Weathering Of Volcanic Rocks Research Articles

Carboniferous–Permian interglacial warming and volcanism temporally linked to the world's oldest alkaline lake deposit of the Fengcheng Formation, NW China

In addition to being an important lacustrine hydrocarbon source rock, the Fengcheng Formation possesses well-preserved sodium‑carbonate evaporite units and tuff beds. Known ancient alkaline salt-lake deposits bearing sodium‑carbonate evaporite minerals like the Late Paleozoic Fengcheng Formation are limited beyond the modern day. However, hitherto the absolute age of the alkaline lacustrine Fengcheng Formation of the Junggar Basin (China) is debated (Late Carboniferous and/or Early Permian), and therefore its temporal link to a specific stage of the Late Paleozoic Ice Age (LPIA) remains unclear. Here, new ReOs geochronology demonstrates that the Fengcheng Formation is predominately of Late Carboniferous-age (304.4 to 297.3 Ma), and therefore its deposition coincides with the interglacial climate warming interval between glaciation C4 and P1 of the LPIA and not the younger interglacial stages as previously proposed. The ReOs isotope systematics indicate that the lake water column during the deposition of the Fengcheng Formation had a relatively unradiogenic Os (187Os/188Os, Osi) isotope composition (0.32 to 0.36), which is in contrast to the typical radiogenic Osi recorded for lacustrine deposits throughout geological time. The unradiogenic Osi for the Fengcheng Formation ties the source of the Os in the lake to the weathering of adjacent mafic volcanic rocks and/or hydrothermal input (∼0.13). As a result, the penecontemporaneous relationship to the Late Paleozoic interglacial climate warming (causing enhanced evaporation) coupled with weathering of volcanic rocks and/or hydrothermal fluid input into the lake is considered to have been mechanistic in the formation of an alkaline salt lake dominated by sodium and carbonate.

Provenance of the Nb-rich bauxite and Li-rich claystone at the base of the Heshan Formation in Pingguo, Guangxi, SW China: Constrained by U–Pb ages and trace element contents of detrital zircon

The base of the Upper Permian Heshan Formation in Pingguo County, Guangxi Province, SW China, can be divided into two layers: a lower Nb-rich bauxite layer and an upper Li-rich claystone layer. The sources of these layers may have played a critical role in their formation. Detrital zircon provenance analysis is an effective method for identifying sediment provenance; however, it has not been applied previously to the Nb-rich bauxite and Li-rich claystone, thus limiting our knowledge of the sources of these two layers. Detrital zircons from the bauxite and the claystone layers yield a single age peak at ∼ 260 Ma, which is consistent with the age of the Emeishan Large Igneous Province (LIP; 263–257 Ma) and the Permian Palaeo-Tethyan magmatic arcs (300–260 Ma). In contrast, detrital zircons from the two layers yield different trace element signatures. Yttrium and Hf contents and U/Yb ratios of detrital zircon grains from the bauxite layer overlap with those of continental and oceanic crust and have similar Hf/Th, Th/Nb, Th/U, and Nb/Hf ratios to those of zircons of within-plate and magmatic arc origins. These results indicate that the bauxite was sourced from volcanic rocks or ash related to the Emeishan LIP and the Permian Palaeo-Tethyan magmatic arcs. However, detrital zircons from the claystone have similar Hf/Th, Th/Nb, Th/U, and Nb/Hf ratios to only those of zircon from magmatic arcs, suggesting a single Permian Palaeo-Tethyan magmatic arc source. Thus, our results reveal an abrupt change in the provenance of zircons at the base of the Heshan Formation. The lower Nb-rich bauxite layer was derived from the weathering of volcanic rocks or ash related to the Emeishan LIP and the Permian Palaeo-Tethyan magmatic arcs, whereas the upper Li-rich claystone layer was derived principally from the weathering of volcanic ash related to the Permian Palaeo-Tethyan magmatic arcs. Since Nb is an immobile element, volcanic rocks/ash related to the Emeishan LIP and the Permian magmatic arc could represent a source for bauxite and Nb. In contrast, Li is chemically highly mobile in the Earth’s surface environment and its origin cannot be constrained easily, further work is needed to identify the sources of Li.

