K-Ar dating of the Lower Palaeozoic K-bentonites from the Baltic Basin and the Baltic Shield: implications for the role of temperature and time in the illitization of smectite

Abstract

Mixed-layer illite-smectite samples from the Ordovician and Silurian K-bentonites of the Baltic Basin and the Baltic Shield (Norway, Sweden, Denmark, Poland and Estonia) were dated by K-Ar on several grain fractions and were studied by X-ray diffraction (XRD), both on oriented and random preparations, in order to reveal the conditions of smectite illitization in the area. Authigenic K-feldspar was also dated. The geographic pattern of the degree of illitization (% smectite in illite-smectite measured by XRD) is consistent with other indicators of palaeotemperatures (acritarchs, conodont alteration index, vitrinite reflectance, apatite fission track ages). It reveals the highest maximum palaeotemperatures (up to at least 200ºC) along the Norwegian and the German-Polish branches of the Caledonides and the lowest palaeotemperatures (120ºC) in the central part of the studied area. The distribution of K-Ar ages is not well correlated with this pattern, revealing a zone of older ages (Lower Devonian-Lower Carboniferous) between Denmark and Estonia, and areas of younger ages (Upper Devonian to Carboniferous/Permian boundary) to the north and south of this zone. The zone of older ages is interpreted as the result of illitization induced by a thermal event in front of the Caledonian orogenic belt (migration of hot metamorphic fluids?). The areas of younger ages are considered as representing deep burial illitization under a thick Silurian-Carboniferous sedimentary cover, perhaps augmented by a tectonic load. The K-Ar dates invalidate the hypothesis of a long-lasting low-temperature illitization as the mechanism of formation of the Estonian Palaeozoic illite-smectite. The ammonium content of illite-smectite from the Baltic K-bentonites reflects the proximity of organic-rich source rocks that underwent thermal alteration at the time of illite crystallization.

Climatic and sea‐level control of Jurassic (Pliensbachian) clay mineral sedimentation in the Cardigan Bay Basin, Llanbedr (Mochras Farm) borehole, Wales

“…Before examining the significance of clay minerals in terms of palaeoclimate and palaeoenvironment, it is important to ensure that they are dominantly detrital in origin without any significant influence of burial diagenesis. Smectites are known to be particularly sensitive to burial diagenesis and transform into R1 I–S by illitization processes with temperatures over 60 to 70°C (Merriman & Frey, ; Lanson et al ., ; Środoń et al ., ; Dellisanti et al ., ). As a result, the abundance of smectite from the lowermost Pliensbachian (base of the jamesoni Zone) and throughout the core suggests that the maximum burial temperatures of the Pliensbachian sediments never exceeded 60 to 70°C.…”

The Timing of Diagenesis and Thermal Maturation of the Cretaceous Marias River Shale, Disturbed Belt, Montana

“…The degree of illitization of smectite, indicated by the type of stacking order and the percentage of illite layers in diagenetic mixed-layered illite-smectite (I-S), is well known as a gauge of thermal maturity in basins where conversion of smectite to illite (smectite illitization) was caused by burial, and the timing of the illitization can be determined by K-Ar dating (Hoffman et al, 1976; influence clay diagenesis in foreland basins adjacent to tectonic highlands (Morton, 1985;Elliott and Aronson, 1987;Elliott and Aronson, 1993;Ziegler and Longstaffe, 2000;Elliott and Haynes, 2002;Ś rodoń et al, 2009). The conversion of smectite to illite proceeds either by solid-state transformation or dissolution-precipitation reactions; the latter mechanism prevails in fluiddominated systems (Altaner and Ylagan, 1997).…”

Clay Mineralogy of Coal-Hosted Nb-Zr-REE-Ga Mineralized Beds from Late Permian Strata, Eastern Yunnan, SW China: Implications for Paleotemperature and Origin of the Micro-Quartz

“…It has been suggested that the temperature that reflects the appearance of R1 I/S during the smectite illitization process is generally around 100 °C while the transition temperature from R1 to R3 I/S can be up to ca. 180 °C [48,50,[53][54][55]. In this study, most of the studied samples have the R1 ordered I/S, except for only one sample that has the R3 ordered I/S (Table 1) indicating that the paleo-diagenetic temperature for the studied samples could have been up to 180 °C but for most of the samples, this value ranges from 100 to 180 °C.…”

“…Temperature and fluid chemistry thus should be the dominant factors that would have influenced the formation of I/S in this study. Many studies have focused on the diagenetic temperature during the burial process, using the evolution of smectite illitization (especially the percentages of smectite layers within I/S) [48,50,[53][54][55]. In the XRD patterns for the EG-saturated clay fractions, as smectite layers decrease in I/S, the peaks at 9 Å, as well as peaks at 5 Å, become sharp and narrow showing a trend towards illite and indicating a progressive increment in temperature (Figure 7).…”