Detrital Illite Research Articles

Complex hydrothermal alteration and illite K–Ar ages in Upper Visean molasse sediments and magmatic rocks of the Variscan Badenweiler-Lenzkirch suture zone, Black Forest, Germany

Post-collisional Upper Visean molasse sediments and magmatic rocks of the Badenweiler-Lenzkirch Zone reveal by microscopy of thin sections different degrees of hydrothermal illitization of feldspar and mica particles, and XRD, IR and XRF data of the <2 µm fractions show illitic material as the dominant clay mineral consisting of a mixture of 1M and 2M1 polytypes. Moreover, small amounts of illite/smectite mixed-layer minerals of R1-ordering are proved in the granites. In the separates, two illite mixing lines with different Fe + Mg contents are verified between authigenic illite from feldspar alteration and detrital illite in the molasse sediments, as well as between authigenic illite from feldspar alteration and altered mica flakes in the granites. Fe-rich detrital chlorite is present within the molasse sediments, while mixtures of high aluminous Fe-poor dioctahedral/di-trioctahedral chlorite with randomly interstratified chlorite/smectite mixed-layer minerals are formed from feldspar alteration in the granites. Illite K–Ar dating of the <2 and <0.63 µm fractions yields hydrothermal illitization of feldspar and partial resetting of the K–Ar system of detrital illite and mica flakes in the molasse sediments at ≥200 °C during Upper Permian to Middle Triassic times, while the granites in the eastern part of the study area were not altered contemporaneously. In contrast, hydrothermal activity at ≤200 °C during Upper Jurassic to Lower Cretaceous times occurred in the granites, whereas these temperatures were too low for resetting the older ‘Permo–Triassic’ illite K–Ar ages in the molasse rocks. Within both K–Ar age clusters, the data are seen to decrease with grain size and portion of illite 2M1 polytype. The alteration phenomena indicate multiple hydrothermal episodes in the study area, and they match those from the Central and Western European crust as fluid supply was controlled geodynamically by episodic break up of Pangea.

Texture development in naturally compacted and experimentally deformed silty clay sediments from the Nankai Trench and Forearc, Japan

The petrophysical properties of fine-grained marine sediments to a large extent depend on the microstructure and crystallographic preferred orientations (CPOs). In this contribution we show that Rietveld-based synchrotron texture analysis is a new and valuable tool to quantify textures of water-saturated fine-grained phyllosilicate-rich sediments, and assess the effects of compaction and tectonic deformation. We studied the CPO of compositionally almost homogeneous silty clay drillcore samples from the Nankai Accretionary Prism slope and the incoming Philippine Sea plate, offshore SW Japan. Basal planes of phyllosilicates show bedding-parallel alignment increasing with drillhole depth, thus reflecting progressive burial and compaction. In some samples calcite and albite display a CPO due to crystallographically controlled non-isometric grain shapes, or nannofossil tests. Consolidated-undrained experimental deformation of a suite of thirteen samples from the prism slope shows that the CPOs of phyllosilicate and calcite basal planes develop normal to the experimental shortening axis. There is at least a qualitative relation between CPO intensity and strain magnitude. Scanning electron micrographs show concurrent evolution of preferred orientations of micropores and detrital illite flakes normal to axial shortening. This indicates that the microfabrics are sensitive strain gauges, and contribute to anisotropic physical properties along with the CPO.

Fault gouge dating in the Southern Appalachians, USA

Illite age analysis (IAA), the method of comparing radiometric ages of successive size fractions with varying percentages of detrital illite, has been successfully applied to several rock types, including fault gouge, shales and argillaceous limestones. IAA results are presented for fi ve fault rocks, including four clay gouges and one cataclasite, from the exhumed Southern Appalachian foreland fold-thrust belt (eastern United States). Determining detrital versus authigenic illite is now an established procedure, utilizing X-ray analysis to quantify illite polytypes. Less established, however, is how to interpret 40 Ar/ 39 Ar ages as a function of diagenetic grade. Both a total gas age, incorporating the recoiled argon fraction after irradiation, and a retention age (omitting the recoiled fraction) are obtained for a sample. We relate their respective use to diagenetic grade and, specifi cally, the crystallite thickness of illite. When measured crystallite sizes are equal to or smaller than that calculated from Ar recoil we use total gas ages (here ~5 nm), while for crystallites greater than ~10 nm we use retention ages. The four clay gouge ages are all the same within error and much younger than that of the cataclasite. We conclude that a major period of frontal Appalachian fault activity occurred during the early Permian (Cisuralian, 276–280 Ma) and that these foreland faults were active simultaneously as part of an internally deforming, regional thrust wedge.

