Kubler Index Research Articles

Timing and Duration of Hydrothermal in the Oligocene Hamad Cauldron, SW Japan: Evidence from K‐Ar Ages of Sericites

Abstract: Crystallinity, chemical compositions and K‐Ar ages of sericites in highly‐sericitized granites and associated fissure‐filling veins were examined to delineate the timing and duration of the hydrothermal activity in the Oligocene Hamada cauldron in the San‐in district, SW Japan. Sericite separates (>2 μm) from the highly‐sericitized granites consist mainly of 2M1 polytype having high crystallinity and low Kübler indices of 0.22–0.35, while those in the fissure‐filling veins have lower 2M1/1Md ratios and crystallinity, and high Kübler indices of 0.29–0.35. This suggests that the sericites in highlysericitized granites were formed at a higher temperature than the vein sericites.Sericites from the highly‐sericitized granites of the Kumogi pluton give K‐Ar ages of 30.0±0.7, 30.4±0.7, 30.6±0.7, 30.6±0.7, 32.1±0.7, 32.3±0.7 and 33.0±0.7 Ma (1s̀), while those of the central plutons, 33.8±0.7 and 33.8±0.7 Ma. Sericites in the fissure‐filling veins of the Kumogi granite give K‐Ar ages of 31.0±0.7, 31.5±0.7, 31.6±0.7, 31.7±0.7 and 32.3±0.7 Ma. Biotite separates from the fresh Kumogi granite give K‐Ar ages of 31.7±0.8, 32.0±0.8, 32.7±0.7 and 33.5±0.7 Ma. The K‐Ar age data revealed that the hydrothermal alteration began at about 33 Ma and ended by about 30 Ma and that the period of sericite alteration was nearly synchronous with the cooling of the granite intrusions in the Hamada cauldron.Despite intense hydrothermal alteration, the Oligocene granitoids have not accompanied with any economic base metal mineralization. The bulk chemical analyses of sericite separates in the veins indicate that the post‐magmatic fluids were originally barren in heavy metals.

Dehydroxylation and Ostwald ripening effects in roofing slates

Roofing slates only open in wet and when they are maintained in wet from quarry. Their irreversible drying could be explained by the closure of porosity following Ostwald ripening processes of illite along the slaty cleavage interface-interphases along with the leaking out water-alkali solutions. Roofing slate drying randomly produces swelling of layers and crystallite coalescence. They have been detected huge deformations of the illite-chlorite XRD 10 A peak by isothermal scanning (i.e., 95°C) of XRD under thermo-photonic stimulation and electronic control of temperature. These large anomalies of the illite crystallinity peak working with aliquots of slate prevent against the petrological use of the Kubler index using the slaty cleavage of micro-mono-blocks. The slate also shows a continuous distribution of thermoluminescence traps with maxima concentration in 300°C, which could be attributed to the creation-annihilation of [AlO 4 /alcali] + y [AlO4]° centres. The convergent trend of thermoluminescence glow curves of preheated aliquots, circa 475°C, coincides with the theoretical maxima of dehydroxylation temperature.

XRD Measurement of Mean Crystallite Thickness of Illite and Illite/Smectite: Reappraisal of the Kubler Index and the Scherrer Equation

AbstractThe standard form of the Scherrer equation, which has been used to calculate the mean thickness of the coherent scattering domain (CSD) of illite crystals from X-ray diffraction (XRD) full width data at half maximum (FWHM) intensity, employs a constant, Ksh, of 0.89. Use of this constant is unjustified, even if swelling has no effect on peak broadening, because this constant is valid only if all CSDs have a single thickness. For different thickness distributions, the Scherrer “constant” has very different values.Analysis of fundamental particle thickness data (transmission electron microscopy, TEM) for samples of authigenic illite and illite/smectite from diagenetically altered pyroclastics and filamentous illites from sandstones reveals a unique family of lognormal thickness distributions for these clays. Experimental relations between the distributions’ lognormal parameters and mean thicknesses are established. These relations then are used to calculate the mean thickness of CSDs for illitic samples from XRD FWHM, or from integral XRD peak widths (integrated intensity/maximum intensity).For mixed-layer illite/smectite, the measured thickness of the CSD corresponds to the mean thickness of the mixed-layer crystal. Using this measurement, the mean thickness of the fundamental particles that compose the mixed-layer crystals can be calculated after XRD determination of percent smectitic interlayers. The effect of mixed layering (swelling) on XRD peak width for these samples is eliminated by using the 003 reflection for glycolated samples, and the 001, 002 or 003 reflection for dehydrated, K-sa-turated samples. If this technique is applied to the 001 reflection of air-dried samples (Kubler index measurement), mean CSD thicknesses are underestimated due to the mixed-layering effect.The technique was calibrated using NEWMOD©-simulated XRD profiles of illite, and then tested on well-characterized illite and illite/smectite samples. The XRD measurements are in good agreement with estimates of the mean thickness of fundamental particles obtained both from TEM measurements and from fixed cations content, up to a mean value of 20 layers. Correction for instrumental broadening under the conditions employed here is unnecessary for this range of thicknesses.

