Rhyolitic Tuff Research Articles

Age, tectonic setting and regional implications of the Chiang Khong volcanic suite, northern Thailand

The Chiang Khong volcanic suite forms the northeastern part of the Tak–Chiang Khong volcanic belt in the Sukhothai terrane of northern Thailand. It consists mainly of subaerial andesitic to rhyolitic tuffaceous rocks. A rhyolitic tuff yielded a Middle Triassic U–Pb (zircon) age of 232.9 ± 0.4 Ma. The suite displays calc-alkalic chemical trends and Nb, P, and Ti depletion characteristic of subduction-related magmatism. Based on petrological similarities, the suite is correlated with the c . 240 Ma Lampang volcanic rocks located farther to the SW in the Tak–Chiang Khong belt, and with volcanic rocks of similar age in the Lincang–Jinghong belt in Yunnan, in the western part of the Simao block. These correlations support the placement of the Palaeo-Tethyan suture to the west of the Sukhothai terrane.

Zeolites and zeolitization of acid pyroclastic rocks from paroxysmal Paleogene volcanism, Eastern Rhodopes, Bulgaria

An extensive bimodal volcanic suite developed in the Late Paleogene in the Eastern Rhodopes Mountains, Southern Bulgaria. Most of the widespread and thick rhyolitic tuffs and ignimbrites, erupted during two major Early Oligocene acid phases, were emplaced in a shallow marine environment and subsequently altered. K-rich Ca-clinoptilolite (in some places K-clinoptilolite, mordenite or analcime), clay minerals (celadonite or/and smectite), adularia (or albite) and opal-CT replace the volcanic glass. The zeolitized pyroclastic rocks are strongly depleted in MnO, Na2O, K2O, and enriched in CaO, Fe2O3, TiO2 and MgO. Regional zoning in distribution of the major glass-replacing minerals has been established: quartz + adularia (or albite) ± zeolites in the proximal to the vent area; zeolites in the medial and clay minerals in the distal area. Vertical zoning has been also recognized because only the basal pyroclastic layer is transformed into adularia + quartz whereas clinoptilolite, accompanied by opal-CT and clay minerals, prevails in the rest of the section. The zeolitized glass shards are characteristically zoned: (1) a thin rim (<5μm thick) of celadonite coats the shard surface retaining their original shape; (2) a 30–50μm-thick zone, made of contiguous platy zeolite crystals, oriented perpendicular to the rim and (3) a central zone of large euhedral crystals, surrounding a central hollow area. Phenocrysts are not affected by the zeolitization and some relict glass is found in the top of the pyroclastic section. According to the semiquantitative X-ray analysis and heat of immersion test of Culfaz et al. (1973) the clinoptilolite content in the zeolitized pyroclastic rocks varies from 38 to 72 wt.%. The cation exchange capacity values range between 40 and 153 meq/100 g and show a significant positive correlation with clinoptilolite content obtained by the heat of immersion test (r = 0.71). The lateral zoning and presence of typical hydrothermal minerals such as celadotite indicate that zeolitization of the thick Eastern Rhodopes pyroclastic series results from the operation of a large low temperature hydrothermal system. The hydrothermal solutions consist of marine water heated by the anomalous geothermal gradient of the active volcanic areas or/and by the hot pyroclastic deposits.

On age of Nazimovskaya and Khasan formations in the Paleogene succession of southwestern Primor’e

Materials of previous studies and new original geological and paleobotanic data are used to substantiate synchronism (late Eocene) of the Khasan and Nazimovskaya formations, the key stratigraphic units of the Paleogene in Primor’e. Coal-bearing sediments of the units rest upon early-middle Eocene volcanics (Kraskino rhyolite tuffs, Zaisanovka basaltic andesites) being overlain by the lower Fatash Subformation of the lower Oligocene with flora of the Kraskino type. The Ust’-Davydovskaya Formation of the Rechnoi Peninsula (outskirts of Vladivostok) is the most probable analogue of the Khasan (=Nazimovskaya) Formation.

