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Diagenetic Alteration of Silicic Ash in Searles Lake, California

Published online by Cambridge University Press:  02 April 2024

Richard L. Hay  and
Sandra G. Guldman
Richard L. Hay
Affiliation:
Department of Geology, University of Illinois, 1301 West Green Street, Urbana, Illinois 61801
Sandra G. Guldman
Affiliation:
Environmental Science Associates, 760 Harrison Street, San Francisco, California 94107
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Abstract

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Ash layers from Searles Lake, California, were sampled in core of drill hole KM-3, which penetrated 693 m of lacustrine sediment deposited in a playa-lake complex over the past 3.2 my. Lake water changed from moderately saline and slightly alkaline (pH ~7.5) to highly saline 2.04 my ago and to highly saline and alkaline (pH ~9.5) 1.28 my ago. As a result of brines flushing downward, the upper 291 m of sediment, spanning the past 1.28 my, contain highly saline, alkaline pore fluid. Silicic ash layers in contact with highly saline, alkaline pore fluid were first altered to phillipsite and meriinoite and then to K-feldspar and searlesite. The transformation of phillipsite and/or meriinoite to K-feldspar required more than 45,000 years and was largely completed in 140,000 years. Tephra layers in contact with moderately saline, slightly alkaline pore fluid vary from uncemented vitric ash containing minor smectite to bentonites in which glass is wholly altered to smectite, clinoptilolite, analcime, and opal. Layers with much fine tephra are more altered than the coarser, better-sorted layers. Alteration is attributed to hydrolysis in essentially a closed hydrologic system, in which the alteration of glass to smectite raised the pH, aSiO2, and (Na+ + K+)/H+ activity ratio to the level where clinoptilolite formed. Some diffusion and/ or fluid flow is, however, indicated by the loss of SiO2 during the alteration of ash layers to smectite and by anhydrite deposited during and after clinoptilolite in some tuffaceous sandstones.

Keywords

AlkalineSaline lakeAnacimeClinoptiloliteDiagenesisK-feldsparMerlinoitePhillipsiteVolcanic ashZeolite
Type
Research Article
Information
Clays and Clay Minerals , Volume 35 , Issue 6 , December 1987 , pp. 449 - 457
Copyright
Copyright © 1987, The Clay Minerals Society

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