Heterogeneous Packing and Quartz Cementation of the Eureka Quartzarenite (Middle Ordovician), Utah and Nevada, U.S.A

Abstract The Eureka Sandstone is a quartzarenite 200 m thick that was deposited on the eastern shelf of the Cordilleran foreland basin from Canada to California. It has many characteristics of other lower Paleozoic quartzarenites in terms of its sheet-like geometry, mineralogical and textural maturity (99.5% detrital quartz, almost free of detrital clay), shallow-marine environment of deposition, and abundance of quartz cement (10–26%). However, it displays wide variations in compaction as measured by intergranular volume (IGV range of 16%) that are not all related to differences in burial depth plus IGV values as low as 10%. Sutured grain contacts and fractured grains cannot account for such abnormally low IGV values, which suggests that dissolution of the edges of grains where they overlapped their neighbors (the Fuchtbauer effect), leaving minimal evidence, was a major process of compaction. The normal evolution of microquartz overgrowths ( 10 mm) with time was retarded; consequently microquartz and mesoquartz cement (80%) dominate over macroquartz (20%) and are the only cements in the poorest-cemented beds and laminations. Illite co-precipitated with micro- and mesoquartz and impeded quartz cementation by coating quartz crystal faces. Typical macrocrystalline quartz was the final cement only in illite-poor samples. Heterogeneous illite distribution may reflect heterogeneous abundance of K-feldspar or detrital illite that possibly served as seed crystals for authigenic illite. In spite of reaching temperatures > 135° C for ~ 100 My, many Eureka sandstones, especially in Utah, remain incompletely and unevenly cemented. Consequently, beds range from weakly friable to pervasively quartz-cemented “quartzite” in the same outcrop. Porosity was generally reduced to less than 2% except in rare laminations where it reaches 22%. Grain size had some influence on the amount of quartz cement locally, but the chief cause of cement heterogeneity appears to have been illite abundance. The most likely major source of silica for quartz cement was quartz grain dissolution at shale beds and clay drapes, although documentation is absent. Intergranular grain dissolution was a minor source of silica; overlap quartz averages less than 4%. Oxygen isotopic data are compatible with the precipitation of quartz either from diagenetically evolved meteoric water that invaded the sandstones from tectonic highlands to the west or evaporative brines.

Another insight into illitization by K-Ar dating of micro- to nano-metric illite-type particles exchanged with alkylammonium cations

AbstractMicrometric and nanometric illite-rich size fractions of claystones, bentonites and shales were exchanged with alkylammonium cations that have the specificity of stoichiometrically replacing K in trioctahedral mica interlayers. The purpose of the study was a separate evaluation of the K-Ar ages of potentially dioctahedral residual particles not affected by alkylammonium leaching and of potentially trioctahedral illites that were exchanged by the organic molecules.The K-Ar ages of micrometric size fractions from an Estonian Blue Clay sample collected next to another studied previously show that, even if identical in their mineralogical characteristics, the fractions contain variable amounts of trioctahedral particles that are of different origins. The alkylammonium treatment modifies slightly, within analytical uncertainty, the K-Ar ages of the &lt;0.1 and 0.1–0.4 mm size fractions from, respectively, 464±13 and 530±14 Ma before organic exchange to 480 ± 11 and 546±12 Ma after. The K-Ar ages of alkylammonium exchanged nanometric size fractions from the rim and centre of a thick Upper Cretaceous bentonite bed in Montana suggest that trioctahedral illite-rich particles mineralogically and chemically homogeneous and about 30 Ma old precipitated next to older dioctahedral particles of ∼60–65 Ma. The untreated mixtures consist of two generations of authigenic illite having apparently different di/trioctahedral layerings. The same type of authigenic di/trioctahedral illite layering could be demonstrated for nanometric illite particles of a bentonite bed from the East Slovak Basin, one size fraction appearing to even consist of a pure trioctahedral illite as the alkylammonium exchange emptied completely the illite interlayers. The nearby shale level consisted of detrital illite particles that were found to be of different ages, di/trioctahedral layerings and therefore varied origin.K-Ar ages of alkylammonium exchanged micrometric to nanometric illite and illite-smectite mixed layers, either increasing or decreasing, appear to outline variable di/trioctahedral layering assemblages or independent particle mixtures resulting from a more complex smectite illitization process than the conventionally assumed homogeneous reaction. It could record changing chemical compositions of the interacting pore fluid during crystallization, even when illitization progressed slowly. Similar ages before and after alkylammonium exchange suggest a constant chemical composition and therefore an homogeneous dioctahedral crystal structure. Alternatively, a changing chemical composition of the fluids during illitization is potentially recorded by variable K-Ar ages of the alkylammonium-leached illite resulting from differentiated ion exchanges.

