Geochemical Mass Balance Research Articles

Pyroclastic component influence on the weathering indices assessment in marine sediments – Lessons from Upper Ordovician of the Baltic Basin

The chemical weathering indices of the sediments can serve as tools for estimating the intensity of weathering in the sediment source rock area and its changes in response to the dynamic interplay of climate, tectonics, and bioevolution. The siliciclastic terrigenous component, comprising sediment particles formed during weathering, carries information regarding weathering. Nevertheless, the presence of volcanic material with silicate composition in the sedimentary record can introduce bias to the weathering estimate by incorporating pyroclastic material that has not undergone weathering but is susceptible to diagenetic alteration. Therefore, a comprehensive understanding of the nature and significance of the volcanic component in sediments is imperative for accurately interpreting weathering derived from sedimentary successions. Our study focuses on investigating the presence of the volcanic component and its influence on weathering indices calculation in the Upper Ordovician sedimentary succession of the Baltic Basin. The results from geochemical massbalance mixing modelling reveal that volcanic material is not confined solely to discrete altered volcanic ash beds (bentonites), and a substantial amount of volcanic material is dispersed or re-deposited into the host rock. Within the interval containing discrete bentonite beds, the volcanic component in the carbonate host rock sections ranges from 50% to 90%, and significantly impacts the terrigenous fraction weathering index calculations. This underscores the necessity of considering the nature and significance of the volcanic component when interpreting geochemical-mineralogical signals.

The “Net” Impact of Hydrothermal Venting on Oceanic Elemental Inventories: Contributions to Plume Geochemistry from the International GEOTRACES Program

Hydrothermal vents serve as a primary interface between the cold deep ocean and the warm oceanic crust. While early research showed that seawater-​rock interactions add to or remove elements from seawater during the generation of hydrothermal fluids, consideration of these fluid fluxes alone does not relay the total impact that hydrothermal systems have on seawater geochemistry. In addition, hydrothermal plumes, areas where hydrothermal fluids mix with ocean waters, are host to a range of particle precipitation and scavenging reactions that further modify gross hydrothermal fluid fluxes to define the total “net” hydrothermal impact on oceanic inventories. Here, we review the major discoveries made by the international GEOTRACES program regarding the geochemical transformations occurring within hydrothermal plumes. We classify each element into one of five categories based on its behavior in hydrothermal plumes, a spectrum spanning the geochemical mass balance between net hydrothermal source fluxes and net hydrothermal plume scavenging sinks. Overall, we celebrate the role that GEOTRACES has played in defining the extent and dynamics of hydrothermal plume geochemistry, which is a crucial lever for determining global hydrothermal impacts.

Microscopic weathering mechanisms of subflorescence and crust patterns in the Nankan Grotto, northern Sichuan, China

The mineralogy, geochemical mass balance, sulfur isotope, and micro-structure of the sandstone in the Nankan Grotto were analyzed to explore the weathering mechanisms of the subflorescence and crust patterns. The results revealed that the mineralogical compositions of the rock samples from the subflorescence and crust sites were quartz, feldspar, calcite, and clay minerals. The calcite content was much higher in the crust site than that in the subflorescence site. Based on the microscopic petrographic evidence, the crystallization stress of the thenardite and calcite and the swelling/shrinking stress of the clay minerals led to the detachment of the outer rock layer, after which subflorescence became visible on the newly exposed surface. Solution migration carried Ca2+ from the interior of the rock to the surface, where it combined with CO32− and SO42− to produce gypsum and calcite on the surface during the drying process, resulting in the formation of the crust. Therefore, the mineralogical composition (especially the calcite content) laid the foundation for the distinct development of the subflorescence and crust. In addition, the microstructure and external environment influenced the amount and location of salt crystallization, eventually leading to thenardite crystallizing within the rock in the subflorescence site while the gypsum precipitated on the surface as a crust. The sulfur isotope analysis revealed that the sulfur in the subflorescence and crust sites was most likely derived from detergents and from the combustion of oil and natural gas. Our research revealed the microscopic weathering mechanisms of the subflorescence and crust patterns in the Nankan Grotto, providing insights on salt weathering mechanisms for stone heritage sites around the world.

