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Alteration of hydrovolcanic basaltic ash - Observations with visible and near-infrared spectrometryAltered basaltic tephras from tuff rings and tuff cones across the Basin and Range were examined using visible and NIR (Vis/IR) reflectance spectrometry and a variety of other techniques. It was found that Vis/IR spectrometry is sensitive to subtle changes in the hydration and oxidation state of these tephras. In the tuffs examined, ferrous and ferric iron minerals produce one composite absorption feature rather than two resolvable bands. Different styles of alteration were noted between thinly bedded tuff ring deposits emplaced by high energy, largely dry, pyroclastic surge and massively bedded tuff cone deposits emplaced by wetter, lower energy pyroclastic flow. The former class of tephras become hydrated and only moderately oxidized with some limited development of dioctehedral smectite clay minerals and minimal palagomitization. The latter class of tephras are highly palagonitized which implies a high degree of hydration and oxidation. In the most of highly altered tephras, all the Fe(2+) that is initially in the volcanic glass is converted to Fe(3+) within nanophase and bulk ferric oxide phases in the palagonite. There is also more extensive development in tuff cone beds of zeolite and phyllosilicate minerals. The differences in reflectance spectra of altered basaltic tephras can be traced to initial differences in the water/magma ratio extant at the volcanic vent.
Document ID
19930032489
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
Authors
Farrand, William H.
(NASA Headquarters Washington, DC United States)
Singer, Robert B.
(Arizona Univ. Tucson, United States)
Date Acquired
August 15, 2013
Publication Date
November 10, 1992
Publication Information
Publication: Journal of Geophysical Research
Volume: 97
Issue: B12
ISSN: 0148-0227
Subject Category
Geophysics
Accession Number
93A16486
Funding Number(s)
CONTRACT_GRANT: NAGW-1059
Distribution Limits
Public
Copyright
Other

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