Characterization of Hungarian Red Soils

Some pedological and micromorphological investigations were carried out on representative red clay samples selected from a large number of profiles. On the basis of conclusions drawn from the analytical results, the red clays can be divided into the following groups: - The red clays of the foothills of the Tokaj Mountains were formed on rhyolite or rhyolite tuff, and are covered by loess in some areas. They are relic soils, older than loess, formed under the warm climate of the Tertiary Period. In addition to quartz they contain feldspars, illite, montmorillonite and a small amount of kaolinite. - The red clays of Aggtelek Karst are Tertiary relic soils formed on Mesozoic limestone. The dominant clay mineral is kaolinite, but they contain a siginificant amount of smectite as well. - The red clays of the Northern periphery of the Hungarian Plain are situated on clay, silt and sand layers of different origin or between loess depositions. They were formed in the Pliocene and at the turn of the Pliocene and Pleistocene. These soils have a medium clay content, with a large quantity of montmorillonite and a small amount of kaolinite. - Red clays formed on Permian sandstones . These rocks were formed in the Permian period, and were issued from a mixture of sediments under tropical climate, tropical weathering. They are the signs of the oldest soil formation in Hungary. They can be characterized by their kaolinite, illite, montmorillonite and hematite contents. - The red clays of the Transdanubian hilly region wereformed by the weathering of thePannonian surface between the end of the Miocene Period and the lower Pleistocene. Medium clay content is characteristic of these red clays. They contain kaolinite, montmorillonite, chlorite and a small amount of hematite. Concerning micromorphological features, speckled and granostriated b-fabrics of the groundmass, mainly due to swelling and shrinking, were observed in some samples. Clay coatings are mainly interpreted as micromorphological features of illuviation. The investigated red clays are similar to tropical and sub-tropical ferrallitic soils in respect of their formation and mineral characteristics.

Middle Archean volcano-hydrothermal sequence: Bacterial microfossil-bearing 3.2 Ga Dixon Island Formation, coastal Pilbara terrane, Australia

The 3.2 Ga Dixon Island Formation in the Cleaverville Group of the coastal Pilbara terrane, Australia, is one of the most complete and best-preserved examples of middle Archean oceanic stratigraphy and contains possible microbial material. Field observations and geochemical evidence suggest that this formation contains a low-temperature hydrothermal vent system with a biogenic microbial colony from the Archean ocean. The Dixon Island Formation is ∼350 m thick and consists of the Rhyolite Tuff, Black Chert, and Varicolored Chert Members, in ascending order. The Rhyolite Tuff Member contains many vein swarms, such as quartz and black chert veins, and highly altered rhyolite tuff layers, which are identified as an underground bypass zone for circulating hydrothermal fluid. Many black chert vein swarms in the Rhyolite Tuff Member imply intensive low-temperature hydrothermal activity during deposition of the Black Chert Member, which is 10–15 m thick. The Black Chert Member is composed of massive black chert, laminated black chert, dark-greenish siliceous shale and tuffaceous laminated chert, which are mainly composed of very fine quartz. Abundant pseudomorphs of silica after aragonite, barite, and gypsum, and a distinctly continuous, stromatolite-like biomat layer (10–20 cm thick), are preserved within the laminated black chert bed. The stromatolite-like biomat bed is formed of fine iron or iron-coated quartz pisolite within fine-grained silica. The absence of detrital sediment of continental origin and the many vein injections imply that this sedimentary facies represents a pelagic hydrothermal environment at ∼500–2000 m paleodepth, and may have been on the slope of an immature island arc. Microbial material has been preserved well in the lower part of Black Chert Member. The massive black chert has carbonaceous peloids (0.3–2.0 mm in diameter), which are similar to those in the black chert veins. The massive black chert contains spiral-, rod, and dendrite-shaped bacterial material. The total organic carbon (TOC) value of massive black chert in the lower part of the Black Chert Member is higher (TOC = 0.15–0.45%) than that of the overlying laminated chert section (TOC = 0.02–0.15%) and the black chert vein (TOC = 0.1–0.13%), and the carbon isotope (δ 13 C) values of this lithology (−33‰ to ∼−27‰) are also lighter than for the black chert veins (−29‰ to ∼−26‰) and the laminated black chert in the upper part of the Black Chert Member and the Varicolored Chert Member (−27‰ to ∼−13‰). This evidence suggests that the carbonaceous grains and bacteria-shaped material in the lower part of the Black Chert Member are of bio-genic origin and were formed close to a low-temperature hydrothermal vent system. The microbial colony may have been rapidly fossilized by silicification related to hydrothermal activity. Laminated black chert in the upper part of the Black Chert and the Varicolored Chert Members may have formed by cyanobacterial sedimentation from the ocean surface.

