Geochemical Domains Research Articles

Mapping geochemical domains using stream sediment geochemistry: An approach based on compositional indicators in the Volturno River basin (South Italy)

When dealing with environmental problems, it is of fundamental importance to establish reference values (geochemical baselines) against which to determine the presence or absence of active contamination processes.In the effort to develop a method to assess the geochemical baselines for territories featuring complex geological settings and a well-established anthropic environmental pressure, we combined compositional data analysis (CoDA) with geolithological information to reduce the degree of uncertainty possibly affecting the results. The proposed approach comprises (1) a knowledge-driven step to select a number of sample subsets from a geochemical dataset each with a high probability of having its composition strongly influenced by only one of the lithologies outcropping in the study area; (2) a data-driven step to compute compositional principal balances and define geochemical indicators to be used to assign each of the observations in the dataset to one of the geochemical domains associated to a mayor lithologies outcropping in the study area; (3) the determination for each geochemical domain of baseline values based on the samples assigned to them by the data-driven step.The method was tested using the geochemical data referring to 887 stream sediment samples collected across the Volturno River catchment basin (Southern Italy), featuring a relevant lithological heterogeneity.The results obtained were easily interpretable as they fitted well with the geomorphological, geochemical, and geodynamic processes characterizing the study area.Despite the use of stream sediments for the specific case study presented, the application principles of the method hold for any environmental media and for any territory for which there is a need to define baseline values. However, for a successful application of the method, it is crucial to have a fair knowledge of the geological settings of the study area.

Mapping the Structure and Metasomatic Enrichment of the Lithospheric Mantle Beneath the Kimberley Craton, Western Australia

AbstractThe lithology, geochemistry, and architecture of the continental lithospheric mantle (CLM) underlying the Kimberley Craton of north‐western Australia has been constrained using pressure‐temperature estimates and mineral compositions for >5,000 newly analyzed and published garnet and chrome (Cr) diopside mantle xenocrysts from 25 kimberlites and lamproites of Mesoproterozoic to Miocene age. Single‐grain Cr diopside paleogeotherms define lithospheric thicknesses of 200–250 km and fall along conductive geotherms corresponding to a surface heat flow of 37–40 mW/m2. Similar geotherms derived from Miocene and Mesoproterozoic intrusions indicate that the lithospheric architecture and thermal state of the CLM has remained stable since at least 1,000 Ma. The chemistry of xenocrysts defines a layered lithosphere with lithological and geochemical domains in the shallow (<100 km) and deep (>150 km) CLM, separated by a diopside‐depleted and seismically slow mid‐lithosphere discontinuity (100–150 km). The shallow CLM is comprised of Cr diopsides derived from depleted garnet‐poor and spinel‐bearing lherzolite that has been weakly metasomatized. This layer may represent an early (Meso to Neoarchean?) nucleus of the craton. The deep CLM is comprised of high Cr2O3garnet lherzolite with lesser harzburgite, and eclogite. The peridotite components are inferred to have formed as residues of polybaric partial mantle melting in the Archean, whereas eclogite likely represents former oceanic crust accreted during Paleoproterozoic subduction. This deep CLM was metasomatized by H2O‐rich melts derived from subducted sediments and high‐temperature FeO‐TiO2melts from the asthenosphere.

Geochemical mapping by unmixing alluvial sediments: An example from northern Australia

