Geochemical Contrast Research Articles

MORPHOGENETIC ABNORMALITIES OF BRYOBIONTS IN GEOCHEMICALLY CONTRASTING CONDITIONS IN DONBASS

The territory of modern central Donbass is considered as an experimental site for the implementation of an ingredient biomonitoring program. Bryophytes were transplanted and exposed in the geolocalities of the direct impact of enterprises of coal mining and processing complexes, metallurgical and chemical industries, ruderal and residential ecotopes. For bryobionts with a wide amplitude of resistance to technogenic pollution (Amblystegium subtile (Hedw.) Schimp., Brachythecium campestre (Müll.Hal.) Bruch et al., Bryum argenteum Hedw., Bryum caespiticium Hedw., Bryum capillare Hedw., Ceratodon purpureus (Hedw.) Brid and Pylaisia polyantha (Hedw.) Schimp.) the following criteria of anthropotolerance were selected: 1) elements accumulation function, 2) the ability to accumulate specific pollutants, 3) structural and functional reactions and manifestation of atypical morphogenesis (teratogenesis) of plant organisms and 4) shifts in the characteristics of the survival strategy of species in the conditions of deep transformation of the landscape systems of Donbass. Specificity of accumulation of Na, Mg, Al, Si, P, S, Cl, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Br, Rb, Sr, Zr, Mo, Cd, Sb, I, Cs, Ba, La, Ce, Nd, Sm, Eu, Tb, Dy, Yb, Hf, Ta, W, Hg, Pb, Th, U in samples of indicator plants used as seasonal phytotest monitors to assess pollution of natural environments have been defined. With a radical violation of geochemical cycles in plant life support systems, cases of specific chlorosis and necrosis of point localization, hypo- and hypergenesis and deformation, as well as fasciation, prolification, dystopia, oligomerization of vegetative organs, features of the development of individual groups of cells in the integumentary and conformational tissues of the leaf apparatus of bryobionts have been recorded. The revealed abnormalities are used as a phyto-indication characteristic when conducting an express analysis of the level of technogenic stress in field diagnostics. The coefficients of biological absorption and technogenic concentration of elements in biosubstrates were calculated when assessing the geochemical contrast of the environment. The difference in the accumulative capacity of bryobionts for individual elements or their associative groups in various ectopes is defined. The related morphogenetic heterogeneity of the structure and elemental composition of plants is an individual case of compliance with V.I. Vernadsky's ideas of the concentration, information and environment-forming functions of living matter.

Elevated uranium concentration and low activity ratio (234U/238U) in the Œuf river as the result of groundwater–surface water interaction (Essonne river valley, South of Paris Basin, France)

Uranium (U) is a naturally occurring radioactive heavy metal widely distributed on Earth. Noticeable elevated U concentration and low activity ratio (AR) were occasionally detected in headwater stream of the Essonne river (Seine Basin, France), the namely Œuf river. This paper aims at providing new insight on geogenic U features in headwater streams and examines the role of river-groundwater interaction. The Œuf river was sampled four times in 2020 to investigate the influence of heterogeneous geology and hydrological seasonality. The dissolved fraction of water samples was analyzed for a variety of chemical parameters (anion, major, minor and trace element concentrations, isotopes 234U and 238U). The Œuf river was shown to exhibit elevated U concentration up to 19.3 μg L−1 (exceeding by 100-fold the value of 0.19 μg L−1 known for riverine average) and low AR down to 0.41 (almost the third of the value expected in surface water, i.e., 1.17). The Œuf river got enriched in U when receiving groundwater from Beauce Limestone Aquifer System. High U concentration (above 15 μg L−1) was found in association with low AR (below 0.5) in the stream water when flowing in the outcrop zone of one BLAS unit. Taking advantage of changes in the stream flow conditions and the geochemical contrast between surface and ground waters, mixing volumes were calculated. This study first examined the potential of using U isotopes in combination with selenium as hydrogeochemical tracers of the river-groundwater continuum. In HWS, the aquifer discharge was shown to supply 12 to 59 % of the river water. This study demonstrates the key role played by the river-groundwater interaction on river water chemistry in small streams draining catchment with various geology setting. It also supports the use of combining redox sensitive trace elements to track the river-groundwater continuum.

