Retrograde Diagenesis Research Articles

Mineralogical and chemical composition of Ediacaran-Cambrian pelitic rocks of The Tamengo and Guaicurus formations, (Corumbá Group - MS, Brazil): Stratigraphic positioning and paleoenvironmental interpretations

The Tamengo and Guaicurus formations, upper units of the Corumbá Group, are placed at Ediacaran-Cambrian transition due to biostratigraphy and chemostratigraphy data. The Tamengo Formation is composed mainly of limestones with interbedded mudrocks (siliciclastic, carbonate and mixed siliciclastic-carbonate mudrocks) and record occurrences of macro-fossils Cloudina and Corumbella. The Guaicurus Formation is characterized as an extensive and homogeneous siliciclastic siltstone package. This paper focus in sedimentary, petrographic, mineral (XRD) and chemical (XRF) characterizations of mudrocks and siltstones in order to define stratigraphic positioning of the Tamengo and Guaicurus formations, and indicate depositional environment. To this purpose, three areas were studied in the vicinity of Corumbá (MS-Brazil): Laginha and Corcal quarries and an outcrop along MS-243 road. The rocks of the Tamengo Formation, under microscope, present fluid diffusion features as well as irregular laminations, calcite laminas as evaporate pseudomorphes and biogenic mats. In contrast, siltstones of the Guaicurus Formation are persistently laminated, well sorted and homogenous. Both mudrocks interbedded in carbonate layers and siltstones are composed of detrital quartz, mica and diagenetic clay minerals. Furthermore, the major oxides compositions are broadly similar, except for Na2O, higher in the Guaicurus Formation. The presence of carbonate rocks and mudrocks within the Tamengo Formation imply environment favorable to carbonate formation with episodically immature siliciclastic supply, from very fine-grained rocks of a nearby area, interrupting carbonate depositions. The Guaicurus Formation, distinctively, is a result of an uninterrupted immature siliciclastic supply from a nearby source area, broadly similar to the Tamengo Formation, and was deposited under a low energy setting in a high water level, below fair-weather wave. Petrographic features and diagenetic clay mineral paragenesis – illite + chlorite + smectite – within the Tamengo Formation indicate fluid circulation event. This event resulted in a complex diagenetic history with a possible retrograde diagenesis recorded in the mudrocks of the Tamengo Formation. The siltstones of the Guaicurus Formation present neither petrographic evidences of fluid diffusion nor presence of smectite along the paragenesis illite + chlorite, constraining the event in the Tamengo Formation. After these characterizations, the research revealed that the superior portion of Corcal quarry (L7) exposes rocks from the Tamengo Formation.

Outlining the thermal posdepositional evolution of the Ordovician successions of northwestern Argentina by clay mineral analysis, chlorite geothermometry and Kübler index.

The thermal post-depositional evolution of metapelitic and metavolcanic rocks of the upper Cambrian-Ordovician succession in the Central Andes of northwestern Argentina, was estimated through X ray diffraction of clay mineral analysis, Kubler Index (KI), SEM-EDS study of selected samples and chlorite geothermometry. The study area comprise five representative regions in Puna and Cordillera Oriental, namely at Sierra de Santa Victoria (Nazareno and Santa Victoria river areas), Cordon de los Siete Hermanos (Yavi), Sierra de Rinconada and Sierra de Cochinoca-Escaya. The paleotemperatures obtained with the chlorite geothermometer, in coincidence with KI values, show an E-W trend from diagenesis/low anchizone in the eastern flank of Sierra de Santa Victoria, to high anchizone/epizone in the Puna, with intermediate values in the western flank of Sierra de Santa Victoria. This trend, in conjunction with the development of slaty cleavage in the Ordovician rocks from Yavi, Sierra de Cochinoca-Escaya and western localities coincide with the limits proposed for the “Ocloyic deformation belt”. The common occurrence of kaolinite in the slates and the metavolcanic rocks of Sierra de Cochinoca-Escaya, coupled with the substitution of chlorite by interstratified lower-temperature phases in most of these rocks and the occurrence of jarosite in a metadacite, indicate presence of hydrothermal fluids with high H+/cations ratios, producing acid type alteration (Utada, 1980), at temperatures between 100 and ~300 °C. The hydrothermal alteration should have been subsequent to the maximum burial and the attainment of epizonal metamorphism, and was very probably related with the posthumous activity of the Ordovician volcanic arc. The widespread occurrence of retrograde diagenesis products (smectite, kaolinite, as well as interstratified Chl/Sm and Chl/Vrm phases) in the anchizonal/epizonal metapelites of Sierra de Cochinoca-Escaya and Sierra de Rinconada constitute, up to our knowledge, the first report of an extensive hydrothermal activity affecting not only the metavolcanic rocks but also the Lower Ordovician sediments of northern Puna.

