2019 Vol. 10, No. 5

Editorial
Groundwater arsenic biogeochemistry-Key questions and use of tracers to understand arsenic-prone groundwater systems
David A. Polya, Charlotte Sparrenbom, Saugata Datta, Huaming Guo
2019, 10(5): 1635-1641. doi: 10.1016/j.gsf.2019.05.004
Abstract(149) HTML PDF(14)
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Over 100,000,000 people worldwide are exposed to high arsenic groundwater utilised for drinking or cooking. The consequent global avoidable disease burden is estimated to be of the order of 100,000 avoidable deaths or more per annum from just direct exposures-i.e. excluding indirect exposure (from rice and other foods) and excluding morbidity. Notwithstanding 1000s of papers published on arsenic (hydro) (bio)geochemistry, there remain a number of key outstanding questions to be addressed in relation to arsenic geoscience-these include questions related to:(i) the role of human activities-irrigation, agriculture and other land uses-on arsenic mobilisation in groundwaters; (ii) the specific sources, nature and role of organics, minerals and microbial communities involved in arsenic mobilisation; (iii) the relationship to microscopic to macroscopic scale geological (including tectonic) and evolution processes; (iv) unravelling the over-printing of multiple processes in complex highly heterogeneous aquifer systems and (v) using increasing understanding of the controls of arsenic mobility in groundwaters systems to informing improved locally-relevant remediation and mitigation approaches. This article further summarises how the 9 further papers in this Special Issue address some of these questions through the use of chemical and/or isotopic tracers.
Research Paper
Use and application of CFC-11, CFC-12, CFC-113 and SF6 as environmental tracers of groundwater residence time: A review
L. A. Chambers, D. C. Gooddy, A. M. Binley
2019, 10(5): 1643-1652. doi: 10.1016/j.gsf.2018.02.017
Abstract(176) HTML PDF(3)
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Groundwater residence time is a fundamental property of groundwater to understand important hydrogeological issues, such as deriving sustainable abstraction volumes, or, the evolution of groundwater quality. The anthropogenic trace gases chlorofluorocarbons (CFC-11, CFC-12 and CFC-113) and sulphur hexafluoride (SF6) are ideal in this regard because they have been released globally at known rates and become dissolved in groundwater following Henry's Law, integrating over large spatial (global) and temporal (decades) scales. The CFCs and SF6 are able to date groundwater up to ~100 years old with the caveat of certain simplifying assumptions. However, the inversion of environmental tracer concentrations (CFCs and SF6) to derive groundwater age rests on the accurate determination of groundwater recharge parameters, namely temperature, elevation, salinity and excess air, in addition to resolving the potential for contamination, degradation and unsaturated zone effects. This review explores the fundamentals of CFC-11, CFC-12, CFC-113 and SF6 as environmental tracers of groundwater age and recommends complementary techniques throughout. Once this relatively simple and inexpensive technique has been used to determine initial concentrations at the recharge zone, setting the groundwater dating ‘clock’ to zero, this review then explores the meaning of groundwater ‘age’ in relation to measured environmental tracer concentrations. It is shown that the CFCs and SF6 may be applied to a wide-range of hydrogeological problems and suggests that environmental tracers are particularly powerful tools when integrated with numerical flow and transport models.
Dissolved organic matter tracers reveal contrasting characteristics across high arsenic aquifers in Cambodia: A fluorescence spectroscopy study
Laura A. Richards, Dan J. Lapworth, Daniel Magnone, Daren C. Gooddy, Lee Chambers, Peter J. Williams, Bart E. van Dongen, David A. Polya
2019, 10(5): 1653-1667. doi: 10.1016/j.gsf.2019.04.009
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Organic matter in the environment is involved in many biogeochemical processes, including the mobilization of geogenic trace elements, such as arsenic, into groundwater. In this paper we present the use of fluorescence spectroscopy to characterize the dissolved organic matter (DOM) pool in heavily arsenic-affected groundwaters in Kandal Province, Cambodia. The fluorescence DOM (fDOM) characteristics between contrasting field areas of differing dominant lithologies were compared and linked to other hydrogeochemical parameters, including arsenic and dissolved methane as well as selected sedimentary characteristics. Absorbance-corrected fluorescence indices were used to characterize depth profiles and compare field areas. Groundwater fDOM was generally dominated by terrestrial humic and fulvic-like components, with relatively small contributions from microbially-derived, tryptophan-like components. Groundwater fDOM from sand-dominated sequences typically contained lower tryptophan-like, lower fulvic-like and lower humic-like components, was less bioavailable, and had higher humification index than clay-dominated sequences. Methane concentrations were strongly correlated with fDOM bioavailability as well as with tryptophan-like components, suggesting that groundwater methane in these arsenic-prone aquifers is likely of biogenic origin. A comparison of fDOM tracers with sedimentary OM tracers is consistent with the hypothesis that external, surface-derived contributions to the aqueous DOM pool are an important control on groundwater hydrogeochemistry.
Arsenic variability and groundwater age in three water supply wells in southeast New Hampshire
Joseph P. Levitt, James R. Degnan, Sarah M. Flanagan, Bryant C. Jurgens
2019, 10(5): 1669-1683. doi: 10.1016/j.gsf.2019.01.002
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Three wells in New Hampshire were sampled bimonthly over three years to evaluate the temporal variability of arsenic concentrations and groundwater age. All samples had measurable concentrations of arsenic throughout the entire sampling period and concentrations in individual wells had a mean variation of more than 7 μg/L. The time series data from this sampling effort showed that arsenic concentrations ranged from a median of 4 μg/L in a glacial aquifer well (SGW-65) to medians of 19 μg/L and 37 μg/L in wells (SGW-93 and KFW-87) screened in the bedrock aquifer, respectively. These high arsenic concentrations were associated with the consistently high pH (median ≥ 8) and low dissolved oxygen (median <0.1 mg/L) in the bedrock aquifer wells, which is typical of fractured crystalline bedrock aquifers in New Hampshire. Groundwater from the glacial aquifer often has high dissolved oxygen, but in this case was consistently low. The pH also is generally acidic in the glacial aquifer but in this case was slightly alkaline (median=7.5). Also, sorption sites may be more abundant in glacial aquifer deposits than in fractured bedrock which may contribute to lower arsenic concentrations.