MINERALOGY AND DIAGENESIS OF INTERSTRATIFIED I/S IN THE TERTIARY YEONIL SEDIMENT, SE KOREA

Mineralogical and chemical examinations were made on interstratified illite/ smectite in the mudstones taken from three drill holes penetrating the Tertiary Yeonil Group to study its reaction during diagenesis. The Yeonil mudstones are dominated by large amounts of the randomly (R = 0) interstratified illite/smectite with a maximum illite layer of up to 38%. Chemically the interstratified I/S was formed by input of K and Al ions originated from the destruction of detrital illite and kaolinite in compensate for the release of water and exchangeable cations from interlayers during burial diagenesis. This can be represented simply by the following reaction. 4.5K + 8Al 3+ + smectite → illite + 3Si 4+ + H 2 O + Na, Ca However, the illitization terminated at the randomly interstratified I/S with illite layers of 38% because the Yeonil Group has shallow burial depth. On the other hand, R = 1 ordered I/S appears in only one sample from the deepest depth of the F hole with different occurrence and organic content from the mudrocks including the randomly interstratified I/S. Judging from the high potassium content and the deeper part of the hole, the illitization may be accelerated further. The Yeonil Group shows a very low degree of the diagenesis, probably result from the shallow burial depth. The maximum temperature of the Yeonil Group is estimated at 50- 70°C based on the illite/smectite diagenesis and organic maturation.

K-Ar dating of illites for time constraint on tectonic burial metamorphism of the Jurassic Nampo Group (West Korea)

The timing of tectonic burial metamorphism of the early Mesozoic Nampo Group (western Korea) related to the Jurassic Daebo orogeny was first constrained by K-Ar dating of illite in sedimentary rocks. Two shale samples show a clear decreasing trend of both the amount of detrital component and K-Ar age of illites with decreasing grain size from >4 μm to 4–2 μm, and to <2 μm fractions. This observation most likely reflects a binary mixture of old detrital 2M1 and young authigenic 1Md illites, usually seen in argillaceous sedimentary rocks. In this sense, the latest illitization attributable to the crustal loading can be estimated to be 157–140 Ma, the extrapolated illitic age inferred from a linear regression between the amount of detrital component and apparent K-Ar age of different size fractions. This result suggests that the maximum tectonic burial metamorphism of the Nampo Group has occurred in the last stage of the Daebo orogeny.

Upper Cretaceous feldspars in the Cenozoic Limagne Basin: A key argument in reconstructing the palaeocover of the Massif Central (France)

The northern part of the N–S-trending basin of the Limagne graben, located in the northern part of the Massif Central (France), contains Oligocene and Miocene calcareous lacustrine deposits. The area is known for its stromatolite reefs surrounded by marl, clay and uncemented carbonate sand with oncolites. Because the origin of the carbonate accumulated by cyanobacteria has long been debated, the clayey deposits associated with the stromatolites were analysed using XRD, SEM and K–Ar dating methods. The results show a monotonous assemblage of carbonates, dioctahedral smectite, illite–smectite (Reichweite) R = 0, glauconitic illite and abundant orthoclase. Detrital minerals such as quartz and detrital illite are found near the basin's borders but are very scarce in its central part, indicating a lack of detrital input from the regional basement. Hence, the abundant orthoclase found in all the clayey deposits of the Limagne Basin cannot be linked to erosion of the surrounding Hercynian basement. The SEM analysis showed the orthoclase crystals to have a tabular morphology with sharp boundaries and to be embedded within a clay matrix, whilst the K–Ar dating of the orthoclase and glauconitic illite yielded ages ranging from 90 to 66 ± 2 Ma, i.e. Turonian to Maastrichtian. These two results clearly indicate that the fill of the Limagne Basin, composed mainly of carbonate, clay and orthoclase, must have been partly due to the erosion of an Upper Cretaceous sedimentary blanket once covering the Massif Central basement. This cover was probably constituted of chalk, dioctahedral smectite, I/S and glauconitic illite, associated with authigenic orthoclase and flint. Flints are found reworked in the alluvial formations of the Late Pliocene Lower Bourbonnais sands and clays (‘Sables et argiles du Bourbonnais’) and in the Pleistocene terraces. Moreover, relicts of the Upper Cretaceous cover were preserved during the sedimentary filling of the Cenozoic Limagne Basin. Authigenic orthoclase of similar shape has been described in the Chalk Group of the Paris Basin and from localities in northern France and Belgium. The mineral is rare in the Turonian chalk but increases rapidly at the base of the Coniacian to Campanian chalk; it has also been noted that the number of orthoclase crystals increases as the quantity of clastic components decreases. Similar orthoclase crystals were found associated with dioctahedral smectite in the decarbonated residues of the Coniacian to Santonian chalk from Fécamp and Etretat (northern France). Moreover, our results are consistent with other data from within and around the Paris Basin, such as the palaeogeography and facies distribution of the Chalk formations, the residual flints in the clay-with-flints of the southern Paris Basin and around the Morvan, and apatite fission-track thermochronology data from the Hercynian basement of the Massif Central and the Morvan. All these data indicate a major connection between the Paris Basin and the Tethys, and an extensive palaeocover on the Massif Central and Morvan basement.

