The isotopic and chemical evolution of Mount St. Helens

Details The isotopic and chemical evolution of Mount St. Helens The isotopic and chemical evolution of Mount St. Helens

May 1983

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1983e%26psl..63..241h&link_type=abstract

Earth and Planetary Science Letters (ISSN 0012-821X), vol. 63, no. 2, May 1983, p. 241-256.

Computer Science

39

Chemical Evolution, Isotopes, Trace Elements, Volcanoes, Volcanology, Lead Isotopes, Mountains, Neodymium Isotopes, Rare Earth Elements, Strontium Isotopes, Tables (Data)

Scientific paper

Strong constraints are placed on the source or sources of the magmas of Mount St. Helens. The lack of an Eu anomaly and the high Sr content rule out plagioclase as a significant source phase. This is seen as suggesting that the source lies in the lowermost crust or mantle. The Sr, Nd, and O isotopes data suggest only very small amounts of contamination with old crustal material or metasediment. Altered oceanic crust or slab-derived fluids did not contribute materially to the magmas. The sequence of eruption and the intimate relationship between the chemical and isotopic compositions indicate that dacitic magma in a deep reservoir was invaded by and mixed with basaltic magma, producing the low-SiO2 dacite and andesite observed in the more recent (within the last 1000 years) eruptions. The uniformity of the Rb/Ba ratio before and after the 2500-1000 year period is seen as indicating that there was one main reservoir for the magmas and that the incompatible trace element concentrations were being buffered by large volumes of material. It is also noted that the light rare earth element (REE) enrichment and the low heavy REE concentrations are suggestive of garnet or zircon as fractionating or residual source phases.

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