Alluvial sediments have long been used in geochemical surveys as their compositions are assumed to be representative of areas upstream. Overbank and floodplain sediments, in particular, are increasingly used for regional to continental-scale geochemical mapping. However, during downstream transport, sediments from heterogeneous source regions are carried away from their source regions and mixed. Consequently, using alluvial sedimentary geochemical data to generate continuous geochemical maps remains challenging. In this study we demonstrate a technique that numerically unmixes alluvial sediments to make a geochemical map of their upstream catchments. The unmixing approach uses a model that predicts the concentration of elements in downstream sediments, given a map of the drainage network and element concentrations in the source region. To unmix sedimentary chemistry, we seek the upstream geochemical map that, when mixed downstream, best fits geochemical observations downstream. To prevent overfitting we penalise the roughness of the geochemical model. To demonstrate our approach we apply it to alluvial samples gathered as part of the Northern Australia Geochemical Survey. This survey gathered samples collected over a ∼500,000 km2 area in northern Australia. We first validate our approach for this sample distribution with synthetic tests, which indicate that we can resolve geochemical variability at scales greater than 0.5–1° in size. We proceed to invert real geochemical data from the total digestion of fine-grained fraction of alluvial sediments. The resulting geochemical maps for two elements of potential economic interest, Cu and Nd, are evaluated in detail. We find that in both cases, our predicted downstream concentrations match well against a held-out, previously ‘unseen’ subset of the data, as well as against data from an independent geochemical survey. By performing principal component analysis on maps generated for all 46 available elements we produce a synthesis map to identify the significant geochemical domains in this part of northern Australia. This map shows strong spatial similarities to the underlying lithological map of the area. Finally, we compare the results from our approach to geochemical maps produced by interpolating alluvial geochemical data using two standard methods: kriging and inverse distance weighting. We find that, unlike the method presented here, both these methods can generate maps that are dampened relative to expected magnitude, as well as being spatially distorted. We argue that the unmixing approach is the most appropriate method for generating geochemical maps from regional-scale alluvial surveys.

A HIMU volcanic belt along the SW African coast (∼83–49 Ma): New geochemical clues to deep mantle dynamics from carbonatite and silica-undersaturated complexes in Namibia

The origin of carbonatitic and highly silica-undersaturated volcanism, common along the SW coast of Africa extending from Angola through Namibia to the tip of South Africa, is still poorly understood. Here we present new geochemical data (major and trace element and Sr-Nd-Pb-Hf-O-C isotopes) from the Agate Mountain calcio- to magnesio‑carbonatites (∼83 Ma), Dicker Willem calcio‑carbonatites (49 Ma) and Swakopmund basanitic plugs (76–72 Ma) along the coast of Namibia that were emplaced after the EMI (enriched mantle one) type Etendeka flood basalts. The trace element and isotopic composition of Agate Mountain carbonatites and Swakopmund basanites indicate that they were derived from a HIMU-type (high time-integrated 238 U/ 204 Pb with radiogenic Pb isotope ratios) magma source, similar to the St. Helena global HIMU endmember in the South Atlantic. The Agate Mountain carbonatites form part of the late-stage Walvis Ridge HIMU hotspot track overlying the EM1-type Walvis Ridge basement forming part of the Tristan-Gough hotspot track. The Dicker Willem carbonatites, however, extend to higher 206 Pb/ 204 Pb than St. Helena, but have similar 206 Pb/ 204 Pb to Mangaia HIMU lavas in the Pacific. Compared to Mangaia HIMU, the Dicker Willem carbonatites with mantle-type O and C isotopes have higher 207 Pb/ 204 Pb and 87 Sr/ 86 Sr but lower 143 Nd/ 144 Nd, suggesting it may represent a new HIMU endmember flavor. The HIMU carbonatitic and silica-undersaturated rocks form a belt of age-progressive volcanic tracks, including: 1) from the Walvis Ridge, through NW Namibia to central Angola, 2) from the Vema Seamount via Dicker Willem carbonatite to Gibeon kimberlites and carbonatites, 3) from the Namaqualand to Bushmanland and to Warmbad volcanic centers in northwestern South Africa, and 4) along the older end of the Shona EMI-type volcanic track extending into South Africa. Geochemical and seismic tomographic data suggest that the EMI and HIMU mantle plumes are generated from different geochemical domains at the base of the lower mantle. The Tristan-Gough, Discovery and Shona EM1 volcanic tracks are derived from a common low-velocity anomaly (superplume-like structure with three branching arms) ascending from the outer margin, possibly lower primoridal layer, of the African large low-shear-velocity province (LLSVP). Seismic low-velocity anomalies can be traced from beneath the belt of HIMU volcanism to an internal and shallower part of the LLSVP, located ∼900–1200 km east of the outer LLSVP margin and suggest that HIMU-type (possibly subducted oceanic lithospheric) material overlies EMI-type (possibly primordial) material in the internal part of the LLSVP. • A HIMU-like volcanism belt along the southwest Africa. • The HIMU-like volcanic complexes form age-progressive volcanic tracks. • EMI and HIMU mantle plumes are from different domains in the lower mantle.

