Geologic Publications for Mount Rainier

Preliminary mineralogic and stable isotope studies of altered summit and flank rocks and Osceola mudflow deposits on Mount Rainier, Washington

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Author(s): Robert O. Rye, George N. Breit, David R. Zimbelman

Category: PUBLICATION
Document Type: Open-File Report 2003-464
Publisher: United States Geological Survey
Published Year: 2008
Volume:
Number:
Pages: 26
DOI Identifier:
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Abstract:
About 5600 years ago part of Mount Rainier's edifice collapsed with the resultant Osceola Mudflow traveling more than 120 km and covering an area of at least 505 km². Mineralogic and stable isotope studies were conducted on altered rocks from outcrops near the summit and east flank of the volcano and samples of clasts and matrix from the Osceola Mudflow. Results of these analyses are used to constrain processes responsible for pre-collapse alteration and provide insight into the role of alteration in edifice instability prior to the Osceola collapse event. Jarosite, pyrite, alunite, and kaolinite occur in hydrothermally altered rock exposed in summit scarps formed by edifice collapse events and in altered rock within the east-west structural zone (EWSZ) of the volcano's east flank. Deposits of the Osceola Mudflow contain clasts of variably altered and unaltered andesite within a clay-rich matrix. Minerals detected in samples from the edifice are also present in many of the clasts. The matrix includes abundant smectite, kaolinite and variably abundant jarosite. Hydrothermal fluid compositions calculated from hydrogen and oxygen isotope data of alunite, and smectite on Mount Rainier reflect mixing of magmatic and meteoric waters. The range in the δD values of modern meteoric water on the volcano (-85 to 155‰) reflect the influence of elevation on the δD of precipitation. The δ³⁴S and δ¹⁸O_SO4 values of alunite, gypsum and jarosite are distinct but together range from 1.7 to 17.6‰ and -12.3 to 15.0‰, respectively; both parameters increase from jarosite to gypsum to alunite. The variations in sulfur isotope composition are attributed to the varying contributions of disproportionation of magmatic SO₂, the supergene oxidation of hydrothermal pyrite and possible oxidation of H2S to the parent aqueous sulfate. The δ¹⁸O_SO4 values of jarosite are the lowest recorded for the mineral, consistent with a supergene origin. The mineralogy and isotope composition of alteration minerals define two and possibly three environments of alteration. At deeper levels magmatic vapor, H₂S, SO₂ and other gases from venting magmas migrated upward and condensed into the meteoric water. Disproportionation of SO₂ into aqueous sulfate and H₂S resulted in acid-sulfate (alunite + kaolinite + pyrite) and related argillic and propylitic alteration envelopes in a magmatic hydrothermal environment. At shallow levels H₂S reacted with andesite to form pyrite that is associated with smectite along fractures on both the flanks and upper edifice. It is not clear to what extent H₂S was oxidized by atmospheric O₂ to form aqueous sulfate in a steam-heated environment. Near the ground surface, pyrite is oxidized by atmospheric oxygen resulting in soluble iron-and aluminum-hydroxysulfates. These supergene hydroxysulfates, which may also form around fumaroles from the oxidation of H₂S, are subject to continuous solution and redeposition.

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In Text Citation:
Rye and others (2008) or (Rye et al., 2008)

References Citation:
Rye, R.O., G.N. Breit, and D.R. Zimbelman, 2008, Preliminary mineralogic and stable isotope studies of altered summit and flank rocks and Osceola mudflow deposits on Mount Rainier, Washington: Open-File Report 2003-464, United States Geological Survey, 26 p..