Illite/Smectite Geothermometry of the Proterozoic Oronto Group, Midcontinent Rift System

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.

Section: Geological Implicationsmentioning

confidence: 99%

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

Chaudhuri

Środoń

Clauer³

1999

“…1993, Pollastro 1993. Price and McDowell (1993) advanced a T-t model for clay transformations observed in 1.1 Ga old pelites of the Nonesuch Formation, Michigan. Pollastro (1993) inferred that different T-t relations apply to sediments of Miocene through Mississippian age and those <3 Ma in age.…”

Section: What Can Clays Contribute To Geothermometry?mentioning

confidence: 99%

Clay Mineral Thermometry—A Critical Perspective

Essene

Peacor

1995

231

129

Abstract--Diagenetic clay minerals usually occur as heterogeneous assemblages of submicroscopic layers consisting of different structure types such as illite, smectite and chlorite, with variable composition within a given structure type, and with highly variable concentrations of imperfections. The dimensions of mixed-layering, the semi-coherent to coherent nature of the structures across the layering, and compositional heterogeneity occur at a scale well below that of an individual thermodynamic phase. These relations imply that most clays are not distinct minerals or phases, and that assemblages of clays in shales and mudstones are incompatible with the phase rule. Such relations are better evaluated in terms of the formation of metastable materials with each small unit having unique chemical properties, rather than as a small number of stable homogeneous phases. Consequently, treatment of most clay minerals in terms of equilibrium stability with either a thermodynamic or experimental approach is subject to error.Chemical reactions involving most clay minerals are best understood with kinetic models. These involve a great variety of parameters such as time, fluid/rock ratio, deformation history, nature of starting materials and transformation mechanisms, as well as the variables, such as temperature, pressure and composition, that are commonly used to define equilibrium. Solubility experiments on the stabilities of clay minerals are unlikely to attain equilibrium at low temperatures. Moreover, the activity of soluble species may be controlled by surface equilibria, or by absorbed or exchangeable cations. Interpretations of available experiments on the solubility of illite vs. other mineral assemblages are in violation of Schreinemakers' rules and indicate lack of equilibrium.Predictable sequences of clay minerals as a function of temperature are best understood through the Ostwald step rule, in which clay mineral assemblages undergo reactions in response to kinetic factors that represent reaction progress rather than an approach to equilibrium. Currently used clay mineral thermometers (illite crystallinity, smectite/illite reaction, chlorite composition) are not based on equilibrium reactions. Such systems are not accurate thermometers and therefore have questionable utility.

“…With increasing depth of burial in sedimentary basins there is a progressive conversion of smectite into illite and the extent of reaction is frequently used as an indicator of diagenetic grade (Hoffman and Hower 1979, H6roux et al 1979, Pollastro 1990, Price and McDowell 1993. However, besides the increase of temperature with depth, many other factors may influence the progress of this reaction complicating attempts to use it as a geothermometer.…”

Section: Introductionmentioning

confidence: 99%

Illite/Smectite Diagenesis and Its Variable Correlation with Vitrinite Reflectance in the Pannonian Basin

Hillier

Mátyás

Matter

et al. 1995

Abstract--The correlation between illite/smectite (I/S) diagenesis and mean vitrinite reflectance (Ro) data is examined in mudrocks from a hydrocarbon exploration well (geothermal gradient 35"C km -1) from the Great Hungarian Plain of the Pannonian Basin System. The expandability of I/S decreases with depth and there is a change from random to ordered mixed-layering at about 2500 m depth. At this depth Ro is about 0.6%. Comparison of the correlation of expandability and Ro from this study to published data for the Vienna Basin and the Transcarpathian Basin, sub-basins of the Pannonian Basin System, shows that the correlation is systematically different for each sub-basin, according to their thermal histories. In the Vienna Basin (geothermal gradient 25"C km-~), for any given value of Ro, the expandability of US is less than in the Transcarpathian Basin (geothermal gradient 55*(2 km -1) and the sediments are older and more deeply buried. Data from the present study are intermediate. This variation is believed to be due to the effect of time on the smectite-to-illite reaction. Results of an optimization procedure to calculate the kinetics of the smectite-to-illite reaction, using as input the expandability depth profiles, and thermal histories constrained by comparison of observed and calculated Ro data, showed that I/S diagenesis in the Pannonian Basin System can be modelled by a single first order rate equation:-dS/dt = e log(A) -E/RT. S where S = fraction of smectite layers in I/S, t = time (Ma), e = exponential function, log(A) = frequency factor = 7.5 (Ma-l), E = activation energy = 31.0 kJ mo1-1, R = universal gas constant, and T is temperature in Kelvin. This result also suggests an important role for time. By combining the kinetics of the smectite-to-illite reaction with a kinetic model ofvitrinite maturation it is possible to define a domain within which all 'normal' (burial diagenesis) correlations between Ro and US diagenesis should lie. Such diagrams can be used to identify different thermal histories related to different geotectonic settings and 'anomalous' data such as that affected by igneous intrusions.