Geochemical Signatures of Bara Basin Aquifers, Kordofan State, Sudan

Purpose: In this paper, geochemical investigation has been carried out to study the concentrations of major cations; Sodium (Na+), Potassium (K+), Magnesium (Mg2+), and Calcium (Ca2+), and major anions; Chloride (Cl-), Bicarbonate (HCO3-), Nitrate (NO3-) and Sulphate (SO42-), other parameters such as (pH), Electric conductivity and T.D.S were also studied in all water samples taken from wells in Bara basin.
 Methodology: Forty five (45) water samples have been collected from wells. Lab works were done at Groundwater and Wadis Directorate (GWD), Khartoum State according to Sudanese Standards and Metrology Organization, (2002), to study the major cations and major anions.
 Findings: The results revealed that the average value of pH was (7.9) E.C values were (843.4 μS/cm) and T.D.S values were (59 mg/l). The average concentrations of the major cations obtained were; Na+ (126.3), K+ (5.22), Mg2+ (28.30), Ca2+(66.41) and the major anions (mg/l) were; Cl- (159.00), HCO3- (165.30), NO3- (156.70) and SO42- (164.70). The (Na/Cl) ratio is (2.47) is also high which reflects the abundance of (Na+) ions relative to the Chloride (Cl-) ions which confirmed the light-salty nature of groundwater. Geochemical mass balance calculations indicate that upper aquifer is mainly affected by the dissolution and dolomitization processes of major anions.
 Unique Contribution to Theory, Policy and Practice: This process is highly developed in the central west part of the basin due to the abundance of Sulphate (SO42-), Bicarbonate (HCO3-), Magnesium (Mg2+), and Calcium (Ca2+) ions. High values of E.C, T.D.S, Ca2+, and SO42- are mainly presents as a saline water resource in some villages. Two types of water are classified here these are : i). Na-HCO3 type and ii). Ca- Cl -SO4 type. The study concluded that water samples from Almakser, Hamdan-2, Um-Zeeraga-2, and Al-Zareeba-2 villages have very high concentrations of E.C, T.D.S Ca2-, Cl-, NO3- and SO42-, which may suggest that water samples are not fit for human consumption, according to guidelins by W.H.O (1984). This paper recommends further study on water quality.

Hydrogeochemical characterization of groundwater in the shallow aquifer system of Middle Ganga Basin, India

Middle Ganga Basin (MGB) is lifeline for millions of inhabitants relying heavily on the groundwater. This has resulted in depletion of water quality and quantity at a very rapid scale. The present study has emphasized on hydrogeochemical evolution of groundwater in the Middle Ganga Basin, covering an area of 99,058 sq. km. Around 400 water samples were evaluated to determine the geochemical evolution of the shallow groundwater in MGB. The weighted average water quality index (WAWQI) shows 20.2% of the groundwater are unsuitable for drinking purposes. The Ca–HCO3 water facies dominates in northern region in the interfluves of Ghaghara and West Rapti rivers whereas more evolved water types such as Mg–HCO3, Na–HCO3 are found in the interfluves area of Ghaghara, Ganga, Yamuna, and Gomati Rivers. The occurrence of more mineralized water with increasing residence time in the flow direction suggests geogenic control and evolution follow the Chebotarev sequence. The saline water type is observed in and around the settlement reflecting the in-situ enrichment due to stagnation and anthropogenic activities. Hierarchical cluster analysis classified the regional groundwater data into three distinct major groups G1, G2, and G3. Factor 1 is attributed to anthropogenic inputs and associated with subgroup G2B and group G3. Factor 2 is attributable to the geogenic factors and is associated with subgroups G1A, G1B, and G2A. The various bivariate plots confirm the dominance of silicate weathering over carbonate weathering in the study area. Geochemical mass balance modeling suggests calcite and dolomite are in saturation to oversaturation conditions, restricting their further dissolution and primary silicate minerals controlling the groundwater chemistry. Our work reveals hydrogeochemical evolution on a regional scale in the shallow groundwater which will help to develop sustainable groundwater management strategies.