Petrogenesis of Neoarchaean volcanic rocks of the MacQuoid supracrustal belt: A back-arc setting for the northwestern Hearne subdomain, western Churchill Province, Canada

Rocks exposed in the MacQuoid-Gibson Lakes region, northwest Hearne subdomain, western Churchill Province, Canada comprise three major lithotectonic assemblages: the Principal volcanic belt; the metasedimentary MacQuoid homocline and; the Cross Bay plutonic complex. Neoarchaean supracrustal rocks of the belt range in age from <2745 to <2672 Ma and were intruded during the interval <2689 to 2655 Ma by diverse plutonic units ranging from gabbro through syenogranite, but greatly dominated by tonalite. Volcanic rocks occur only in the Principal volcanic belt and the MacQuoid homocline, are metamorphosed to amphibolite facies and vary from rare pillowed to common massive basalt and andesite, intercalated with less abundant, thin, dacitic to rhyolitic tuffs, lavas and volcaniclastic rocks. Basalt and andesite are dominated by subalkaline, FeO T-rich tholeiites with less common calc-alkaline rocks with higher SiO 2 contents and variable trace element contents. Felsic volcanic rocks exhibit calc-alkaline affinities and similarly diverse trace element abundances. The diverse trace element chemistry of the basalt and andesite supports their derivation from a heterogeneous mantle source(s) capable of generating MORB-, Arc-, BABB- and boninite-like rocks. Two geochemically distinct, arc-like suites were generated through contamination of the primary mantle-derived magmas either via assimilation of lower or middle tonalitic crust, or through contamination of their mantle source through subduction. Geochemical features of the felsic volcanic rocks indicate that these formed via both anatexis of crust in the amphibolite ± garnet stability field and via fractionation of more primitive progenitors in mid-upper crustal magma chambers. ɛNd t = 2680 Ma isotopic compositions cluster near depleted mantle, indicating that significant incorporation of older, >2700 Ma crust likely did not occur. ɛNd t = 2680 Ma values for three specimens, one from each of the Arc-like suites and one BABB-like basalt are slightly lower than the remainder, suggesting very minor incorporation of slightly older crust. These features imply that the processes that generated the MacQuoid supracrustal belt required simultaneous tapping of geochemically distinct mantle reservoirs with concomitant anatexis of sialic crust (garnet stability field) and fractionation of felsic magmas in upper crustal magma chambers. Shallow water deposition of abundant volcaniclastic rocks and semipelite along with minor conglomerate and quartzite was broadly contemporaneous with this magmatism. We envisage a geodynamic setting characterized by tectonomagmatic processes similar to those of modern supra-subduction zone back-arc marginal basins such as the Sea of Japan. Therein, an extensional, back-arc setting, likely proximal to continental crust, provides an explanation for a broad swath of diverse mantle-derived rocks intercalated with less common felsic rocks as well as an abundance of immature clastic metasedimentary rocks.

Contrasting processes in silicic magma chambers: evidence from very large volume ignimbrites

Very large volume (&gt;1000 km3 of magma) crystal-rich dacitic ignimbrites that lack pronounced evidence of fractional crystallization or vertical zonation erupt in some continental magmatic arcs (e.g. the Lund Tuff of the Great Basin and the Fish Canyon Tuff of Colorado in western USA). Apparently, their magma chambers were only modestly heterogeneous and not systematically zoned from top to bottom. These ignimbrites have 40 to 50% phenocrysts set in a high-silica rhyolite glass. Mineral assemblages and mineral compositions suggest pre-eruption temperatures were 730 to 820°C and water and oxygen fugacities were relatively high. We have speculated that these very large volume ignimbrites are unzoned because crystallization and convection in slab-shaped magma chambers inhibited separation of crystals from liquids and resulted in a chamber filled with compositionally heterogeneous magma that lacked systematic chemical zonation or strong fractionation. However, many other very large volume silicic ignimbrites are strongly fractionated and may be vertically zoned (e.g. tuffs related to the Yellowstone hotspot). These rhyolitic tuffs typically have few phenocrysts, anhydrous mineral assemblages, low oxygen fugacities, crystallization temperatures of 830 to 1050°C, and a strong imprint of fractional crystallization. Yet these Yellowstone-type rhyolites are derived from chambers 40 to 70 km across which have sill-like shapes (depth/diameter ratios much less than 1). Thus, factors other than chamber shape must be important for establishing the degree of evolution and nature of zonation in silicic magma chambers. Here, the role of crystallinity-dependent viscosity on the evolution of these two types of contrasting magmas is explored. Calculated magma viscosities for the hot, dry, crystal-poor rhyolites are significantly lower than for the cooler, wetter, crystal-rich dacites. Perhaps these hot rhyolites had low enough crystal contents and viscosities to allow efficient crystal–liquid separation, probably by a combination of unhindered crystal-settling, floor crystallization (including compaction), and crystallization on the walls of large chambers. Clean separation of melt from residual solids at their sources may have been promoted by their high temperatures and low viscosities (&lt;104.5 Pa s). In contrast, monotonous dacitic magmas may never have been crystal-free near-liquidus magmas. Their large magma chambers may have developed by progressive growth at a shallow level with repeated input of intermediate to silicic magma. Crystallization of the water-enriched dacitic magmas occurred at lower temperatures (&lt;800 °C) where crystallinity and hence magma viscosity (&gt;106.5 Pa s) were significantly higher. These characteristics inhibited all forms of crystal–liquid separation, hindered development of systematic vertical zonation, and promoted quasi-equilibrium crystallization in small domains within large heterogeneous magma chambers. Eruptions of these crystal-rich dacites may only occur if the roof fails over a growing magma chamber that is becoming increasingly molten.