Mean groundwater ages were less than 50 years old in all three wells and correlated with conservative tracer concentrations, such as chloride; however, mean age was not directly correlated with arsenic concentrations. Arsenic concentrations at KFW-87 did correlate with water levels, in addition, there was a seasonal pattern, which suggests that either the timing of or multiple sampling efforts may be important to define the full range of arsenic concentrations in domestic bedrock wells.
Since geochemically reduced conditions and alkaline pHs are common to both bedrock and glacial aquifer wells in this study, groundwater age correlates less strongly with arsenic concentrations than geochemical conditions. There also is evidence of direct hydraulic connection between the glacial and bedrock aquifers, which can influence arsenic concentrations. Correlations between arsenic concentrations and the age of the old fraction of water in SGW-65 and the age of the young fraction of water in SGW-93 suggest that water in the two aquifers may be mixing or at least some of the deeper, older water captured by the glacial aquifer well may be from a similar source as the shallow young groundwater from the bedrock aquifer. The contrast in arsenic concentrations in the two aquifers may be because of increased adsorption capacity of glacio-fluvial sediments, which can limit contaminants more than fractured rock. In addition, this study illustrates that long residence times are not necessary to achieve more geochemically evolved conditions such as high pH and reduced conditions as is typically found with older water in other regions.
Hydrogeochemical and multi-tracer investigations of arsenic-affected aquifers in semi-arid West Africa
Anja Bretzler, Lucien Stolze, Julien Nikiema, Franck Lalanne, Elaheh Ghadiri, Matthias S. Brennwald, Massimo Rolle, Mario Schirmer
2019, 10(5): 1685-1699. doi: 10.1016/j.gsf.2018.06.004
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The semi-arid Sahel regions of West Africa rely heavily on groundwater from shallow to moderately deep (<100 m b.g.l.) crystalline bedrock aquifers for drinking water production. Groundwater quality may be affected by high geogenic arsenic (As) concentrations (>10 μg/L) stemming from the oxidation of sulphide minerals (pyrite, arsenopyrite) in mineralised zones. These aquifers are still little investigated, especially concerning groundwater residence times and the influence of the annual monsoon season on groundwater chemistry. To gain insights on the temporal aspects of As contamination, we have used isotope tracers (noble gases, 3H, stable water isotopes (2H, 18O)) and performed hydrochemical analyses on groundwater abstracted from tube wells and dug wells in a small study area in southwestern Burkina Faso. Results revealed a great variability in groundwater properties (e.g. redox conditions, As concentrations, water level, residence time) over spatial scales of only a few hundred metres, characteristic of the highly heterogeneous fractured underground. Elevated As levels are found in oxic groundwater of circum-neutral pH and show little relation with any of the measured parameters. Arsenic concentrations are relatively stable over the course of the year, with little effect seen by the monsoon. Groundwater residence time does not seem to have an influence on As concentrations, as elevated As can be found both in groundwater with short (<50 a) and long (>103 a) residence times as indicated by 3He/4He ratios spanning three orders of magnitude. These results support the hypothesis that the proximity to mineralised zones is the most crucial factor controlling As concentrations in the observed redox/pH conditions. The existence of very old water portions with residence times >103 years already at depths of <50 m b.g.l. is a new finding for the shallow fractured bedrock aquifers of Burkina Faso, suggesting that overexploitation of these relatively low-yielding aquifers may be an issue in the future.
Contrasting sorption behaviours affecting groundwater arsenic concentration in Kandal Province, Cambodia
Laura A. Richards, Maria J. Casanueva-Marenco, Daniel Magnone, Chansopheaktra Sovann, Bart E. van Dongen, David A. Polya
2019, 10(5): 1701-1713. doi: 10.1016/j.gsf.2019.02.010
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Natural arsenic (As) contamination of groundwater which provides drinking water and/or irrigation supplies remains a major public health issue, particularly in South and Southeast Asia. A number of studies have evaluated various aspects of the biogeochemical controls on As mobilization in aquifers typical to this region, however many are predicated on the assumption that key biogeochemical processes may be deduced by sampled water chemistry. The validity of this assumption has not been clearly established even though the role of sorption/desorption of As and other heavy metals onto Fe/Mn (hydr)oxides is an important control in As mobilization. Here, selective chemical extractions of sand-rich and clay-rich sediments from an As-affected aquifer in Kandal Province, Cambodia, were undertaken to explore the potential role of partial re-equilibrium through sorption/desorption reactions of As and related solutes (Fe, Mn and P) between groundwater and the associated solid aquifer matrix. In general, groundwater As is strongly affected by both pH and Eh throughout the study area. However, contrasting sorption behaviour is observed in two distinct sand-dominated (T-Sand) and clay dominated (T-Clay) transects, and plausibly attributed to differing dominant lithologies, biogeochemical and/or hydrogeological conditions. Sorption/desorption processes appear to be re-setting groundwater As concentrations in both transects, but to varying extents and in different ways. In T-Sand, which is typically highly reducing, correlations suggest that dissolved As may be sequestered by sorption/re-adsorption to Fe-bearing mineral phases and/or sedimentary organic matter; in T-Clay Eh is a major control on As mobilization although binding/occlusion of Fe-bearing minerals to sedimentary organic matter may also occur. Multiple linear regression analysis was conducted with groups categorised by transect and by Eh, and the output correlations support the contrasting sorption behaviours encountered in this study area. Irrespective of transect, however, the key biogeochemical processes which initially control As mobilization in such aquifers, may be "masked" by the re-setting of As concentrations through in-aquifer sorption/desorption processes.
