Illite Crystallinity Research Articles

Clay mineralogy and geochemistry of pelitic rocks in the middle Permian Fanjiatun formation, East Changchun, China: implications for metamorphism and provenance

Metamorphic pressure–temperature conditions and sedimentary provenance were determined for the middle Permian Fanjiatun Formation in east Changchun, China, based on clay mineralogy, vitrinite reflectance and geochemical data. The pelitic rocks in the Fanjiatun Formation contain illite + kaolinite + chlorite ± mixed-layer chlorite/smectite and detrital quartz + plagioclase. Illite crystallinity in the formation varies from 0.38 to 0.49 (average = 0.44), and consists dominantly of the 2M1 and mixed 2M1 and 1Md polytypes. The illite b0 dimension is 8.9860–9.0130 A (average = 9.0010 A), indicating the Fanjiatun Formation experienced very-low-grade metamorphism at <1.5 kbar. The metamorphic temperatures obtained based on vitrinite reflectance are 191–308 °C, indicating the formation experienced anchizone-zone facies metamorphism. Al2O3/TiO2 ratios vary from 23.93 to 24.22 (average = 24.11), and the chemical index of alteration is 64–69 (average = 67). Rare earth element contents are high and exhibit enrichment in light rare earth elements and negative Eu anomalies. Trace element characteristics indicate that the source of the pelitic rocks was moderately weathered silicic rocks. The sediments were deposited in a reducing continental setting.

Marine sedimentary records of chemical weathering evolution in the western Himalaya since 17 Ma

AbstractThe Indus Fan derives sediment from the western Himalaya and Karakoram. Sediment from International Ocean Discovery Program drill sites in the eastern part of the fan coupled with data from an industrial well near the river mouth allow the weathering history of the region since ca. 16 Ma to be reconstructed. Clay minerals, bulk sediment geochemistry, and magnetic susceptibility were used to constrain degrees of chemical alteration. Diffuse reflectance spectroscopy was used to measure the abundance of moisture-sensitive minerals hematite and goethite. Indus Fan sediment is more weathered than Bengal Fan material, probably reflecting slow transport, despite the drier climate, which slows chemical weathering rates. Some chemical weathering proxies, such as K/Si or kaolinite/(illite + chlorite), show no temporal evolution, but illite crystallinity and the chemical index of alteration do have statistically measurable decreases over long time periods. Using these proxies, we suggest that sediment alteration was moderate and then increased from 13 to 11 Ma, remained high until 9 Ma, and then reduced from that time until 6 Ma in the context of reduced physical erosion during a time of increasing aridity as tracked by hematite/goethite values. The poorly defined reducing trend in weathering intensity is not clearly linked to global cooling and at least partly reflects regional climate change. Since 6 Ma, weathering has been weak but variable since a final reduction in alteration state after 3.5 Ma that correlates with the onset of Northern Hemispheric glaciation. Reduced or stable chemical weathering at a time of falling sedimentation rates is not consistent with models for Cenozoic global climate change that invoke greater Himalayan weathering fluxes drawing down atmospheric CO2 but are in accord with the idea of greater surface reactivity to weathering.

Late Paleozoic Gondwanide deformation in the Central Andes: Insights from RSCM thermometry and thermal modeling, southern Bolivia

A widespread unconformity between Ordovician metasedimentary rocks and nonmetamorphosed Mesozoic sedimentary rocks across the Eastern Cordillera of Bolivia provides evidence for significant pre-Late Cretaceous deformation and erosion. In this paper, we refine the middle to late Paleozoic tectonic history of the central Andean segment of Gondwana's western margin. We combine Raman spectroscopy of carbonaceous material (RSCM) thermometry with semi-quantitative deformation temperature estimates from quartz recrystallization microstructures and published illite crystallinity values from Cambrian-Devonian sedimentary rocks that span the Eastern Cordillera of southern Bolivia at ~21°S. Estimates of peak temperature and deformation temperature range primarily between ~220 and ~ 400 °C and show a general increase with depth that defines a metamorphic field gradient between ~37 and ~ 47 °C/km. Temperatures obtained from Ordovician rocks directly below the unconformity require erosion of a > ~5 km overburden prior to the Late Cretaceous, which was likely accomplished by a combination of distributed cleavage development and Paleozoic folding and thrust faulting revealed from cross-section reconstructions. The peak temperature data are incorporated into thermal models along with published illite KAr, zircon (UTh)/He, and zircon fission-track cooling ages that refine the timing of Paleozoic orogenesis in the central Andes. We interpret shortening-related flexural subsidence driven by cratonward growth of a pre-Andean orogenic wedge along the western margin of Gondwana, with peak temperature conditions attained during eastward advance of a foreland basin between ~420 and ~ 318 Ma during the Devonian-Carboniferous Gondwanide Orogeny. Erosional exhumation between ~352 and ~ 294 Ma was the result of continued growth and eastward expansion of the associated Transpampean tectonic highland across the Eastern Cordillera. The growth of this pre-Andean contractional orogen coincided with global plate reorganization and the reestablishment of subduction and arc magmatism along the western margin of Gondwana, as indicated by a significant influx of Carboniferous arc-related detritus into the foreland basin system.