Origin of kaolin deposits in the ‘Los Menucos’ area, Río Negro Province, Argentina

AbstractKaolinite deposits, hosted by rhyolitic tuffs, are found in the SE of the ‘Los Menucos’ area (Province of Rio Negro, Argentina). The most representative examples are from the Blanquita and Equivocada mines. To discover the origin of these deposits, a mineralogical, geochemical (major and trace element) and stable isotope study (O and H) of kaolinite was carried out. Mineralogically, Blanquita mine is characterized by the presence of dickite, kaolinite, alunite and pyrophyllite, whereas in Equivocada mine, kaolinite is accompanied by dickite and traces of alunite (without pyrophyllite). The trace element contents and ratios of kaolin samples (P vs. S, Zr vs. Ti, (Cr + Nb) vs. (Ti + Fe), and (Ce + Y + La) vs. (Ba + Sr)) suggest that they formed from the hydrothermal alteration of the enclosing rhyolites although a possible contribution of kaolinite from residual processes cannot be discounted. δ18O values of kaolinite range from +4.8 to +10.3‰, and δD varies from −88 to −116‰. The presence of pyrophyllite in Blanquita mine suggests temperatures of formation &gt;350°C. The calculated isotopic composition of the fluid in equilibrium with kaolinite at 350°C varies from +3.6 to +9.2‰ for O and from −74 to −103‰ for H. These values are compatible with fluids of magmatic origin or with fluids of surficial origin isotopically equilibrated with a rhyolitic magma at high temperatures. The isotope composition of kaolinite from the Blanquita and Equivocada mines is clearly different from kaolinite of residual origin from deposits in the Chubut and Santa Cruz provinces. The isotopic signature has proved to be a useful tool in discriminating between different genetic processes in the formation of kaolin deposits.

Subaqueous eruption and emplacement of OT2 in the Middle Miocene Iizuka Formation, Noto Peninsula, Japan

OT2 (Okada Tuff 2) is one of six rhyolitic tuff beds that occur in a Middle Miocene marine diatomite (lower Iizuka Formation) and are known as the Okada Tuff. OT2 is well-sorted, and thins and thickens infilling channels 1–3 m across and 0–30 cm deep with a thickness up to 30 cm. Small channels 10–30 cm across and 1–10 cm deep are commonly superimposed within wider channels. A thin diatomite layer is ripped from the substrate up to a level immediately above the small channels. The basal part beneath the ripped-up diatomite layer is dominated by ash components with vague wavy stratifications. Above the ripped-up diatomite layer, diatomite clasts are dispersed, and pumice lapilli increase upward in amount and size with local interruption at a middle lower level by a poorly defined, pumice lapilli poor thin layer. Many glass and pumice shards are blocky with curviplanar surfaces and jigsaw cracks, presumably produced by phreatomagmatic explosions. The poorly defined two layers of OT2 are interpreted to be ash turbidites individually produced by discrete phreatomagmatic explosions. Lenticular bedforms and channels indicate that the currents were fully turbulent and unsteady, fed by the submarine eruptions. Upward coarsening of pumice probably represents settling lag of hot pumice. OT2 perhaps provides potential diagnostic features to identify eruption-fed turbidites.

