled as many composition of all magmas erupted since May 18, 1980. significant dacitic eruptive units as possible. To avoid We have investigated the petrology and geochemistry of possible reworked material, we collected samples of large all pre-1980 magma types, but here we focus on samples pumice clasts from pyroclastic air-fall deposits and representing most of the documented dacite eruptions interior samples from domes and lava flows. There is no (Table 1). doubt that the samples studied are of primary magmatic The origin of dacitic magmas erupted in volcanic arcs origin. It was impossible to sample some key units after is problematic and may involve complex interplay of the 1980 eruptions; for these we obtained samples from W. several processes. Various petrogenetic models have been G. Melson and C. A. Hopson (Goat Rocks dome, Summit proposed to account for the formation of dacitic magmas dome, and Summit avalanche deposits) and from D. R. including (1) closed-system crystal fractionation of Mullineaux (Sugar Blast deposit). All dacite samples basaltic or andesitic magmas [e.g., Lopez-Escobar, 1984; studied in detail are listed in Table 1. Thin sections were Berman, 1981; Reid and Cole, 1983], (2) mixing of examined for all samples except those collected by Drs. rhyolitic and basaltic magmas [e.g., Eichelberger, 1975; Melson and Hopson that were provided to us as small Gerlach and Grove, 1982], (3) crustal contamination of chips. Analytical data for andesitic and basaltic eruptive mafic magma, with or without concomitant fractional products, shown for reference in diagrams in this paper, crystallization [e.g., Grant et al., 1984; Francis et al., 1980; are from Smith [1984]; these eruptive units will be Harmon et al., 1981; Deruelle et al., 1983; Matsuhisa and described in more detail elsewhere. Mineral and glass analyses were determined for Copyright 1987 by the American Geophysical Union. selected samples by electron microprobe techniques (EMP) as discussed by Smith and Leeman [1982]. Whole rock Paper number 6B5948. samples selected for chemical analysis were ground in a 0148-0227/87/006B-5948505.00 steel rather than tungsten carbide ball mill to avoid Co