A Batch Experiment of Cesium Uptake Using Illitic Clays with Different Degrees of Crystallinity

Radiocesium released by the severe nuclear accident and nuclear weapon test is a hazardous material. Illitic clays play a key role in the spatial distribution of radiocesium in groundwater environments due to selective uptake sites at the illite mineral, such as frayed edge sites. However, the cesium uptake capabilities of illitic clays are diverse, which could be associated with the illite crystallinity. This study was performed to determine the cesium uptake of illitic clays and evaluate the crystallinity effects on cesium uptake using statistical approaches. A total of 10 illitic clays showed various crystallinity, which was parameterized by the full width at half maximum (FWHM) at 10 Å XRD peak ranging from 0.15 to 0.64. The uptake behavior of illitic clays was well fitted with the Freundlich model (i.e., r2 &gt; 0.946). The uptake efficiency of illitic clays increased with the decrease in dissolved cesium concentrations. The cesium uptake was significantly correlated with the FWHM and cation exchange capacity, suggesting that the uptake becomes higher with decreasing crystallinity through expansion of the edge site and/or formation of ion-exchangeable sites.

Paleotemperature investigation of the Variscan southern external domain: the case of the Montagne Noire (France)

The Montagne Noire located in the southern part of the French Massif Central represents the northern part of the South-Variscan Foreland. It is subdivided into three parts. The granite-migmatite Axial Zone dome is surrounded by non- or weakly metamorphosed Paleozoic sedimentary series. Both northern and southern flanks of the Montagne Noire dome are deformed by km-scale, south to southeast facing recumbent folds and thrusts sheets. The Raman Spectroscopy of Carbonaceous Material (RSCM) method, carried out in the low-grade metamorphic rocks of the southern flank of the Montagne Noire, yielded temperatures comprised between 400 °C near the dome, and 230 °C in the southern domain. Three Raman geothermometers were used to cover this temperature range. RSCM temperatures comply qualitatively with previous estimates based on illite crystallinity, conodont color alteration, and fluid inclusions carried out in the same area, which document a metamorphic temperature increase towards the dome. The isotherms cut across the different nappe contacts and are oriented parallel to the southern margin of the Axial Zone. This temperature distribution supports the idea that the thermal structure was acquired during the Axial Zone dome emplacement. The thermal structure acquired during the recumbent folds emplacement and burial of the sedimentary series is totally overprinted by the doming event. In addition, in a domain relatively remote from the Axial Zone dome, the RSCM measurements yielded significantly higher temperatures than illite crystallinity. This discrepancy points to a higher sensitivity of RSCM to short-lived thermal events than illite crystallinity, possibly because of more efficient kinetics of the carbonization reaction. On the other hand, high RSCM temperatures analysed far from the Axial Zone, between 300 °C and 360 °C could be explained by the presence of granitic plutons under the foreland basin.