Eustatic and Tectonic/Volcanic Control in Sedimentary Bentonite Formation — A Case Study of Miocene Bentonite Deposits from the Pannonian Basin

AbstractSeven sedimentary bentonite deposits were investigated in the Miocene series of the Pannonian Basin. The following stratigraphic and genetic characteristics were significant: (1) all deposits were formed within a transgressive series of a given Miocene sequence; and (2) it is possible that the source material of the bentonites is rhyolitic, confirmed by radiometric data proving simultaneous rhyolite tuff volcanism.A detailed investigation on three lithologically different bentonite horizons within the same transgressive series was made at Sajoábaábony to determine the source material and to determine the causes of the differences. X-ray diffraction, differential thermal analysis and geochemical data of the different lithological types show that they all have rhyolitic source material, although in the case of the lowermost horizon the existence of reworked material from an underlying andesite tuff series is also presumed. The main difference is the degree of weathering. Considering the ratio between the amorphous phase and the montmorillonite, the amorphous volcanic glass can be regarded as the main source of the montmorillonite formation. The differences in the degree of alteration can be related to the changing characteristics of the tuff accumulation and the sedimentation. Transgression decreases the sedimentation rate allowing the optimal alteration of the amorphous phase. The increasing intensity of the tuff accumulation can also limit the bentonite formation because rapid deposition and burial present too little time for the optimal alteration of the amorphous phase.Summarizing the results from the stratigraphic interpretation of the bentonite deposits and from the comparative analyses of the different bentonite horizons within the same transgressive systems tract, we can state that the relationship of the tectonic-related tuff accumulation and the eustasy-related sedimentation rate can affect both the possibility of bentonite formation in macro-scale and the degree of bentonitization in micro-scale.

Weathering forms of rhyolite tuffs

Weathering forms of rhyolite tuffs

Large-volume, low-δ18O rhyolites of the central Snake River Plain, Idaho, USA

The Miocene Bruneau-Jarbidge and adjacent volcanic fields of the central Snake River Plain, southwest Idaho, are dominated by high-temperature rhyolitic tuffs and lavas having an aggregate volume estimated as 7000 km3. Samples from units representing at least 50% of this volume are strongly depleted in 18O, with magmatic feldspar δ18OVSMOW (Vienna standard mean ocean water) values between −1.4‰ and 3.8‰. The magnitude of the 18O depletion and the complete lack of any rhyolites with normal values (7‰–10‰) combine to suggest that assimilation or melting of a caldera block altered by near- contemporaneous hydrothermal activity is unlikely. Instead, we envisage generation of the high-temperature rhyolites by shallow melting of Idaho Batholith rocks, under the influence of the Yellowstone hotspot, affected by Eocene meteoric-hydrothermal events. The seeming worldwide scarcity of strongly 18O-depleted rhyolites may simply reflect a similar scarcity of suitable crustal protoliths.

Provenance history of a Late Triassic‐Jurassic Gondwana margin forearc basin, Murihiku Terrane, North Island, New Zealand: Petrographic and geochemical constraints

The Murihiku Terrane in the North Island was a forearc basin adjacent to a volcanic arc along the eastern margin of Gondwana during the Mesozoic. The rocks that infill the basin are mainly volcaniclastic sandstones and mudstones, often turbiditic, with sparse shellbeds, rhyolitic tuffs, carbonaceous sandstones, plant beds, concretionary horizons, and rare thick granitoid‐rich conglomerates. Petrographic studies of the rock fragments in the sandstones show that andesites are the dominant lithic type, but there is a wide range of other lithologies, including dacites, rhyolites, ignimbrites, granitoids, quartzofeldspathic mica schists, rare amphibolites, and reworked mudstones and sandstones. The sandstones are texturally and mineralogically immature and suggest deposition relatively close to a source of high relief, undergoing physical rather than chemical weathering in cool‐ to cold‐temperate conditions. Geochemical analyses of 67 whole‐rock volcaniclastic sandstones and siltstones indicate that they were derived from an active and dissected volcanic arc in a convergent margin setting built upon relatively thin continental crust. Modal petrographic data and whole‐rock geochemistry both confirm that there were systematic variations with time in the composition of clastic material being supplied to the basin. From the Late Triassic to Middle Jurassic, there was a decrease in silicic volcanic material, plutonics, and metamorphics, and an increase in the supply of andesitic detritus. This was followed in the Late Jurassic by a broader range of volcanic detritus, varying from basaltic andesite to rhyolite, which may have been caused by progressive extension of the volcanic arc and thinning of the crust, a precursor to the breakup of Gondwana in the Early‐Middle Cretaceous. Comparison with the Southland segment of the Murihiku Terrane in the South Island suggests that there were significant along‐arc source variations, with relatively less silicic but greater andesitic and continental crust contributions in the North Island than in Southland. This may be analogous to the modern Taupo‐Kermadec arc where there is a south—north along‐arc transition from a continental to an oceanic arc.