A precise geochemical volcano-stratigraphy of the Deccan traps

This geochemical study is based on 69 volcanic rocks representing the entire volcano-stratigraphy of the 12 lava formations of the Deccan Traps (DT) of the Indian peninsula. The DT constitutes one of the largest continental flood basalt province (CFB) in the world. More than 90% of the lavas of this province are considered to have erupted in less than one million years by plume volcanism, coinciding with the Cretaceous-Paleogene (K-Pg) boundary, leading to the speculation that this volcanism had a major role in the end-Cretaceous mass extinction. The current study was undertaken to fully geochemically characterize the DT plume with respect to major-trace element composition as well as their correlated Sr-Nd-Pb and Hf isotope geochemistry of each lava formation in step-by-step sampling the entire Deccan vertical stratigraphy of over 60 samples mostly on the Western Ghat Escarpment as collected by Beane (1988).The upper lava formations from Poladpur to Panhala-Desur, the Wai subgroup, are sharply distinguishable geochemically by all the geochemical parameters from the lower lava formations of the Lonavala and Kalsubai subgroups. This distinction is remarkable as they are on samples in a continuous supposedly vertical sequence in space and time. The Wai subgroup shows restricted range in FeO/MgO ratios at a given MgO wt% but indicating a more Fe-rich mantle source than many other CFB (average Mg# 46.7 as opposed to the 53.3 Mg# for the lower subgroups) province; the lower subgroups show a widely variable FeO/MgO ratios, interpreted as results of partial melting of subducted eclogitic slab. This interpretation is supported by the trace elements including the rare earths, and the isotopic ratios of Nd, Sr, Pb and Hf that were all measured in the same lava samples. Our high precision Pb isotopic measurements reveal the gist of the Deccan plume characteristics as Pb content in the mantle is low in contrast with their higher abundance in the continental crust. The Wai subgroup demonstrates a relatively consistent initial 206Pb/204Pb ratios as opposed to their heterogeneous compositions in the lower formations of Lonavala and Kalsubai. In addition, the Pb isotopes in a 206Pb/204Pb vs 207Pb/204Pb plot define an age of 2604 ± 140 Ma that we interpret as the “provenance” age of the subducted slab that sourced the lower formation lavas. Interestingly, this “errorchron” is also consistent with the “Dupal anomaly” concept developed on the sub Indian Oceanic mantle derived basalts.The combined multiple isotopic volcano-stratigraphy shows the geochemical contrast more dramatically than any other previous study on the Deccan. This high precision multiple isotopic study of the Deccan is consistent with recent U-Pb and Ar-Ar geochronologic studies on the rapid eruption of the Deccan, mostly the plume head, interpreted here constituting the upper lava formations of the Wai subgroup as without much interaction with the continental crust.

Using magmatic biotite chemistry to differentiate barren and mineralized Silurian–Devonian granitoids of New Brunswick, Canada