Hydrothermal alteration of Britain’s oldest palaeosols: saddle dolomite and smectite at the Lewisian–Torridon Group (early Neoproterozoic) unconformity, NW Scotland

Weathering profiles 2–3 m thick mantle a palaeoplain formed on the Lewisian Complex and are overlain unconformably by fluvial conglomerates of the c . 980 Ma Applecross Formation (Torridon Group) in NW Scotland. Geological and palaeomagnetic data indicate that the profiles formed penecontemporaneously with deposition of the Applecross Formation and constitute the oldest known palaeosols in Britain. However, their degree of later alteration and burial depth are contentious. The weathering profiles are characterized by the presence of mixed-layer illite–smectite and smectite, which is anomalous for Precambrian palaeosols, raising doubts regarding a Precambrian age for the clay suite. Estimates of burial depth range from <1.5 km, based on the apparently limited burial alteration with the preservation of smectite, to c . 6 km and possibly c . 12 km following Caledonian tectonism. Thin (1–10 mm) veins of saddle (baroque) dolomite are present in weathered amphibolite ≤1 m below the Lewisian–Torridon Group unconformity and also in overlying conglomerate 15 km SSW of Cape Wrath, indicating the flow of heated ( c . 90–160 °C) saline fluids. The anomalous occurrence of mixed-layer illite–smectite and smectite in these Precambrian palaeosols implies ‘retrograde diagenesis’ of diagenetic illite and chlorite likewise induced by hydrothermal fluids. Alteration of the palaeosols is attributed to hydrothermal fluid flow along the Lewisian–Torridon Group unconformity that was related to fluid movement in the Moine Thrust zone during Silurian or Permian regional tectonism. Radiometric dating of the illite may illuminate the timing of alteration of the palaeosols and their burial history.

Manganese deposits: Communication 2. Major epochs and phases of manganese accumulation in the Earth’s history

Accumulation of manganiferous rocks in the history of the Earth’s lithosphere evolution began not later than the end of the Middle Archean. Primary manganese sediments were accumulated at that time in shallow-water sedimentation basins with the active participation of organic matter. The concentration of Mn in the primary sediments usually did not reach economic values. The formation of genuine manganese ores is related to later processes of the transformation of primary ores—diagenesis, catagenesis, metamorphism, and retrograde diagenesis. Types of basins of manganese ore sedimentation and character of processes of the formation of manganese sediments during the Earth’s shell evolution changed appreciably and correlated with the general evolution of paleocontinents. Major periods, epochs, and phases of manganese ore genesis are defined. At the early stages of lithosphere formation (Archean-Proterozoic), manganese was deposited in basins commonly confined to the central part of Western Gondwana and western part of Eastern Gondwana, as well as the western part of the Ur paleocontinent. Basins of manganese ore sedimentation were characterized by the ferruginous-siliceous, carbonaceous-clayey, and carbonaceous-carbonate-clayey composition. The Early-Middle Paleozoic epoch of manganiferous sediment accumulation was characterized by the presence of several small sedimentation basins with active manifestation of volcanic and hydrothermal activity. Since the formation of Pangea in the Late Paleozoic until its breakup, accumulation of Mn was closely associated with processes of diagenesis and active participation of the oxidized organic matter.