Secular geochemical variations of the Lower Cretaceous siliciclastic rocks from central Tibet (China) indicate a tectonic transition from continental collision to back-arc rifting

Fifty-five Lower Cretaceous siliciclastic rocks from central Tibet were analyzed for major and trace elements, in an attempt to evaluate their provenance and the type of tectonic setting in which they were deposited. The spatial–temporal variation of both sediment maturity indices (SiO 2/Al 2O 3, La/V, and Zr/TiO 2), the ratios of immobile trace elements (Cr/Th, Th/Sc, Th/Co, La/Sc, and La/Co), and SiO 2, K 2O/Na 2O, Eu/Eu*, (Gd/Yb) n , and ferromagnesian contents, displays that the Berriasian–Valanginian (BV) sediment could have been derived from northern orogenic source areas, whereas the Hauterivian–Lower Barremian (HB) sediment could have been closely related to the syndepositional volcanism in the northern Lhasa block itself. The Na 2O/Al 2O 3–K 2O/Al 2O 3 relations also place approximately half of HB samples in the field of volcanic rocks of back-arc rifting origin, whereas little volcanic clasts exist in the BV rocks. There are distinct signatures that the mafic/ultramafic materials in the BV rocks were derived from oceanic island basalts (ophiolites) in the Bangong–Nujiang suture. Therefore, the collisional orogen due to the Qiangtang–Lhasa collision should have played a key role in supply of the sediments during BV time. Bimodal sedimentary basaltic and rhyolitic tuffs, with respective distinguished geochemical features, exist in the HB sequence. Therefore, a distinct tectonic transition from continental collision to back-arc rifting in central Tibet is believed to have occurred during Early Cretaceous time. Extreme depletion of Na of most Lower Cretaceous samples compared to the upper crust, and their high average chemical index alteration (CIA) values and Th/U ratios, point out the source rocks formed in tropical climatic regions with little tectonic uplift, although the weathering degree of the source rocks for HB samples could likely have been affected by the syndepositional volcanism. During Middle Cretaceous time, the entire southern Eurasian margin could have been characterized by a series of back-arc rift basins (including those in Kohistan and Ladakh).

Thermal and thermo-catalytic degradation of high-density polyethylene waste

Thermal and thermo-catalytic cracking behaviour of a commercial high-density polyethylene (HDPE) waste was investigated in the temperature range of 400–450 °C in a laboratory scale pyrex batch reactor. An equilibrium fluid catalytic cracking catalyst (FCC), HZSM-5 and a clinoptilolite containing rhyolite tuff were used as catalysts. The presence of a catalyst significantly altered both the yield and composition of the products and differences were found between catalysts having different acidities, micro- and macropore surface areas. The yield of gases increased in the order: non-thermo-catalytic cracking < clinoptilolite < FCC < HZSM-5 catalysed cracking, while the yields of liquid products increased in the sequence of non-thermo-catalytic cracking < clinoptilolite < HZSM-5 < FCC catalysed cracking. The catalysts significantly affected the isomerization of the olefin double bond and of the carbon-chain of the cracked products, the olefin content of liquids did not differ considerably. The effect of the catalysts on the yields and structure of products were getting less significant with increasing cracking temperature. The range of the carbon atom number in liquids was C 5–C 28 in non-thermo-catalytic cases, whereas using catalysts it was C 5–C 25, but the temperature of cracking had a noticeable effect on the distribution of carbon atom. Each liquid fraction is of low sulphur content, which may be advantageous for application as fuel.