Interplay of S and As in Mekong Delta sediments during redox oscillations
Van T. H. Phan, Fabrizio Bardelli, Pierre Le Pape, Raoul-Marie Couture, Alejandro Fernandez-Martinez, Delphine Tisserand, Rizlan Bernier-Latmani, Laurent Charlet
2019, 10(5): 1715-1729. doi: 10.1016/j.gsf.2018.03.008
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The cumulative effects of periodic redox cycling on the mobility of As, Fe, and S from alluvial sediment to groundwater were investigated in bioreactor experiments. Two particular sediments from the alluvial floodplain of the Mekong Delta River were investigated:Matrix A (14 m deep) had a higher pyrite concentration than matrix B (7 m deep) sediments. Gypsum was present in matrix B but absent in matrix A. In the reactors, the sediment suspensions were supplemented with As(III) and SO42-, and were subjected to three full-redox cycles entailing phases of nitrogen/CO2, compressed air sparging, and cellobiose addition. Major differences in As concentration and speciation were observed upon redox cycling. Evidences support the fact that initial sediment composition is the main factor controlling arsenic release and its speciation during the redox cycles. Indeed, a high pyrite content associated with a low SO42- content resulted in an increase in dissolved As concentrations, mainly in the form of As(III), after anoxic half-cycles; whereas a decrease in As concentrations mainly in the form of As(V), was instead observed after oxic half-cycles. In addition, oxic conditions were found to be responsible for pyrite and arsenian pyrite oxidation, increasing the As pool available for mobilization. The same processes seem to occur in sediment with the presence of gypsum, but, in this case, dissolved As were sequestered by biotic or abiotic redox reactions occurring in the Fe-S system, and by specific physico-chemical condition (e.g. pH). The contrasting results obtained for two sediments sampled from the same core show that many complexes and entangled factors are at work, and further refinement is needed to explain the spatial and temporal variability of As release to groundwater of the Mekong River Delta (Vietnam).
A geologically-based approach to map arsenic risk in crystalline aquifers: Analysis of the Tampere region, Finland
Daniele Pedretti, Samrit Luoma, Timo Ruskeeniemi, Birgitta Backman
2019, 10(5): 1731-1741. doi: 10.1016/j.gsf.2018.12.004
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The study illustrates the critical role of accurate geological structural mapping to delineate crystalline aquifer zones more prone to high health risk due to elevated dissolved As in drinking wells. The analysis revisits the results from more than 1200 groundwater samples collected over ten years from domestic wells across the Tampere region (Finland). It is demonstrated that the highest dissolved As concentrations in the region (up to 2230 μg/L) are exclusively found near major faults and deformation zones (FDZs) detected via geophysical and geological surveys, and that a clear correlation exists between dissolved concentrations and the distance from the FDZs (r). Almost all values exceeding the drinking water limit (10 μg/L) occur at r< 8 km, while concentrations above 100 μg/L occur at r< 4 km. Solid-phase As concentrations in bedrock show less dependency on FDZ than aqueous concentrations. This behavior is explained considering different mechanisms, which include enhanced sulfide oxidation and fracture connectivity, promoting preferential transport of dissolved As to FDZs and mixing of waters from different redox zones, mobilizing preferentially As(III) or As(V). Fe hydro-oxides may also precipitate/dissolve preferentially because of FDZs, while residence time may influence the contact time between water and As-bearing minerals. It is concluded that the accurate mapping of FDZs, and in general of structural geology, provides an important preliminary information to identify where localized, site-specific characterization of hydrogeology and geochemistry is more urgent to reduce As-related health risk from groundwater intake.
Validation of basaltic glass adsorption capabilities from geothermal arsenic in a basaltic aquifer: A case study from Bjarnarflag power Station, Iceland
K. C. Weaver, M. A. Hoque, S. M. Amin, S. H. Markússon, A. P. Butler
2019, 10(5): 1743-1753. doi: 10.1016/j.gsf.2019.01.001
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Arsenic is a carcinogen known for its acute toxicity to organisms. Geothermal waters are commonly high in arsenic, as shown at the Bjarnarflag Power Plant, Iceland (~224 μg/kg of solvent). Development of geothermal energy requires adequate disposal of arsenic-rich waters into groundwater/geothermal systems. The outcome of arsenic transport models that assess the effect of geothermal effluent on the environment and ecosystems may be influenced by the sensitivity of hydraulic parameters. However, previous such studies in Iceland do not consider the sensitivity of hydraulic parameters and thereby the interpretations remain unreliable. Here we used the Lake Mývatn basaltic aquifer system as a case study to identify the sensitive hydraulic parameters and assess their role in arsenic transport. We develop a one-dimensional reactive transport model (PHREEQC ver. 2.), using geochemical data from Bjarnarflag, Iceland.
In our model, arsenite (H3AsO3) was predicted to be the dominant species of inorganic arsenic in both groundwater and geothermal water. Dilution reduced arsenic concentration below ~5 μg/kg. Adsorption reduced the residual contamination below ~0.4 μg/kg at 250 m along transect. Based on our modelling, we found volumetric input to be the most sensitive parameter in the model. In addition, the adsorption strength of basaltic glass was such that the physical hydrogeological parameters, namely:groundwater velocity and longitudinal dispersivity had little influence on the concentration profile.
Modeling transport of arsenic through modified granular natural siderite filters for arsenic removal
Fulan Li, Huaming Guo, Kai Zhao, Wei Xiu, Jiaxing Shen, Yi Chen
2019, 10(5): 1755-1764. doi: 10.1016/j.gsf.2018.12.002
Abstract(191) HTML PDF(1)
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Groundwater arsenic (As) contamination is a hot issue, which is severe health concern worldwide. Recently, many Fe-based adsorbents have been used for As removal from solutions. Modified granular natural siderite (MGNS), a special hybrid Fe(II)/Fe(III) system, had higher adsorption capacity for As(III) than As(V), but the feasibility of its application in treating high-As groundwater is still unclear. In combination with transport modeling, laboratory column studies and field pilot tests were performed to reveal both mechanisms and factors controlling As removal by MGNS-filled filters. Results show that weakly acid pH and discontinuous treatment enhanced As(III) removal, with a throughput of 8700 bed volumes (BV) of 1.0 mg/L As(III) water at breakthrough of 10 μg/L As at pH 6. Influent HCO3- inhibited As removal by the filters. Iron mineral species, SEM and XRD patterns of As-loading MGNS show that the important process contributing to high As(III) removal was the mineral transformation from siderite to goethite in the filter. The homogeneous surface diffusion modeling (HSDM) shows that competition between As(III) and HCO3- with adsorption sites on MGNS was negligible. The inhibition of HCO3- on As(III) removal was connected to inhibition of siderite dissolution and mineral transformation. Arsenic loadings were lower in field pilot tests than those in the laboratory experiments, showing that high concentrations of coexisting anions (especially HCO3- and SiO44-), high pH, low EBCT, and low groundwater temperature decreased As removal. It was suggested that acidification and aeration of high-As groundwater and discontinuous treatment would improve the MGNS filter performance of As removal from real high-As groundwater.