The influence of erosion and vegetation on soil production and chemical weathering rates in the Southern Alps, New Zealand

Chemical weathering influences many aspects of the Earth system, including biogeochemical cycling, climate, and ecosystem function. Physical erosion influences chemical weathering rates by setting the supply of fresh minerals to the critical zone. Vegetation also influences chemical weathering rates, both by physical processes that expose mineral surfaces and via production of acids that contribute to mineral dissolution. However, the role of vegetation in setting surface process rates in different landscapes is unclear. Here we use 10Be and geochemical mass balance to quantify soil production, physical erosion, and chemical weathering rates in a landscape where a migrating drainage divide separates catchments with an order-of magnitude contrast in erosion rates and where vegetation spans temperate rainforest, tussock grassland, and unvegetated alpine ecosystems in the western Southern Alps of New Zealand. Soil production, physical erosion, and chemical weathering rates are significantly higher on the rapidly eroding versus the slowly eroding side of the drainage divide. However, chemical weathering intensity does not vary significantly across the divide or as a function of vegetation type. Soil production rates are correlated with ridgetop curvature, and ridgetops are more convex on the rapidly eroding side of the divide, where soil mineral residence times are lowest. Hence our findings suggest fluvially-driven erosion rates control soil production and soil chemical weathering rates by influencing the relationship between hillslope topography and mineral residence times. In the western Southern Alps, soil production and chemical weathering rates are more strongly mediated by physical rock breakdown driven by landscape response to tectonics, than by vegetation.

Pre-agricultural soil erosion rates in the midwestern United States

Abstract Erosion degrades soils and undermines agricultural productivity. For agriculture to be sustainable, soil erosion rates must be low enough to maintain fertile soil. Hence, quantifying both pre-agricultural and agricultural erosion rates is vital for determining whether farming practices are sustainable. However, there have been few measurements of pre-agricultural erosion rates in major farming areas where soils form from Pleistocene deposits. We quantified pre-agricultural erosion rates in the midwestern United States, one of the world's most productive agricultural regions. We sampled soil profiles from 14 native prairies and used in situ–produced 10Be and geochemical mass balance to calculate physical erosion rates. The median pre-agricultural erosion rate of 0.04 mm yr–1 is orders of magnitude lower than agricultural values previously measured in adjacent fields, as is a site-averaged diffusion coefficient (0.005 m2 yr–1) calculated from erosion rate and topographic curvature data. The long-term erosion rates are also one to four orders of magnitude lower than the assumed 1 mm yr–1 soil loss tolerance value assigned to these locations by the U.S. Department of Agriculture. Hence, quantifying long-term erosion rates using cosmogenic nuclides provides a means for more robustly defining rates of tolerable erosion and for developing management guidelines that promote soil sustainability.

Monitoring and inverse modelling of hydrogeochemical processes during managed aquifer recharge in Southwestern Bangladesh

Managed Aquifer Recharge (MAR) has been applied as Aquifer Storage, Transfer, and Recovery (ASTR) to provide fresh drinking water for local communities at 99 locations in southwest Bangladesh since 2009. Aerobic freshwater from ponds is filtered and subsequently infiltrated into anaerobic shallow brackish aquifers. At approximately 45% of these sites, relatively higher levels of Fe and As were observed in recovered water, which requires a better understanding of the hydrogeochemical processes that govern the Fe, Mn, and As levels in these MAR systems. Therefore, two representative sites with As above (74 ± 11 μg/L at site GMF11) and below (19 ± 6 μg/L at site JJS91), the Bangladesh drinking water standard of 50 μg/L were weekly monitored on hydrochemical changes from Dec 2017 to Dec 2018. Hydrogeochemical processes occurring during storage were quantified with inverse and forward geochemical mass balance models developed with PHREEQC. The following processes explained the changes in water quality: 1) mixing of infiltration water with native groundwater (∼90%:∼10%); 2) consumption of O2 by a) dissolved Fe2+ that subsequently precipitated as Fe(OH)3 at GMF11 and by b) dissolved and sedimentary organic matter (OM) at site JJS91; 3) reduction of SO4 coupled to the oxidation of OM at both sites; and 4) mixing corrosion and freshening induced cation-exchange (Ca sorption; Na desorption) triggering calcite and siderite dissolution at GMF11. Dissolution of these carbonate minerals occurred to a lesser extent at JJS91, while cation exchange (Na sorption; Ca desorption) suggested that the freshwater was displaced by brackish groundwater because of inadequate infiltration at JJS91. Distinct pH values in recovered water reflected the dominance of Fe2+ versus OM oxidation. Siderite dissolution led to 4.3 ± 3.1 and 1.0 ± 0.5 mg/L Fe in recovered water at GMF11 and JJS91, respectively. Elevated As and Mn levels in recovered water were caused for max. 20% by mixing with native groundwater and for min. 80% by mobilization processes, mainly by desorption of As from Fe-oxides and by the dissolution of Mn-bearing siderite. Recommendations are provided to improve recovered water quality.