Interpretation of geochemical anomalies and domains using Gaussian mixture models

The element's concentrations in stream sediments are frequently the product of complex geological processes and a variety of lithological sources. This variety of processes dilutions or concentrates the trace elements in the sediments concerning the original concentration presented in the source rock. When analyzing the geochemical data of stream sediments, it is common to find skewed distributions. This skewness can be attributed to multiplicative effects involved in dilution processes that result in lognormal distributions. It also can imply a mixture of populations that results from the variety of geological processes and/or the diversity of lithological sources. In this study, a method is proposed to classify geochemical samples of sediments into data sets or geochemical domains that follow normal, lognormal or log-ratio normal distributions. This methodology can be based on the classification of samples according to geological features or use Gaussian mixture models to determine the geochemical domains. To mixture modeling, CLR transformation and the expectation-maximization (EM) algorithm were used. The results obtained include mixture models of 2–3 populations associated with main geochemical processes and/or dominant lithological sources. The proposed methodology defines threshold values for each domain and allows to set a more appropriate background. An analysis carried out with data from northern Mexico identified anomalies that would not be defined without the separation of previous domains.

Geochemical domains and their characteristics in the middle-southern part of the Sanjiang orogenic belt in southwestern China

Principal component analysis (PCA) was used to extract geological information about regional strata and mineralization from many high-quality stream sediment geochemical data in the middle-southern part of the Sanjiang orogenic belt. According to the geological reality, the 32 main ore-forming elements in the study area were grouped into seven element associations that reflected the stratum features and mineralization. The study area was further divided into six geochemical domains that reflected the geological background. The strata and structure were combined with mineral data and revealed that the geological domains proposed in this paper represented the geological background of the study area. The corresponding elements within these geochemical domains have higher concentrations and enrichment factors, showing that the geological domains in the study area are reasonable. In the study area, the observed anomalies in the geochemical domains highlighted effective anomalies and reduced false anomalies. These anomalies can not only be used to identify potential metallogenic areas, but also provide information on tectonics; for instance, the middle axial fault of the Lanping-Simao basin was reflected in the linear anomalies of the LiB element association. The results of this study can help understand the combination and distribution characteristics of ore-forming elements and the regional geological structure of the middle-southern part of the Sanjiang orogenic belt.

Spatial Characteristics of Recycled and Primordial Reservoirs in the Deep Mantle

AbstractThe spatial distribution of the geochemical domains hosting recycled crust and primordial (high‐3He/4He) reservoirs, and how they are linked to mantle convection, are poorly understood. Two continent‐sized seismic anomalies located near the core‐mantle boundary—called the Large Low Shear Wave Velocity Provinces (LLSVPs)—are potential geochemical reservoir hosts. It has been suggested that high‐3He/4He hotspots are spatially confined to the LLSVPs, hotspots sampling recycled continental crust are associated with only one of the LLSVPs, and recycled continental crust shows no relationship with latitude. We reevaluate the links between LLSVPs and isotopic signatures of hotspot lavas using improved mantle flow models including plume conduit advection. While most hotspots with the highest‐3He/4He can indeed be traced to the LLSVP interiors, at least one high‐3He/4He hotspot, Yellowstone, is located outside of the LLSVPs. This suggests high‐3He/4He is not geographically confined to the LLSVPs. Instead, a positive correlation between hotspot buoyancy flux and maximum hotspot 3He/4He suggests that it is plume dynamics (i.e., buoyancy), not geography, which determines whether a dense, deep, and possibly widespread high‐3He/4He reservoir is entrained. We also show that plume‐fed EM hotspots (enriched mantle, with low‐143Nd/144Nd), signaling recycled continental crust, are spatially linked to both LLSVPs, and located primarily in the southern hemisphere. Lastly, we confirm that hotspots sampling HIMU (“high‐μ,” or high 238U/204Pb) domains are not spatially limited to the LLSVPs. These findings clarify and advance our understanding of deep mantle reservoir distributions, and we discuss how continental and oceanic crust subduction is consistent with the spatial decoupling of EM and HIMU.