Weathering of Volcanic Rocks in Baturappe Formation and Its Implications for Foundation Planning of Pamukkulu Dam, South Sulawesi, Indonesia

The presence of weathered volcanic rocks in Pamukkulu area is important in determining the location and construction of the Pamukkulu main dam and plinth structure. In general, volcanic rocks have poor geotechnical characteristics due to varying degree of weathering, influence of groundwater, and systematic joints produced by cooling lava. This study aims to study the type and quality of weathered rock around the main dam and plinth area. First, mapping of surface geology was carried out. To evaluate the degree of weathering, rock resistivity data is used around the main dam area. Whereas for the plinth area, the bearing capacity of soil/rock is evaluated using Plate Bearing Test (PBT) and Standard Penetration Test (SPT). Bearing capacity obtained from PBT that were carried out on rock surface can be used to interpret rock weathering. In addition to SPT, the coring from rock drilling was also evaluated using Rock Quality Designation (RQD). It was found that the rock up to 20 m depth are of poor quality. It is recommended that the poor-quality rock is removed until moderately dense rock layer is found.

Soilscapes and geoenvironments at Stansbury Peninsula, Nelson Island, maritime Antarctica

Considering that the extreme variation in Antarctica’s climate has important effects on soil properties and distribution, characterizing and mapping surface landforms in ice-free areas is strategic. We aimed to evaluate the pedogenetic processes, as well as classify and map the different types of soils at Stansbury Peninsula (ST), Nelson Island. In order to stratify the whole environment area of the Peninsula, we used the geosystems’ approach, identifying the landscape unit of geoenvironment. We mapped geoenvironments and soils, performing physical–chemical and mineralogical characterizations. Our results suggest that compared to other islands in the South Shetlands, the active layer at Stansbury is thin, so that it is more susceptible to climate change, representing a Key monitoring site for climate change studies. The northwestern area of Stansbury Peninsula is the preferential area for marine birds, mammals nesting and resting places. Hence, higher values of nutrients are recorded in their soils. The climate at ST is harsh due to strong northwest polar winds, which promote widespread wind ablation, erosion and contribute to physical weathering, with a high prevalence of periglacial processes under wet conditions, which is typical of this zone of South Shetlands areas, exposed to the Drake passage. Soils at ST are predominantly skeletal, gravelly, with little organic matter and little fauna activity, poor vegetation cover, and high levels of base saturation suggesting limited weathering of volcanic rocks. Soils display a strong characteristic of physical weathering due freezing and thawing process, resulting in typical patterned ground (cryoturbation forming Cryosols), under active periglacial processes. The unique proglacial and periglacial geoenvironments of Stansbury are influenced by melting water and widespread snow banks, under wet polar oceanic climate condition.

Why was there a Neoproterozoic Snowball Earth?

Snowball Earth is a nickname for the Cryogenian Period, 717 to 635 million years ago, when glaciers extended well into tropical latitudes. Such extreme planetary chilling has been blamed on increased carbon sequestration by life in the sea, by chemical weathering of volcanic rocks, and by life on land. Discovery of pre-Cryogenian paleosols containing a diverse eukaryotic soil microbiome including fungi and testate amoebae suggests that glacial expansion was caused by increased carbon consumption due to evolutionary advances in life on land. Microfossils in paleosols of the 776–729 Ma Chuar Group of Arizona include cyanobacterial microfossils (Siphonophycus typicum and Polytrichoides linearis), fungal hyphae (Tortotubus protuberans and Mucorites ripheicus), fungal vesicles (Palaeoglomus strotheri), and testate amoebae (Cycliocyrillium torquatum, Trigonocyrillium horodyskii, Melanocyrillium hexadiadema, Bonneia dachrychares, and Carabinia granosa). Also found were fragments of enigmatic lichen-like, multicellular thalli described here as Porterra dehlerae gen. et sp. nov. A few fossils by themselves did not change the world, but Chuar and other paleosols reveal transition from low productivity Gypsids to high productivity Calcids. Increased biomass, soil respiration, and nutrient depletion documented from chemical analyses of paleosols accelerated silicate weathering and carbon consumption, diminishing CO2 greenhouse, and ushering in Snowball Earth.