Groundwater discharge locally shapes the rocky shore macroinvertebrate community in South-Southwest Portugal

Groundwater discharge is an essential process in the functioning of coastal aquatic ecosystems due to its significant role in nutrient cycling, geochemical mass balances and primary productivity. However, the occurrence patterns, importance, and effects of this discharge on rocky shores communities remain largely unknown. We assessed the importance of groundwater discharge into the highly ecologically important intertidal ecosystems. We compared the benthic macroinvertebrate composition and abundance between discharge and no-discharge sites, replicated for five shores in South and Southwest Portugal. This robust replicated feature across shores and regions is a particularly novel contribution to the field. Groundwater discharge significantly affected the biological communities’ abundance across all shores, but not biodiversity patterns. The algae Enteromorpha sp., snail Melaraphe neritoides and lichen Verrucaria maura can potentially be used as bioindication tools for shifts in groundwater discharge quantity and qualitative patterns. Our study validates the importance of this commonly overlooked local disturbance factor in regulating intertidal communities.

Nickel pollution pathways in small ecosystem, Egypt

Nickel has negative human health effects depending on dose and length of exposure; it is classified as a class 1 carcinogen. Nickel pollution threatens Saft Rashin village, Egypt. The area’s crops are highly contaminated with nickel, suggesting them as bioaccumulators. Nickel daily intake exceeds twofold the permissible limits. To fully understand the pollution process, Ni concentrations in every input/output were determined and have been normalized to grams per hectare per year (g ha−1 year−1). Geochemical mass balance (GMB) revealed that dustfall is the major Ni input (88.8%) followed by P-fertilizers (11.2% of the total inputs). Nickel in dust is mainly related to traffic emissions. On the other hand, plant uptake removes about 90% of the total nickel input. The remaining 10% accumulates in the soil with a retention rate of 44.7 g ha−1 year−1. Accordingly, using new diesel models that are more efficient and less polluting can reduce pollution levels; likewise, use eco-friendly fertilizers. Egyptian clover is recommended as a phytoremediator to remove Ni from growth media for its high efficiency and low planting cost.

Drainage chemistry of mine tailings from a carbonatite-hosted Nb-REE deposit, Oka, Québec, Canada

Potential environmental issues associated with the mining of carbonatites are receiving increased attention due to the importance of critical metals for green technologies. This study investigates the chemistry of tailings seepage at the former Saint Lawrence Columbium mine near Oka, Québec, Canada, which produced pyrochlore concentrate and ferroniobium from a carbonatite-hosted Nb-REE deposit. Detailed field sampling and laboratory methods were used to characterize the hydraulic properties of the tailings, their bulk chemistry, mineralogy, pore water and effluent chemistries. The tailings are composed of REE-enriched calcite (64–89 wt %) and fluorapatite (2–22 wt %), as well as biotite (6–17 wt %) and chlorite (0–7 wt %). Minor minerals include ankerite, pyrite, sphalerite, molybdenite, magnetite and unrecovered pyrochlore. Secondary minerals include gypsum, barite, strontianite and rhodochrosite. Geochemical mass balance modeling, constrained by speciation modeling, was used to identify dissolution, precipitation and exchange reactions controlling the chemical evolution of pore water along its flow path through the tailings impoundment. In the unsaturated zone, these reactions include sulfide oxidation and calcite dissolution with acid neutralization. Below the water table, gypsum dissolution is followed by sulfate reduction and FeS precipitation driven by the oxidation of organic carbon in the tailings. Incongruent dissolution of biotite and chlorite releases K, Mg, Fe, Mn, Ba and F and forms kaolinite and Ca-smectite. Cation exchange reactions further remove Ca from solution, increasing concentrations of Na and K. Fluoride concentrations reach 23 mg/L and 8 mg/L in tailings pore water and effluent, respectively. These values exceed Canadian guidelines for the protection of aquatic life. In the mildly alkaline (pH 8.3) pore waters, Mo is highly mobile and reaches an average concentration of 83 μg/L in tailings effluent, which slightly exceeds environmental guidelines. Concentrations (unfiltered) of Zn reach 1702 μg/L in tailings pore water although values in effluent are usually less than 20 μg/L. At the ambient pH, Zn is strongly adsorbed by Fe–Mn oxyhydroxides. Although U forms mobile complexes in tailings pore water, concentrations do not exceed 16 μg/L due to the low solubility of its pyrochlore host. Adsorption and the low solubility of pyrochlore limit concentrations of Nb to less than 49 μg/L. Cerium, from calcite dissolution, is strongly adsorbed although it reaches concentrations (unfiltered) in excess of 1 mg/L and 100 μg/L in pore water and effluent, respectively. Results of this study show that mine tailings from carbonatite deposits are enriched in a wide variety of incompatible elements with multiple mineral hosts of varying solubility. Some of these elements, such as F and Mo, may represent contaminants of concern because of their mobility in alkaline tailings waters.