Manganese deposits: Communication 1. Genetic models of manganese ore formation

Major regularities in the formation of manganese rocks and ores have been established on the basis of available published and original data. The proposed genetic classification of main manganese deposits (with model examples) is as follows: sedimentary-diagenetic (Nikopol, Bol’she-Tokmak; Ukraine), (volcanogenic) hydrothermal-sedimentary (deposits of the Atasui area, Kazakhstan; Magnitogorsk Trough, South Urals), epigenetic (catagenetic) (deposits of the Kalahari manganese ore field, South Africa; Usinsk deposit, Kuznetsk Alatau), and supergene (residual, infiltrational, cavern filling, and pisolitic deposits in India, Brazil, South Africa, and Australia). The results suggest the following conclusions: (1) all primary manganese rocks and ores at the known deposits are hydrothermal- and diagenetic-sedimentary formations of marine environments; (2) manganese concentrations achieve the size of deposits at postsedimentary stages of the initial manganiferous sediment and manganese rock transformation (diagenesis, catagenesis, and retrograde diagenesis); (3) indispensable participation of the isotopically light carbon dioxide related to the destruction of organic matter (OM) is a characteristic feature of manganese carbonate formation during diagenesis; and (4) the role of organic carbon in manganese ore formation becomes notable since early stages of Mn accumulation in the Precambrian sedimentary basins (terminal Archean-initial Early Proterozoic).

Influence of the surface energy on the kinetics of chemical reactions of mineral nanoparticles

The surface energy of mineral nanoparticles is a significant factor that affects the thermodynamic and kinetic parameters of their chemical reactions. It plays a particularly important role in lowtemperature and lowbaric processes in retrograde diagenesis zones or nearsurface settings: the temperature growth is accompanied by a gradual decease in its influence on the properties of minerals and the kinetics of reac� tions. This problem is insufficiently studied due to dif� ficulties connected with synthesis and selection of nanoparticles with the assigned size and the lack of a database of their surface properties, the surface energy included. The purpose of this communication is to shed light on these aspects and fill the gap in the knowledge of the influence of surface energy on the kinetic parameters of chemical reactions of mineral nanoparticles. Following J. Gibbs, we characterize energy properties by the surface enthalpy HF, surface entropy SF, and surface energy σ: .( 1)

Conodont palaeothermometry of contact metamorphism in Middle Ordovician rocks from the Precordillera of western Argentina

AbstractThe Yerba Loca Formation (Middle–Upper Ordovician), exposed in the Western Precordillera, San Juan, Argentina, is made up of clastic–carbonate turbidites, and basic–ultrabasic rocks. It is affected by regional Siluro-Devonian very low-grade metamorphism that locally reaches greenschist facies. At Ancaucha creek, 45 conodont samples were taken from two sections that include 30 to 50 m thick sills. In order to analyse the thermal alteration patterns produced by these intrusive bodies, conodont Colour Alteration Index (CAI) is contrasted with optical petrography and X-ray diffraction analyses of clay minerals. The intrusions are dated as post-Darriwilian (Da2), as determined by conodont biostratigraphy of the host rock, which indicates theParoistodus horridusSubzone of theLenodus variabilisZone. The distribution of CAI values defines a thermal aureole of about 2.5 times intrusion thickness that prevailed over the later very low-grade metamorphism. Metasomatism at Ancaucha creek is recorded by CAI values of 4 to 7, particularly restricted to a few layers close to the intrusions, as indicated by conodont textures and rock fabric. One-dimensional thermal computer simulation conforms to empirical data indicating temperatures greater than 600 °C for the contact zone, although it points out slightly narrower thermal aureoles. The clay mineral assemblage of most of the analysed samples (chlorite, illite, smectite and I/S mixed-layers) is complex and probably derives from several superimposed processes, thus representing non-equilibrium assemblages. In turn, KI values (0.27–0.32) indicate anchizone metamorphism, in agreement with regional CAI values of 4; consequently, the occurrence of smectite and I/S probably resulted from retrograde diagenesis processes.