Unsupervised clustering of LA-ICP-MS raster map data for geological interpretation: A case study using epidote from the Yerington district, Nevada

Raster element concentration maps created using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) can be used to interpret microscale compositional and textural domains within mineral grains. Raster maps are typically evaluated element by element; however, application of statistical techniques (such as cluster analysis) can enhance the generation of geochemical domains to support interpretation of growth zones, core-rim relationships, sector zones, and compositional-textural associations. Clustered LA-ICP-MS map data can be assessed within individual samples and between multiple samples, and can extend insight from the microscopic scale to the regional scale to better understand geological paragenesis of an area.Our workflow (1) applies a centred log transformation to selected elements in a raster map dataset; (2) uses principal component analysis (PCA) applied to the multi-sample, mono-mineralic dataset to group similar elements in epidote based on geochemical character; (3) applies unsupervised clustering to separate different types and generations of epidote in chemical feature space; (4) presents clustered LA-ICP-MS raster map results for interpretation of inter- and intra-mineral chemical zones; and (5) plots results spatially, across a regional map area, to investigate geological paragenesis.The workflow is illustrated using samples of epidote from the Yerington porphyry-skarn Cu (Mo–Au) district. In the case study area, six clusters are defined by unique mineral compositions: (1) low U; (2) elevated Pb, Mn and low Fe and Sr; (3) elevated Ce, U and low Mn and Pb; (4) elevated U, Ce; low Mn, Pb; (5) elevated Sr, Fe and low Mn, Pb; and (6) elevated Mn, Sr, and Fe and low Ce and U. The regional distribution of these groups is presented as indicating proximity to the porphyry environment (lower concentrations of Ce, U, As and Sb and higher concentrations of Mn, Sr and Fe) versus retrograde skarn (elevated Ce, U, As and Sb).

Contrasting Old and Young Volcanism from Aitutaki, Cook Islands: Implications for the Origins of the Cook–Austral Volcanic Chain

AbstractThe Cook–Austral volcanic lineament is thought to be the product of three overlapping South Pacific hotspot tracks: Macdonald, Arago, and Rarotonga. However, the entire Rarotonga hotspot track consists of volcanism on just two islands, Rarotonga and Aitutaki, making it difficult to evaluate a hotspot origin for the young volcanism on these islands. We present new Sr–Nd–Hf–Pb–He isotopic and major and trace element data on Aitutaki—which has two periods of volcanism, older (9·39 Ma) and younger (1·382–1·941 Ma)—and Atiu in the Cook Islands to evaluate a hotspot origin. Like Atiu, the ‘older volcanic series’ at Aitutaki was tentatively linked with the Arago hotspot based on its age, but this link has never been tested with geochemistry. New geochemical data on the older Aitutaki volcanic series show clear affinities to the Arago hotspot volcano Atiu. In contrast, the younger Aitutaki volcanic series, erupted on a near-atoll, exhibits evidence for a rejuvenated, non-plume volcanic origin. If Rarotonga (1·157–1·697 Ma) and the young series at Aitutaki share a common heritage and mechanism for formation as previously proposed—they erupted only 260 km apart over an overlapping time period and exhibit overlapping radiogenic isotopic compositions—then Rarotonga lavas may also represent a subaerial rejuvenated volcanic stage capping an older volcano related to a different hotspot, consistent with no direct involvement from a mantle plume in the origin of subaerial lavas at Rarotonga. Rarotonga, Aitutaki, and Atiu exhibit some of the most extreme enriched mantle (EM) signatures in the Cook–Austral volcanic lineament. There are three geographically separated geochemical domains along the Cook–Austral volcanic lineament—a northern geochemical domain with EM geochemical signatures is separated from a southern EM domain by a 1000 km long central HIMU (‘high μ’, or high 238U/204Pb) domain—and Rarotonga, Aitutaki, and Atiu are part of the northernmost geochemical domain. There is a possible role for lithospheric thickness in controlling geochemical compositions of melt extracted from the mantle along the Cook–Austral volcanic lineament, particularly across the Austral Fracture Zone. However, this is complicated by the lineament’s juxtaposition with the South Pacific Superswell, which may modify lithospheric behavior and complicate a simple lithospheric thickness model describing variable hotspot volcano compositions in the region.