A model for fibrous illite nucleation and growth in sandstones

We have developed a model for the formation of fibrous illite in sandstones where kaolinite is a primary reactant and potassium is derived from in-situ K-feldspar grain dissolution or imported into the model reference frame. Illite fiber nucleation and growth are modeled using Arrhenius expressions that consider saturation state in addition to temperature and time. Nucleation occurs on pore walls, and muscovite and detrital illite may be defined as energetically favorable substrates. The model is integrated with other Touchstone™ models to account for the influence of other diagenetic processes on surface area and reactant volumes and to provide input for permeability simulations. We evaluated the illite model performance on two data sets: (1) Jurassic quartzose samples from offshore mid-Norway with maximum temperatures ranging from 108 to 173C (226 to 343F) and (2) Miocene lithic samples from offshore Southeast Asia that have maximum temperatures ranging from 157 to 182C (315 to 360F). The model matches measured abundances of illite, kaolinite, and K-feldspar in both data sets using identical kinetic parameters. Predicted K-Ar ages are consistent with available data given uncertainties associated with detrital contaminants. Although no illite particle-size data are available from the analyzed samples, modeled crystallite thicknesses from the Norway data set are comparable to published measurements of 0.004 to 0.012 m from North Sea samples with similar temperature histories.

Closing the Clymene ocean and bending a Brasiliano belt: Evidence for the Cambrian formation of Gondwana, southeast Amazon craton

Research Article| March 01, 2010 Closing the Clymene ocean and bending a Brasiliano belt: Evidence for the Cambrian formation of Gondwana, southeast Amazon craton E. Tohver; E. Tohver * 1School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia *E-mail: etohver@cyllene.uwa.edu.au. Search for other works by this author on: GSW Google Scholar R.I.F. Trindade; R.I.F. Trindade 2Instituto de Astronomia e Geofísica, Universidade de São Paulo, Rua Maria Antônia, 294 São Paulo, SP 01222010, Brazil Search for other works by this author on: GSW Google Scholar J.G. Solum; J.G. Solum 3Shell Exploration, Bellaire Technology Center, Shell International Exploration and Production, 3737 Bellaire Blvd, Houston, Texas 77025, USA Search for other works by this author on: GSW Google Scholar C.M. Hall; C.M. Hall 4Department of Geological Sciences, University of Michigan, 2534 C.C. Little Bldg., Ann Arbor, Michigan 48104, USA Search for other works by this author on: GSW Google Scholar C. Riccomini; C. Riccomini 5Instituto de Geociências, Universidade de São Paulo, Rua Maria Antônia, 294 São Paulo, SP 01222010, Brazil Search for other works by this author on: GSW Google Scholar A.C. Nogueira A.C. Nogueira 6Instituto de Geociências, Universidade de Pará, Rua Augusto Corrêa, 01-Guamá, CEP 66075-110, Brazil Search for other works by this author on: GSW Google Scholar Geology (2010) 38 (3): 267–270. https://doi.org/10.1130/G30510.1 Article history received: 08 Jul 2009 rev-recd: 19 Oct 2009 accepted: 20 Oct 2009 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation E. Tohver, R.I.F. Trindade, J.G. Solum, C.M. Hall, C. Riccomini, A.C. Nogueira; Closing the Clymene ocean and bending a Brasiliano belt: Evidence for the Cambrian formation of Gondwana, southeast Amazon craton. Geology 2010;; 38 (3): 267–270. doi: https://doi.org/10.1130/G30510.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We report new paleomagnetic and geochronological data from Ediacaran rift-drift carbonates in the Paraguai belt at the southern end of the suture zone between the Amazon craton and the São Francisco and Rio de Plata cratons, South America. Early thrusting resulted in remagnetization ca. 528 ± 36 Ma or later; the mean age is established by 40Ar/39Ar encapsulation dating of mixed authigenic and detrital illite from remagnetized carbonates from the unmetamorphosed fold-thrust belt. This remagnetization overlaps with a 525 Ma Gondwana reference pole. Metamorphic illite from the slate belt yields 40Ar/39Ar ages of 496–484 Ma, the timing of peak regional metamorphism. Oroclinal bending of the Paraguai belt was caused by a 90° clockwise rotation of the east-west limb after ca. 528 Ma, probably reflecting the irregular margin of the southeast Amazon craton. The age of the Paraguai belt overlaps with that of the Pampean orogeny farther south along the western margin of the Rio de Plata craton, suggesting a coeval closure for the Clymene ocean separating the Amazon craton from the São Francisco and Rio de Plata cratons. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

K-Ar dating of extensional fault gouge from the Yi-Shu segment of the Tan-Lu fault zone