Retrograde diagenesis, a widespread process on a regional scale

AbstractPelitic and basic rocks occurring within prograde sequences in Portugal, Spain and Hungary have been studied by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The minerals formed in typical prograde reactions define the general sequences, but smectite, chlorite-smectite (corrensite) and/or berthierine were found to have replaced chlorite, whereas kaolinite and mixed-layer illite-smectite replaced illite-muscovite. Alteration occurred under conditions normally associated with diagenesis, subsequent to regional metamorphism, and we therefore refer to such processes with the term “retrograde diagenesis”. In the cases studied and in other cited examples, reactions occurred on a regional basis via pervasive fluids under open-system conditions inferred to be related to tectonic stress. The observed alterations could generally not be inferred from XRD data, although the presence of pure smectite in sediments other than bentonite is suggestive of retrograde relations, especially where other minerals are consistent with a higher grade of diagenesis. Retrograde diagenesis is readily observed through imaging of textures by TEM, however. Textural features show that retrograde reactions are more common than generally assumed, and that care should be used in interpreting geological events where appropriate textural relations are not seen.

Clay minerals as provenance indicators in continental lacustrine sequences: the Leza Formation, early Cretaceous, Cameros Basin, northern Spain

AbstractVariations in clay mineral assemblages, changes in Kübler index (KI), and the chemical composition of chlorites are used to identify source areas in the lacustrine materials in the Lower Cretaceous Leza Limestone Formation of the Cameros Basin, northern Spain. This formation has fairly homogeneous lithological characteristics and facies associations which do not allow for identification and characterization of local source areas. The Arnedillo lithosome of the Leza Limestone Formation contains a clay mineral association (Mg-chlorite, illite and smectite) indicative of its provenance. Chlorite composition and illite KI values indicate that these minerals were formed at temperatures higher than those reached by the Leza Formation which indicates its detrital origin. The similarity in the Mg-chlorite composition between the Arnedillo lithosome and the Keuper sediments of the area indicates that these materials acted as a local source area. This implies that Triassic sediments were exposed, at least locally, at the time of deposition of the Leza Formation. The presence of smectite in the Leza Formation is related to a retrograde diagenesis event that altered the Mg-chlorites in some samples.

Prograde and retrograde diagenetic and metamorphic evolution in metapelitic rocks of Sierra Espuña (Spain)

AbstractAn unusually complete sequence of pelitic rocks ranging from diagenetic to greenschist-facies metamorphic grades occurs in southern Sierra Espun˜a, Spain. Prograde and retrograde reactions have been studied by X-ray diffraction and electron microscopy (SEM, TEM and AEM). The prograde reaction series, with reactions facilitated by tectonic stress, includes: (1) R4 interstratified illite-smectite in the diagenetic Malaguide Complex that preserves the variable orientation of original smectite packets, and has 1Md polytypism; (2) chemically heterogeneous illite and Na-K dioctahedral white micas that progressively evolve toward chemical and textural equilibrium in the anchizonal Intermediate Units; and (3) thick, defect-free packets of phengite, paragonite and clinochlore which have a typical metamorphic texture, in the Alpujarride Complex. Two superimposed retrograde episodes produced: (a) sudoite at near-peak metamorphic conditions and (b) dioctahedral smectite during low-temperature retrograde diagenesis.