Metamorphism and exhumation of basement gneiss domes in the Quadrilátero Ferrífero: Two stage dome-and-keel evolution?
Kathryn Cutts, Cristiano Lana, Fernando Alkmim, Federico Farina, Hugo Moreira, Viviane Coelho
2019, 10(5): 1765-1787. doi: 10.1016/j.gsf.2019.02.009
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The presence of dome-and-keel provinces in Archean cratons has been connected with the initiation of plate tectonics on Earth as these features are most commonly observed in Archean rocks. The Quadrilátero Ferrífero in Brazil has been identified as a Paleoproterozoic dome-and-keel province for more than three decades. The prevailing model suggests that it formed during the Rhyacian Transamazonian orogeny, making it unique among dome-and-keel provinces. However, a lack of appropriate lithologies, datable minerals and the metamorphic overprint of later orogenesis has resulted in a cryptic metamorphic record for the formation of this dome-and-keel province. A clinopyroxene-bearing migmatite from the core of the Bação dome has peak P-T conditions of 5-7 kbar and 700-750℃ and a published age of ca. 2730 Ma based on U-Pb ages of zircon from leucosomes, suggesting that this age represents the migmatisation event. A fine-grained epidote-albite-titanite assemblage overprints the coarse-grained clinopyroxene and amphibole, giving P-T conditions of 8-9 kbar and 550℃ with an associated titanite age of ca. 2050 Ma. A garnet-bearing amphibolite sample also from the core of the dome has peak P-T conditions of 7-8 kbar and 650-700℃, and texturally late titanite from this sample produces an age of ca. 2060 Ma. Three additional samples were collected from the edges of the dome. A garnet-gedrite bearing felsic schist produces peak P-T conditions of 8-9 kbar and 650-700℃ on a clockwise P-T evolution. This sample has a U-Pb zircon age of ca. 2775 Ma, which could date metamorphism or be the age of its volcaniclastic protolith. Texturally unconstrained titanite from the sample gives an age of ca. 2040 Ma. A garnet-bearing amphibolite that occurs as a boudin within the felsic schist gives both zircon and titanite ages of ca. 2050 Ma and has peak P-T conditions of 5-6 kbar and 650-700℃ on a near isobaric P-T path. An amphibolite dike, observed to cross-cut the felsic schist produces a zircon U-Pb age of ca. 2760 Ma. Altogether this data suggests that the samples were metamorphosed in the Archean (ca. 2775-2730 Ma) and again during the Transamazonian event. The most plausible explanation for this data is that dome-and-keel formation occurred in the Archean with migmatisation and high-temperature metamorphism occurring at this time. The Paleoproterozoic event is interpreted as a reactivation of the dome-and-keel formation structures, with Paleoproterozoic keels crosscutting Archean keels and producing metamorphic aureoles. The high radiogenic heat production and the presence of dense sedimentary successions in Archean terranes make dome-and-keel provinces a uniquely Archean feature, but they are susceptible to reworking, resulting in an enigmatic record of formation.
Incipient charnockites from southern India: The role of brines
Jacques L. R. Touret, Robert C. Newton, Michel Cuney
2019, 10(5): 1789-1801. doi: 10.1016/j.gsf.2019.03.004
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Southern India and Sri-Lanka are the places where "incipient charnockites", i.e. the local transformation of amphibolite-facies gneisses into orthopyroxene-bearing, igneous looking charnockites, have been discovered in the early sixties. The fact that some incipient charnockites occur along a network of brittle fractures, together with CO2 remnants preserved in mineral inclusions, had called for the role of fluids during charnockite alteration. The present work presents new observations on fluid inclusions and microtextures of incipient charnockites from type localities in southern India. In addition to CO2-rich fluid inclusions in quartz and feldspar, all of the occurrences have disrupted remnants of concentrated aqueous alkali chloride solutions. CO2 inclusions are more abundant in paragneiss (Kerala) than in orthogneiss (Karnataka/Tamil Nadu). The finding of disrupted brine inclusions in the Kabbal charnockite is a key link between closely associated massive charnockites and Closepet Granite, both of which also share the brine remnants. All of the occurrences studied here have feldspar or feldspar-quartz microvein networks along grain boundaries of recrystallized quartz, feldspar and orthopyroxene. These metasomatic veins again indicate the action of alkali-exchanging fluids (i.e., saline solutions). Feldspar microveins, which have been found in most "massive" charnockites, along with the CO2-rich fluid inclusions, suggest a commonality of incipient charnockite and massive charnockite, both types differing in intensity of interaction with metasomatizing pore fluids.