Two grabens were developed in the Yi-Shu segment of the Tan-Lu fault zone (TLFZ) during its extensional activities, and are now confined by four major NNE-trending normal faults and filled with Cretaceous sediments. These faults were developed due to their reactivities, containing gouge and cutting the graben sediments. Detailed fieldwork demonstrates that the faults experienced sinistral transtensional moment related to regional NE-SW extension during the reactivity. X-ray diffraction (XRD) analysis of the finest gouge samples gives illite crystallinity values higher than 0.42°Δ2θ, indicating temperatures experienced by the gouge samples were less than 150°C. From the relation between K-Ar data and proportions of detrital illite in different size fractions of the gouge samples, we conclude that refaulting for the western boundary fault of the TLFZ, abbreviated to F4, took place at ca. 90 Ma and for the eastern boundary fault, abbreviated to F1, happened from 70 to 60 Ma. During the two phases of reactivity imposed by the same NE-SW extension, the TLFZ experienced uplifting and no sediments were deposited in the two grabens. It is suggested that the TLFZ experienced extension during the Late Cretaceous, which supports the inference that lithospheric thinning was still undergoing in the east of the North China Craton during the Late Cretaceous magmatic hiatus.

Extraction of diagenetic and detrital ages and of the 40Kdetrital/40Kdiagenetic ratio from K-Ar dates of clay fractions

AbstractIllite age analysis (IAA) is a classical method for extracting diagenetic and detrital ages from mixed ages measured by K-Ar. This approach is based on measuring the masses of diagenetic and detrital illitic components in a few different grain-size fractions of one rock sample and measuring the mixed ages of these fractions. The 1Md illitic polytype is usually considered to be diagenetic, while 2M1 is considered detrital. A plot of the function: exp(λt)−1 (where t is time and λ is the decay constant) vs. weight percent of the detrital fraction is constructed. On the basis of linear extrapolation to end-member fractions, the diagenetic and the detrital age is obtained. This approach does not take into account various K contents in different polytypes (%Kdetrital and %Kdiagenetic). In order to do that, the detrital mass fraction (wt.%detrital) should be recalculated into the percentage of detrital K (%Id(K)): Analytical constraint of the K content of different polytypes is very difficult, so a new approach to this problem has been developed. In the present study, the plot of 40Ar*/40K vs. %Id(K) for a precisely determined ratio of 40Kdetrital/40Kdiagenetic was observed to be linear. On the basis of this observation, a computer program, MODELAGE, was written in the Java programming language using as input a few measured detrital illite mass fractions along with the mixed K-Ar ages of the relevant grain fractions. It then calculates the end-member ages and the 40Kdetrital/40Kdiagenetic ratio using genetic algorithms.The errors in diagenetic and detrital illite mass-fraction determination mean that the 40Kdetrital/40Kdiagenetic ratio and the end-member ages can be evaluated only with some uncertainty. The best results are obtained if the measured mass fractions represent a relatively broad range. Constraining one of the unknowns (particularly the 40Kdetrital/40Kdiagenetic ratio) improves the results significantly.Evaluation of data obtained from the literature using the proposed approach leads to the conclusion that the 40Kdetrital/40Kdiagenetic ratio is often &gt;1.00, and some of 1Md illite polytype materials may be of detrital origin. If this is not the case, if a broad range of mass fractions is covered, and if the differences between end-member ages are relatively small, IAA analysis still gives appropriate results, even if the true 40Kdetrital/40Kdiagenetic ratio is different from 1.00.

Evidence from Middle Ordovician paleosols for the predominance of alkaline groundwater at the dawn of land plant radiation