“Retrograde Diagenesis” of Clay Minerals in the Precambrian Freda Sandstone, Wisconsin

AbstractSiltstones from the Precambrian Freda Formation in Wisconsin have been studied by scanning, scanning-transmission and analytical electron microscopy (SEM, STEM and AEM), and X-ray diffraction (XRD). XRD data for drill core samples show a change from smectite-rich Illite-Smectite (I-S) in shallow samples to illite in deeper samples, implying a transition during burial diagenesis.Transmission electron microscopy (TEM) observations of shallow samples reveal the presence of three clearly distinguishable kinds of dioctahedral clay minerals: (1) detrital grains of micrometer-sized, mature muscovite; (2) small packets in the matrix consisting of dominant (Reichweite) R1 I-S or (3) small packets of illite. The illite and I-S stacks occur separately and have similar textures, with packet thicknesses averaging ∼400 Å, within the range of anchizonal illite. Illite and detrital muscovite commonly display strain features typical of the effects of tectonic stress. Void space within I-S or illite packets is inferred to be a strain feature, and to have served as pathways for fluids. Detrital muscovite shows abundant alteration features including (001) boundaries which are continuous with parallel packets of I-S; individual layers commonly show along-layer transitions of muscovite to smectite or I-S. Trioctahedral clays consist primarily of detrital chlorite which commonly shows direct alteration to R1 I-S and smectite, as with detrital muscovite.Deep samples contain only unaltered, coarse detrital muscovite, and thin packets of illite forming stacks and comprising most of the matrix. The texture of the illite appears to be identical to that of shallow samples, with characteristics such as packet size typical of anchizonal illite. Trioctahedral clays consist almost entirely of detrital grains of chlorite and corrensite. They occur as separate grains with rather constant composition, without signs of alteration.The data imply that all of the studied rocks have been subjected to a uniform anchizonal grade of metamorphism in which detrital grains were largely unchanged but matrix clays were transformed to packets of illite. The unusually abrupt transition with depth from highly expandable I-S to illite is inferred to actually be the result of subsequent alteration of authigenic illite and detrital chlorite and muscovite to R1 I-S and smectite in shallow rocks. This late overprinting of the anchimetamorphic clay mineral assemblage is inferred to have been locally caused by fluids with temperatures less than those of peak metamorphism. This process, called “retrograde diagenesis”, gave rise to a sequence of dioctahedral I-S and illite which mimics classic prograde sequences. Interpretations of such sequences as being prograde, especially in cases of ancient rocks, should be interpreted with caution when high-resolution images of textures are not available.

Modes of Diagenesis Responsible for Observed Succession of Potash Evaporites in the Salado Formation, Delaware Basin, New Mexico

ABSTRACT Clays are a prevalent constituent of the McNutt Potash Member of the upper Salado Formation (Permian), New Mexico. The mode of clay distribution is varied and is an important indicator of fluid movement. Clays in the study area occur as thick clay seams (up to 4 cm), disseminated deposits, and as linings on crystal contacts of evaporite minerals. These different occurrences represent different conditions during evaporite deposition: 1) Thick clay seams represent major incursions of fresh seawater diluting the brine in the shallow Delaware basin; 2) disseminated deposits represent undisturbed portions of primary evaporite deposition; and 3) clay linings on langbeinite and sylvite depict percolation of solutions that dissolve preexisting minerals, resulting in deposition of clays during downward migration of the solution front. Although the clay chemistry varies little, the clay mineralogy differs with respect to its associated evaporite mineralogy. Low-charge corrensite is associated with potassium-deficient salts like halite, anhydrite, and polyhalite. High-charge corrensite is associated with potassium-rich salts like sylvite and langbeinite. Three modes of evaporite diagenesis are invoked to explain the major evaporite mineral assemblages in the McNutt Potash Member. Syndepositional alteration best explains the occurrence of polyhalite. Burial diagenesis fits well in explaining the minor amounts of kainite replacing langbeinite in (or near) clay-rich areas. Retrograde diagenesis seems to be the dominant mechanism responsible for formation of the potash bitter salts. The retrograde process involves percolation of fresh seawater into poorly crystallized strata, causing the replacement of primary salts like kainite, kieserite, and carnallite with secondary salts like langbeinite, sylvite, and loewite.