The geochemistry of biotite crystals from thirty fertile and barren Silurian–Devonian granitoids of New Brunswick, Canada, was studied in situ using electron microprobe and laser ablation inductively coupled plasma-mass spectrometry to investigate the suitability of biotite geochemistry as a diagnostic fertility index among these intrusions. The Fe2+/(Fe2+ + Mg2+) ratio of biotite varies as a function of intrusion metal affinity, increasing from Cu–Mo-related (mean of 0.56 ± 0.12), to Mo-related (mean of 0.69 ± 0.06) to Sn–W-related (mean of 0.77 ± 0.16), with barren granitoids lying between Cu–Mo and Mo types (mean of 0.66 ± 0.06). The results show a distinctive geochemical contrast between mineralized and barren samples. Compatible elements (Ti, Mg, Co, Ni, V, Cr, Ba, and Sr) decrease from barren to Cu–Mo, Mo, and Sn–W granitoids, whereas incompatible elements (Mn, Zn, Sn, W, Rb, Cs, and Li) show the opposite trend. These two trends might indicate higher degree of fractionation indicated by biotite chemistry in Sn–W-related granites. Furthermore, barren intrusions have the lowest water content (1–3 wt.% H2O), whereas Sn–W and Cu–Mo-related intrusions have between 3 and 6 wt.% H2O. Mo-bearing intrusions have a limited range of H2O contents (4–4.5 wt.%). A high degree of halogen enrichment related to degree of fractional crystallization results in enrichment of incompatible elements in the magmas associated with Sn–W mineralization and is reflected by the geochemical characteristics of biotite from these systems. New metallogenic classifications are introduced using ternary V–Na–Li (ppm) and Sn + W (ppm) versus Ga (ppm) to differentiate barren and mineralized granitic systems in New Brunswick.

Time-resolved porosity changes at cement-clay interfaces derived from neutron imaging

Interfaces between cement and clay are common in engineered barrier systems for the underground disposal of radioactive waste. The geochemical contrast between clay and cement leads to alterations of both materials following dissolution and precipitation reactions. Porosity is a key parameter for, and a key outcome of, such reactive transport. Here we report on the evolution of porosity within small samples made from ordinary portland cement and Na montmorillonite. Water contents, a proxy for porosity in saturated samples, were derived from neutron imaging. Specific calibration procedures were developed to account for multiple scattering effects. An increase in cement porosity was observed propagating ∼2 mm away from the interface within 600 days, and a considerable decrease in clay porosity within ∼2 mm. Our experiments provide unique information on the dynamics of the processes up to nearly two years. The data set provides valuable input to better constrain reactive transport models.

Apatite Chemical Compositions from Acadian-Related Granitoids of New Brunswick, Canada: Implications for Petrogenesis and Metallogenesis

The geochemistry of apatite crystals from fifteen fertile and infertile Acadian-related granitoids of New Brunswick (Canada) was studied in situ, using electron microprobe and laser ablation-inductively coupled plasma-mass spectrometry to further investigate petrogenesis and fertility index among these intrusions. The results indicate a clear geochemical contrast between barren and mineralized samples where apatite grains from barren intrusions are the most hydrous (OH > 0.3 wt. %), with lowest Mn (<1700 ppm), Fe (<800 ppm), and Sn (<0.01 ppm). In contrast, apatite grains from Cu-Mo related intrusions are distinguished by higher Cl (>0.1 wt. %), (La/Yb)N ratios of 21.17, (Eu/Eu*)N ratios of 0.30, and LREE/HREE ratios of 6.03. Apatites from Sn-W related magmatic suites have the highest F (>3 wt. %), Mn (>5350 ppm), Fe (>2200 ppm), Y (>4900 ppm), Sn (>2 ppm), and the lowest Cl (<0.01 wt. %), Sr (<60 ppm), U (<18 ppm), Th (<29 ppm), (Eu/Eu*)N ratios (<0.01), and (La/Yb)N ratios (<0.88). Lastly, apatite grains from Mo-bearing systems have the lowest SiO2 (<0.4 wt. %), Sr (<33 ppm), Th (<28 ppm), a moderate Mn (~3800 ppm), Y (~3500 ppm), and highest FeOt (<0.9 wt. %). However, the results indicated apatite Mn, Sr, LREE/HREE, and (Eu/Eu*)N ratios as the best fertility indices used for discriminating barren from fertile granite intrusions.

