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Illite K–Ar Dating and Crystal Growth Processes in Diagenetic Environments: a Critical Review

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Illite K–Ar Dating and Crystal Growth Processes in Diagenetic Environments: a Critical Review View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Centro de Servicios en Gestión de Información doi: 10.1111/j.1365-3121.2004.00563.x Illite K–Ar dating and crystal growth processes in diagenetic environments: a critical review Alain Meunier,1 Bruce Velde2 and Patricia Zalba3 1HYDRASA UMR 6532 CNRS University of Poitiers, 40 av. Recteur Pineau, 86022 Poitiers Cedex, France; 2Laboratory Ge´ologie, E.N.S., 24 rue Lhomond 75231 Paris Cedex 05, France; 3CETMIC – University La Plata, Camino Centenario entre 505 y 508 (1897), M.B. Gonnet, Prov. BS. AS, Argentina ABSTRACT K–Ar dating of illitic minerals is commonly used in studies of ageK–Arfinest), it is shown that the K–Ar age significance depends diagenetic series applied to oil prospecting. In spite of a great on the illite nucleation–growth processes. A ‘diagenetic age’ is number of specialized papers, some problems remain unre- obtained when these processes are rapid (the K2O accumulation solved. These are mostly due to a misunderstanding of the period is shorter than 2r). If lower than this value, the K–Ar argon accumulation process during illitization. Criteria for ratio depends on the proportions of new and old particles, identifying detrital–authigenic mineral mixtures, crystal ripen- respectively, which are controlled by the relative rates of ing, fast precipitation or continuous nucleation-growth proces- nucleation, crystal growth and ripening. ses are discussed using K–Ar data available in the literature. Using different parameters, such as Dage (ageK–Ar ) agestrati), Terra Nova, 16, 296–304, 2004 Dcryst (diagenetic ageK–Ar ) agestrati)orDfrac (ageK–Arfraction ) Whatever the mixing law used by the the illitization process in diagenetic Introduction different authors, detrital and authi- environments using selected published Illite contains potassium, which is a genic phases are considered to have a data. part of its definition. This potassium fixed K–Ar age. This assumption is can be used to obtain an age, or acceptable only if, first, the source of Diagenetic illite: what do we period of existence of potassium in an the detrital K-bearing phases is the know? illite crystal. The estimation of time is same in all the studied samples and, obtained through the identification second, if the authigenic phases have a What is illite and how does it form in and measurement of radioactive decay given age. This latter point is debat- diagenetic environments? and decay products. A radiometric able because it refers to the duration age is obtained by determining the of K accumulation compared with the Illite is commonly identified in dia- amount of potassium in a mineral and experimental dating error (2r). In genetic sediments using X-ray diffrac- the amount of radio-decay argon (see other words the crystal growth pro- tion (XRD) by a peak near 10 A˚ the review by Clauer and Chaudhuri, cesses of illite and I ⁄ S MLM phases whose width at half maximum inten- 1995). However, this is not as easy as should be taken into account before sity (FWHM) is higher than that of it sounds for the following reasons: any interpretation of K–Ar ratio in micas (Meunier and Velde, 2004). The terms of geological dating. shape of the peak is asymmetrical and 1 mixing with other K-bearing phases How can one study the effect of the biased towards small angles. Accord- such as detrital mica or K-feldspar growth processes related to illitization ing to Lanson (1997), mathematical (contamination effect), in diagenetic environments on K–Ar decomposition procedures may be 2 Ar loss or capture during crystal- dating? Analysing the shapes of crys- used to analyse the peak shape and lization, tal size distributions, Eberl et al. intensity. It has been shown that the 3 duration of K accumulation (1998) showed that they are controlled ÔilliteÕ 10-A˚ peak results from the (growth stage) in the illitic phases, by three different mechanisms: nucle- contribution of three different particle i.e. illite and illite–smectite mixed ation + growth, surface-controlled populations: I ⁄S, PCI and WCI. layer particles (I ⁄S MLM) com- growth and supply-controlled ripen- (Fig. 1). The relative intensity of pared with the dating experimental ing. However, in spite of this recent the WCI peak increases with depth error (2r). progress, a theoretical approach re- in diagenetic series. Thus, the 10-A˚ The contamination effect due to mains unclear because the growth peak becomes sharper and the Ku¨bler authigenic–detrital phase mixing has processes are still not fully understood index decreases (Ku¨bler and Jaboye- been discussed in numerous papers (see Srodon et al., 2002). Indeed, the doff, 2000). Illite is considered to be (Pevear, 1992; Srodon, 1999; Ylagan size of illite particles is difficult to pure when the