Late Mesozoic magmatism in the East Qinling Orogen, China and its tectonic implications
Fan Yang, Fei Xue, M. Santosh, Gongwen Wang, Sung Won Kim, Zhiwei Shen, Wenjuan Jia, Xuhuang Zhang
2019, 10(5): 1803-1821. doi: 10.1016/j.gsf.2019.03.003
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The Qinling Orogen in Central China records the history of a complex geological evolution and tectonic transition from compression to extension during the Late Mesozoic, with concomitant voluminous granitoids formation. In this study, we present results from petrological, geochemical, zircon U-Pb-Lu-Hf isotopic studies on the Lengshui felsic dykes from Luanchuan region in the East Qinling Orogen. We also compile published geochronological, geochemical, and Hf isotopic data from Luanchuan region and present zircon Hf isotopic contour maps. The newly obtained age data yield two group of ages at ~145 Ma and 140 Ma for two granite porphyries from the Lengshui felsic dykes, with the ~145 Ma interpreted as response to the peak of magmatism in the region, and the ~140 Ma as the timing of formation of the felsic dykes. The corresponding Hf isotopic data of the granite porphyries display negative εHf(t) values of -16.67 to -4.61, and Hf crustal model ages (TDMC) of 2255-1490 Ma, indicating magma sourced from the melting of Paleo-to Mesoproterozoic crustal materials. The compiled age data display two major magmatic pulses at 160-130 Ma and 111-108 Ma with magmatic quiescence in between, and the zircon Hf isotopic data display εHf(t) values ranging from -41.9 to 2.1 and TDMC values of 3387-1033 Ma, suggesting mixed crustal and mantle-derived components in the magma source, and correspond to multiple tectonic events during the Late Mesozoic. The Luanchuan granitoids are identified as I-type granites and most of these are highly fractionated granites, involving magma mixing and mingling and crystal fractionation. The tectonic setting in the region transformed from the Late Jurassic syn-collision setting to Early Cretaceous within-plate setting, with E-W extension in the Early Cretaceous. This extension is correlated with the N-S trending post-collisional extension between the North China Craton and Yangtze Craton as well as the E-W trending back-arc extension triggered by the westward Paleo-Pacific Plate subduction, eventually leading to lithospheric thinning, asthenospheric upwelling, mafic magma underplating, and crustal melting in the East Qinling Orogen.
Provenance and tectonic setting transition as recorded in the Neoproterozoic strata, western Jiangnan Orogen: Implications for South China within Rodinia
Jiawei Zhang, Taiping Ye, Yaran Dai, Jianshu Chen, Hui Zhang, Chuangu Dai, Guohua Yuan, Kaiyuan Jiang
2019, 10(5): 1823-1839. doi: 10.1016/j.gsf.2018.10.009
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The Neoproterozoic Tonian strata (ca. 870-725 Ma) in the western Jiangnan Orogen archive the records of sedimentary provenance and tectonic setting which can be used to understand the geological evolution of the South China Continent. These strata are separated into the basement and cover sequences by a regional angular unconformity. The basement sequence can be subdivided into the lower and the upper parts by the widespread interbedded ca. 840 Ma basalt with pillow structure. In the present work, 234 concordant detrital zircon analyses are obtained from three Tonian sandstone samples in the Fanjingshan district, Guizhou Province. Combined with previous results, a total of 1736 analyses of detrital zircon U-Pb ages derived from 12 formations of Tonian strata in the western Jiangnan Orogen are used to decipher the integrated sedimentary and tectonic histories. The zircons from the lowermost part of the basement sequence (the Yujiagou Formation) show oval morphology and display two Paleoproterozoic age peaks at 2325 Ma and 1845 Ma which are similar with the detrital zircon age peaks from the Late Paleoproterozoic to Early Mesoproterozoic Dongchuan/Dahongshan/Hekou groups, suggesting a passive margin basin in which the sediments were mainly sourced from the southwestern Yangtze Block. However, the zircon age population of the lower part of the basement sequence (the Xiaojiahe, Huixiangping formations and their equivalents) indicates the sedimentary derivation from bidirectional sources (the ca. 870 Ma arc materials in the south and the old detritus from the southwestern Yangtze Block) which is consistent with a back arc setting for the deposition of the sediments. Zircons from the upper part of the basement sequence (the Duyantang Formation and its equivalent) show euhedral and subangular morphology and display a unimodal age peak at ca. 835 Ma. This sequence was possibly deposited in a convergent setting and the detritus were came from the locally distributed syn-collisional igneous rocks. The lower part of the cover sequence (the Xinzhai and Wuye formations and their equivalents) shows a distinct zircon age peak at 815-809 Ma and two subordinate peaks at 2485 Ma and 2018 Ma, suggesting that the basin had gradually transformed into a continental rift basin and received the detritus from the ca. 815 Ma post-collisional magmatic rocks as well as from different Paleoproterozoic source rocks in the northern Yangtze Block. We propose a tectonic evolution model that envisages eruption of ca. 840 Ma basalt in a back arc basin that existed during ca. 870-835 Ma, an angular unconformity was formed during amalgamation of the Yangtze Block and the Cathaysia Block at ca. 835-820 Ma and the rifting of the South China Continent was initiated at ca. 800 Ma. Our study concludes that the South China Continent was formed on the periphery of the Rodinia supercontinent.
Crystallization and impact history of a meteoritic sample of early lunar crust (NWA 3163) refined by atom probe geochronology
L. F. White, D. E. Moser, K. T. Tait, B. Langelier, I. Barker, J. R. Darling
2019, 10(5): 1841-1848. doi: 10.1016/j.gsf.2018.11.005
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Granulitic lunar meteorites offer rare insights into the timing and nature of igneous, metamorphic and impact processes in the lunar crust. Accurately dating the different events recorded by these materials is very challenging, however, due to low trace element abundances (e.g. Sm, Nd, Lu, Hf), rare micrometer-scale U-Th-bearing accessory minerals, and disturbed Ar-Ar systematics following a multi-stage history of shock and thermal metamorphism. Here we report on micro-baddeleyite grains in granulitic mafic breccia NWA 3163 for the first time and show that targeted microstructural analysis (electron backscatter diffraction) and nanoscale geochronology (atom probe tomography) can overcome these barriers to lunar chronology. A twinned (~90°/<401>) baddeleyite domain yields a 232Th/208Pb age of 4328±309 Ma, which overlaps with a robust secondary ion mass spectrometry (SIMS) 207Pb/206Pb age of 4308±18.6 Ma and is interpreted here as the crystallization age for the igneous protolith of NWA 3163. A second microstructural domain, <2 μm in width, contains patchy overprinting baddeleyite and yields a Th-Pb age of 2175±143 Ma, interpreted as dating the last substantial impact event to affect the sample. This finding demonstrates the potential of combining microstructural characterization with nanoscale geochronology when resolving complex P-T-t histories in planetary materials, here yielding the oldest measured crystallization age for components of lunar granulite NWA 3163 and placing further constraints on the formation and evolution of lunar crust.