Birth and closure of the Kallipetra Basin: Late Cretaceous reworking of the Jurassic Pelagonian–Axios/Vardar contact (northern Greece)

Abstract. Some 20 Myr after the Late Jurassic to Early Cretaceous obduction and collision at the eastern margin of Adria, the eroded Pelagonia (Adria)–Axios/Vardar (oceanic complex) contact collapsed, forming the Kallipetra Basin, described around the Aliakmon River near Veroia (northern Greece). Clastic and carbonate marine sediments deposited from the early Cenomanian to the end of the Turonian, with abundant olistoliths and slope failures at the base due to active normal faults. The middle part of the series is characterized by red and green pelagic limestones, with a minimal contribution of terrigenous debris. Rudist mounds in the upper part of the basin started forming on the southwestern slope, and their growth competed with a flux of ophiolitic debris, documenting the new fault scarps affecting the Vardar oceanic complex (VOC). Eventually, the basin was closed by overthrusting of the VOC towards the northeast and was buried and heated up to ∼ 180 ∘C. A strong reverse geothermal gradient with temperatures increasing up-section to near 300 ∘C is recorded beneath the VOC by illite crystallinity and by the crystallization of chlorite during deformation. This syntectonic heat partially reset the zircon fission track ages bracketing the timing of closure just after the deposition of the ophiolitic debris in the Turonian. This study documents the reworking of the Pelagonian–Axios/Vardar contact, with Cenomanian extension and basin widening followed by Turonian compression and basin inversion. Thrusting occurred earlier than previously reported in the literature for the eastern Adria and shows a vergence toward the northeast, at odds with the regional southwest vergence of the whole margin but in accordance to some reports about 50 km north.

Origin of clay minerals on section of Luochuan loesspalaeosol in Shaanxi Province, northwest China

Crystallinity, polytype, and morphology of clay minerals in the Luochuan loess-palaeosols in Shaanxi Province, northwest China were studied in order to have an insight into their origin using X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) methods. The SEM observations showed that the morphology of some illites seems to be lath-shaped crystals. An analysis of illite crystallinity (IC) on the Luochuan section indicated that the origin of illite was related to the rocks of an anchizone. Most illite in the Luochuan loess-palaeosol section were of 2M1 polytype, but some were of 1M polytype formed by weathering of feldspar in the process of pedogenesis. Illite in the Luochuan section has undergone both physical and chemical weathering. These results revealed that most illite were of detrital origin related to the source area of an anchizone, but parts of the illite were of an authigenic origin formed during pedogenesis after sedimentation. Chlorites in the samples of the Luochuan section were mainly composed of irregular flaky grains and their crystallinity was good. These showed that chlorite had the detrital origin formed by physical weathering. Kaolinite crystallinity was relatively good. The value of CIA on the Luochuan section ranges from 61.9 to 69, and therefore kaolinite could not be formed during weathering and pedogenesis. These results indicated that the kaolinite had a detrital origin. Morphology of smectite seems to be capillaceous. The XRD patterns of all samples contained diffraction peaks at 1.5218 A (nontronite) and 1.5052 A (montmorillonite), thus indicating an intermediate composition between trioctahedral and dioctahedral smectite. The smectite crystallinity was very poor. These results revealed that smectite in the study area was authigenic rather than detrital origin.

Mineralogical and geochemical imprints to determine the provenance, depositional history and tectonic settings of Triassic turbidites in the Semanggol and Semantan Basins, Peninsular Malaysia

• Semanggol and Semantan formations are part of Triassic successions in SE Asia. • Triassic turbidites exemplify inner to outer fan in a deep-marine fan system. • Semanggol turbidites were sourced from felsic rocks and represent passive margin. • Semantan rocks have felsic-intermediate origin and show active-continental margin. • Based on redox-sensitive metals, depositional conditions fluctuate from anoxic-oxic. • Type-III kerogen, mature-overmature proposing terrestrial input of organic matter. The present study provides systematic mineralogical, inorganic geochemical and Rock-Eval analyses of turbidite deposits of the Semanggol and Semantan formations to investigate their provenance, depositional environment and tectonic setting as well as type, origin and maturation of organic matter in black shale. These deposits are extensively distributed in Peninsular Malaysia and are observed in the field as conglomerate, pebbly sandstone, thick-bedded sandstone, interbedded sandstone and shale, laminated black shale, and tuffaceous sandstone. These sedimentary sequences are predominantly deep-marine and occasional shallow-marine deposits characterizing a deep-marine fan system. Based on Al 2 O 3 /TiO 2 ratios, more felsic rocks for the Semanggol Formation and felsic-intermediate rocks for the Semantan Formation are proposed as sources for their detrital fractions and this interpretation is also substantiated by the binary plot of Zr vs Ti. V/(V + Ni), V/Ni and Ni/Co analyses indicate anoxic/oxic conditions. Ternary diagram of immobile- trace-elements inferred passive margin for Semanggol turbidites and an active-continental margin for Semantan turbidites. The dissimilarity between geochemically and geologically derived tectonic settings, mainly for the Semanggol turbidites, is because of changing tectonic setting and recycled nature of sediment sources that were deposited in diverse tectonic settings. The black shale clay fractions from both formations dominantly contain illite and kaolininte with abundant traces of quartz. The illite content varies from 43 to 56% and illite crystallinity index (IC) from 0.2° to 0.4° Δ2θ (average: 0.28° Δ2θ), showing studied black shale fall in anchizone. Rock-Eval pyrolysis reveals Type-III kerogens, suggesting a terrestrial source of organic matter in black shale of both formations.