Application of prompt gamma activation analysis to provenance study of the Korean obsidian artefacts

Prompt gamma activation analysis (PGAA) is a non-destructive nuclear analytical method and is useful to obtain the bulk elemental composition of materials. We conducted PGAA for the source rocks of Korean obsidian artefacts. It has been reported that there exist lots of microlites inside the obsidian artefacts. When the uneven distribution of microlites is prominent, bulk analysis rather than spot analysis is preferable to the provenance study of obsidian artefacts. We compared two bulk analytical methods for obsidian: the correlation between the analytical results of PGAA and wavelength dispersive X-ray fluorescence analysis (WDXRF) has been examined. We have found that the major oxide contents from the PGAA are in good accordance with those from the WDXRF except for MnO. Using the PGAA data, we investigated the geochemical contrast between the two kinds of source rocks of the Korean obsidian artefacts and found out that the bulk PGAA data can be used as a good discriminator of the two distinct sources.

Mineral composition and geochemistry of the Upper Cretaceous siliciclastics (Nubia Group), Aswan District, south Egypt: Implications for provenance and weathering

The Upper-Cretaceous clastic succession (Nubia Group) in the area northeast of Aswan includes three rock units, from base upwards: Abu Aggag Formation (Turonian), Timsah Formation (Coniacian -Santonian) and Um Barmil Formation (Santonian – Campanian). Quartz and clay minerals are the predominant phases throughout the whole succession while feldspars are very rare. Kaolinite is overwhelming among the clay minerals, in addition to less important amounts of illite and illite/smectite. The ultrastable heavy minerals are the prevailing non- opaque phases and they significantly change in relative abundance upsection. SiO2, Al2O3 and Fe2O3 are the dominant chemical components.The statistical examination had revealed that the chemical constituents are loaded on three main geochemical trends; the siliceous, the argillaceous and the ferruginous. The enrichment factor shows that the geochemical behavior of the major and trace elements is uniform throughout the sandstones and mudstones of the Abu Aggag Formation. A significant geochemical contrast is documented between the sandstones and mudstones of both the Timsah and the Um Barmil formations. The provenance – critical elemental ratios Ti/Nb and Ti/Y are nearly constant throughout the sandstones and mudstones of the Abu Aggag Formation, and they fall within the range of granitic-granodioritic composition. The Timsah sandstones have Ti/Nb and Ti/Y ratios that are consistent with those of the Abu Aggag rocks, suggesting a similar provenance. These elemental ratios are extremely higher in the Timsah mudstones, reflecting a great influx of mafic material. The Um Barmil sandstones exhibit exceedingly dispersed values of Ti/Nb and Ti/Y, reflecting their derivation from large catchment's areas of different rock types with the eroded products being mixed in various relative proportions. The Ti/Nb and Ti/Y of the Um Barmil mudstones are closely akin to those of the Timsah mudstones suggesting analogous source. The chemical index of alteration (CIA) and the A-CN-K relation, together with the mineral composition, indicate that the siliciclastic rocks throughout the whole succession were derived mainly from deeply weathered detritus under steady-state weathering conditions. Recycling is limited in the Abu Aggag Formation and increases upwards to be more common in the Um Barmil rocks.

Application of high‐resolution Mobile Metal Ion (MMI) soil geochemistry to archaeological investigations: An example from a Roman metal working site, Somerset, United Kingdom

AbstractAn innovative application of Mobile Metal Ion (MMI) partial extraction soil geochemistry is used to identify below‐surface archaeological features, using a previously incompletely surveyed Roman metal‐working site at St. Algar's Farm, Somerset, as a case study. Soil samples were taken and analyzed for 53 elements by the MMI geochemical method. Lead, Tl, Ba, and Zn were found in very high concentrations and the sensitivity of the technique also enabled Ag, Au, and Sn to be measured in anomalous concentrations. Elemental maps accurately outlined known metal working areas. Principal component analysis and bivariate correlations identified two suites of associated elements: Pb, Ba, Tl, Ag, Au, Cu, Sb, the base and noble metal group (BNM), and Fe, Ti, Nb, Mn, Co, Cu, P, Li, Rb, Sc, Cs, K, Ga, P, Zr, Th, and Sn, the pegmatite (PEG) group. These were used to form indices that delineate the metal working area and areas possibly related to the processing of pegmatite containing Sn. The high‐sensitivity MMI data were compared with strong acid digest results from a limited number of the MMI samples; the MMI data showed better geochemical contrast than the strong acid results. Multielement statistical similarity comparisons with off‐site samples suggest likely sources for the Pb and Sn used at the St. Algar's site. The increased sensitivity of MMI soil analysis combined with the multielement capacity allows a more detailed archaeological interpretation.