Alteration and mineralization styles of the orogenic disseminated Zhenyuan gold deposit, southeastern Tibet: Contrast with carlin gold deposit
Huajian Li, Qingfei Wang, Jun Deng, Lin Yang, Chaoyi Dong, Huazhi Yu
2019, 10(5): 1849-1862. doi: 10.1016/j.gsf.2019.01.008
Abstract(182) HTML PDF(9)
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Orogenic disseminated and Carlin gold deposits share much similarity in alteration and mineralization. The disseminated orogenic Zhenyuan Au deposit along the Ailaoshan shear zone, southeastern Tibet, was selected to clarify their difference. The alteration and mineralization from the different lithologies, including meta-quartz sandstone, carbonaceous slate, meta-(ultra)mafic rock, quartz porphyry and lamprophyre were researched. According to the mineral assemblage and replacement relationship in all types of host rocks, two reactions show general control on gold deposition:(1) replacement of earlier magnetite by pyrite and carbonaceous material; (2) alteration of biotite and phlogopite phenocrysts in quartz porphyry and lamprophyre into dolomite/ankerite and sericite. Despite the lamprophyre is volumetrically minor and much less fractured than other host rocks, it contains a large portion of Au reserve, indicating that the chemically active lithology has played a more important role in gold precipitation compared to structure. LA-ICP-MS analysis shows that Au mainly occurs as invisible gold in fine-grained pyrite disseminated in the host rocks, with Au content reaching to 258.95 ppm. The diagenetic core of pyrite in meta-quartz sandstone enriched in Co, Ni, Mo, Ag and Hg is wrapped by hydrothermal pyrite enriched in Cu, As, Sb, Au, Tl, Pb and Bi.
Different host rock lithology has much impact on the alteration and mineralization features. Carbonate and sericite in altered lamprophyre show they have higher Mg than those developed in other of host rocks denoting that the carbonate and sericite incorporated Mg from phlogopite phenocrysts in the primary lamprophyre during alteration. The ore fluid activated the diagenetic pyrite in meta-quartz sandstone leading the hydrothermal pyrite enriched in Cu, Mo, Ag, Sb, Te, Hg, Tl, Pb and Bi, but the hydrothermal pyrite in meta-(ultra)mafic rock is enriched in Co and Ni as the meta-(ultra)mafic rock host rock contain high content of Co and Ni. However, Au and As shear similar range in both types of host rocks indicating that these two elements most likely come from the deep source fluid rather than the host rocks. It was shown in the disseminated orogenic gold deposit that similar hydrothermal alteration with mineral assemblage of carbonate (mainly dolomite and ankerite), sericite, pyrite and arsenopyrite develops in all types of host rocks. This is different from the Nevada Carlin type, in which alteration is mainly dissolution and silicification of carbonate host rock. On the other hand, Au mainly occur as invisible gold in both disseminated orogenic and Carlin gold deposits.
Genesis of the Bianjiadayuan Pb-Zn polymetallic deposit, Inner Mongolia, China: Constraints from in-situ sulfur isotope and trace element geochemistry of pyrite
Kai-Rui Song, Li Tang, Shou-Ting Zhang, M. Santosh, Christopher J. Spencer, Yu Zhao, Hao-Xing Li, Liang Wang, An-Li Zhang, Yin-Qiang Sun
2019, 10(5): 1863-1877. doi: 10.1016/j.gsf.2019.02.004
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The Southern Great Xing'an Range (SGXR) which forms part of the eastern segment of the Central Asian Orogenic Belt (CAOB) is known as one of the most important Cu-Mo-Pb-Zn-Ag-Au metallogenic belts in China, hosting a number of porphyry Mo (Cu), skarn Fe (Sn), epithermal Au-Ag, and hydrothermal vein-type Ag-Pb-Zn ore deposits. Here we investigate the Bianjiadayuan hydrothermal vein-type Ag-Pb-Zn ore deposit in the southern part of the SGXR. Porphyry Sn±Cu±Mo mineralization is also developed to the west of the Ag-Pb-Zn veins in the ore field. We identify a five-stage mineralization process based on field and petrologic studies including (i) the early porphyry mineralization stage, (ii) main porphyry mineralization stage, (iii) transition mineralization stage, (iv) vein-type mineralization stage and (v) late mineralization stage. Pyrite is the predominant sulfide mineral in all stages except in the late mineralization stage, and we identify corresponding four types of pyrites:Py1 is medium-grained subhedral to euhedral occurring in the early barren quartz vein; Py2 is medium-to fine-grained euhedral pyrite mainly coexisting with molybdenite, chalcopyrite, minor sphalerite and galena; Py3 is fine-grained, subhedral to irregular pyrite and displays cataclastic textures with micro-fractures; Py4 occurs as euhedral microcrystals and forms irregularly shaped aggregate with sphalerite and galena. LA-ICP-MS trace element analyses of pyrite show that Cu, Pb, Zn, Ag, Sn, Cd and Sb are partitioned into pyrite as structurally bound metals or mineral micro/nano-inclusions, whereas Co, Ni, As and Se enter the lattice via isomorphism in all types of pyrite. The Cu, Zn, Ag, Cd concentrations gradually increase from Py1 to Py4, which we correlate with cooling and mixing of ore-forming fluid with meteoric water. Py2 contains the highest contents of Co, Ni, Se, Te and Bi, suggesting high temperature conditions for the porphyry mineralization stage. Ratios of Co/Ni (0.03-10.79, average 2.13) and sulphur isotope composition of sulfide indicate typical hydrothermal origin for pyrites. The δ34SCDT values of Py1 (0.42‰-1.61‰, average 1.16‰), Py2 (-1.23‰ to 0.82‰, average 0.35‰), Py3 (-0.36‰ to 2.47‰, average 0.97‰), Py4 (2.51‰-3.72‰, average 3.06‰), and other sulfides are consistent with those of typical porphyry deposit (-5‰ to 5‰), indicating that the Pb-Zn polymetallic mineralization in the Bianjiadayuan deposit is genetically linked to the Yanshanian (Jurassic-Cretaceous) magmatic-hydrothermal events. Variations of δ34S values are ascribed to the changes in physical and chemical conditions during the evolution and migration of the ore-forming fluid. We propose that the high Sn content of pyrite in the Bianjiadayuan hydrothermal vein-type Pb-Zn polymetallic deposit can be used as a possible pathfinder to prospect for Sn mineralization in the surrounding area or deeper level of the ore field in this region.