Occurrence of a Likely Tuff Bed between the Middle and Upper Siwaliks, Taunsa area, Dera Ghazi Khan, Eastern Sulaiman Range, Pakistan

A likely tuff bed lies along the gradational contact of the Middle and Upper Siwaliks in eastern Sulaiman Range, Taunsa area of Dera Ghazi Khan district, Pakistan. This tuffaceous unit is 0.5–3 m thick and extends for 10 km along the north-south strike in the eastern limb of the Zindapir anticline. It is greyish white to white on fresh surface, fine-grained to silty at the bottom and clayey at the top and thus shows a fining upward grain-size grading. The lower part of the ash bed shows a prominent lamination defined by megascopically visible abundant biotite, while the central and upper parts are so fine-grained that the individual minerals cannot be seen in hand sample. Unlike the lower well-laminated part, the central and upper parts are crudely laminated to apparently massive. The bulk samples analysed with X-ray diffraction consist of quartz, feldspar (plagioclase), biotite, clays, calcite and some ore mineral likely spinel, while the clay-size fractions contain illite, chlorite, biotite and probably their mixed-layered varieties. The colour, texture, presence of abundant biotite and stratigraphic position of the Taunsa tuff correlate with those reported from Potwar plateau and from Kashmir basin. However, the apparent absence of smectite from the XRD pattern makes the Taunsa ash bed different from both Potwar and Kashmir tuffs. The present stratigraphic position of the tuff bed corresponds to shallow diagenetic zone, while the absence of smectite in the tuff and crystallinity of illite suggest that the tuff is probably derived upon reworking from a deeper diagenetic zone belonging to a lower stratigraphic level. The Eocene or other older pre-Siwalik units in Pakistan may have or had some primary ashfall deposits as reported in the northwestern Himalayas of India. This older volcanic ash may have been reworked to its present site of occurrence along the gradational contact of the Middle and the Upper Siwaliks in Taunsa area of Dera Ghazi Khan. However, the primary source of the Taunsa tuff may belong more likely to Chagai arc in Pakistan than to Dacht-e-Nawar volcanic complex in Afghanistan.

Occurrence of a Likely Tuff Bed between the Middle and Upper Siwaliks, Taunsa area, Dera Ghazi Khan, Eastern Sulaiman Range, Pakistan

A likely tuff bed lies along the gradational contact of the Middle and Upper Siwaliks in eastern Sulaiman Range, Taunsa area of Dera Ghazi Khan district, Pakistan. This tuffaceous unit is 0.5–3 m thick and extends for 10 km along the north-south strike in the eastern limb of the Zindapir anticline. It is greyish white to white on fresh surface, fine-grained to silty at the bottom and clayey at the top and thus shows a fining upward grain-size grading. The lower part of the ash bed shows a prominent lamination defined by megascopically visible abundant biotite, while the central and upper parts are so fine-grained that the individual minerals cannot be seen in hand sample. Unlike the lower well-laminated part, the central and upper parts are crudely laminated to apparently massive. The bulk samples analysed with X-ray diffraction consist of quartz, feldspar (plagioclase), biotite, clays, calcite and some ore mineral likely spinel, while the clay-size fractions contain illite, chlorite, biotite and probably their mixed-layered varieties. The colour, texture, presence of abundant biotite and stratigraphic position of the Taunsa tuff correlate with those reported from Potwar plateau and from Kashmir basin. However, the apparent absence of smectite from the XRD pattern makes the Taunsa ash bed different from both Potwar and Kashmir tuffs. The present stratigraphic position of the tuff bed corresponds to shallow diagenetic zone, while the absence of smectite in the tuff and crystallinity of illite suggest that the tuff is probably derived upon reworking from a deeper diagenetic zone belonging to a lower stratigraphic level. The Eocene or other older pre-Siwalik units in Pakistan may have or had some primary ashfall deposits as reported in the northwestern Himalayas of India. This older volcanic ash may have been reworked to its present site of occurrence along the gradational contact of the Middle and the Upper Siwaliks in Taunsa area of Dera Ghazi Khan. However, the primary source of the Taunsa tuff may belong more likely to Chagai arc in Pakistan than to Dacht-e-Nawar volcanic complex in Afghanistan.