Thermal structure and uplift of the Cretaceous Shimanto Belt, Kii Peninsula, Southwest Japan: An illite crystallinity and Mite bo, lattice spacing study

Abstract Illite crystallinity (IC) and illite b, lattice spacing were measured across the Cretaceous Shimanto Belt, Kii Peninsula, Southwest Japan. For the IC survey, 103 samples of argillaceous rocks were analyzed from the central area and the western area of the belt. Values of IC (Kubler Index) vary between 0.28 and 0.71 Δ°2θ and indicate diagenetic and anchizone metamorphism respectively. The IC distribution reveals two contrasting patterns of thermal maturity. The Hanazono Formation, exposed in the northern area of the belt, generally dips north, but IC values increase systematically from 0.28 Δ°2θ in the north to 0.54 Δ°2θ in the south and indicate an inverted thermal structure. Values in other formations vary widely in the southern area of the belt ranging between 0.45 and 0.71 Δ°2θ, but the values do not show any systematic change from north to south and on average remain almost constant. Illite bo, lattice spacing values for 56 samples vary between 9.006 and 9.041 Å corresponding to intermediate pressure conditions of the metamorphic facies. These values, combined with paleotemperatures estimated from IC, indicate paleogeothermal gradients of 22 and 31°C/km for the northern and southern areas of the belt, respectively. The inverted thermal structure of the Hanazono Formation, together with a lower paleogeothermal gradient, possibly is a result of the subduction of a relatively cold oceanic plate during the Late Cretaceous. The higher geothermal gradient could be a product of late thermal overprinting caused by the later subduction of a comparatively younger and hotter oceanic plate during the Eocene.

Characterization and dating of argillic alteration in the Mercur gold district, Utah

The Mercur gold district of north-central Utah includes several sediment-hosted disseminated gold deposits which are located in the lower member of the Mississippian Great Blue Limestone. Argillic alteration of host limestone consists of illite (R3 illite-smectite <10% S) + kaolinite + quartz + or - Fe oxides or pyrite. Argillized limestone has identical clay mineralogy in both oxidized and unoxidized rock. Unlike some other sediment-hosted disseminated gold deposits, variations in the Kubler index and illite/kaolinite ratios show no spatial relationship to faults or to gold distribution within the mineralized areas. Hydrothermally altered Long Trail Shale, the middle member of the Great Blue Limestone, also occurs within the Mercur district and surrounding region. The shale varies in mineralogy from an assemblage consisting of R1-type illite-smectite (20-35% smectite) + chlorite + kaolinite + quartz + pyrite or Fe oxides to an assemblage consisting of R3-type illite-smectite (5-10% smectite) + kaolinite + quartz + pyrite or Fe oxides. Shale with R3 illite-smectite is most highly altered and is mineralogically and chemically very similar to the highly altered limestone within the gold deposits. Both the altered shale and the altered limestone contain veins of R3 illite-smectite + kaolinite + or - quartz and pyrite or Fe oxides. Additional hydrothermal features exhibited by both shale and limestone are felted and interlocking clay mineral textures, radiating clay mineral habits, crosscutting and replacement clay mineral textures, and fossil replacement by clay minerals and quartz. Ten K-Ar ages of illite-rich clay-size separates of argillically altered and mineralized limestone from the Mercur deposits range from 98.4 to 226 Ma. None of the hydrothermal illites in the deposit have Tertiary ages. These ages are similar to a range of Mesozoic ages previously obtained for vein and matrix clay in the Long Trail Shale. Most of these ages are interpreted as mixed ages resulting from overprinted thermal and/ or mass transfer events. Completely argillized and highly mineralized limestone obtained from the Carrie Steele fault yields an age of 152 + or - 4 Ma (0.03-0.3-mu m-size fraction) that is interpreted as the best estimate...