THE IMPACT OF GEOLOGICAL AND RAINFALL CHARACTERISTICS ON SLOPE STABILITY ANALYSIS IN SHALLOW LANDSLIDE MODELLING USING THE TRIGRS MODEL

West Java Province has a high potential for susceptibility to landslides. West Bandung Regency is one of the regions in West Java which has high occurrences of landslides triggered by rainfalls. Rainfall-induced landslides in this region have occurred 165 times during rainfall in the last ten (10) years. An effort to predict the landslide occurrences in this region requires the knowledge of factors affecting the hillslope stability. This paper aims to (1) evaluate the parameters affecting slope stability, (2) determine rainfall characteristics affecting slope, and (3) evaluate the performance of TRIGRS modelling. This study applies a deterministic method using the model of TRIGRS to analyze the effect of one-day antecedent rainfall intensity on the instability of hillslopes. The parameters used in this study are based on field investigation, geological and topographical maps, soil engineering properties, and historical rainfall data. The effect of one-day antecedent rainfall intensity was considered in two scenarios, i.e. 6 hours and 12 hours of antecedent rainfall. Modelling results show that hillslope instability in the study areas is affected by many factors, such as hilly morphology, steep slopes, low shear strength parameters, weathering of volcanic rocks, and the lineament density of geological structures. The modelling simulation also indicates that landslide occurrences in this study area are initiated by a long antecedent rainfall period. Based on ROC analysis, the TRIGRS model shows that the model performance evaluation is in good agreement for modelling slope stability against actual landslide events.

Controls on distributions of sulphate, fluoride, and salinity in aquitard porewater from the North China Plain: Long-term implications for groundwater quality

Groundwater interacts with porewater within clay-rich sequences. However, the effects of aquitard porewater on groundwater quality are generally poorly understood. This study explores potential links between porewater chemistry in a thick clay-rich sequence of Quaternary sediments, and groundwater quality in the North China Plain (NCP), one of China’s most important water supply aquifers. Previous studies have hypothesised the release of salinity, fluoride, and sulphate from aquitards in response to intensive pumping, without explicitly analysing the chemistry of aquitard porewaters. This study further investigated this hypothesis through targeted analysis of porewater chemistry profiles through the NCP sedimentary sequence. Two cores were analysed – one in the central plain (Hengshui, HS) and the other in the coastal plain (Cangzhou, CZ). Both areas have experienced significant groundwater extraction over recent decades from confined aquifers inter-layered with clays. Aqueous extraction (AE) and squeezing (SE) methods were both used to assess porewater chemistry. The hydrochemical characteristics of groundwater are closer to those of the squeezed porewater, which can be regarded as more closely corresponding to in-situ porewater compositions than AE profiles. Chloride was generally consistent between the two methods, while sulphate and fluoride concentrations were greater using AE, ranging from 340 to 48,700 mg/L and 8.8 to 144 mg/L, compared to 350 to 8860 mg/L and 0.2 to 4.3 mg/L using SE, respectively. The much greater concentrations of fluoride derived from the AE method indicates a high susceptibility to release during changes to the ambient geochemical environment. While the primary fluoride sources in groundwater and porewater are likely minerals from weathering of volcanic rocks in the sedimentary sequence, release into groundwater appears linked to common geochemical influences such as high Na/Ca ratio (increased during cation exchange) and the presence of iron and aluminium oxides (providing sorption sites). The primary source of solutes is interpreted to be different in each profile, with evapotranspiration important in the central plain and marine solutes important in the coastal profile, indicating significant storage of salinity, fluoride, and sulphate within low permeability sediments. Our data supports the hypothesis that release of these ions is driving degradation of groundwater quality, in response to decades of intensive groundwater extraction, which is an ongoing threat to the quality of groundwater in the NCP.

Processes explaining the origin and evolution of groundwater composition in the Andean Precordillera and Altiplano of the Tarapacá Region of northern Chile

In the arid area of northern Chile, groundwater resources in the Andean formations are essential for native populations, ecological services, mining, and other human activities. Validated conceptual hydrogeological models are required for current and future water and land management. This work aims to explain the processes controlling the origin and distribution of recharge and groundwater composition in the Andean Precordillera and Altiplano of the Tarapacá Region of northern Chile, using major solutes in spring, river, and well water, and the stable and radioactive isotopes of water oxygen, hydrogen, and dissolved inorganic carbon. The waters are mainly of the Na-Ca-SO4 type. Processes controlling the chemical evolution of waters are atmospheric dust contribution, evapo-concentration, and enhanced volcanic rock weathering, as well as halite dissolution in some locations. The isotopic composition of Precordillera eastern flank water samples follows an evaporation line, while those in the western flank, in the Altiplano, follow a line that is parallel to the local meteoric line, suggesting unsaturated zone evaporation processes of infiltrated rainfall. δ13CDIC contents (-2 to -27‰) indicate mixing processes, volcanic CO2 in the Altiplano, and calcite dissolution in some sectors. In the western depression, the only recharge is due to water infiltration in creek channels. In the highland areas, 5–25% of precipitation produces recharge. The estimated groundwater renewal time in the Precordillera was 3–14 kyr. The piezometric elevation in the Precordillera due to low-permeability intrusive rocks and local recharge prevents the east-west groundwater transfer from the Altiplano to the western depression and explains why the volcanic CO2 in the Altiplano basins is not observed on the western flank. These results provide new insights for the evolution of water quality in volcanic aquifers in arid environments and provide considerations for estimating groundwater residence times using radiocarbon in areas influenced by volcanic CO2.