I ⁄ S MLM contribution et al., 2000; Srodon et al., 2002). measure in three dimensions. How- becomes negligible compared with the ever, some published K–Ar data con- PCI and WCI peaks. Correspondence: Alain Meunier, HY- cerning different size fractions of clay Consequently, even if no expandable DRASA UMR 6532 CNRS University of samples can be used to estimate the layers are detected in XRD patterns, Poitiers, 40 av. Recteur Pineau, 86022 effect of crystal growth. The present any K–Ar dating of illite in sediments Poitiers Cedex, France. E-mail: alain. contribution aims to examine how is based on an average composition of [email protected] argon is accumulated or lost during a composite population of particles 296 Ó 2004 Blackwell Publishing Ltd Terra Nova, Vol 16, No. 5, 296–304 A. Meunier et al. • Illite K–Ar dating ............................................................................................................................................................. a hairy illite 1996; Berger et al., 1997). The size and Summarizing, the increasing illite shape of crystals depends on tempera- content of I ⁄S MLM up to 95% is ture (Lanson et al., 1996) rather than related to crystal growth controlled by 10.02 Å FWHM=0.23 on chemical constraints (Small et al., a ripening process while pure illite 1992). The morphological evolution particles nucleate and grow concom- WCI from hairy habit through lath-shaped itantly (Wilkinson and Haszeldine, to isometric or hexagonal plates oc- 2002). Significant progresses has been intensity curs simultaneously with an increase made since these first analyses with in ÔcrystallinityÕ. In such a case, crys- regard to the distribution of particle PCI 10.21 Å FWHM=0.5 0 tallinity depends on the relative pro- size in diagenetic sediments. Indeed, portions of PCI and WCI as indicated the distribution of particle size chan- by XRD decomposition (Fig. 1a,b). ges according to the relative import- 77.588.5 9 9.51 0 The illitization of smectite-rich ance of nucleation on the one hand θ α °2 Cu K sediments in diagenetic conditions and of ripening on the other. Eberl was shown to be controlled by two et al. (1998, 2002) showed that the b hexagons concomitant mineral reactions: the distribution function is asymptotic or increase of illite content of I ⁄S MLM log-normal if nucleation or ripening and the nucleation and growth of pure dominate, respectively (Fig. 2a,b). 10.01 Å FWHM=0.17 WCI illite particles. These two reactions Three general trends have been dis- result from the dissolution of unstable criminated in an a–b2 plot (Fig. 2c). particles and the growth of the stable We must keep in mind that K–Ar ones. Concomitantly with an illite dating of diagenetic illite aims to intensity content increase of I ⁄S MLMs up to provide absolute ages in the complex PCI 10.10 Å FWHM=0.35 95%, the shape and size of the parti- geological history of sedimentary ba- cles change: small-sized lath-shaped sins. These ages are useful data in oil smectite-rich I ⁄S particles transform prospecting for example. Unfortu- I/S10.56 Å progressively into large-sized more nately, illite is frequently mixed with 7 7.5 8 8.5 9 9.5 10 isotropic particles. Thus, illitization detrital micas whose age is much °2θ Cu Kα proceeds through crystal growth. older. Thus, the above theory, which Fig. 1 Example of decomposition of was successfully applied to hydrother- mal systems (Eberl et al., 2002), can- XRD patterns of the illitic phases in How do illite crystals grow in not be directly used for illite age the Proterozoic sandstones of the Atha- diagenetic environments? basca basin (Canada). (a) Illite was calculation because of this contamin- formed by direct precipitation from Because illitization is an addition of ation effect. Besides, K–Ar dating is potassium-rich solutions. (b) The illitic illite layers on I ⁄S MLMs, these par- obtained from amounts of argon and phases have been matured by diagenetic ticles always have a small fraction of potassium not from crystal growth processes. smectite present (less than 5%). By models. Consequently, the qualitative contrast, the illite crystallites appear models presented below are based on to have no smectite present regardless K ⁄Ar mass-balance and do not give having different size, shape and thick- of their size. This pure illite is nor- documented information on the actual ness. This average crystalline state mally well crystallized (WCI) and has growth processes. changes with depth as a result of a crystal shape reasonably close to different processes (Altaner and Yla- hexagonal. Observations carried out The K–Ar apparent age of gan, 1997). It is important to identify on several diagenetic series (Eberl and authigenic–detrital mineral the process at work in order to under- 1Srodon, 1988; Inoue et al., 1988; Eberl mixtures stand how potassium becomes incor- et al., 1990; Lanson and Champion, porated during the change of 21991; Varajao and Meunier, 1995) Evidence of mixtures of detrital and crystalline state.
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