Petrological perspectives on the magmatic evolution of Mount St. Helens (Washington, US)
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2019
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Doctoral Thesis
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Abstract
Over the last 3900 years, Mount St. Helens has been the most active volcano in the Cascade Arc. Erupted compositions vary from a heterogeneous suite of basalts to rhyodacite with the majority (> 90%) formed by relatively uniform dacite. Mount St. Helens experienced significant variations in its eruptive style and compositional output, particularly during the last 2600 years. Since its cataclysmic reawakening in 1980, it has become one of the world’s best-studied volcanoes. Yet controversies remain about magma generation processes acting at depth. This thesis addresses the geochemical and petrological heterogeneity of mafic magmas at Mount St. Helens based on a recently refined stratigraphy. It further elucidates the relative contributions of fractional crystallization of mantle-derived parental magma versus assimilation of pre-existing crustal material in the petrogenesis of the dacites, and provides an ‘amphibolitic perspective’ on the magmatic evolution of the volcano.
At the end of the Castle Creek eruptive period (1700-1900 years B.P.), Mount St. Helens erupted a variety of basalts and basaltic andesites. These include three mafic endmembers that each resemble more primitive variants of the same rock type in the Cascades: (1) high field strength element (HFSE)-rich basalts enriched in K, Ti, P and incompatible trace elements, (2) low-K olivine tholeiites (LKOT) with lower amounts of incompatible trace elements, and (3) calc-alkaline (arc-type) basaltic andesites with a typical arc signature, i.e., enrichment in fluid-mobile elements and depletion of immobile Nb and Ta. Several units form geochemical trends from the basaltic endmembers to basaltic andesitic compositions that are formed by assimilation of calc-alkaline, more evolved material (and in one case mixing of different types of mafic magma). Most of the erupted basalts are porphyritic (10-30 vol% crystals) with an assemblage dominated by olivine and plagioclase. Although typical arc basalt produced by flux melting in the mantle is absent in the eruptive products of Mount St. Helens, arc-type basaltic andesite with a characteristic subduction signature suggests its presence at depth. Primitive spinel compositions and whole-rock trace element variations indicate at least two distinct, fertile lherzolite sources for LKOT and HFSE-rich basalts. Both types are interpreted as relatively water-poor, decompression-induced melts.
In contrast to the established view that the voluminous dacites of Mount St. Helens dominantly formed by partial melting of metabasaltic lower crust, the geochemical and thermal models of this thesis indicate that the dacites are derived by fractional crystallization from a hydrous mantle-derived endmember with a maximum of 20-30% assimilation of pre-existing crustal material. Mass balance and trace element modeling reproduce compositional trends from arc-type basaltic andesite to dacite by means of about 45% fractionation of an assemblage composed of 58% amphibole, 35% plagioclase, 5.8% magnetite, 0.6% apatite, and 0.5% ilmenite. Amphibole textures and compositions support such a differentiation trend in a polybaric mush column. Combined with recent geophysical imaging and constraints from petrologic experiments, these data suggest that Mount St. Helens dacites are generated by (1) mantle-derived arc magma evolving to dacitic compositions through assimilation and fractional crystallization (AFC) in a lower crustal magma generation zone, and (2) ascent of these magmas to a mid to upper crustal reservoir, where they reach high crystallinity without significant further differentiation.
Amphiboles from dacitic tephras and magmatic inclusions from Mount St. Helens archive major changes in the magmatic system throughout the Spirit Lake stage (the current active volcanic stage that extended over the last 3900 years). The amphibole assemblages diversified with time, changing from a bimodal distribution with high-Al2O3 and low-Al2O3 compositions towards a continuous distribution centered at medium Al2O3 contents. Such medium-Al amphiboles are variably enriched in trace elements. They first appeared about 2600 years B.P., along with a compositional change in low-Al amphiboles, increasing pre-eruptive temperatures, and decreasing pre-eruptive oxygen fugacities and water contents. These correlations and their occurrence in a certain type of magmatic inclusion indicate that they result from mixing of HFSE-rich basalts with the upper crustal dacite. Up to the most recent eruptions (2004-08), the presence of such medium-Al amphiboles reflects some contribution of HFSE-rich basalt to Mount St. Helens dacites. At the same time, relics of high-Al amphibole cores record melt evolution from basaltic andesite to dacite in the mid to lower crust. They remain largely unaffected by HFSE-rich basalts, suggesting that these mainly bypass the major part of the lower crustal plumbing system. Such isolated pathways through the lower crust might be aided by the complex fault system beneath Mount St. Helens and a potential offset structure of the magmatic system suggested by seismic and magnetotelluric anomalies in the crust.
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ETH Zurich
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Magmatic petrology; Volcanology; Mount St. Helens; Dacite; Basalt; Amphibole; AFC processes
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03958 - Bachmann, Olivier / Bachmann, Olivier