3. RESULTS: FIELD

WATER CHEMISTRY

Regolith waters (rock pit) have much lower alkalinity than well waters, presumably a result of shorter residence time. Another striking difference between rock pit and well waters is the K⁺/SiO₂ ratio. Rock pit waters have an order of magnitude higher ratio of K⁺/SiO₂ than well waters. This oberservation is consistent with the depletion of K₂O in the bulk regolith with respect to unweathered tuff. Precipitation of Seven Parks has no detectable K⁺ or SiO₂.

Infiltrating water has distinctly different chemistry than well water.

REGOLITH TEMPERATURE

Soil probes continuously monitored soil temperatures at the Seven Parks soil lysimeter site. During the period when the regolith is frozen, temperatures remain at about -4^o C. The duration between thawing and freezing represents a maximum time for wetting of regolith minerals. Well level records of ground water near the soil temperature probes suggest that actual time of water infiltration is between 40 and 60 days in the early summer. There is late-summer precipitation but these events are short-lived and probably do not uniformly saturate the regolith, unlike the slower, more voluminous snowmelt event.

Conditions favoring chemical weathering are relatively short-lived.

Rock fragments in regolith disaggregate with little or no mineral alteration.

THIN SECTION PETROGRAPHY
Thin section examination of smaller (2 cm diameter) regolith rock fragments show that phenocrysts at or near the surface are unaltered. Blue epoxy within fractures shows that the fractures permeate phenocrysts and matrix alike. Secondary clay minerals are notably absent from the near-surface fractures.

SEM IMAGES OF GRAIN MORPHOLOGY
Aggregate grain from regolith exhibiting well-defined phenocrysts and microphenocrysts.

Regolith material in various stages of disaggregation. Note lack of dissolution pitting on mineral grains.

Regolith forms by mechanical disintegration of the tuff

SEM IMAGES OF RESIDUAL INTERSTITIAL GLASS
In the finest fraction of the regolith (less than 63um), crystal aggregates are observed to be bound together by an amorphous substance.

Numerous qualitative X-ray spectral analyses on the amorphous interstitial substance in different aggregates yield very similar spectra (Si>>K>Al>Fe>Ti), suggesting that this is a distinct phase; probably volcanic glass.

Trace amounts of glass are observed in regolith grain aggregates.

USGS home page || USGS WRD National Research Program Web Page
The URL of this page is: http://wwwbrr.cr.usgs.gov/projects/SW_corrosion/GSA_poster/index.html
Comments and feedback: cgunther@usgs.gov
Last modified: Thursday, 16-July-98 15:50:14 MST