Research Article| January 01, 2009 Evidence from Middle Ordovician paleosols for the predominance of alkaline groundwater at the dawn of land plant radiation Pierre Jutras; Pierre Jutras * 1Department of Geology, Saint Mary's University, Halifax, Nova Scotia B3H 3C3, Canada *E-mail: pierre.jutras@smu.ca Search for other works by this author on: GSW Google Scholar Ryan S. Quillan; Ryan S. Quillan 1Department of Geology, Saint Mary's University, Halifax, Nova Scotia B3H 3C3, Canada Search for other works by this author on: GSW Google Scholar Matthew J. LeForte Matthew J. LeForte 1Department of Geology, Saint Mary's University, Halifax, Nova Scotia B3H 3C3, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information Pierre Jutras * 1Department of Geology, Saint Mary's University, Halifax, Nova Scotia B3H 3C3, Canada Ryan S. Quillan 1Department of Geology, Saint Mary's University, Halifax, Nova Scotia B3H 3C3, Canada Matthew J. LeForte 1Department of Geology, Saint Mary's University, Halifax, Nova Scotia B3H 3C3, Canada *E-mail: pierre.jutras@smu.ca Publisher: Geological Society of America Received: 13 Apr 2008 Revision Received: 18 Aug 2008 Accepted: 09 Sep 2008 First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2009 Geological Society of America Geology (2009) 37 (1): 91–94. https://doi.org/10.1130/G25447A.1 Article history Received: 13 Apr 2008 Revision Received: 18 Aug 2008 Accepted: 09 Sep 2008 First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Pierre Jutras, Ryan S. Quillan, Matthew J. LeForte; Evidence from Middle Ordovician paleosols for the predominance of alkaline groundwater at the dawn of land plant radiation. Geology 2009;; 37 (1): 91–94. doi: https://doi.org/10.1130/G25447A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Atmospheric carbon is estimated to have been ~500 times more abundant in Hadean time than at present, and its concentration has been gradually decreasing since then due to its storage in sedimentary rocks. Consequently, rain pH has been gradually increasing through geologic time, leading to the common assumption that groundwaters are less acidic today than they were in the distant past. However, this assumption overlooks the fact that root-forming land plants increase the carbonic acid concentration in soils by one or two orders of magnitude. In the absence of rooted land plants, reactions between minerals and rainwater are known to promote alkalinity. It is hypothesized that groundwater pH must have been, on average, highest shortly before the Late Ordovician to Silurian proliferation of root-forming land plants. To verify this hypothesis, we studied the mineralogy and geochemistry of the youngest known pre-Silurian paleosols to have developed on primary rocks, and provide evidence that they have evolved in predominantly alkaline groundwaters despite warm and humid paleo-environmental conditions. Today, a lush vegetation cover would thrive in such a climate, and the system would be necessarily acidic due to large inputs of organic acids. Paired with previous observations indicating that early Paleozoic sedimentary rocks are especially rich in detrital illite and K-feldspar, there is now enough evidence to believe that there was a greater tendency for alkalinity during this time period than during previous and subsequent geologic periods. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Na-bearing white micas from Triassic rocks of the transition between the Maláguide and Alpujárride complexes (Betic Cordillera, Spain)

AbstractThe structural significance of micas with Na-K intermediate composition, and their chemical and structural evolution at increasing metamorphic grade have been investigated in Triassic rocks from the transition between the Maláguide and Alpujárride complexes (Internal zones of the Betic Cordillera, Spain). Micas were studied by X-ray diffraction (XRD) and by scanning and transmission electron microscopy (SEM/TEM). Three samples, belonging to the late diagenesis and to the low and medium anchizone, were selected for this study. Na-bearing mica appears as submicroscopic packets intergrown in parallel with K-mica, becoming more compositionally uniform with increasing grade. The diagenetic sample contains illite, minor paragonite, and two main populations of intermediate Na-K micas, with average compositions Ms60Prg40 and Ms35Prg65, respectively, where Ms represents muscovite and Prg, paragonite. The lattice-fringe images of mica packets with intermediate compositions suggest the presence of random mixed-layered paragonite-muscovite. Under low anchizonal conditions the amount of discrete paragonite increases and the Na-K intermediate mica has a mean composition of Ms40Prg60. The TEM images suggest that the packets with intermediate composition are solid solutions of paragonite and illite. Micas with Na-K intermediate composition are lacking in the sample with the highest metamorphic grade. In this sample, paragonite and muscovite coexist with mica, with composition intermediate between paragonite and margarite. The lattice-fringe images of these Na-Ca-bearing packets suggest that they consist of irregularly shaped domains enriched either in Na or in Ca.Our data indicate that Na+K-bearing micas have several origins: detrital stacks of K- and Na-bearing micas coexist with authigenic phases, formed from dickite in the diagenetic, coarse-grained samples, and perhaps from smectite-bearing mixed-layers or detrital illite, in the fine-grained rocks. The changes observed at increasing metamorphic grade can be related to the influence of the lithology, the metamorphic grade, and the different geological settings. Intermediate Na-Ca mica appears to have grown from paragonite, with calcite as the source of Ca.

Clay mineralogy across the P-T boundary of the Xiakou section, China: Evidence of clay provenance and environment

AbstractThe provenance of clays in shaley intervals across the Permian-Triassic boundary (PTB) in the Xiakou section was investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM), and the results suggest that the layers have three different provenances. The layer P267-b has a loose texture with an oriented arrangement of detrital clay particles, consisting mainly of illite and minor chlorite with irregular outlines or ragged edges. The dehydroxylation reaction of the clays in this layer is characterized by an intense overlapping endothermic effect at ∼600°C, produced by mixed-layer illite-smectite (I-S) consisting of a mixture of cis-vacant (cv)and trans-vacant (tv) octahedral sheets derived from weathering of detrital illite. Layer P259-b shows a more condensed texture with a dark color, and is composed mainly of I-S and minor illite and chlorite. Evidence for alteration of detrital materials to clay mineral aggregates was observed under SEM. Similar to layer P267-b, an intense dehydroxylation reaction occurs at ∼600°C, indicating clays consisting of a mixture of tv and cv sheets and, therefore, that the sediments were derived from a mixture of terrigenous and volcanic sources, combining the texture and the clay-mineral composition of those sediments. However, the undisturbed lamination and relatively small grain size in this bed indicate a low-energy depositional environment. The clay-mineral compositions of the other layers are mainly of I-S with minor amounts of illite and chlorite. Their endothermic dehydroxylation reaction, however, occurs mainly at ∼660°C, indicating that cv sheets are dominant in the clays, and thus, are derived from smectites of volcanic origin. Observations by SEM show that clay minerals grow at the expense of detrital materials, confirming the diagenetic alteration of volcanic ashes in marine sediments. Illite and chlorite are the detrital clay minerals in the clay layers across the PTB in the Xiakou section. The presence of detrital illite and chlorite in the sediments means that an arid climate prevailed in the region during the end-Permian and early Triassic period.