Challenges of identifying putative planetary-origin meteorites of non-igneous material
Yana Anfinogenova, John Anfinogenov
2019, 10(5): 1879-1890. doi: 10.1016/j.gsf.2018.11.009
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This paper summarizes the challenges of identifying planetary-origin meteorites of non-igneous composition-particularly those of sedimentary origin. Evidence for putative sedimentary-origin (sed-type) meteorites and their potential parent bodies is reviewed, suggesting that the list of candidate parent bodies for sed-type meteorites includes, but is not limited to, Mars, Enceladus, Ganymede, Europa, Ceres, Vesta, and other hypothetical planets that existed between the orbits of Mars and Jupiter in the past. The extraterrestrial origin and probable parent body for sed-type meteorites should be assessed based on multiple lines of evidence, and not solely limited to tests of oxygen and noble gas isotopes, whose signatures may undergo terrestrial contamination and which may exhibit significant heterogeneity within both the Solar System and parent cosmic bodies. The observed fall of a cosmic body, evidence of hypervelocity fall, signs of impact, presence of fusion crust, melting, and/or shock deformation features in impactor fragments should be considered as priority signs of meteoritic origin.
Origin of trondhjemite and albitite at the expense of A-type granite, Aravalli orogen, India: Evidence from new metasomatic replacement fronts
Parampreet Kaur, Naveen Chaudhri, Nusrat Eliyas
2019, 10(5): 1891-1913. doi: 10.1016/j.gsf.2018.09.019
Abstract(127) HTML PDF(1)
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The A-type Harsora-Dadikar granites in the Alwar complex of northern Aravalli orogen, NW India provide evidence for subsolidus-requilibration of feldspars. They record three new discrete stages of albitisation, producing trondhjemite and albitite sequentially at the expense of original granite. Stage-I metasomatism deanorthised the magmatic oligoclase and transformed the grey least-albitised granite to pinkish grey microcline-oligoclase granite. Stage-II converted the latter to trondhjemite by replacement of microcline to oligoclase. Stage-III metasomatism led to the formation of albitite/albite granite from trondhjemite, where the metasomatically formed oligoclase was replaced by albite. This stage of metasomatism resulted in nearly complete disappearance of amphibole and biotite, producing a monomineralic rock (albitite), which is consistent with Korzhinskii theory of infiltration metasomatism. The reaction fronts delineating the Stage-II and Stage-III are sharp and easily discernible by their prominent color differences in Harsora on the outcrop scale. Chemically, the mineral transformations during three stages are manifested by the differential gains/losses in Na, K, Ca, Rb, Ba, Sr, Fe and Mg. The formation of albite, Cl-rich marialitic scapolite and Cl-rich amphibole in the albitised granites are suggestive of Na- and Cl-brines as the metasomatising fluids. The fluid-rock interactions, which can significantly transform the pristine mineralogy of granitoids, should be carefully considered to avoid any misinterpretations about their petrological history.
Magnetotelluric investigation of lithospheric electrical structure beneath the Dharwar Craton in south India: Evidence for mantle suture and plume-continental interaction
D. Malleswari, K. Veeraswamy, K. K. Abdul Azeez, A. K. Gupta, Narendra Babu, Prasanta K. Patro, T. Harinarayana
2019, 10(5): 1915-1930. doi: 10.1016/j.gsf.2018.10.011
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Broad-band and long period magnetotelluric measurements made at 63 locations along ~500 km long Chikmagalur-Kavali profile, that cut across the Dharwar craton (DC) and Eastern Ghat Mobile Belt (EGMB) in south India, is modelled to examine the lithosphere architecture of the cratonic domain and define tectonic boundaries. The 2-D resistivity model shows moderately conductive features that intersperse a highly resistive background of crystalline rocks and spatially connect to the exposed schist belts or granitic intrusions in the DC. These features are therefore interpreted as images of fossil path-ways of the volcanic emplacements associated with the greenstone belt and granite suite formation exposed in the region. A near vertical conductive feature in the upper mantle under the Chitradurga Shear Zone represents the Archean suture between the western and eastern blocks of DC. Although thick (~200 km) cratonic (highly resistive) lithosphere is preserved, significant part of the cratonic lithosphere below the western DC is modified due to plume-continental lithosphere interactions during the CretaceouseTertiary period. A west-verging moderately conductive feature imaged beneath EGMB lithosphere is interpreted as the remnant of the Proterozoic collision process between the Indian land mass and East Antarctica. Thin (~120 km) lithosphere is seen below the EGMB, which form the exterior margin of the India shield subsequent to its separation from East Antarctica through rifting and opening of the Indian Ocean in the Cretaceous.
