Abstract

The principal components of secondary mineral assemblages found in Pasco Basin basalts are iron-rich smectite (nontronite), clinoptilolite, and silica. Silica occurs as quartz, cristobalite, tridymite, and opal-CT. Extractable iron within the nontronite suggests the presence of an iron-bearing oxyhydroxide phase intercalated with the nontronite. Other components present in minor or trace amounts are mordenite, celadonite, apatite, pyrite, phillipsite, gypsum, erionite, and chabazite. The generalized precipitation sequence with time and/or depth was found to be clay (usually nontronite) --> clinoptilolite --> silica and/ or clay. Nontronite, the first phase to form, is present at nearly all sampled depths. Clinoptilolite is apparently restricted to depths below about 350 m. Quartz is ubiquitous whereas opal and cristobalite appear to be abundant only below 600 m. Mordenite occurs only at depths below about 900 m, which correlates roughly with the first occurrence of dissolution-etched clinoptilolite. These observations as well as comparisons with data on secondary mineral assemblages from other basaltic and felsic systems suggest that the geochemical evolution of Pasco Basin basalts probably occurred under conditions similar to those existing today.

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