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

Abstract

Abstract--Two clay fractions of a Cambrian claystone from Estonia, consisting essentially of illite and 20% expandable illite-smectite, (I-S) were treated with C~2 and C]s alkylammonium cations for K-exchange. Both the untreated and treated samples were dated by the K-Ar method. The treated clays lost several percent of their original K20 , with greater losses for longer-chain cations and for longer reaction time, in accordance with previously published studies. The dates of the treated clay fractions were 20-30 Ma lower than those of the untreated clays. The decrease in the dates suggests preferential opening of older, detrital clays. The K-At dates of the illite layers susceptible to K extraction by the various treatments were calculated by subtracting K~O and radiogenic 4~Ar values of the consecutive step products, and they were plotted against the total % KzO removed, used as an indicator of the reaction progress. Extrapolation of the plot revealed a detrital (1550 Ma) and a diagenetic (380 Ma) age for the 2 illitic minerals present in the investigated shale sample. The inferred Devonian age of diagenesis of the Estonian clay corresponds to the period of massive dolomitization in the area. Both alteration processes can be related to a Devonian incursion of hot or alkaline fluids, which helps to explain the occurrence of 20% expandable I-S in claystones that have never been buried more than 1000 m.Extrapolated K-Ar ages and K20 contents of the illitic minerals, estimated from the X-ray diffraction (XRD) data, were used to model the experimental data. A good agreement was reached when dilution effects (chlorite and expanded illite) were taken into account.

Diagenetic history of Lower Palaeozoic sediments in Pomerania (northern Poland), traced across the Teisseyre–Tornquist tectonic zone using mixed– layer illite– smectite

“…An age for diagenesis on the craton (390 Ma), that is very similar to the results of this study (370 390 Ma) has been inferred by a new K-Ar dating technique, based on alkylammonium cation exchange, for the Cambrian claystones of Estonia (Chaudhuri et al, 1999). The extrapolation of Rb-Sr dates for various grain-size fractions of the Ca mbrian clayst ones from the same area (Kirsimäe et al, 1999, Fig.…”

“…On one hand, this led Kirsimäe et al (1999) to propose low-temperature illitization for Cambrian shales of Estonia. On the other hand,~20%S measured for these shales (Chaudhuri et al, 1999) indicates a level of diagenesis similar to the top of the Lower Palaeozoic in the study area and in the rest of the craton, which corresponds to~1208C palaeotemperatures. Also, the age of diagenesis in this area coincides with the rest of the craton.…”

“…The most complete literature data, which allow us to view our findings in a broader perspective, are available for the Cambrian rocks. Chaudhuri et al (1999) measured 20%S in the I-S of the Cambrian 'Blue Clay' from Estonia (craton, NE of the study area). Lindgreen et al (2000) also measured %S in I-S from the Cambrian Alum Shales which cover a large portion in the northwest of the craton (Denmark and Sweden).…”

Stable Isotopes in the Sedimentary Record

“…The linear regression is shown next to each line. Data: East Slovak Basin from Clauer et al (1997); Estonian 'blue clay' from Chaudhuri et al (1999); all the rest referenced and discussed in Lerman and Clauer (2005).…”

Multiphase Silurian bentonites in the Baltic Palaeobasin

“…In addition, the Ar/Ar data of the zoned K-feldspar overgrowths from the same sandstone hosted Pb-Zn bodies suggest two discrete events -early burial diagenesis (∼528-567 Ma) and a later tectonically induced fluid flow event related to the collapse of the Caledonian orogen (425-400 Ma) (Sherlock et al, 2005). Moreover, Chaudhuri et al (1999) associated illitization of Cambrian claystones in Baltic Palaeobasin with the expulsion of hot hydrothermal and/or highly alkaline solutions during the Early Devonian. The established diagenetic ages for Cambrian claystones generally agree with the diagenetic age of Ordovician and Silurian K-bentonites in southern Baltic Palaeobasin and Pomerania (Środoń and Caluer, 2001).…”