Isotopic insights into the degassing and secondary hydration of volcanic glass from the 1980 eruptions of Mount St. Helens

Abstract The Butcher Ridge igneous complex, Antarctica, is an ∼6000 km3 hypabyssal silicic intrusion containing rhythmically layered glassy rocks. Baddeleyite U-Pb geochronologic analysis on a sample of the Butcher Ridge igneous complex yielded an age of ca. 182.4 Ma, which confirms that it was emplaced synchronously with the Ferrar large igneous province. Rocks of the Butcher Ridge igneous complex vary from basaltic andesite to rhyolite, and so the inferred volume of the Butcher Ridge igneous complex makes it the most voluminous silicic component of the Ferrar large igneous province. Major-element, trace-element, and isotopic data combined with binary mixing, assimilation-fractional crystallization (AFC), and energy-constrained AFC models are consistent with formation of Butcher Ridge igneous complex silicic rocks by contamination of mafic Ferrar parental magma(s) with local Paleozoic plutonic basement rocks. Field and petrographic observations and evidence for alkali ion exchange suggest that the kilometer-long, meter-thick enigmatic rhythmic layering formed as a result of secondary hydration and devitrification of volcanic glass along parallel fracture networks. The regularity and scale of fracturing/layering imply a thermally driven process that occurred during shallow emplacement and supercooling of the intrusion in the upper crust. We suggest that layering observed in the Butcher Ridge igneous complex is analogous to that reported from terrestrial and Martian cryptodomes, and therefore it is an ideal locality at which to study layering processes in igneous bodies.

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Correlation of Distal Tephra Layers in Loess in the Channeled Scabland and Palouse of Washington state

Quaternary Research ◽

10.1016/0033-5894(92)90067-s ◽

1992 ◽

Vol 37 (3) ◽

pp. 281-303 ◽

Cited By ~ 45

Author(s):

Alan J. Busacca ◽

Kevin T. Nelstead ◽

Eric V. McDonald ◽

Michael D. Purser

Keyword(s):

Late Quaternary ◽

Volcanic Glass ◽

Similarity Coefficients ◽

Regional Study ◽

Radiocarbon Age ◽

Stratigraphic Position ◽

Normal Polarity ◽

Tephra Layers ◽

Future Work ◽

Mount St Helens

AbstractThirty-two distal tephra layers that are interbedded in Quaternary loess at 13 sites in the Channeled Scabland and Palouse were sampled as part of a regional study of the stratigraphy and chronology of dominantly windblown sediments on the Columbia Plateau. An electron microprobe was used to determine the elemental composition of volcanic glass in all of the samples and also to determine the composition of ilmenite in 14 of them. Two of the distal tephra layers correlate with Glacier Peak eruptions (11,200 yr B.P.), five with Mount St. Helens tephra set S (13,000 yr B.P.), and nine with Mount St. Helens tephra set C (ca. 36,000 yr B.P.) based on analysis of glass and ilmenite in reference pumices from Glacier Peak, Mount St. Helens, Mount Mazama, Mount Rainier, and Mount Jefferson, on the calculation of similarity coefficients for comparisons of both glass and ilmenite reference compositions with those of distal tephras, and on considerations of stratigraphic position. The composition of glass and ilmenite and the stratigraphic position of one distal tephra layer in the loess suggests that it is from an eruption of Mount St. Helens at least several thousand years older than the set C eruptions. Glass composition and stratigraphic position of a distal tephra at another site in loess suggested a possible correlation with some layers of the Pumice Castle eruptive sequence at Mount Mazama (ca. 70,000 yr B.P.), but similarity coefficients on ilmenite of only 45 and 48 fail to support the correlation and show why multiple correlation methods should be used. Similarity coefficients higher than 96 for both glass and ilmenite establish a correlation with Mount St. Helens layer Cw for distal layers in two widely separated sites. These layers are in sedimentary successions that are closely associated with giant floods in the Channeled Scabland. The 36,000 yr B.P. radiocarbon age of the Mount St. Helens set C establishes a minimum limiting date for an episode of flooding that predates the widespread late Wisconsin floods. A correlation of distal tephra layers at two other sites in the Scabland and Palouse establishes a chronostratigraphic link to a still-older episode of flooding within the Brunhes Normal Polarity Chron. Six distal tephra layers in pre-late Quaternary loess that are not correlated with known or dated eruptions have compositions and distinctive stratigraphic positions relative to magnetic reversal boundaries that make them key markers for future work.

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