CLAY MINERAL DEVELOPMENT ON TERRESTRIALLY DEPOSITED TEPHRAS

Nine explosively erupted Holocene tephras from seven volcanoes were studied to determine the relationship between the age, environmental setting, and degree of clay formation. The volcanoes are located in the western United States, Canada, and the Kamchatka Peninsula, Russia, and range in age from 20 to 15,000 years B.P. All of the sampled tephras were deposited within terrestrial or lacustrine environments.

The samples were analyzed using X-ray Diffraction, X-ray Fluorescence, Scanning Electron Microscopy, and Inductively Coupled Plasma Spectroscopy. The tephras consist mainly of glass shards, feldspars and micas. Chemically, the nine samples range from andesite to dacite (total alkali-silica plot). Analyses also indicate light REE enrichment, suggestive of intermediate to felsic volcanics. This is consistent with explosive volcanism associated with subduction at convergent plate boundaries.

Clay minerals developing on the tephras are mainly chlorite, derived from the alteration of biotite, and halloysite, from the alteration of glass. The glass phase is fairly pristine in most of the samples despite the variation in ages. The combined effects of high latitude, low weathering rates, and terrestrial deposition appear to be the primary factors in retarding clay mineral formation.