AMS studies on a 450 km long 2216 Ma dyke from Dharwar craton, India: Implications to magma flow
E. Nagaraju, V. Parashuramulu
2019, 10(5): 1931-1939. doi: 10.1016/j.gsf.2018.12.003
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Anisotropy of magnetic susceptibility (AMS) studies were carried out on a precisely dated (2216.0±0.9 Ma), 450 km long N-S striking dyke in the Dharwar Craton, to determine the magma flow direction along the dyke length. In order to use the imbrication of the magnetic foliation, forty eight samples were collected from 13 locations along the length of the dyke. Magnetogranulometry studies show that AMS fabric is dominated by medium grained interstitial Ti-poor multidomain magnetite. The corrected anisotropy degree (Pj) of the samples was found to be low to moderate, between 1.007 and 1.072, which indicates primary magnetic fabric. The magnetic ellipsoid is either triaxial, prolate or oblate and clearly defines normal, intermediate and inverse magnetic fabrics related to magma flow during the dyke emplacement. The maximum susceptibility axes (Kmax) of the AMS tensor of the dyke is predom inantly inclined at low angles (<30°), with no systematic variation in depth along the N-S profile, indicating sub-horizontal flow even at mid crustal levels which could probably be governed by location of the focal region of the magma source (mantle plume?), flow dynamics together with the compressive stresses exerted by the overlying crust.
Phlogopite in mantle xenoliths and kimberlite from the Grib pipe, Arkhangelsk province, Russia: Evidence for multi-stage mantle metasomatism and origin of phlogopite in kimberlite
A. V. Kargin, L. V. Sazonova, A. A. Nosova, N. M. Lebedeva, Yu. A. Kostitsyn, E. V. Kovalchuk, V. V. Tretyachenko, Ya. S. Tikhomirova
2019, 10(5): 1941-1959. doi: 10.1016/j.gsf.2018.12.006
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We present petrography and mineral chemistry for both phlogopite, from mantle-derived xenoliths (garnet peridotite, eclogite and clinopyroxeneephlogopite rocks) and for megacryst, macrocryst and groundmass flakes from the Grib kimberlite in the Arkhangelsk diamond province of Russia to provide new insights into multi-stage metasomatism in the subcratonic lithospheric mantle (SCLM) and the origin of phlogopite in kimberlite. Based on the analysed xenoliths, phlogopite is characterized by several generations. The first generation (Phl1) occurs as coarse, discrete grains within garnet peridotite and eclogite xenoliths and as a rock-forming mineral within clinopyroxeneephlogopite xenoliths. The second phlogopite generation (Phl2) occurs as rims and outer zones that surround the Phl1 grains and as fine flakes within kimberlite-related veinlets filled with carbonate, serpentine, chlorite and spinel. In garnet peridotite xenoliths, phlogopite occurs as overgrowths surrounding garnet porphyroblasts, within which phlogopite is associated with Cr-spinel and minor carbonate. In eclogite xenoliths, phlogopite occa-sionally associates with carbonate bearing veinlet networks. Phlogopite, from the kimberlite, occurs as megacrysts, macrocrysts, microcrysts and fine flakes in the groundmass and matrix of kimberlitic pyroclasts. Most phlogopite grains within the kimberlite are characterised by signs of deformation and form partly fragmented grains, which indicates that they are the disintegrated fragments of previously larger grains.
Origin and evolution of ore-forming fluid for the Gaosongshan gold deposit, Lesser Xing'an Range: Evidence from fluid inclusions, H-O-S-Pb isotopes
Yang Liu, Jinggui Sun, Jilong Han, Liang Ren, Alei Gu, Keqiang Zhao, Changshen Wang
2019, 10(5): 1961-1980. doi: 10.1016/j.gsf.2019.01.006
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Epithermal gold deposits are typical precious metal deposits related to volcanic and subvolcanic magmatism. Due to the lack of direct geological and petrographic evidences, the origin of the ore-forming fluid is deduced from the spatial diagenesis-mineralization relationship, chronological data, physicochemical characteristics of mineral fluid inclusions, mineral or rock elements and isotopic geochemical characteristics. By objectively examining this scientific problem via a geological field survey and petrographic analysis of the Gaosongshan epithermal gold deposit, we recently discovered and verified the following points:(1) Pyrite-bearing spherical quartz aggregates (PSQA) occur in the rhyolitic porphyry; (2) the mineralization is structurally dominated by WNW- and ENE-trending systems and occurs mostly in hydrothermal breccias and pyrite-quartz veins, and the ore types are mainly hematite-crusted quartz, hydrothermal breccia, massive pyrite-quartz, etc.; (3) the alteration types consist of prevalent silicification, sericitization, propylitization and carbonation, with local adularization and illitization. The ore minerals are mainly pyrite, primary hematite, native gold, and electrum, with lesser amounts of chalcopyrite, magnetite, sphalerite, and galena, indicating a characteristic epithermal low-sulfidation deposit. The ore-forming fluid may have been primarily derived from magmatic fluid exsolved from a crystallizing rhyolitic porphyry magma. Further zircon U-Pb geochronology, fluid inclusion, physicochemical and isotopic geochemical analyses revealed that (1) rhyolitic porphyry magmatism occurred at 104.6±1.0 Ma, whereas the crystallization of the PSQA occurred at 100.8±2.1 Ma; (2) the hydrothermal fluid of the pre-ore stage was an exsolved CO2-bearing H2O-NaCl magmatic fluid that produced inclusions mainly composed of pure vapor (PV), vapor-rich (WV) and liquid-rich (WL) inclusions with a small number of melt-(M) and solid-bearing (S) inclusions; mineralization-stage quartz contains WL and rare PV, WV and pure liquid (PL) inclusions characterized by the H2O-NaCl system with low formation temperatures and low salinities; (3) the characteristics of hydrogen, oxygen, sulfur, and lead isotopes and those of rare earth elements (REEs) provide insight into the affinity between PSQA and orebodies resulting from juvenile crust or enriched mantle. Combined with previous research on the mineralogenetic epoch (99.32±0.01 Ma), we further confirm that the mineralization of the deposit occurred in the late Early Cretaceous, which coincides with the extension of the continental margin induced by subduction of the Pacific Plate beneath the Eurasian Plate. The formation of the ore deposit was proceeded by a series of magmatic and hydrothermal events, including melting of enriched juvenile crust, upwelling, the eruption and emplacement of the rhyolitic magma, the exsolution and accumulation of magmatic hydrothermal fluid, decompression, the cooling and immiscibility/boiling of the fluid, and mixing of the magmatic fluid with meteoric water, in association with water-rock interaction.