Mercury abundance and isotopic composition indicate subaerial volcanism prior to the end-Archean “whiff” of oxygen

Earth's early atmosphere witnessed multiple transient episodes of oxygenation before the Great Oxidation Event 2.4 billion years ago (Ga) [e.g., A. D. Anbar et al., Science 317, 1903-1906 (2007); M. C. Koehler, R. Buick, M. E. Barley, Precambrian Res. 320, 281-290 (2019)], but the triggers for these short-lived events are so far unknown. Here, we use mercury (Hg) abundance and stable isotope composition to investigate atmospheric evolution and its driving mechanisms across the well-studied "whiff" of O2 recorded in the ∼2.5-Ga Mt. McRae Shale from the Pilbara Craton in Western Australia [A. D. Anbar et al., Science 317, 1903-1906 (2007)]. Our data from the oxygenated interval show strong Hg enrichment paired with slightly negative ∆199Hg and near-zero ∆200Hg, suggestive of increased oxidative weathering. In contrast, slightly older beds, which were evidently deposited under an anoxic atmosphere in ferruginous waters [C. T. Reinhard, R. Raiswell, C. Scott, A. D. Anbar, T. W. Lyons, Science 326, 713-716 (2009)], show Hg enrichment coupled with positive ∆199Hg and slightly negative ∆200Hg values. This pattern is consistent with photochemical reactions associated with subaerial volcanism under intense UV radiation. Our results therefore suggest that the whiff of O2 was preceded by subaerial volcanism. The transient interval of O2 accumulation may thus have been triggered by diminished volcanic O2 sinks, followed by enhanced nutrient supply to the ocean from weathering of volcanic rocks causing increased biological productivity.

Landscape modification due to agricultural irrigation: Carbonate tufa formation on Tenerife, Canary Islands, Spain

The use of irrigation water on Tenerife has modified the hydrogeological system resulting in the formation of an anthropogenic carbonate tufa . The processes involved in its formation are: 1) infiltration of rainwater and mixing with groundwater rich in volcanic carbon dioxide (CO 2 ), 2) weathering of volcanic rocks increasing the ion content of the water, 3) extraction of this water from subterranean galleries and channels and spilling it onto the Lomo Morin slope, 4) mechanical CO 2 degassing and photosynthetic activity driving calcite precipitation. The tufa consists of boundstones of algae and coated stems of reeds, along with laminated microbial crusts. Textures range from microspar to coarse mosaics to fibrous-radial calcite, all containing biogenic features. The presence of the green macroalga Blidingia is noticeable. Carbon stable isotope ratio ( δ 13 C) varies between -3.58 and +1.03‰ VPDB (Vienna Pee Dee Belemnite) and oxygen stable isotope ratio ( δ 18 O) between -6.77 and -5.00‰ VPDB, highlighting the largely meteoric origin of the water with some addition of deep CO 2 . A strontium isotope ratio ( 87 Sr/ 86 Sr) of 0.7030 confirms the weathering of volcanic rocks as the source of calcium. The Lomo Morin tufa provides a scale model of how water use contributes to rapid landscape change through alterations in the hydrogeological system. It also provides a good example of a CO 2 sink in a continental setting, and is comparable to the global carbon cycle, which mostly involves marine limestones. Although anthropogenic processes are commonly viewed negatively, the human modification of the hydrogeological system reported in this study has produced changes to geological and ecological conditions that have increased the geo - and biodiversity of the island.