Petrology of weathering materials developed on granites in Biou area, North-Cameroon: implication for rare-earth elements (REE) exploration in semi-arid regions

Due to the increasing demand resulting from the use of REE in many fields of human life, a weathering profile developed on granites in the semi-arid region of Biou area (North-Cameroon) has been characterized for rare-earth elements (REE) exploration. The mineralogical compositions of weathered materials were revealed by X-ray diffraction (XRD). X-ray Fluorescence (XRF) and Inductively Coupled Plasmas-Mass Spectrometry (ICP-MS) have been used to determine the geochemical composition of granites and the overlying weathered materials. The S-type and peraluminous granites are constituted by quartz, orthoclase, microcline, plagioclase, biotite, muscovite, pyroxene and opaque minerals. Accessory minerals are probably responsible for the interesting contents in REE + Y and some trace elements (e.g., Zr, Zn, Ba, Rb). The weathering profile show from the bottom to the top: (i) saprolitic horizons; (ii) lower loose clayey horizon; (iii) iron duricrust horizon; (iv) upper loose clayey horizon; (v) and organo-mineral horizon. Some weathered rock fragments remain in the loose clayey and organo-mineral horizons. The mineralogical composition of the weathering materials is dominated by illite, muscovite and feldspar. The low weathering degree of the materials is justified by the climatic and reducing conditions. The large ion lithophiles and ferromagnesian elements (Mg, Fe, V, Cu, Co, Cs Cr, Ni, Sc and Li) which are supposed to be mobile are so much accumulated in the weathering materials. REE show very low degree of fractionation in weathering profile due probably to the lack of good drainage. The whole weathering materials shows high REE + Y contents as its parent rock. Geochemical mass balance and enrichment factor reveal that REE, especially light REE, are so much enriched in the iron duricrust horizon (ion-adsorption REE deposit type). Some heavy REE are also enriched in the lower loose clayey horizon. This first survey has revealed that the weathering materials developed on granites in Biou area are favorable for further REE exploration.

Weathering profiles developed on gneisses from Batchenga and Doua areas, central Cameroon: Climate and topography controls

Two weathering profiles from Batchenga and Doua areas in Central Cameroun developed under contrasting equatorial climate conditions and located in different morphological units have been selected to constraint the variability of their morphology, mineralogy, and geochemistry. The parent rock is a biotite and amphibole gneiss with a heterogranular granoblastic texture. The weathering profiles are typical lateritic materials formed in a humid tropical climate zone. They comprise, from bottom to top, coarse saprolite, fine saprolite, nodular horizon and loose clayey horizon. The Doua weathered materials are more clay-rich as a result of intense chemical weathering (CIA: 95.0–99.5%) than those of Batchenga (CIA: 51.3–87.8%). The weathering profile of Doua is thicker (12.00 m) than that of Batchenga (6.30 m). The intense chemical weathering and the total leaching of mobile elements as a consequence of good drainage led to the formation of a hard nodular horizon in the Doua weathering profile. The mineral composition is dominated by quartz, kaolinite, muscovite, illite, hematite/goethite, gibbsite and anatase. Illite is particularly abundant in the Batchenga weathering profile. Among trace elements, Ba, V, Zr, Sr, and Cr have high contents while REE show moderate contents with the highest contents in the Batchenga weathering profile. The oxidizing conditions favor positive Ce anomalies while positive Eu anomalies are due to the resistance of Eu mineral bearers. Geochemical mass balance results suggest that climate and topographic position which influence the quality of drainage greatly influence the distribution patterns of major, trace and rare earth elements in the weathering profiles. Apart from Al, Fe, and P, major elements have been much leached than trace elements during chemical weathering. Thus, the important variations are explained by the variation in the annual rainfall and topographic conditions.

