Clay Diagenesis and Mass Balance—The Forest and the Trees

233

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.

Section: What Can Clays Contribute To Geothermometry?mentioning

confidence: 99%

Clay Mineral Thermometry—A Critical Perspective

Essene

Peacor

1995

233

129

“…Finally, the chemical properties of porewater, with which the clays interact, seldom can be characterized fully. Yet, if mineral reaction mechanisms and controls are to be unraveled, the whole bulk-rock system must be documented and the variables within the system assessed for their contribution to the control on mineral reaction mechanisms (Abercrombie et al 1994;Pollastro 1994).…”

Section: Background and Approachmentioning

confidence: 99%

“…The data collected from the USGC by Burst (1969) and subsequently by Flower and coworkers were the basis for the concept of smectite illitization. This fundamental concept was laid down first by the original authors themselves, and was subsequently refined by many clay scientists on the basis of the same central data set or new data from the same area (Awwiller 1993;Eberl 1993;Pollastro 1994). However, a consensus on the governing parameters and the detailed reaction mechanism(s) is yet to be reached (for example, replacement vs. dissolution-precipitation; importance of K, Si and AI in porewater; open vs. closed system diagenesis; and channelized vs. pervasive vs. minor porewater flow).…”

Section: The Usgc Shalementioning

confidence: 99%

Comparison of the Mineralogical and Chemical Composition of 2 Shales from the Western Canada Sedimentary Basin and the United States Gulf Coast

Caritat

Bloch²,

Hutcheon

et al. 1997

Abstract--The mineralogy and geochemistry of shales reflect the composition of the initially deposited precursor mud, subsequently modified by diagenetic processes. To see if significant geochemical differences exist between shales that mainly owe their present-day composition to either deposition or diagenesis, we compare the published mineralogical, bulk and clay-fraction geochemical, and clay-fraction O-isotopic compositions of 2 shales. One shale is from the Western Canada Sedimentary Basin (WCSB), and its composition mainly reflects primary (depositional) chemical and mineralogical variations (smectitic to illitic illite/smectite) within this unit. The other shale is from the United States Gulf Coast (USGC), and its composition mainly reflects mixed-layer illite/smectite (US) diagenesis of deposited smectitic clay material. The chemical and mineralogical trends of WCSB and USGC shales, including one of increasing illite content in US with depth or maturity, are essentially indistinguishable, in both bulk shale and clay fraction, despite the contrasting genetic interpretations for the origin of the contained US. Thus, similar mineralogical and chemical trends with depth or temperature can result either from inherited depositional compositional heterogeneity of the sediment, from burial metamorphism of shale or a combination of both. Interestingly, the O-isotopic compositions of the clay fractions from the WCSB and USGC are significantly different, a fact that reflects original clay formation from source material and water of quite different isotopic compositions. The discrimination between depositional and diagenetic contributions to shale composition continues to pose challenges, but a combination of bentonite, illite polytype, clay isotopic and trace and rare earth elemental analyses together with illite age analysis holds promise for future work;

“…Time, temperature, chemical composition of the diagenetic fluids, availability of K, crystallography and chemistry of the parent minerals and fluid dynamics in the diagenetic zone were suggested as various important factors that can influence the diagenetic evolution. Opinions are still varied about the relative importance of these factors, leading to varied views about the mechanism of the change from smectite to illite in the Gulf Coast area, on the basis of either mineralogical and geochemical studies (Boles and Franks 1979;Freed 1981;Ahn and Peacor 1985Pollastro 1985Pollastro , 1993Pollastro , 1994Fisher and Land 1986;Freed andPeacor 1989, 1992;Shaw and Primmer 1989;Primmer and Shaw 1991;Milliken 1992;Awwiller 1993), or isotopic studies (Yeh and Savin 1977;Yeh 1980;Land 1984;N~lorton 1985;Land and Fisher 1987;Ohr et al 1991;Eberl 1993;. Consequently, the various studies of Gulf Coast sediments have significantly influenced thoughts about burial-induced illitization of smectite in sediments in other basins.…”

Section: Introductionmentioning

confidence: 99%

Inter-Basinal Comparison of the Diagenetic Evolution of Illite/Smectite Minerals in Buried Shales on the Basis of K-Ar Systematics

Clauer

1996

Abstract--The K-Ar systematics of illite/smectite (I/S) mixed layers in deeply buried shales from the Gulf Coast, the North Sea and the Mahakam Delta basins have been compared to provide additional perspectives on the diagenetic evolution of these minerals. Comparison of the results suggests that illitization proceeds similarly in the 3 basins, at least for the increase in the illite-layer and K contents, despite differences in the provenance of the detrital components, the ages of deposition, the depths of burial and the tectonic history of the basins. Analysis of the trends with depth in the illite-layer and K contents of I/S-enriched size fractions of shales in the North Sea and the Mahakam Delta basins shows that these trends represent segments of the more complete trends from I/S minerals of the Gulf Coast area.The trends with depth in the radiogenic 4~ contents and in the K-Ar ages of the I/S-rich fractions in the North Sea and Mahakam Delta basins suggest that, relative to the reference trends of the Gulf Coast area, the K-Ar system of the clay material is more dependent on the behavior of the radiogenic 4~ than on the occurrence or non-occurrence of detrital grains in the size fractions. Recasting of the available data suggests that retention of radiogenic 4~ by the illite-type minerals occurs in the intense illitization zone and release occurs in the deeper part of the basins. We therefore speculate that the illitization process of the I/S mixed layers of progressively buried shale-type sediments could be controlled by a transformation process integrating dissolution of detrital components in poral rock environments relatively impermeable to radiogenic 4~ These excesses, which might be partly or completely erased in deeper parts of the sedimentary basins, question the application of the K-Ar dating method on clay minerals extracted from shales.