Ge and Si Isotope Behavior During Intense Tropical Weathering and Ecosystem Cycling

AbstractChemical weathering of volcanic rocks in warm and humid climates contributes disproportionately to global solute fluxes. Geochemical signatures of solutes and solids formed during this process can help quantify and reconstruct weathering intensity in the past. Here, we measured silicon (Si) and germanium (Ge) isotope ratios of the soils, clays, and fluids from a tropical lowland rainforest in Costa Rica. The bulk topsoil is intensely weathered and isotopically light (mean ± 1σ: δ30Si = −2.1 ± 0.3‰, δ74Ge = −0.13 ± 0.12‰) compared to the parent rock (δ30Si = −0.11 ± 0.05‰, δ74Ge = 0.59 ± 0.07‰). Neoforming clays have even lower values (δ30Si = −2.5 ± 0.2‰, δ74Ge = −0.16 ± 0.09‰), demonstrating a whole‐system isotopic shift in extremely weathered systems. The lowland streams represent mixing of dilute local fluids (δ30Si = 0.2 − 0.6‰, δ74Ge = 2.2 − 2.6‰) with solute‐rich interbasin groundwater (δ30Si = 1.0 ± 0.2‰, δ74Ge = 4.0‰). Using a Ge‐Si isotope mass balance model, we calculate that 91 ± 9% of Ge released via weathering of lowland soils is sequestered by neoforming clays, 9 ± 9% by vegetation, and only 0.2 ± 0.2% remains dissolved. Vegetation plays an important role in the Si cycle, directly sequestering 39 ± 14% of released Si and enhancing clay neoformation in surface soils via the addition of amorphous phytolith silica. Globally, volcanic soil δ74Ge closely tracks the depletion of Ge by chemical weathering (τGe), whereas δ30Si and Ge/Si both reflect the loss of Si (τSi). Because of the different chemical mobilities of Ge and Si, a δ74Ge‐δ30Si multiproxy system is sensitive to a wider range of weathering intensities than each isotopic system in isolation.

Decoding the origins and sources of clay minerals in the Upper Cretaceous Tununk Shale of south‐central Utah: Implications for the pursuit of climate and burial histories

AbstractClay minerals in fine‐grained marine sedimentary successions are most commonly considered to be detrital in origin and have been used extensively by geologists as indicators of palaeoclimate conditions in the hinterland. Most of these previous studies, however, were not designed to address in depth the potential effects of mixing clay minerals from multiple sources and the formation of authigenic clay minerals during early diagenesis on the ultimately observed clay mineral assemblages of fine‐grained marine sedimentary successions. Herein, clay minerals in shales and bentonites of the Tununk Shale Member in south‐central Utah were examined through integrated X‐ray diffraction and petrographic (scanning electron microscopy) analysis, to evaluate the various origins of clay minerals in this offshore mudstone succession. Clays in Tununk bentonites contain dominantly smectite (>80%) and a minor amount of kaolinite. Clays in Tununk shale samples consist dominantly of mixed‐layer illite/smectite with up to 45% illite‐like layers, small amounts of kaolinite and mica, and in places trace amounts of chlorite. Clay minerals in Tununk shale samples occur in the following three forms: (a) in clay‐dominated aggregates (i.e. smectite‐dominated altered volcanic rock fragments and illite/smectite‐dominated shale lithics); (b) in the fine‐grained matrix (mostly illite/smectite and minor amounts of mica and kaolinite); and (c) in intergranular and intragranular pore spaces (authigenic smectite, kaolinite and chlorite). Possible sources for the mixed‐layer illite/smectite in shales include (a) erosion of older smectite‐bearing mudstone successions and (b) weathering of volcanic rocks or volcanic debris that had been deposited on land. Most of the kaolinite and chlorite in the Tununk Shale were precipitated as pore‐filling cements, rather than having a terrigenous source (as weathering products). A comprehensive understanding of the multiple origins of clay minerals (e.g. terrigenous input, volcanic input, recycled sediments and diagenesis) in marine mudstone successions is critical when attempting to use clay mineral data for reconstructions of palaeoclimate and burial and thermal histories.