Hydrocarbon potential of the Silurian and Carboniferous Shales in the Western and Central Taurus Region of Turkey

Organic maturity levels of the Silurian and Carboniferous shales in the western and central Taurus regions are thermally over mature and therefore they can still contain shale gas. A total of 43 samples were determined using TOC analysis and Rock-Eval Pyrolysis to indicate the first basic hydrocarbon source rock characteristics. Average TOC values of Silurian samples range from 0.61 to 2.56% in wt. However, Carboniferous samples have low TOC values of less than 1.0%. Present low hydrogen index values between 2 and 14 mg HC/g TOC and average oil saturation index values ranging from 8 to 17 indicate that there is no hydrocarbon generation. X-Ray Diffraction data for the samples show the presence of quartz, feldspar, calcite, dolomite, mica and clay minerals. The calculated illite crystallinity values of the samples usually fall into the diagenetic zone and wet gas generation zone corresponding to the late catagenesis stage. Sharpness ratio or Weaver Index values of the samples, except for Kayrak formation samples, fall into the diagenetic zone. On the other hand, the calculated sharpness ratios of the Kayrak Formation samples between 2.35 and 5.15 indicate anchizone. These results show that there is a consistency between the IC values. Sharpness Ratios and Rock-Eval pyrolysis data. The data obtained in this study would be important during the evaluation of unconventional hydrocarbon resources in the future.

Illite crystallinity index an indicator of physical weathering of the Sediments of the Tista River, Rangpur, Bangladesh

The Tista River is a tributary of the Brahmaputra River. The deposits that exposed along the both banks of the Tista River are characterized mainly by sand, sand laden with gravel and pebble with minor amounts of silt and clay. The X-ray Diffraction (XRD) of the clay sized sediments of the Tista River reveals that illite (and/or mica), chlorite, kaolinite, quartz and feldspar are the principal mineral constituents. The minor to trace amounts of lavendulan, glauconite lepidolite, enstatite, sekaninaite and ferrierite minerals were identified in the investigated area. Illite constitutes 36% of the total minerals of the Tista River basin. The values of the illite crystallinity index varies from 0.228 to 0.345, indicating that the illites are relatively well crystallized and derived from the mechanical weathering of pre-existing rocks. The presence of illite and kaolinite suggests their derivation from the crystalline rocks that contain feldspar and mica, as well as from the pre-existing soils and sedimentary rocks. Glauconite forms in the sediments of continental shelf in the marine environment. The accessory minerals like enstatite, sekanianite and ferrierite are derived from basic igneous rocks. The dissolution of copper arsenate mineral, lavendulan might be a source of arsenic in the sediments of the study area.

Mineralogical and geochemical characteristics of the Paleozoic source rocks, Akkas gas field, Western Desert of Iraq: Implications for their origin, maturation and Ordovician-Silurian transition