Diagenetic reorientation of phyllosilicate minerals in Paleogene mudstones of the Podhale Basin, southern Poland

AbstractWe used high-resolution X-ray texture goniometry to quantify changes in the mm-scale orientation of phyllosilicate minerals in a suite of Paleogene mudstones from the Podhale Basin in southern Poland. The sample set covers an estimated range of burial depths between 2.4 and 7.0 km, corresponding to a temperature range of 60–160°C. Although mechanical compaction has reduced porosities to ∼10% in the shallowest samples, the phyllosilicate fabric is only modestly aligned. Coarser-grained (&gt;10 µm) detrital chlorite and mica appear to be more strongly aligned with (001) parallel to bedding, suggesting their deposition as single grains rather than as isotropic flocs or aggregates. From 2.4 to 4.6 km, R0 illite-smectite with 40–50% illite layers changes to R1 illite-smectite with 70–80%) illite layers. At the same time kaolinite is lost and diagenetic chlorite is formed. The mineralogical changes are accompanied by a strong increase in the alignment of illite-smectite, chlorite, and detrital illite, parallel to bedding and normal to the presumed principal effective stress. We propose that the development of a more aligned I-S fabric results from the dissolution of smectite and the growth of illite with (001) normal to the maximum effective stress. Water released by illitization may act as a lubricant for the rotation of all platy minerals into nanoporosity transiently formed by the illitization reaction. At greater depths and temperatures, further illitization is inhibited through the exhaustion of K-feldspar. After the cessation of illitization, a further 2.4 km of burial only results in a small increase in phyllosilicate alignment. At such small values for porosity and pore size, increasing stress does not substantially reorient phyllosilicates in the absence of mineralogical change.

Response of clastic sediments to episodic hydrothermal fluid flows in intramontane troughs: a case study from Black Forest, Germany

Permo-Triassic sandstones of the Breisgau and Schramberg troughs from the central Black Forest and the adjoining Rhine Graben were hydrothermally illitized during post-Variscan times. Illite K-Ar dating outlines a multi-episodic activity during Late Jurassic (about 145 Ma ago), Early Cretaceous (about 130 Ma ago) and Tertiary (about 60 Ma ago). The last episode appears to be restricted to the foothill zone along the Rhine Graben boundary fault. Detrital feldspars, micas, illite and authigenic kaolinite are the percursors of the hydrothermal illite that is Al-rich when derived from feldspars and authigenic kaolinite, or Al-depleted when formed from micas and detrital illite. The chemical composition of authigenic illite is therefore not an appropriate geothermometer to be used on immature sandstones. Furthermore, the occurrence of mixed-layered illite/smectite and the limited extent of intensive alteration suggest low fluid-rock ratios. Illitization was induced by hot, low saline, metamorphic and/or formation fluids (150– 250 °C, eq ). A pulse of high saline and low temperature fluids (about 100 °C, 20–25 wt% NaCl eq ) derived from mixtures of deep seated saline basement brines and meteoric waters caused quartz overgrowths only in the Triassic sandstones. In addition, kaolinite deriving from detrital feldspars in the Rhine Graben area, could have resulted from interaction with thermal, acidic and low-saline fluids. The results indicate tectonically controlled fluid transport, mainly along reactivated NE and NNE trending faults, due to large-scale tectonic processes, namely: (1) rifting in the central Atlantic and western Tethys oceans, (2) Alpine orogenesis, and (3) rifting of the Rhine Graben.

