Clay Mineral Diagenesis in Core KM-3 of Searles Lake, California

Abstract

Abstract--Core KM-3 at Searles Lake, California, comprises 693.4 m of lacustrine sediments deposited over the past 3.2 m.y. The lake water evolved from moderately saline, slightly alkaline, and dominated by Na, Ca, C1, SO4, and HCO3 + CO3 to a highly alkaline brine dominated by Na and CO3 ions. Sediments are chiefly muds and evaporites. Montmorillonite and illite are the principal detrital clay minerals supplied to Searles Lake at present and probably during the late Pliocene and Pleistocene.The drill core is divided into three diagenetic zones on the basis of clay-mineral reactions. The upper zone (0-291.1 m) contains authigenic Fe-illite, Mg-smectite, K-feldspar, and analcime, which average 60-70% of the <2-tim silicate fraction of mud samples. The principal silicate reactants are detrital montmorillonite and kaolinite, which have been largely consumed. The middle zone (291.1-541.6 m) also contains authigenic Fe-illite, Mg-smectite, K-feldspar, and analcime, but they form only 20 to 30% of the < 2-~m silicate fraction, and considerable detrital montmorillonite and kaolinite remain unaltered. In muds of the lower zone (541.6-693.4 m), a small amount of clinoptilolite is the only authigenic silicate mineral identified, although authigenie montmoriUonite probably coexists here as well. Vitric ash is the silicate reactant, and detrital clay minerals apparently remain unaltered in the lower zone.Diagenetic zoning reflects the pore-water chemistry, in which pH may have been the most important parameter. Sediments of the upper zone were deposited in highly alkaline lakes of variable salinity, and sinking brines with a pH of 9.0-10.0 have saturated all sediments. Sediments of the middle zone were deposited in lakes of moderate to high salinity. The pH was overall lower than in the upper zone, although it probably exceeded 9.0, at least locally, during silicate diagenesis. Moderate salinity and a slightly alkaline pH (-7.5-8.0) are inferred for both the lake and pore water of the lower zone.Oxygen-isotope values of authigenic Fe-illite, Mg-smectite, and K-feldspar, and phillipsite in the upper diagenetic zone reflect a high degree of evaporative concentration and presumably of salinity. Equilibrium water values calculated for 22~ from the oxygen-isotopic composition of authigenic phyllosilicates range from -3.5 to + 1.9~ averaging -1.207o0 (SMOW). Higher salinities are suggested for K-feldspar, for which water values range from + 1.8 to +4.807~, averaging +3.4%o. The water value for phillipsite is +0.307o0. By comparison, rainfall at Searles Lake has an average fi~80 value of about -9.80700, and a brine sample has a value of +4.007o0.Mud samples of the middle and upper diagenetic zones tend to be rich in Fe-illite or Mg-smectite, but not in both, indicating that the two minerals are favored to some extent by different environments. Feillite seems to be generally favored by oxidizing conditions and probably by a playa environment, and Mg-smectite seems to be favored by reducing conditions and an open-water environment. ...

Geochemical constraints on the stability of zeolites and C–S–H in the high pH reaction of bentonite

“…Saline, alkaline lakes provide perhaps the most relevant natural system evidence for the alteration of bentonite by Na-K-OH alkaline fluids. These environments show marked mineral zoning in which the end products for newly formed Na and K silicates are commonly K-feldspar and analcime (Hay et al, 1991;Savage et al, 2007). It is remarkable that in these alkaline environments, Mg-silicates, as stevensite or sepiolite, are common, in accordance with the experimental reactivity of natural bentonites.…”

K-Ar Dating of Illitic Fractions of Estonian “Blue Clay” Treated With Alkylammonium Cations

“…Based on the current knowledge of the illitization mechanism, 2 processes could produce the observed degree of diagenetic illitization in Lontova clays: 1) a Devonian thermal event or the penetration of hot fluids in the study area, that is, processes which would significantly raise the rock temperatures at shallow depths, and 2) a Devonian incursion of highly alkaline fluids (pH -->10). It has been shown by field evidence (Hay et al 1991) and by laboratory studies (Eberl et al , 1993) that alkaline fluids are capable of promoting illitization at low temperatures.…”

Characterization, pillaring and catalytic properties of a saponite from Vicálvaro, Madrid, Spain

“…These clays were formed by diagenesis of sediments in a lacustrine environment. According to Hay et al (1991), smectites are formed in high salinity waters at pH 9-10, while sepiolites develop in low salinity waters at lower pH. Ram/rez et al (1995) agree that sepiolites were formed by a progressive replacement of the trioctahedral smectites, and in the presence of Li § Fluorine contents of 1-1.5% are closely related with the sepiolite levels, part of this F being incorporated in the sepiolite structure where it replaces structural OH groups (Santaren et al, 1990).…”