Regional geochemical zonation of cultivated floodplains–Application of multi-element associations for soil quality evaluation along the Ohře (Eger) River, Czech Republic

Floodplains are dynamic ecosystems wherein intensive farming can meet increased environmental concentrations and bioavailability of a number of trace elements. Therefore, the primary objective of this study was to investigate the partial digestion of selected trace elements in surficial soil samples (depth up to 40 cm) across a gradient of environmental conditions along the Ohře River and determining the elements of high priority, while also exploring their geographic variation. The established agrochemical testing of Czech soils provides a valuable resource to determine the geochemical differences of multi-element associations in cultivated areas (arable crops and hop fields) in periodically inundated areas in the catchment with a number of potential pollution sources. The topsoil samples were digested using the nitric acid (2 mol/L HNO3), analysed for trace elements (Be, Cd, Co, Cr, Cu, Ni, Pb, V, and Zn) using inductively coupled plasma mass spectrometry (ICP-MS) and atomic absorption spectroscopy (AAS), and for their total Hg content. Data (n = 130) were modelled against the sample location and cultivation using the log-ratio approach. The explanatory and inferential statistical tools (the Mantel's correlation test, PCA, MANOVA) adapted for compositions, combined with a simple spatial ranking and land-use information defined effective discriminating subcomposition (Cu–Hg–Ni) that were able to describe the changes of geochemical domains in floodplains and retained a decent representation (46%) of the entire compositional variability in our dataset. Mapping the Cu–Hg–Ni subcomposition using the RGB composite enabled to display the high-dimensional multivariate dataset with reduced information and enabled to discriminate spatial domains representing geochemical signatures related to changes in element inputs along the watercourse. The analysis of our results within the compositional framework showed a changing pattern of relative enrichment/depletion of the HNO3–extractable trace elements in which a regional contrast between groups of the chalcophile elements (Cu, Pb, Zn, and partly Cd), siderophile transition metals (Cr, Co, Ni, and V) and single elements (Hg, Be) was source-apportioned to the effect of point pollution sources (Hg, Ni, Cr, Co) as well as diffuse sources (Cu, Zn, Pb, Cd).

Evidence for a deep mantle source for EM and HIMU domains from integrated geochemical and geophysical constraints

Subduction of oceanic and continental crust (and associated sediments) into the mantle over geologic time generates mantle domains with geochemically distinct signatures, referred to as HIMU (high “μ”, where μ=238U/204Pb) and EM (enriched mantle) domains. Identification of EM and HIMU geochemical signatures in hotspot lavas provides evidence that subducted crustal materials are recycled into the source of hotspots. It remains uncertain where these materials are located in the mantle, and a key question is whether upwelling mantle plumes are required to transport mantle domains with EM and HIMU signatures to the shallow mantle beneath hotspots. Therefore, this study evaluates relationships between extreme EM and HIMU compositions at oceanic hotspots and the presence (or absence) of seismically-constrained mantle plumes beneath the hotspots. We draw on three existing plume catalogs based on global seismic shear-wave velocity models, and these plume catalogs indicate the presence or absence of a plume beneath each of 42 oceanic hotspots. From each hotspot, we select a lava with the highest 206Pb/204Pb composition and one with the lowest 143Nd/144Nd composition.We show that hotspots associated with seismically defined plumes show a greater likelihood of hosting lavas with either extreme EM (143Nd/144Nd ≤ 0.512630) or extreme HIMU (206Pb/204Pb ≥ 20.0) compositions than hotspots not associated with plumes, but HIMU hotspots show a stronger association with plumes than EM hotspots. The significance of the relationship between plumes and extreme geochemical signatures at hotspots improves if extreme EM and HIMU compositions are considered together instead of separately: hotspots sourced by mantle plumes are even more likely to exhibit extreme EM or extreme HIMU signatures than hotspots not sourced by plumes. The significance tests also show that hotspots with extreme EM or HIMU compositions are more likely to be associated with mantle plumes than hotspots that lack extreme geochemical signatures. A relationship between seismically detected deep mantle plumes and the presence of extreme EM or HIMU compositions at hotspots provides evidence for a deep mantle source for these geochemical domains.

A 1.5 Ma record of plume-ridge interaction at the Western Galápagos Spreading Center (91°40′–92°00′W)