Comparison of K-Ar Ages of Diagenetic Illite-Smectite to the Age of a Chemical Remanent Magnetization (CRM): An Example from the Isle of Skye, Scotland

AbstractThe clay fractions of Jurassic marls in the Great Estuarine Group in southern Isle of Skye are composed of mixed-layered illite-smectite (I-S) with large percentages (&gt;85%) of illite layers, kaolinite, and generally smaller amounts of chlorite. These marls have not been buried to the depths normally required to convert smectite to illite-rich I-S, so it is possible that the conversion was in response to heat and hydrothermal fluids from nearby early Tertiary igneous activity ∼55 Ma ago. The large percentages of illite layers in I-S, the Środoń intensity ratios, and the Kübler index values appear to be consistent with the formation of diagenetic I-S as a result of relatively brief heating caused by igneous activity. The Jurassic rocks in southern Skye contain a secondary chemical remanent magnetization (CRM) that resides in magnetite and formed at approximately the same time as the Tertiary igneous rocks on Skye. K-Ar age values for I-S based on illite age analysis have been determined to test the hypothesis that the CRM was acquired coincidently with the smectite-to-illite conversion. However, linear extrapolation of K-Ar age vs. percentage of 2M1 polytype (detrital illite) from one marl (EL-6) yields an estimate for the age of diagenetic illite of 106 Ma, which is close to the measured age of the finest subfraction (108 Ma). These estimated and measured age values, however, could be substantially greater than the true age of the diagenetic illite in I-S because of the presence of detrital 1Md illite that was recycled from early Paleozoic shales and whose abundance relative to the diagenetic I-S may have been enhanced because the diagenetic fluid had a low K/Na ratio, limiting the amount of diagenetic illite formed. Nevertheless, most of the illite in the Elgol marls (80% or more in the finest fractions) must be diagenetic and probably formed in response to the early Tertiary magmatism.

A km-scale illite alteration zone in sedimentary wall rocks adjacent to a hydrothermal fluorite vein deposit

AbstractIn a study of wall-rock alteration in the 1.4 km long Würmtal adit at the Käfersteige hydrothermal vein deposit (northern Black Forest, Germany) illite was found to be the only clay mineral within the Bunter sandstone and intercalated claystone. Illite occurs mainly as a detrital mineral in the claystone, whereas it is hydrothermally neoformed in the sandstone, either in the pores or as an alteration product of K-feldspar. The extensive occurrence of authigenic illite along the entire 1.4 km long profile confirms that the fluids migrated far into the sandstone.The authigenic illite formed during a first pulse of high-temperature fluids (Th of ~220°C) with a low salinity (~1 wt.% NaCleq). These fluids also dissolved Sr and Rb from detrital illite of the claystones at the vein. A later hydrothermal pulse with a lower temperature (~70–150°C) and higher salinity (21–28 wt.% NaCleq) silicified the sandstone adjacent to the vein and caused partial substitution of OH- by F- in the structure of the detrital and neoformed illite along the profile.Within analytical error, the K-Ar dates for the neoformed illite of the &lt;2 µm fraction are the same along the profile (~150 Ma). During this hydrothermal process, the age of the detrital illite within the claystone was reset from 310 to 190 Ma. The illite-rich &lt;0.2 µm fractions yield ages of ~142 Ma (sandstone) indicating a Jurassic origin. The uniform age data for illite in the sandstone and in the claystone are probably due to extensive migration of hot fluids through the wall rocks.The hydrothermal fluids are attributed to recycled meteoric water and brines that ascended from the basement into the cover rocks during the opening of the North Atlantic and/or the nearby Tethys area.

Detrital illite crystals identified from crystallite thickness measurements in siliciclastic sediments

Illite crystals in siliciclastic sediments are heterogeneous assemblages of detrital material coming from various source rocks and, at paleotemperatures >70 °C, of superimposed diagenetic modification in the parent sediment. We distinguished the relative proportions of 2M1 detrital illite and possible diagenetic 1Md + 1M illite by a combined analysis of crystal-size distribution and illite polytype quantification. We found that the proportions of 1Md + 1M and 2M1 illite could be determined from crystallite thickness measurements (BWA method, using the MudMaster program) by unmixing measured crystallite thickness distributions using theoretical and calculated log-normal and/or asymptotic distributions. The end-member components that we used to unmix the measured distributions were three asymptotic-shaped distributions (assumed to be the diagenetic component of the mixture, the 1Md + 1M polytypes) calculated using the Galoper program (Phase A was simulated using 500 crystals per cycle of nucleation and growth, Phase B = 333/cycle, and Phase C = 250/cycle), and one theoretical log-normal distribution (Phase D, assumed to approximate the detrital 2M1 component of the mixture). In addition, quantitative polytype analysis was carried out using the RockJock software for comparison. The two techniques gave comparable results ( r 2 = 0.93), which indicates that the unmixing method permits one to calculate the proportion of illite polytypes and, therefore, the proportion of 2M1 detrital illite, from crystallite thickness measurements. The overall illite crystallite thicknesses in the samples were found to be a function of the relative proportions of thick 2M1 and thin 1Md + 1M illite. The percentage of illite layers in I-S mixed layers correlates with the mean crystallite thickness of the 1Md + 1M polytypes, indicating that these polytypes, rather than the 2M1 polytype, participate in I-S mixed layering.