Timing of the cataclastic deformation along the Akaishi Tectonic Line, central Japan

This paper describes K-Ar ages of cataclasites and fault gouges from the Akaishi Tectonic Line (ATL), central Japan. Petrological and mineralogical features of these rocks are also examined. Based on the results, we test the hypothesis that the K-Ar ages of these rocks represent the age of hydrothermal alteration associated with the fault movement. Intensity of deformation and alteration increase passing from host rock into cataclasite and finally into fault gouge. This increase corresponds to an increase of the value of crystallinity index (Kubler index) of the micaceous minerals contained in these rocks. Furthermore, the degree of rejuvention of K-Ar ages of the micaceous minerals increases in the same sense. A correlation of K-Ar ages (plotted on the y axis) versus Kubler index (plotted on the x axis) yields a concave curve asymptotically parallel with the x axis at approximately 15 Ma. This curve is interpreted to represent the mode of decrease of the relative amounts of inherited argon in the K-Ar system of the micaceous minerals, corresponding to an increase in the intensity of deformation and alteration. Inclination of the curve becomes zero if all the inherited argon is lost from the K-Ar system of the micaceous minerals. Thus, it is concluded that the hydrothermal alteration occurred at approximately 15 Ma. The strike-slip basin along the ATL formed the middle Miocene. The K-Ar dating of ATL gouges indicates that the ATL was active simultaneously with formation of the strike-slip basin along its trace during the middle Miocene.

Effect of the shape of fundamental particles on XRD characteristics of illitic minerals

Chemical and electron-microscope data (Pt-shadowing and HRTEM) on monomineral illitic samples are used to show that XRD peak positions and peak intensities related to the swelling characteristics of the samples strongly depend on the size of the mixed-layer crystals, while the Kubler index (the half-height width of the 001 reflection) of the samples recorded in Na-form is related to the thickness of fundamental particles. The ability of fundamental particles to associate face-to-face into mixed-layer crystals depends on geometric factors: particles of a given thickness are more prone to form mixed-layer crystals inducing more smectitic XRD characteristics if they have flat surfaces, a large diameter and are not elongated. The variability of particle morphologies detected in natural samples explains the lack of a good correlation between the Kubler index and the parameters reflecting the swelling ability of illites such as the percentage of smectite layers or the Ir index of Srodon (1984). In swelling clays, the thickness of coherent scattering domains, calculated from the Kubler index by means of the Scherrer equation, is larger than the mean thickness of the fundamental particles

Mineralogía de arcillas y grado de diagénesis del Cretácico inferior, en el flanco sur del anticlinorio de Bilbao

The lower Cretaceous of the Bilbao Anticline is built up of a thick sequence of sediments. Three main sedimentary systems are been defined traditionally: Wealdian, Urgonian, and Supraurgonian ones. Pelitic facies were collected from four crossing sections to the structure. XRD was used to analyse the distribution of clay minerals, Kubler index (KI) and some illite crystallochemical parameters. Clay minerals are strongly source controlled. Illite/smectite interstratified distribution and KI, indicated a diagenetic grade which is increasing with deep of buria!' Middle and deep diagenetic grades are been identified and anchimetamophism in the most deeply buried samples of the central area, with no schistosity. Bearing in mind the maximum burial depth and vitrinite reflectance values, unusual presence of a R1 illite/smectite interstratified mineral has been detected in some samples.

The influence of strain, lithology and stratigraphical depth on white mica (illite) crystallinity in mudrocks from the vicinity of the Corris Slate Belt, Wales: implications for the timing of metamorphism in the Welsh Basin

AbstractLow and very low grade metamorphism in a mudrock-dominated succession, ranging from Upper Cambrian to Wenlock, is characterized by the white mica (illite) crystallinity technique. Kubler indices indicate that grade ranges from late diagenetic to low epizonal. The prograde reaction most easily observed by XRD study is the progressive ordering of K/Na micas; diagnostic2M1polytypic reflections become recognizable at 0.4 Δ 2θ and strengthen as grade advances. Grade generally correlates with thickness of overburden in the &gt; 9 km thick succession. However, strain exercises additionally a profound influence on grade, and the distribution of strain was related to décollement resulting from inhomogeneities and ductility contrasts within the succession. Strain energy is seen as an additional and important variable in the development of white mica crystallinity in slates. The distribution of metamorphic grade implies that the Lower Palaeozoic succession was augmented by several kilometres of Lower Devonian (Old Red Sandstone) strata at the metamorphic culmination. The culmination was syntectonic and represents an Acadian rather than an early diastathermal event. Subsequent dextral movement on the Bala Fault displaced the regional isocryst pattern byc.4 km. Retrogressive smectite appears to be localized to the vicinity of the Bala Fault and is probably of Variscan age.