Rock-Sourced Nitrogen in Semi-Arid, Shale-Derived California Soils

Models suggest that rock-derived nitrogen (N) inputs are of global importance to ecosystem N budgets; however, field studies demonstrating the significance of rock N inputs are rare. We examined rock-derived N fluxes in soils derived from sedimentary rocks along a catena formed under a semi-arid climate. Our measurements demonstrate that there are distinct and traceable pools of N in the soil and bedrock and that the fraction of rock-derived N declines downslope along the catena. We used geochemical mass balance weathering flux measurements to estimate a rock-derived N flux of 0.145 to 0.896 kg ha–1 yr–1 at the ridgecrest. We also developed independent N flux estimates using a 15N-based isotope mixing model. While geochemical mass-balance-based estimates fell within the 95% confidence range derived from the isotope mixing model (−1.1 to 44.3 kg ha–1 yr–1), this range was large due to uncertainty in values for atmospheric 15N deposition. Along the catena, N isotopes suggest a diminishing effect of rock-derived N downslope. Overall, we found that despite relatively large N pools within the saprolite and bedrock, slow chemical weathering and landscape denudation limit the influence of rock-derived N, letting atmospheric N deposition (7.1 kg ha–1 yr–1) and N fixation (0.9–3.1 kg ha–1 yr–1) dominate N inputs to this grassland ecosystem.

Petrogenesis of coeval shoshonitic and high-K calc-alkaline igneous suites in the Eopyeong granitoids, Taebaeksan Basin, South Korea: Lithospheric thinning-related Early Cretaceous magmatism in the Korean Peninsula

The Early Cretaceous was the initial period of prolonged Cretaceous-Tertiary magmatic episode in the Korean Peninsula, during which a tectonic transition from compression to extension occurred. The Early Cretaceous Eopyeong granitoids in the Taebaeksan Basin consist of two distinct series of magmas: shoshonitic and high-K calc-alkaline series. We investigated the petrology, whole-rock major and trace element geochemistry, and Sr-Nd-Pb isotope compositions of the Eopyeong granitoids to understand their magma evolution processes and source characteristics. Sensitive high-resolution ion microprobe zircon U-Pb analysis was performed on the Eopyeong granitoids to constrain their intrusion ages. Zircon U-Pb ages of the shoshonitic and high-K calc-alkaline series are 110.63 ± 0.52 and 110.65 ± 0.66 Ma, respectively, indicating that the two series formed during the same magmatic event. The mafic mineral assemblages of the shoshonitic series change from two-pyroxene+biotite to clinopyroxene+amphibole+biotite through magma evolution, whereas those of the high-K calc-alkaline series consist of more hydrous assemblages of amphibole+biotite. The shoshonitic series is also distinguished from the high-K calc-alkaline series by its higher La/Yb ratio and negative anomalies of Rb, Ba, and Eu. Mineral geothermobarometry and hygrometry data and geochemical mass balance modeling results show that the two series evolved mainly by fractional crystallization from two distinct parental magmas containing different water contents. Primitive rocks from each series have high compatible element contents such as MgO, Cr, and Ni with high Mg# compared to crustal melts in a similar range of SiO2, suggesting their mantle origin. They also have abundant incompatible elements and ‘crust-like’ isotopic compositions [(87Sr/86Sr)i = 0.7093–0.7098; εNd(t) = −7.1 to −8.8; (206Pb/204Pb)i = 18.133–18.327; (207Pb/204Pb)i = 15.621–15.665; (208Pb/204Pb)i = 38.691–38.862] with negative Nb-Ta anomalies in the primitive mantle-normalized patterns. The enrichment in both compatible and incompatible elements can be best explained by partial melting of phlogopite-bearing lithospheric mantle metasomatized by isotopically enriched upper crustal sediment-derived melt or fluid. The two magma series may have been generated by different degrees of partial melting of the mantle sources with varying water contents. Changes in subduction angle and direction of the paleo-Pacific plate during the Early Cretaceous may have led to lithospheric thinning and upwelling of asthenospheric mantle, which triggered the partial melting of fusible domains of metasomatized lithospheric mantle. The small igneous bodies of the Early Cretaceous shoshonitic to alkaline mafic rocks occur sporadically throughout the southern part of the Korean Peninsula in close spatial association with the Cretaceous basins. They also show crust-like geochemistry and isotopic compositions, similar to the Eopyeong granitoids, implying that the lithospheric thinning-related melting of the metasomatized lithospheric mantle may have been widespread in the southern part of the Korean Peninsula.