Ordovician-Silurian black shales are the source rocks in the Palaeozoic petroleum system in Iraq. However, their source and depositional conditions from their mineralogy and inorganic geochemistry are quite lacking in previous works. These shales also provide an opportunity to examine the changes in paleoenvironment, paleoclimate and paleoredox conditions across the Ordovician-Silurian transition.The Ordovician black shales comprise of kaolinite, illite, chlorite, quartz, dolomite, calcite, and traces of feldspars and siderite, while the Silurian black shales comprise of illite, quartz, calcite, dolomite and pyrite. Calculated SiO2 (wt %) and Al2O3/TiO2 ratios suggest felsic source rocks for the Ordovician black shales and intermediate source rocks for Silurian hot shales. The shallow marine setting for the Ordovician black shales that changed to a deeper environment during the Silurian time was interpreted from the V, P2O5, and Al2O3 diagrams is consistent with the abundance of trace fossils Monocraterion sp., Cruziana problematica, Diplichnites sp., and Skolithos sp. which refer to proximal shallow marine conditions and abundance of graptolites monograptus in the Silurian black shales, which refer to an open-marine environment. Paleoredox indicators such as TOC and Mn contents as well as Ni/Co, V/Ni, U/Th, and V/Cr ratios prove fluctuation of redox conditions from oxic in the Ordovician shales to anoxic in the Silurian black shales. Late Ordovician glaciation in Iraq can be indicated from the low values of chemical index of weathering (CIA = 52-58) in the samples from the upper part of the upper part of the Khabour Formation that refer to cold and arid climate conditions that prevailed during the late Ordovician time. Abundance of extrabasinal and poor-sorted mud clasts in this interval might support the glaciogenic origin of this part. The black shales in the Khabour Formation is considered as fair source rock (average TOC = 0.9 wt%), while that in the Akkas Formation is an excellent source rock (average TOC = 13.2 wt%) for hydrocarbon generation. Kerogen in the Ordovician and Silurian black shales were derived from marine organic matter as indicated by the binary plot and total C1-C15-n-C6-n-C14-n-C15+ ternary diagram. Organic matter in the Ordovician black shales is highly matured compared to that in the Silurian black shales due to higher illite crystallinity (IC) values in the Ordovician (0.61–0.75° D2θ) relative to that in the Silurian (0.45–0.55° D2θ) black shales.

Geological anatomy of the Upper Triassic sequence in southeastern Tibet: Implication for tectonic evolution of the eastern Himalayan Orogen

Located south of the Yarlung‐Tsangpo Suture Zone (YTSZ), the Upper Triassic rocks of the Tethyan Himalayan Sequence (THS) in the Shannan area, southeastern Tibet, serve as an ideal geologic entity to investigate the post‐collisional evolution of the eastern Tethyan Himalayan realm. Detailed field observations suggest that D2 north‐verging folds and thrusts dominate the deformational pattern of the newly named Luolin Group, while D1 south‐verging imbricated structures characterize the Langjiexue Group and the Nieru Unit. Illite crystallinity measurements in pelitic samples indicate that the metamorphic grade decreases from epizonal in the Luolin Group to anchizonal facies in the Langjiexue‐Nieru Unit. Fluid inclusions from two stages of syn‐deformational quartz veins are characterized by moderate homogenization temperature (~260°C) and low salinity (5% NaCleqv). Electron microprobe analyses (EMPA) of garnet amphibolites show metamorphic temperature of ~510°C. The age of D1 event is determined by 40Ar–39Ar dating of phyllite sample at ca. 47 Ma. Correlations with regional deformational events through the THS demonstrate that the D2 event occurred between ca. 30 and 22 Ma. These data imply that the Upper Triassic sequence is part of the fold‐thrust belt of the Himalayan Orogen, rather than the accretionary mélange within the YTSZ.

Time‐Dependent Heat Budget of a Thrust from Geological Records and Numerical Experiments

AbstractWe present thermometric measurements and numerical simulations of the thermal budget of a shallow thrust (5–7 km of depth). We determine the temperatures of synkinematic minerals by chlorite geothermometer and illite crystallinity index and sheared veins with stable isotope and fluid inclusion microthemometry. The fault zone attained temperature higher than the wall rocks (ΔT of ~50 °C). Some heavily damaged rocks retain a minor but systematic record of a further temperature increase up to 150 °C, and the isotopic signal of fluids indicates a proximal source localized close to the fault zone. We simulated the geological evolution with a suite of elasto‐visco‐plastic thermomechanical models using a 2‐D finite difference code that reproduces the stress, strain, temperature, and pressure variations occurring within the thrust zone. Results indicate that shear heating during distributed cataclastic flow can account for a moderate and persistent temperature increment (ΔT 5–30 °C). Transient higher temperature pulses (50–90 °C) are reproduced simulating coseismic slip along narrow fault planes localized within the broad cataclastic zone. Integration of analytical data and numerical results supports the slow creeping frictional‐viscous deformation as effective mechanism explaining the low‐T part of the thermal history. Seismic deformation equivalent to moment magnitude Mw &gt; 6.5–7.0 earthquakes is required to fully account for the thermal signal, including the extreme peak temperature.