Enrichment of ion-exchangeable rare earth elements by felsic volcanic rock weathering in South China: Genetic mechanism and formation preference

Felsic volcanic rocks are an important source for generating ion adsorption-type rare earth element (REE) resources. To better understand the supergene enrichment of REEs related to felsic volcanic rock weathering, this study conducted an investigation of Indosinian felsic volcanic rocks from Guangxi, southwest China, which have been subjected to deep weathering. The weathering has formed large areas of thick regolith with the potential to host ion adsorption-type REE mineralization. The felsic volcanic rocks belong to the dacite–rhyolite series and are peraluminous, high-K, calc-alkaline, and REE-rich (325–376 ppm). Abundant REE-bearing accessory minerals are present in the felsic volcanic rocks, including titanite (average ΣREE = 15.1 wt%), allanite (average ΣREE = 14.9 wt%), and apatite (average ΣREE = 1510 ppm), and these three minerals contain an estimated 88.7% of the whole-rock REEs content. Significant REE enrichment is present in the felsic volcanic rock-derived regolith. A typical rhyolite-derived regolith profile has REE contents that increase from 376 ppm in the rhyolite to 1737 ppm in the regolith, representing an almost five-fold enrichment due to weathering. The REEs in the regolith are present in an ion-exchangeable form (iREEs), which accounts for 52%–87% of the total REEs present (TREEs). The occurrence of iREEs is closely linked to clay minerals, showing an affinity in the order halloysite > kaolinite > illite. Continuous operations of REEs by an eluviation–illuviation process from the source minerals (titanite + allanite + apatite) to sink minerals (kaolinite + halloysite + illite) results in an iREE-enriched zone in the middle and lower parts of the rhyolitic regolith. Notably, besides the studied Indosinian felsic volcanic rocks, there are multi-epochs of such lithology outcropped in South China. Their related iREEs mineralizations, however, are preferentially developed in the weathered terrains of early Yanshanian and Indosinian felsic volcanic rocks, and not in those of the Yanshanian and late Yanshanian felsic volcanic rocks. A comparison of the mineralized and barren units indicates that the iREEs mineralization hosted in felsic volcanic rocks regolith is controlled by some key endogenic and exogenic ore-forming factors. High initial REE concentrations in the unaltered felsic volcanic rocks, a suitable climate, and a relatively quiescent tectonic setting are favorable for the formation and preservation of iREEs mineralization.

Anomalous concentrations of arsenic, fluoride and radon in volcanic-sedimentary aquifers from central Italy: Quality indexes for management of the water resource

659 water samples from springs and wells in the Sabatini and Vicano-Cimino Volcanic Districts (central Italy) were analyzed for arsenic (As), fluoride (F−) and radon (222Rn) concentrations. Waters mostly sourced from a shallow and cold aquifer hosted within volcanic rocks, which represents the main public drinking water supply. Cold waters from perched aquifers within sedimentary formations and thermal waters related to a deep hydrothermal reservoir were also analyzed. The highest concentrations of As and F− were measured in the thermal waters and attributed to their enhanced mobility during water-rock interaction processes at hydrothermal temperatures. Relatively high concentrations of As and F− were also recorded in those springs and wells discharging from the volcanic aquifer, whereas waters hosted in the sedimentary units showed significantly lower contents. About 60% (As) and 25% (F−) of cold waters from the volcanic aquifer exceeded the maximum allowable concentrations for human consumption. Such anomalously high levels of geogenic pollutants were caused by mixing with fluids upwelling through faulted zones from the hydrothermal reservoir. Chemical weathering of volcanic rocks and groundwater flow path were also considered to contribute to the observed concentrations. Cold waters from the volcanic aquifer showed the highest 222Rn concentrations, resulting from the high contents of Rn-generating radionuclides in the volcanic units. Approximately 22% of these waters exceeded the recommended value for human consumption. A specific Quality Index (QI), comprised between 1 (very low) and 4 (very high), was computed for each water on the basis of As, F− and 222Rn concentrations and visualized through a spatial distribution map processed by means of geostatistical techniques. This map and the specific As, F− and 222Rn maps can be regarded as useful tools for water management by local authorities to both improve intervention plans in contaminated sectors and identify new water resources suitable for human consumption.

Behavior of the 238U, 235U, and 234U isotopes at weathering of volcanic rocks with u mineralization: a case study at the Tulukuevskoe deposit, Eastern Transbaikalia