The Ixtacamaxtitlán kaolinite deposit and sinter (Puebla State, Mexico): a magmatic‐hydrothermal system telescoped by a shallow paleoaquifer

AbstractThe Ixtacamaxtitlán hydrothermal deposit is made up of a succession, from bottom to top, of: (1) a porphyritic subvolcanic body, crosscut by quartz veins, and a stockwork with subordinate sulfides (pyrite and chalcopyrite), showing propylitic alteration haloes overprinting a previous potassic alteration event (biotitization); (2) an overlying, kaolinized lithic‐rich rhyolitic tuff; and (3) a layered opal deposit with preserved sedimentary structures. This vertical zonation, coupled with the distribution of the alteration assemblages, lead us to the interpretation of the whole as a porphyry‐type deposit grading upwards to a barren, steam‐heated, acid‐leached, kaolinite blanket with a partially preserved silica sinter on top. Both the fluid inclusion study carried out on the veins and stockwork, and the stable isotopic analyses of the kaolinized bodies, suggest the presence of two major hydrothermal events. The older event is characterized by the occurrence of hot hypersaline fluids (up to 320°C and 36 wt% NaCl equivalents), likely of magmatic origin, closely associated with the emplacement of the underlying early Miocene porphyry‐type deposit. The later event is characterized by the presence of cooler and dilute fluids (up to 140°C and 4 wt% NaCl equivalents) and by advanced argillic alteration close to the paleosurface. The calculated isotopic composition of water in equilibrium with the kaolinitic sequence plots close to and underneath the meteoric water line, partially overlapping the Los Humeros present‐day geothermal fluids. This evidence coupled with the petrographic observations suggests that steam‐heated phreatic waters altered the lithic‐rich rhyolitic tuff. This would have occurred when acid vapors, exsolved from deeper hydrothermal fluids by boiling, reached the local paleowater table and condensed, after a sector collapse that changed the system from lithostatic to hydrostatic conditions.

Kinetic Model of the Chemical Recycling of Waste Polyethylene into Fuels

Tertiary recycling methods offer potential possibilities for the utilization of waste plastics. Thermal and catalytic degradation of low-density polyethylene wastes were studied in a batch reactor under mild cracking conditions to derive liquid cracking products of olefins and paraffins. Zeolite catalysts with different activities were studied: a commercial equilibrium FCC catalyst, an H form ZSM-5 catalyst, and a natural clinoptilolite containing rhyolite tuff. The catalytic processes could convert waste polymers into valuable hydrocarbons. The properties of products could be modified by the use of catalysts. The olefin content and the position of the olefinic double bonds were investigated with IR spectrometry. In the presence of catalysts the olefin content increased, and the terminal bonds migrated into internal positions. Furthermore the liquid products had lower average molecular weights than those obtained without catalysts. The activation energies of the cracking reactions with or without catalysts were determined. The apparent activation energy was lower in catalytic than that in thermal degradation. The cracking reaction rates were modelled and the reaction rate constant of each hydrocarbon type was calculated. The model gives a good approximation of the experimental results obtained from the degradation of LDPE waste in the presence and absence of catalysts. Under the studied parameters, liquids were formed with suitable properties for further uses (e.g. low sulphur content, low pour point).

Vestiges of an Iapetan rift basin in the New Jersey Highlands: implications for the Neoproterozoic Laurentian margin

Thin, discontinuous remnants of Neoproterozoic intracratonic rift-basin deposits of the Chestnut Hill Formation occur in the western New Jersey Highlands. These deposits form an important link between well-documented Iapetan rift-basins in both the northern and southern Appalachians. The close spatial relations of Chestnut Hill rocks to Paleozoic sedimentary rocks open the possibility that additional Iapetan rift-basins could be concealed beneath the rocks of the Valley and Ridge Province to the west indicating a much broader zone of rifting than has been previously proposed. The Chestnut Hill Formation is intermittently exposed along a 100 km-long band that extends northeast from Pennsylvania nearly to New York State. The lower part of the Chestnut Hill Formation is composed of interbedded lithic pebble- to boulder-conglomerate and feldspathic sandstone grading upward into interbedded phyllite, feldspathic and quartz sandstone, local paleosaprolite, quartz-pebble conglomerate, thin limestone lenses, volcanic, and volcaniclasic rocks, abundant bedded ironstone (hematite ore), and ultimately into diamictites that are interpreted as possible tilloids and containing rounded intra and extrabasinal clasts of the other lithologies. Extensive soft-sediment deformation, cross bedding, and clastic dikes are common in all but the lowest and upper facies. Banded hematite layers occur preferentially in fine-grained tuffs and tuffaceous sediments, but hematitization has affected most lithologies. Volcanic rocks consist of altered rhyolitic tuffs and lapilli tuffs that are interbedded with sediments. The Chestnut Hill Formation is interpreted to have been deposited in early alluvial, and later a complex of fluvial, lacustrine and deltaic environments. Provenance studies based upon petrographic and geochemical analysis of clastic rocks indicate that the sediments are predominantly immature and reflect derivation from local uplifted felsic basement sources in a rifted-margin tectonic setting. Low to moderate weathering of the source rocks is indicated by the geochemistry of most samples, as is the locally intense effect of hydrothermal alteration. Most occurrences of the Chestnut Hill Formation are associated with major faults that exhibit normal movement of apparent Neoproterozoic age. Rocks from the Morgan Hill fault near Easton, Pennsylvania display consistent normal shear sense and vary from low temperature S-C mylonites to breccia that contains deformed pieces of Chestnut Hill Formation.