A transmission electron microscope study of white mica crystallite size distribution in a mudstone to slate transitional sequence, North Wales, UK

High-resolution transmission electron microscopy (HRTEM) measurements of the thickness of white mica crystallites were made on three pelite samples that represented a prograde transition from diagenetic mudstone though anchizonal slate to epizonal slate. Crystallite thickness, measured normal to (001), increases as grade increases, whereas the XRD measured 10 Å peak-profile, the Kubler index, decreases. The mode of the TEM-measured size population can be correlated with the effective crystallite size N(001) determined by XRD. The results indicate that the Kubler index of white mica crystallinity measures changes in the crystallite size population that result from prograde increases in the size of coherent X-ray scattering domains. These changes conform to the Scherrer relationship between XRD peak broadening and small crystallite size. Lattice ‘strain’ broadening is relatively unimportant, and is confined to white mica populations in the diagenetic mudstone. Rapid increases in crystallite size occur in the anchizone, coincident with cleavage development. Changes in the distribution of crystallite thickness with advancing grade and cleavage development are characteristic of grain-growth by Ostwald ripening. The Kubler index rapidly loses sensitivity as an indicator of metapelitic grade within the epizone.

Low grade metamorphism of the Manx Group, Isle of Man: a comparative study of white mica ‘crystallinity’ techniques

Studies have been made in two laboratories on metapelites from the Manx Group, Isle of Man, showing the distribution of white mica ‘crystallinity’, using the Kubler index (Δ°2Θ) and the Weber index (Hb rel ). Two isocryst maps, which show broadly similar distributions of metamorphic grade, are illustrated and a correlation between the laboratories is presented. The mineralogy of the &lt;2 m fraction in relation to ‘crystallinity’ has been determined. The distribution of metamorphic grade over central and eastern parts of the Isle of Man, as depicted by the Kubler index isocryst map, is related to subjacent granite plutons, but these contact metamorphic effects overprint an earlier, largely anchizonal, regional metamorphism. Localized, late, retrograde effects are related to low temperature hydrothermal mineralization and to cold re-working in a large fault zone. Differences between the maps are largely in the degrees of resolution which stem directly from the different sampling intervals employed. Comparisons between the maps suggest a sampling interval of 1-2 km is adequate in regions of simpler geology; but that an interval of &lt;1 km may be necessary in more complex regions. Well resolved maps can be produced only when the sampling interval is linked to geology. The changes in the mineralogy of the &lt;2 m fraction in relation to grade are closely similar to those described from Lower Palaeozoic metapelites from North Wales, although higher grades have been reached in the Manx Group with the development of biotite.

Beyond the Kubler Index

AbstractThe value of peak width at half-height for the illite 001 XRD reflection is known as the Kubler index or the illite ‘crystallinity’ index. This measurement, which has been related to the degree of metamorphism of very low-grade, pelitic rocks, is a function of at least two crystal-chemical factors: (1) illite X-ray scattering domain size; and (2) illite structural distortions (especially swelling). Reynolds' NEWMOD computer program is used to construct a grid with which these two contributions to illite peak width can be determined independently from measurements of the 001 peak width at half-height and the Środoń intensity ratio. This method yields more information about changes undergone by illite during metamorphism than application of the Kubler index method alone.

Low grade metamorphism in the Lower Ordovician Skiddaw Group of the Lake District, England

SUMMARY A white-mica crystallinity (illite crystallinity) survey of the Lower Ordovician Skiddaw Group of the Lake District has revealed a complex pattern in which Kubler Index values range from &lt;0.21 Δ°2θ (low epizone) to &gt;0.42 Δ°2θ (diagenetic), large areas of the outcrop having anchizonal values (0.21–0.42 Δ°2θ). The &lt;2 μ m mineralogy of the higher grade rocks is dominated by 2M 1 muscovite and chlorite, and interlayered paragonite-muscovite is common; lower grade rocks contain 1Md illite, chlorite and interlayered illite-smectite, in some cases accompanied by pyrophyllite and, rarely, kaolinite. The pattern of variation can be related to the metamorphic history of the area in which anchizonal burial metamorphism during the Silurian was superseded by Lower Devonian (Acadian) upper anchizonal to epizonal regional metamorphism in areas in which slaty cleavage is relatively well developed. The contrast between these latter areas and those retaining the burial signature was heightened by late Devonian uplift and faulting. Sub-volcanic metamorphism during Caradocian volcanism is also considered likely to have attained anchizonal grades. Numerous additional variations within the pattern are related to interference with the 10Å muscovite diffraction peaks by interlayered paragonite and smectite, induration by minor intrusions and movement on Caledonian and post-Carboniferous faults.