The trend fractionation of the 238U and 235U isotopes and the extent of this process at the oxidative weathering of uranium minerals were evaluated by studying the variations in the U isotope composition of rocks, minerals, and fracture waters sampled in the quarry of the broadly known Tulukuevskoe uranium deposit in the Streltsovskoe ore field, eastern Transbaikalia. In the rock block in question, fine uranium minerals disseminated in the rocks were weathered under the effect of oxidizing fracture waters. Uranium isotope composition was measured in 22 water samples, eleven samples of the mineralized rocks, and eight uranium minerals. High-precision (±0.07‰, 2SD) measurements of the 238U/235U were carried out by MC-ICP-MS, using a 233U–236U double spike. The results involve the 238U/235U and 234U/238U ratios and the overall range of the δ238U variations determined in the rocks and waters (from –0.13 to –1.0‰ and from –0.22 to –0.59‰, respectively). Interaction between the waters and rocks induces U(IV) → U(VI) oxidation, U(VI) transfer into the aqueous phase, and 0.15–0.28‰ enrichment of U dissolved in the water in the 235U isotope. When the pitchblende is replaced by U(VI) minerals, the 238U and 235U isotopes also fractionate with ~0.3‰ enrichment of the younger U(VI) mineral phases in the light 235U isotope. The 238U/235U and 234U/238U ratios are proved to correlate, and hence, the fractionation of the 238U and 235U isotopes and the enrichment of the aqueous phase in the light 235U isotope proceed simultaneously with the well known shift in equilibrium the 238U–234U system with the accumulation of excess amounts of the 234U in the aqueous phase. Uranium leaching from uranium minerals, which is associated with the enrichment of the aqueous phase in excess amounts of the 234U isotope, can be viewed as a process that controls isotope fractionation in the 238U–235U system. This should be taken into account in describing the fractionation mechanism of the 238U and 235U isotopes at U(IV) → U(VI) oxidation. The fractionation of the 238U and 235U isotopes, which results in the isotopic "lightening" of U in the aqueous phase, largely controlled the complicated distribution pattern of the 238U/235U ratio in the quarry. In addition to isotope fractionation, this distribution was likely also affected by isotope exchange between uranium dissolved in the water and uranium in the finely dispersed minerals. The isotopically light uranium of the water could participate in forming U(VI) minerals at lower levels of the quarry.

Responses of the East Asian Summer Monsoon in the Low‐Latitude South China Sea to High‐Latitude Millennial‐Scale Climatic Changes During the Last Glaciation: Evidence From a High‐Resolution Clay Mineralogical Record

AbstractHigh‐resolution clay mineral assemblage combined with Nd and Sr isotopic compositions of Core MD12‐3434 located in the northern South China Sea was investigated to reveal terrigenous sediment response to the East Asian monsoon evolution during the last glaciation. Clay mineralogical variations indicate clear millennial‐scale oscillations that are mainly due to rapid changes in the proportion of smectite to illite and chlorite. Smectite is derived predominantly from rapid chemical weathering of volcanic rocks in Luzon under strong summer monsoon, while illite and chlorite are mainly sourced from Taiwan through reinforced physical erosion. Thus, the smectite/(illite + chlorite) ratio is adopted to reconstruct the East Asian summer monsoon evolution during the last glaciation. Rapid increases in the smectite/(illite + chlorite) ratio imply strengthened summer monsoon occurred during Dansgaard‐Oeschger and Bølling‐Allerød interstadials. In contrast, rapid decreases in the ratio indicate relatively weakened summer monsoon happened during Heinrich and Younger Dryas stadials. These millennial‐scale climatic signals documented by clay mineralogical compositions of deep‐sea sediments in the South China Sea can be better preserved in calm deeper‐water sedimentary environments. Our study highlights the prompt responses of the East Asian monsoon system to millennial‐scale climatic changes occurred in high‐latitude Northern Hemisphere through contemporaneous chemical weathering of volcanic rocks and/or sediment supply variations under strong physical erosion in the low‐latitude South China Sea, implying an atmospheric teleconnection from the North Atlantic to the Asian monsoon region.

Mineralogical signatures and sources of recent sediment in a large tropical lake

Two lead-210 (210Pb) dated sediment cores from Chapala Lake were studied to identify the mineral composition of the sediments and to discuss its relation with trace metals. Minerals and trace metals reflected the domain of volcanic rocks (i.e. basalts, rhyolite and andesite) that characterize Chapala Lake and the Lerma River watershed. Redundancy analyses (RDA) were used to identify the sedimentary variables (magnetic susceptibility, organic matter, sand content, and mineral composition) that could be related to elemental composition. Despite the distance between the two cores (7.3km) and hydrodynamic circulation the RDA showed that the main mechanism that controls the input of mineral species, and, therefore, element distributions in the lacustrine sediment of Chapala Lake, is related to the weathering of volcanic rocks in the Lerma-Chapala watershed, the consequent runoff and transport of fine grained catchment materials, and later in-lake processes. These findings highlight the importance of controlling watershed erosion to contribute to the improvement of the environmental quality of the lake.