Weathering, recycling, hydraulic sorting and metamorphism/metasomatism implications of the NE Gondwana lower Cambrian-Lower Cretaceous siliciclastic succession of Jordan

Whole rock major elements, detrital and authigenic light, heavy, and clay minerals, and neoformed zeolite minerals of the NE Gondwana lower Cambrian-Lower Cretaceous siliciclastic succession of Jordan were investigated to evaluate the factors controlling the chemical composition of the sandstone. The provenance source rocks, controlled by the tectonic setting at the time of deposition of the subarkosic arenites and the quartzarenites were the principal mineral- and chemical-governing factor, followed by an interplay of chemical weathering, recycling, hydraulic sorting, diagenesis, and very-low grade metamorphism/hydrothermal metasomatism. The sandstone provenance was mainly granitoids of the Arabian Shield formed during the Neoproterozoic Gondwana amalgamation, and minor Paleozoic and Mesozoic siliciclastic strata. High CIA and low WIP weathering index values indicate an intensive chemical weathering event that affected the provenance during the early Cambrian-Middle Ordovician, and gave rise to mature first-cycle quartzarenites. High CIA/WIP and SiO2/Al2O3 ratios indicate a pronounced recycling in the Early Cretaceous responsible for super mature second-poly-cycled quartzarenites. The subarkosic arenites were formed either under a thorough chemical weathering, coupled with a high rate of erosion and a rapid deposition in adjacent braidplains, or an arid, glacial climate devoid of chemical weathering. Hydraulic sorting caused enrichment of Al and K in the detrital clay minerals, Ti in the silt-sized Ti-bearing minerals and Si in the sand-sized quartz. The very low-grade metamorphism/hydrothermal metasomatism, recorded here for the first time, is indicated by illite crystallinity (IC = 0.36° 2θ), illite-sericite-muscovite transformation, zeolites-apatite-authigenesis, and alteration of monazite and titanite-ilmenite into thorite and leucoxene/hematite.

Boron paleosalinity proxy for deeply buried Paleozoic and Ediacaran fossils

Boron content is an indicator of paleosalinity for Mesozoic and Cenozoic clayey sediments, but is compromised by clay diagenesis in deeply-buried sediments of Paleozoic and Ediacaran age. This study of North American, Russian, and Australian, Paleozoic and Ediacaran fossils showed tight covariance of B (ppm) and K (wt%), confirming common wisdom that boron is carried largely by illlite-smectite clays. Ratios of B/K >40 μg/g distinguish modern marine and freshwater sediments, but this discrimination threshold declines to 21 μg/g for sequences with bituminous coal just below the anchimetamorphic zone, and to 5 μg/g for sequences within the anchimetamorphic zone toward greenschist facies. Marine and non-marine threshold B/K values can be predicted from Weaver and Kübler indices of illite crystallinity, as a measure of B depletion during deep burial and metamorphism. Deviations from that expectation (ΔB/K-Weaver and ΔB/K-Kübler) are positive in marine rocks, and negative in non-marine rocks. Trilobites, stromatolites and other marine fossils were correctly identified as marine by this proxy, and fossil land plants identified as non-marine, though in some cases by small margins because of declining B content with increased clay crystallinity. Cryogenian to Devonian problematic quilted fossils (Dickinsonia, Arumberia, Rangea, Pteridinium, Ernietta, Aspidella, Rutgersella, Protonympha) are indistinguishable from Devonian to Triassic plants using this proxy, and significantly different from trilobites, brachiopods, ammonites and other securely marine fossils.