Formation of Nontronite from Oxidative Dissolution of Pyrite Disseminated in Precambrian Felsic Metavolcanics of the Southern Iberian Massif (Spain)

AbstractThis paper describes a rare occurrence of nontronite associated with sulfide-bearing felsic metavolcanics, providing evidence of colloidal deposition in open spaces as result of a low-temperature water-rock interaction. Microbotryoidal masses of green nontronite with impurities of kaolinite, illite, barite, amorphous silica and iron oxyhydroxides are found as vein and cavity fillings in deeply kaolinized rhyolites and rhyolitic tuffs of Precambrian age, at Oliva de Merida in SW Spain. Clay mineral characterization has been carried out by X-ray diffraction, infrared spectroscopy, thermal analysis, analytical electron microscopy and stable isotope (oxygen and hydrogen) analysis. Nontronite was formed under low-temperature alteration conditions, from a continuous sequence of reactions and aqueous solution compositions, involving two basic processes that acted in concert: oxidative dissolution of pyrite and hydrolysis of K-feldspar. After acidity neutralization, dissolved silica released by incongruent dissolution of K-feldspar reacted with ferric sulfate derived from pyrite oxidation to form nontronite under oxidizing conditions, in the presence of relatively warm meteoric water.

Structural geology of the proposed site area for a high-level radioactive waste repository, Yucca Mountain, Nevada

Geologic mapping and fracture studies have documented the fundamental patterns of joints and faults in the thick sequence of rhyolite tuffs at Yucca Mountain, Nevada, the proposed site of an underground repository for high-level radioactive waste. The largest structures are north-striking, block-bounding normal faults (with a subordinate left-lateral component) that divide the mountain into numerous 1–4-km-wide panels of gently east-dipping strata. Block-bounding faults, which underwent Quaternary movement as well as earlier Neogene movement, are linked by dominantly northwest-striking relay faults, especially in the more extended southern part of Yucca Mountain. Intrablock faults are commonly short and discontinuous, except those on the more intensely deformed margins of the blocks. Lithologic properties of the local tuff stratigraphy strongly control the mesoscale fracture network, and locally the fracture network has a strong influence on the nature of intrablock faulting. The least faulted part of Yucca Mountain is the north-central part, the site of the proposed repository. Although bounded by complex normal-fault systems, the 4-km-wide central block contains only sparse intrablock faults. Locally intense jointing appears to be strata-bound. The complexity of deformation and the magnitude of extension increase in all directions away from the proposed repository volume, especially in the southern part of the mountain where the intensity of deformation and the amount of vertical-axis rotation increase markedly. Block-bounding faults were active at Yucca Mountain during and after eruption of the 12.8–12.7 Ma Paintbrush Group, and significant motion on these faults postdated the 11.6 Ma Rainier Mesa Tuff. Diminished fault activity continued into Quaternary time. Roughly half of the stratal tilting in the site area occurred after 11.6 Ma, probably synchronous with the main pulse of vertical-axis rotation, which occurred between 11.6 and 11.45 Ma. Studies of sequential formation of tectonic joints, in the context of regional paleo-stress studies, indicate that north- and northwest-striking joint sets formed coevally with the main faulting episode during regional east-northeast–west-southwest extension and that a prominent northeast-striking joint set formed later, probably after 9 Ma. These structural analyses contribute to the understanding of several important issues at Yucca Mountain, including potential hydrologic pathways, seismic hazards, and fault-displacement hazards.