MICROFOSSILS FROM RELICT MESOARCHEAN PALEOSOL DEVELOPED ON KOMATIITES FROM KOIKARY AREA, CENTRAL KARELIA

Relics of the paleosol of Mesoarchean age (3000–2930 Ma) developed on tuffs and lavas of komatiites from Koikary Area (Central Karelia) were studied. Microscopic tubular and spherical structures morphologically similar to fossilized remnants of microorganisms were found in the preserved part of the crust profile, represented by a layer of chloritite. The nature of these micro structures and the possibility of their interpretation as acidophilic bacteria inhabiting the land in the zone of chemical weathering of volcanic rocks are discussed.

Statistical analysis and source rock of the Minim-Martap plateau bauxite, Cameroon

The Minim-Martap plateau bauxite deposit, located between the Minim and the Martap villages, is one of the 11 plateaus within the Minim-Martap bauxite region. The plateau has an elevation of 1294 m above sea level, with three to more 30 m thickness of bauxite horizon. These plateaus were formed as result of supergene weathering of volcanic rocks occurring as dissected flow basalt landscapes that form relatively flat plateau rising steeply from the surrounding granites. The bauxite deposit of the plateau is lateritic, with the surface of the plateau been completely covered by indurated caps. Seventeen bauxite samples were collected from the plateau and prepared for geochemical analysis. Whole rock analysis was carried out using the X-ray Fluorescence technique and ICP-MS was used for trace elements investigation. Statistical analysis reveals that average values of Al2O3 (54.87%), Fe2O3 (7.17%), SiO2 (2.44%), and TiO2 (4.54%) indicate the plateau bauxite deposit is an of a world class standard with very little impurities compared to the standard major element contents of bauxite (> 40% A12O3, less than < 20% Fe2O3, and less than < 8% combined SiO2). Abundant trace elements include Zr, Ce, Sr, V, Ba, La, Nd, Ga, and Nb. Weathering due to chemical alteration indices using the Ruxton ratio and CIA approaches revealed the plateau have undergone intense weathering process that formed the bauxite deposit. Three different classification systems indicate it as a low iron-rich bauxite deposit. Precursor rock investigation indicates the origin of the bauxite is mafic, basaltic andesite igneous rocks with intermediate pH (basic–acidic characteristic).

Co-evolution of monsoonal precipitation in East Asia and the tropical Pacific ENSO system since 2.36 Ma: New insights from high-resolution clay mineral records in the West Philippine Sea

Clay mineralogical analysis and scanning electron microscope (SEM) analysis were performed on deep-sea sediments cored on the Benham Rise (core MD06-3050) in order to reconstruct long-term evolution of East Asian Summer Monsoon (EASM) rainfall in the period since 2.36 Ma. Clay mineralogical variations are due to changes in the ratios of smectite, which derive from weathering of volcanic rocks in Luzon Island during intervals of intensive monsoon rainfall, and illite- and chlorite-rich dusts, which are transported from East Asia by winds associated with the East Asian Winter Monsoon (EAWM). Since Luzon is the main source of smectite to the Benham Rise, long-term consistent variations in the smectite/(illite + chlorite) ratio in core MD06-3050 as well as ODP site 1146 in the Northern South China Sea suggest that minor contributions of eolian dust played a role in the variability of this mineralogical ratio and indicate strengthening EASM precipitation in SE Asia during time intervals from 2360 to 1900 kyr, 1200 to 600 kyr, and after 200 kyr. The EASM rainfall record displays a 30 kyr periodicity suggesting the influence of El Nino–Southern Oscillation (ENSO). These intervals of rainfall intensification on Luzon Island are coeval with a reduction in precipitation over central China and an increase in zonal SST gradient in the equatorial Pacific Ocean, implying a reinforcement of La Nina-like conditions. In contrast, periods of reduced rainfall on Luzon Island are associated with higher precipitation in central China and a weakening zonal SST gradient in the equatorial Pacific Ocean, thereby suggesting the development of dominant El Nino-like conditions. Our study, therefore, highlights for the first time a long-term temporal and spatial co-evolution of monsoonal precipitation in East Asia and of the tropical Pacific ENSO system over the past 2.36 Ma.