Alteration of hydrovolcanic basaltic ash: Observations with visible and near‐infrared spectrometry

Abstract

Ash of basaltic composition that is produced by hydrovolcanic activity occurs frequently on Earth and may be much more widespread on Mars. Altered basaltic tephras from tuff rings and tuff cones across the Basin and Range were examined using visible and near infrared (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. Hydration produces an increase in overall reflectance with gradual development of water and hydroxyl absorptions at 1.4 μm and 1.9 μm. The oxidation of basaltic tephras can be observed spectrally through changes in the position and slope of the Fe3+ ‐ O2− charge transfer edge, inflections on that edge due to ferric oxide minerals and changes in the position and depth of the 1 μm iron crystal‐field band. Significantly 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 dioctahedral smectite clay minerals and minimal palagonitization. The latter class of tephras are highly palagonitized which implies a high degree of hydration and oxidation. In fact, in the most highly altered tephras, all the Fe2+ that is initially in the volcanic glass is converted to Fe3+ within nanophase and bulk ferric oxide phases in the palagonite. There is also more extensive development in the tuff cone beds of zeolite and phyllosilicate minerals. The differences in reflectance spectra of altered basaltic tephras can be traced back to initial differences in the water/magma ratio extant at the volcanic vent; thus, Vis/IR spectrometry holds the potential to be used as a tool for investigations into the volcanic history and paleoclimate of volcanic terrains.