Composite soil-geochemical halos delineating carbonate-hosted zinc–lead–barium mineralization in the Irankuh district, Isfahan, west-central Iran

The Irankuh district in west-central Iran hosts several largely unexposed Lower Cretaceous carbonate-hosted Zn–Pb deposits whose host rocks are overlain by soil cover. This paper documents a detailed soil-geochemical exploration program, comprising 804 residual soil samples that were treated with a microwave acid digestion and analyzed by ICP-MS for a multi-element package. The soils overlying the Zn–Pb–Ba veins and mineralized zones have geochemical contrast and element enrichments that are more than two to twenty-one times greater than the non-mineralized background soil values. The most diagnostic elements are Zn, Pb, Ag, Ba, Hg and Sb which show multi-point anomalies across the entire mineralized zone with concentration ranges of 282–10,000ppm Zn, 73–10,000ppm Pb, 2.2–71.9ppm Ag, 1500–3400ppm Ba, 0.89–36ppm Hg and 11–164ppm Sb, respectively. The newly proposed composite geochemical halos defined by Ag*Cd/Pb*Zn, Ag*As/Pb*Zn, Ca*Mg/Pb*Zn, Sb*As/Pb*Zn, Hg*Sb/Pb*Zn, Sb*Ba/Pb*Zn, Ba*Ca/Pb*Zn and Ag*As* Sb/Pb*Zn*Bi correlate well with the anomalous patterns of the individual elements. The anomalies are coincident with the main faults and fractures, in particular Gushfil and Rumarmar faults of the area. On the basis of the low contents of Cu and Fe and the variodiagram of log (Pb+Zn) versus Pb/Pb+Zn, the composition of the samples suggests epigenetic stratabound Zn–Pb–Ba mineralization of the Mississippi-Valley-Type (MVT). The mode of occurrences of the anomalies is likely controlled by structural replacement in sphalerite, galena, carbonates and barite or adsorption by iron–manganese oxides and clay minerals. Follow-up sampling of gossans in highly faulted-fractured zones may indicate more concealed Zn–Pb mineralization.

Quantification of Water Content Across a Cement-clay Interface Using High Resolution Neutron Radiography

In many designs for radioactive waste repositories, cement and clay will come into direct contact. The geochemical contrast between cement and clay will lead to mass fluxes across the interface, which consequently results in alteration of structural and transport properties of both materials that may affect the performance of the multi-barrier system. We present an experimental approach to study cement-clay interactions with a cell to accommodate small samples of cement and clay. The cell design allows both in situ measurement of water content across the sample using neutron radiography and measurement of transport parameters using through-diffusion tracer experiments. The aim of the high-resolution neutron radiography experiments was to monitor changes in water content (porosity) and their spatial extent. Neutron radiographs of several evolving cement-clay interfaces delivered quantitative data which allow resolving local water contents within the sample domain. In the present work we explored the uncertainties of the derived water contents with regard to various input parameters and with regard to the applied image correction procedures. Temporal variation of measurement conditions created absolute uncertainty of the water content in the order of ±0.1 (m3/m3), which could not be fully accounted for by correction procedures. Smaller relative changes in water content between two images can be derived by specific calibrations to two sample regions with different, invariant water contents.