The influence of water\u2013rock interactions on household well water in an area of high prevalence chronic kidney disease of unknown aetiology (CKDu)

Poor drinking water quality in household wells is hypothesised as being a potential contributor to the high prevalence of chronic kidney disease of uncertain aetiology (CKDu) among the farming communities of the Medawachchiya area, Anuradhapura, Sri Lanka. One of the natural processes that can affect water quality is the dissolution of minerals contained within an aquifer by water–rock interactions (WRIs). Here we present a comprehensive assessment of WRIs and their influence on the water chemistry in household wells and spring waters in the Medawachchiya area by combining measurements of environmental isotopes, such as strontium, lithium and stable carbon isotopes and inorganic chemistry parameters, and modelling geochemical mass balance reactions between rainfall and groundwater samples. Our results reveal the presence of strontium, dissolved from both silicate and carbonate minerals, with high isotopic (87Sr/86Sr) ratios of up to 0.7316. Geochemical mass balance modelling and prior 87Sr/86Sr studies on the Wanni Complex bedrock suggest these strontium values may be the result of biotite dissolution. We also identify lithium and uranium contributed from the dissolution of silicates, albeit at concentrations too low to constitute a known health risk. In contrast, the levels of magnesium and calcium in our samples are high and demonstrate that, despite the felsic bedrock, well water chemistry in the Medawachchiya area is dominated by carbonate dissolution.

Mineral chemistry of hydrothermal alteration assemblage in hanging wall Shahapur granite associated with vein-type Gogi uranium deposit, Bhima Basin, Eastern Dharwar Craton, India: Implications for physico-chemical conditions of ore formation

Eastern Dharwar craton (EDC) is a major uranium province in India hosting number of unconformity related, vein-type and carbonate hosted strata-bound uranium deposits. Gogi uranium deposit (GUD) is a structurally controlled hydrothermal ore deposit (~1.35 Ga), hosted in a reverse fault zone between the foot-wall Mesoproterozoic Bhima Group carbonates and the hanging wall basement granites (~2.5 Ga) of the EDC. The hanging wall granites, locally known as Shahapur granite show extensive chloritisation and epidotisation, and constitute the propylitic outer zone of the main hydrothermal system of the GUD. We conducted systematic mineralogical and geochemical studies on hydrothermally altered assemblages to understand physico-chemical conditions of hydrothermal fluid. Quartz, epidote, chlorite, titanite, fluorite and hematite are the main hydrothermal mineral phases. Chlorites occur in two different forms, as pseudomorphic replacement of hornblende and as vein/cavity fillings. Epidotes occur as coarse prismatic grains in the main veins, or as replacement of albite in the wall rock. Mineralogical, textural data and geochemical mass balance calculation indicate that about 17.9% of total albites (Modal abundance = ~48%) and all the hornblendes (modal abundance = ~7.5%) of the parent rock were broken down and reacted to form quartz, chlorite, epidote, secondary titanite and hematite. The hydrothermal reaction resulted in creation of 13 vol% secondary porosity in the host rock and acidification of the evolved fluid. Thermodynamic calculation on chlorite compositions indicates ~ 250 °C temperature of formation and oxidising nature of ambient fluid, with log fO2 ranging between −37 and −31 (avg. −35). Available fluid inclusion data, in conjunction with chlorite geothermometric results, show fluid entrapment pressure between 625 bar and 792 bar, which is consistent with emplacement of hydrothermal fluid in low-pressure brittle deformation zone. We surmise that U6+ was liberated from basement granite by reaction with oxidised acidic fluid, transported as chloride, carbonate complexes and precipitated in high permeability fault-zone, possibly because of mixing with reduced- basin derived fluids.

Diagenetic and supergene ore forming processes in the iron formation of the Neoproterozoic Jacadigo Group, Corumbá, Brazil