Shallow (elevated) portions of mid-ocean ridges with enriched geochemical compositions near hotspots document the interaction of hot, geochemically-enriched plume mantle with shallow depleted upper mantle. Whereas the spatial variations in geochemical composition of ocean crust along the ridge axis in areas where plume-ridge interaction is taking place have been studied globally, only restricted information exists concerning temporal variations in geochemistry of ocean crust formed through plume-ridge interaction. Here we present a detailed geochemical study of 0–1.5Ma ocean crust sampled from the Western Galápagos Spreading Center (WGSC) axis to 50km north of the axis, an area that is presently experiencing a high influx of mantle material from the Galápagos hotspot. The tholeiitic to basaltic andesitic fresh glass and few bulk rock samples have incompatible element abundances and Sr–Nd–Pb isotopic compositions intermediate between depleted normal mid-ocean-ridge basalt (N-MORB) from >95.5°W along the WGSC and enriched lavas from the Galápagos Archipelago, displaying enriched (E-)MORB type compositions. Only limited and no systematic geochemical variations are observed with distance from the ridge axis for <1.0Ma old WGSC crust, whereas 1.0–1.5Ma old crust trends to more enriched isotopic compositions in 87Sr/86Sr, 143Nd/144Nd, 207Pb/204Pb and 208Pb/204Pb isotope ratios. On isotope correlation diagrams, the data set displays correlations between depleted MORB and two enriched components. Neither the geographically referenced geochemical domains of the Galápagos Archipelago nor the end members used for principal component analysis can successfully describe the observed mixing relations. Notably an off-axis volcanic cone at site DR63 has the appropriate composition to serve as the enriched component for the younger WGSC and could represent a portion of the northern part of the Galápagos plume not sampled south of the WGSC. Similar compositions to samples from volcanic cone DR63 have been found in the northern part of the 11–14Ma Galápagos hotspot track offshore Costa Rica, indicating that this composition is derived from the northern portion of the Galápagos plume. The older WGSC requires involvement of an enriched mantle two (EMII) type source, not recognized thus far in the Galápagos system, and is interpreted to reflect entrained material either from small-scale heterogeneities within the upper mantle or from the mantle transition zone. Overall the source material for the 0–1.5Ma WGSC ocean crust appears to represent mixing of depleted upper mantle with Northern Galápagos Plume material of relatively uniform composition in relatively constant proportions.

Do mantle plumes preserve the heterogeneous structure of their deep-mantle source?

It has been proposed that the spatial variations recorded in the geochemistry of hotspot lavas, such as the bilateral asymmetry recorded at Hawaii, can be directly mapped as the heterogeneous structure and composition of their deep-mantle source. This would imply that source-region heterogeneities are transported into, and preserved within, a plume conduit, as the plume rises from the deep-mantle to Earth's surface. Previous laboratory and numerical studies, which neglect density and rheological variations between different chemical components, support this view. However, in this paper, we demonstrate that this interpretation cannot be extended to distinct chemical domains that differ from surrounding mantle in their density and viscosity. By numerically simulating thermo-chemical mantle plumes across a broad parameter space, in 2-D and 3-D, we identify two conduit structures: (i) bilaterally asymmetric conduits, which occur exclusively for cases where the chemical effect on buoyancy is negligible, in which the spatial distribution of deep-mantle heterogeneities is preserved during plume ascent; and (ii) concentric conduits, which occur for all other cases, with dense material preferentially sampled within the conduit's centre. In the latter regime, the spatial distribution of geochemical domains in the lowermost mantle is not preserved during plume ascent. Our results imply that the heterogeneous structure and composition of Earth's lowermost mantle can only be mapped from geochemical observations at Earth's surface if chemical heterogeneity is a passive component of lowermost mantle dynamics (i.e. its effect on density is outweighed by, or is secondary to, the effect of temperature). The implications of our results for: (i) why oceanic crust should be the prevalent component of ocean island basalts; and (ii) how we interpret the geochemical evolution of Earth's deep-mantle are also discussed.

High Spatial Resolution Quantitative Imaging by Cross-calibration Using Laser Ablation Inductively Coupled Plasma Mass Spectrometry and Synchrotron Micro-X-ray Fluorescence Technique

High spatial resolution, quantitative chemical imaging is of importance to various scientific communities, however high spatial resolution and robust quantification are not trivial to attain at the same time. In order to achieve microscopic chemical imaging with enhanced quantification capabilities, the current study links the independent and complementary advantages of two micro-analytical techniques - Synchrotron Radiation-based micro X-ray Fluorescence (SR-microXRF) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS). A cross-calibration approach is established between these two techniques and validated by one experimental demonstration. In the presented test case, the diffusion pattern of trace level Cs migrating into a heterogeneous geological medium is imaged quantitatively with high spatial resolution. The one-dimensional line scans and the two-dimensional chemical images reveal two distinct types of geochemical domains: calcium carbonate rich domains and clay rich domains. During the diffusion, Cs shows a much higher interfacial reactivity within the clay rich domain, and turns out to be nearly non-reactive in the calcium carbonate domains. Such information obtained on the micrometer scale improves our chemical knowledge concerning reactive solute transport mechanism in heterogeneous media. Related to the chosen demonstration study, the outcome of the quantitative, microscopic chemical imaging contributes to a refined safety assessment of potential host rock materials for deep-geological nuclear waste storage repositories.