Clay mineralogy of the upper Miocene-Pliocene red clay from the central Chinese Loess Plateau and its paleoclimate implications

Study of clay mineral is one of effective methods for reconstruction of the paleoclimate and paleoenvironment. Here we represent clay mineral records from an upper Miocene–Pliocene red clay succession in the central Chinese Loess Plateau (CLP). On the basis of the existing age model, we employ X-ray diffraction and scanning electron microscopy to carry out clay mineral analyses, the results indicated that illite was the dominant clay mineral and the contents of smectite, chlorite and kaolinite were minor. From the analysis of contents of clay minerals, illite crystallinity, illite 5 Å/10 Å, smectite/(illite+chlorite) ratio, magnetic susceptibility, summer monsoon intensity and deep-sea δ18O values, the paleoclimatic evolution between 7.5 and 2.6 Ma can be divided into three intervals: interval I, from 7.5 to 6.9 Ma, it was cool and dry; interval II, from 6.9 to 4.1 Ma, was cool-warm oscillations; interval III, from 4.1 to 2.6 Ma, was significantly warm and humid stage of the seasonal climate conditions. The proxy records display a large amplitude transition in the later interval III, from 2.8 to 2.6 Ma, which implied an abrupt climate and environment variability. Through comparison the local and global paleoclimatic records, we found that clay minerals reflect regional paleoclimatic change superimposing global climatic records during the late Miocene–Pliocene period. We conclude that the regional tectonic activity and global cooling had affected East Asia Monsoon, which was characterized by the different temperature and precipitation in the central CLP.

Characterization of Illite Clays associated with the Sinongduo low sulfidation epithermal deposit, Central Tibet using field SWIR spectrometry

Field measurements using shortwave infrared (SWIR) reflectance from drill hole samples provided an effective method to map variations in illite crystallinity and the illite assemblage minerals associated with hydrothermal alteration zonation of the Sinongduo low sulfidation epithermal Ag-Pb-Zn deposit, Tibetan Plateau, China. The SWIR mineral spectral information was combined with electron probe micro-analysis (EPMA) measurements, and microscope observations to provide geological insights into: (a) the likely metallogenic fluid nature of the mixture of magmatic fluids and a cooler water source; (b) the illite physico-chemical properties of the associated hydrothermal fluids; (c) the illite, montmorillonite and sericite evolutionary interactions among these fluids, country rocks, and alteration minerals; and (d) the alteration minerals relationships and illite crystallinity to the economic mineralization. The results suggest that illite characterizations can be used as an indicator for epithermal deposit prospecting.

Mineralogy and geochemistry of atypical reduction spheroids from the Tumblagooda Sandstone, Western Australia

AbstractReduction spheroids are small‐scale, biogenic, redox‐controlled, metal enrichments that occur within red beds globally. This study provides the first analysis of the compositionally unique reduction spheroids of the Tumblagooda Sandstone. The work aims to account for their composition and consequently improve existing models for reduction spheroids generally, which presently fail to account for the mineralogy of the Tumblagooda Sandstone reduction spheroids. Interstitial areas between detrital grains contained in the cores of these reduction spheroids are dominated by microplaty haematite, in addition to minor amounts of svanbergite, gorceixite, anatase, uraninite, monazite and illite. The haematite‐rich composition, along with an absence of base metal phases and the vanadiferous mica roscoelite, makes these reduction spheroids notable in comparison to other global reduction spheroid occurrences. Analyses of illite crystallinity provide values for samples of the Tumblagooda Sandstone host rock corresponding to heating temperatures of ca 200°C. Consequently, while Tumblagooda Sandstone reduction spheroids formed via the typical metabolic processes of dissimilatory metal‐reducing bacteria, the combination of a unique mineralogy and illite crystallinity analysis provides evidence of more complex late‐stage heating and reoxidation. This has not previously been recognised in other reduction spheroids and therefore expands the existing model for reduction spheroid genesis by also considering the potential for late‐stage alteration. As such, future reduction spheroid studies should consider the potential impact of post‐formation modification, particularly where they are to be used as evidence of ancient microbial processes; such as in the search for early evidence of life in the geological record on Earth or other planets. Additionally, because of their potential for modification, reduction spheroids serve as a record of the redox history of red beds and their study could provide insights into the evolution of redox conditions within a given red bed during its diagenesis. Finally, this paper also provides insights into the relatively understudied diagenetic history of the Tumblagooda Sandstone; supplying the first reliable and narrow constraints on its thermal history. This has important implications for the thermal history of the Carnarvon Basin and its petroleum prospectivity more broadly.