The ~450 m thick banded chert-hematite and dolomite-chert-hematite body within the Neoproterozoic Jacadigo Group, located near Corumbá, Mato Grosso do Sul, Brazil, represents one of the most prominent “Rapitan type” iron formation (IF). New petrographical, whole-rock geochemical mass balance calculations, and oxygen isotope data from the Monjolinho (formerly Santa Cruz) medium-to high-grade (40–64 wt% Fe) hematite iron ore deposit are used to propose a novel diagenetic and supergene Fe-upgrade model.Burial diagenesis in an extensional setting with a differential stress field (vertical σ1 > σ2 ≥ σ3) at up to 6 km depth and temperatures of 71°–173 °C (based on disequilibrium hematite-chert oxygen isotope exchange) led to vertical shortening (D1a and D1b) by rock compaction. Burial diagenesis was accompanied by ubiquitous growth of lepidoblastic-microplaty hematite, parallel S1, and locally granoblastic hematite, as well as anhedral hematite replacing chert and dolomite. Coeval interaction with high volumes of fluids facilitated volume reduction by differential dissolution of chert layers. This post-lithification dissolution-podding mechanism increased the iron content by an average of 11 wt%, relative to the primary layered IF (from 40 to 50 wt% to 50–58 wt% Fe), and is thus one reason for the exceptionally high Fe grade in unweathered IF. The critical setting for this hypogene, dissolution-podding Fe upgrade is the coincidence of extensional (graben) tectonics and associated infiltration of down flowing, Si-undersaturated fluid that most likely had interacted with overlying carbonates of the Corumbá Group. Minor magnetite in thrust faults and specular hematite veinlets in IF formed during regional D2 and/or D3 shortening. From the Palaeogene period onward, reactivation of the graben system, exhumation, and development of the present topography triggered supergene upgrade of the IF (up to 64 wt% Fe). Main ore forming processes within the deep supergene zone (i.e., the uppermost 180 m below the present surface) were dissolution of dolomite and its local replacement by goethite. In contrast, chert mobilisation takes place only in the uppermost 30m thick eluvial and colluvial zones.The results advance the research on iron ore genesis: Late diagenetic, pressure solution driven silica removal triggered by intense hydrothermal fluid flow is a newly described Fe-upgrade mechanism. Such mechanism can be considered as an endmember in the larger group of hydrothermal Fe upgrade models. Furthermore, crucial evidence is provided for differential gangue leaching within the supergene zone, with wide-reaching dolomite dissolution and chert mobilisation restricted to the uppermost weathering crust.

Differentiated Neogene bauxitization of volcanic rocks (western Cameroon): Morpho-geological constraints on chemical erosion

Lateritic weathering of Miocene volcanic rocks from western Cameroon highlands formed duricrusted bauxitic profiles. Two weathering profiles on ca. 14 Ma basalt and ca. 16 Ma trachyte were studied using geochemical mass balance functions. Less mobile elements Ti and Zr were used as references to quantify volumetric change (strain, ε), element transfer rate (τ) and geochemical mass transfers during the bauxitization process of basalt and trachyte. Conversion of parent rocks to kaolinite and goethite rich saprolites evolved to Al-Fe rich bauxites, mostly composed of gibbsite and iron oxy-hydroxides (goethite and hematite). However, formation of Al-Fe bauxitic profiles required higher Si leaching on trachyte than on basalt. Our results document that chemical weathering of a larger thickness of trachyte than basalt has been required to form a unit meter of weathering profile, implying differential rates of rock chemical erosion and topographic decay of landscapes. Rates of chemical erosion and formation of lateritic weathering profiles in western Cameroon have been mostly controlled by drainage conditions and volcanic rocks composition (mostly SiO2 content differences), that also resulted in contrasted landscapes evolution during the Neogene.

Concentrations of major and trace elements in soil profiles developed over granites across a climosequence in northeastern Brazil

Soils formed from granites are distributed worldwide, therefore, studies regarding their geochemical signature on tropical soils are important to understand key processes of soil weathering and soil chemistry. This paper reports the effect of S- and I-type granitic rocks on the content and distribution of trace and major elements in soil profiles developed across a climosequence in Northeastern Brazil. In general, soils developed from I-types show greater trace and major element concentrations than those originated from S-types, due to their higher proportion of accessory and mafic minerals. Geochemical mass balance and enrichment factor data suggest that climatic effects showed a less evident effect on the distribution pattern of trace and major elements from soil profiles underlain by S-type granitic rocks. The highest enrichment factor results evidenced in soils formed on I-types are explained by the rock mineralogy, which imparts higher CEC values and proportion of clay to soils. Trace elements were less leached than major elements during chemical weathering, except for Al, Fe, and P. Geochemical mass balance showed that, regardless of the granite type, the highest depletion of trace and major elements in the pedosphere took place in the humid zones as a consequence of weathering intensity. In this zone, except for quartz, the primary minerals were altered to form secondary minerals, giving rise to kaolinitic soils. These results can be used as reference contents of trace and major elements developed from different granitic rocks in environments with tropical climate. Besides, this research sheds some light on the effect of contrasting granites and a climosequence on soil geochemistry.