Geochemical tracing and spatial evolution of the sediment bed load of the Romaine River, Québec, Canada

Abstract A four-reservoir hydroelectric complex which is projected on the Romaine River, in eastern Quebec, Canada, will likely lead to the complete settling of the sand-size bed load of the river. In order to characterise the present river bed load sources, sediment transport and sedimentary connectivity, a study was conducted to document the compositional continuum of the lower 300 km portion of the river. Bed load samples were collected during the late summer low water period and ICP-MS geochemical analyses of the light and heavy mineral portions were conducted. Principal component analysis of the results successfully identified a series of distinctive geochemical domains. Multivariate Euclidean distance coefficients were also calculated between consecutive samples along the river revealing major bed load compositional breaks. These results indicate a longitudinal fractionation of the bed load along the 300 km course of the river. This is further supported by the sedimentary budget calculated from the annual sedimentary input derived from erosion rates of the river banks, from sediment bed load measurements at three gauging stations and from calculated bed load transport capacity at these three locations. It appears that significant sediment accommodation space is available on the upper course (250 km) of the Romaine River, in spite of the fact that this river has drained highly sediment-laden meltwaters from the receding Laurentide Ice Sheet, some 10 ka ago. This accommodation space leads to the renewal of the bed load due to recurrent partial to complete sedimentation. The major part of the bed load reaching the Romaine River mouth originates from the lower 50 km of the river course.

Isotopic portrayal of the Earth’s upper mantle flow field

It is now well established that oceanic plates sink into the lower mantle at subduction zones, but the reverse process of replacing lost upper-mantle material is not well constrained. Even whether the return flow is strongly localized as narrow upwellings or more broadly distributed remains uncertain. Here we show that the distribution of long-lived radiogenic isotopes along the world's mid-ocean ridges can be used to map geochemical domains, which reflect contrasting refilling modes of the upper mantle. New hafnium isotopic data along the Southwest Indian Ridge delineate a sharp transition between an Indian province with a strong lower-mantle isotopic flavour and a South Atlantic province contaminated by advection of upper-mantle material beneath the lithospheric roots of the Archaean African craton. The upper mantle of both domains appears to be refilled through the seismically defined anomaly underlying South Africa and the Afar plume. Because of the viscous drag exerted by the continental keels, refilling of the upper mantle in the Atlantic and Indian domains appears to be slow and confined to localized upwellings. By contrast, in the unencumbered Pacific domain, upwellings seem comparatively much wider and more rapid.

Geochemical blocks for predicting large ore deposits—concept and methodology

Regional geochemical data for 39 elements from several millions of stream sediment samples were obtained in China's national geochemical mapping program. This program was carried out over a 24-year period and covered more than 6 million km 2 of China. By examining this large data set, broad geochemical patterns other than dispersion halos and dispersion trains can be discerned. These broad geochemical patterns, such as geochemical regions, geochemical provinces, geochemical megaprovinces and geochemical domains, are the surface expressions of large blocks of rock with higher metal contents. These large “geochemical blocks” are the net results of the earth's original heterogeneity and the distribution and redistribution of metals during the evolution of earth. Large geochemical blocks will provide large metal endowments necessary for the formation of large to giant ore deposits. The geochemical block concept has the capacity to change mineral exploration strategy and methodology in China, providing a powerful tool for global mineral potential assessment and inspiring new ideas for exploration geochemistry, ore-genesis and metallogeny.

Geochemical blocks for predicting large ore deposits?concept and methodology

Regional geochemical data for 39 elements from several millions of stream sediment samples were obtained in China's national geochemical mapping program. This program was carried out over a 24-year period and covered more than 6 million km2 of China. By examining this large data set, broad geochemical patterns other than dispersion halos and dispersion trains can be discerned. These broad geochemical patterns, such as geochemical regions, geochemical provinces, geochemical megaprovinces and geochemical domains, are the surface expressions of large blocks of rock with higher metal contents. These large “geochemical blocks” are the net results of the earth's original heterogeneity and the distribution and redistribution of metals during the evolution of earth. Large geochemical blocks will provide large metal endowments necessary for the formation of large to giant ore deposits. The geochemical block concept has the capacity to change mineral exploration strategy and methodology in China, providing a powerful tool for global mineral potential assessment and inspiring new ideas for exploration geochemistry, ore-genesis and metallogeny.

