Bimodal density distribution of cryptodome dacite from the 1980 eruption of Mount St. Helens, Washington

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Authors:Hoblitt, Richard P.; Harmon, Russell S.
Author Affiliations:Primary:
U. S. Geological Survey, Vancouver, MA, United States
NERC Isotope Geosciences Laboratory, United Kingdom
Volume Title:Bulletin of Volcanology
Source:for the [Bulletin of Volcanology, 55(6), p.421-437. Publisher: Springer International] International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI), Heidelberg, International. ISSN: 0258-8900
Publication Date:1993
Note:In English. 56 refs.; illus. incl. 7 tables, sketch map
Summary:The explosion of a cryptodome at this volcano produced two juvenile rock types that are derived from the same source magma. Their differences (colour, texture and density) are due only to vesicularity differences. The vesicular grey dacite comprises approx 72% of the juvenile materials; the black dacite makes up the rest. The density of juvenile dacite is bimodally distributed, with peaks at 1.6 g/cm3 (grey) and 2.3 g/cm3 (black). Water contents, D abundances, and the relationship of petrographic structures to vapour-phase crystals indicate both rock types underwent pre-explosion subsurface vesiculation and degassing. The grey dacite underwent a second vesiculation event, probably during the 18 May explosion. In the subsurface, gases probably escaped into the permeable volcanic edifice. Non-uniform degassing of an initially homogeneous magma produced volatile gradients in the cryptodome and these were responsible for the density bimodality. In effect, water contents < 0.2-0.4 wt.% produced vesicle growth rates that were slow in comparison to the pyroclast cooling rates; greater water contents produced the opposite effects. In this scheme, the dacite densities are bimodally distributed simply because, following decompression on 18 May, one clast population vesiculated while the other did not. Clast vesicle growth continued until it was arrested by fragmentation. [R.E.S.]
Subjects:Dacites; Degassing; Density; Geochemistry; Igneous rocks; Isotope ratios; Isotopes; Magmas; O-18/O-16; Oxygen; Stable isotopes; Volcanic rocks; Volcanoes; Cascade Range; Mount Saint Helens; Skamania County Washington; United States; Washington
Abstract Numbers:94M/3752
Record ID:1994024300
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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