Role of monsoon rain on concentrations and dispersion patterns of metal pollutants in sediments and soils of the Ganga Plain, India

Monsoon rain causes large scale sediment-water movement and reworking of sediments of the Ganga Plain which is one of the largest fluvial systems on Earth. Geomorphology and drainage type combined with sedimentation processes play a substantial role on dispersion and transport patterns of metals bound to sediments and soils. The study area of Kanpur-Unnao industrial region in the Ganga Plain has been divided into five independent geochemical domains on the basis of sediment-geomorphic, hydrological and geochemical characters. The monsoon hydrography and physico-chemical parameters (pH, conductivity) of the river and urban drain waters play a prominent role in regulating the concentrations and behaviour of the metals in the aquatic system of the Ganga Plain. Values of pH and specific electrical conductivity of the river water of the study area decrease whereas those of the urban drain water increase in post-monsoon period. The monsoon rain reduces the contents of Co, C-org, Cr, Fe and Ni and enhances the contents of Cd, Sn and Zn in sediments of post-monsoon period. In soils, it reduces the contents of Al, Co, Fe, Mn and Ni and enhances the contents of Cd, Sn and Zn in the post-monsoon period. These changes in concentrations vary from metal to metal and from one geochemical domain to the other. An increase in the concentrations of few metals in the soils from pre- to post-monsoon periods indicates that these metals were mobilized from the overflooding of metal rich waste-water onto the fields during high water stage and also by reworking of the soils through sheet floods during the monsoon time. Despite the changes in concentrations, metal dispersion patterns in each domain remain similar both in pre- and post-monsoon periods which indicate that the geochemical and sediment-geomorphic processes operating for the metal dispersion and mobilization in sediments are persistent even after large scale sediment-water movement and reworking of the sediments during the monsoon period.

Importance of geomorphology and sedimentation processes for metal dispersion in sediments and soils of the Ganga Plain: identification of geochemical domains

The Ganga Plain, one of the largest alluvial plains of the earth is under severe environmental stress due to rapid industrial and urban development and exploding population growth. Disposal of industrial and urban wastes and the ignorance of their management have resulted severe environmental problems, especially metal pollution. In the present study, an attempt is made to understand the transport and dispersion processes of metal contaminants in sediments and soils in Kanpur–Unnao industrial region of the Ganga Plain. The significance of geomorphology and monsoon rain is assessed in determining the distribution of contaminated sediments in this area. Three regional geomorphic surfaces (T 0, T 1 and T 2) are identified, each with its characteristic river channel patterns and sediment distribution system which influence the dispersion and accumulation of the metal contaminants. The sediment and water distribution regime depends on the nature of geomorphic features and character of the river channels. Geomorphic features, sedimentation processes, the nature of the river channels and distribution of excess rain water of the monsoon season have given the bases to delineate five geochemical domains in the study area. Each geochemical domain is an independently operating system with its own geomorphic and geochemical characters. Chemical parameters (e.g., pH), metal concentrations, and dispersion of metal-contaminated sediments and soils vary from domain to domain. In comparison to reference values, each domain exhibits its own enrichment of elements, namely the Loni River domain has high contents of C-org, Cd, Cr, Zn; the Ganda Nala domain of C-org, Cu, Cr; the Ganga River domain of C-org, Cr, Cu, Pb, Sn, Zn; the Sewage Network domain of C-org, Cd, Cr, Cu, Pb, Ni, Sn, Zn; and the Pandu River domain of C-org, Cd, Cu, Ni, Zn. Patterns of changes in metal concentrations from one monsoon year to the other and in different seasons of a monsoon flood cycle are also domain specific. Each geochemical domain exhibits its own degree of metal enrichment in sediments and soils by anthropogenic activities and physico-chemical processes. Identification of geochemical domains with their specific physico-chemical processes in the fluvial system of the Ganga Plain will certainly help in assessing the environmental problems and land contamination of this region.