Geochemical underpinnings of Australia's offshore hydrocarbon prospectivity

Molecular and isotopic analyses of oils and gases can provide information on the depositional environment, maturation and age of their source rocks, and the post expulsion history of the hydrocarbons generated. Source rock analyses can determine their potential to generate hydrocarbons of varying type over specific thermal ranges, as well as demonstrating the strength of oil- or gas-to-source correlations. Together, this geochemical interpretation can provide insights about the extent of petroleum systems and can help delineate the relationships between hydrocarbon occurrences in a basin and across the continent. Oils that do not fit the well-established framework of oil families and Australian petroleum systems point to new source rock fairways. Examples include vagrant oils with lacustrine affinities found at various locations on the western Australian margin. Other examples are oil occurrences in the Gippsland Basin whose geochemical signatures contrast with the dominant non-marine oils, supporting the existence of a viable marine source rock facies. In under-explored and frontier basins, geochemical analyses of potential source rocks can provide key evidence to underpin new exploration efforts. For example, the recent acreage uptake in the Bight Basin was supported by Geoscience Australia’s recovery and analysis of oil-prone marine source rocks, and in the northern Perth Basin by new geochemical analysis extending the distribution of Lower Triassic Hovea marine source rocks offshore. Geoscience Australia has now embarked on a regional petroleum geological program that includes a national source rock study aimed at identifying and characterising Australia’s hydrocarbon sources, families and systems.

Hydrochemical variability at the Upper Paraguay Basin and Pantanal wetland

Abstract. Compartmentalization is a prerequisite to understand large wetlands that receive water from several sources. However, it faces the heterogeneity in space and time, resulting from physical, chemical and biological processes that are specific to wetlands. The Pantanal is a vast seasonally flooded continental wetland located in the centre of South America. The chemical composition of the waters that supply the Pantanal (70 rivers) has been studied in order to establish a compartmentalization of the wetland based on soil-water interactions. A PCA-based EMMA (End-Members Mixing Analysis) procedure shows that the chemistry of the rivers can be viewed as a mixture of 3 end-members, influenced by lithology and land use, and delimiting large regions. Although the chemical composition of the end-members changed between dry and wet seasons, their spatial distribution was maintained. The results were extended to the floodplain by simple tributary mixing calculation according to the hydrographical network and to the areas of influence for each river when in overflow conditions. The resulting map highlights areas of high geochemical contrast on either side of the river Cuiaba in the north, and of the rivers Aquidauana and Abobral in the south. The PCA-based treatment on a sampling conducted in the Nhecolândia, a large sub region of the Pantanal, allowed the identification and ordering of the processes that control the geochemical variability of the surface waters. Despite an enormous variability in electrical conductivity and pH, all data collected were in agreement with an evaporation process of the Taquari River water, which supplies the region. Evaporation and associated saline precipitations (Mg-calcite, Mg-silicates K-silicates) explained more than 77% of the total variability in the chemistry of the regional surface water sampling.

Geochemistry and petrogenesis of the Mesoarchean granites from the Canaã dos Carajás area, Carajás Province, Brazil: Implications for the origin of Archean granites

Four Mesoarchean (2.93 to 2.83Ga) granite units, which encompass the Canaã dos Carajás, Bom Jesus, Cruzadão and Serra Dourada granites, were recognized in the Canaã dos Carajás area of the Archean Carajás Province. The Mesoarchean units are composed dominantly of biotite leucomonzogranites. They are compared with the Neoarchean Planalto suite (2.73Ga) which encompasses biotite–hornblende monzogranites to syenogranites. The Canaã dos Carajás, Bom Jesus and the variety of the Cruzadão granite with higher (La/Yb)N are geochemically more akin to the calc-alkaline granites, whereas the other varieties of the Cruzadão granite are transitional between calc-alkaline and alkaline granites. The Serra Dourada granite has an ambiguous geochemical character with some features similar to those of calc-alkaline granites and other peraluminous granites. The Planalto granites have ferroan character, are similar geochemically to reduced A-type granites and show a strong geochemical contrast with the Mesoarchean studied granites. The Mesoarchean granites described in the Canaã dos Carajás area are geochemically distinct to those of the Rio Maria domain of the Carajás Province. The Canaã dos Carajás and Bom Jesus granites are similar to the high-Ca granites, whereas the Cruzadão and Serra Dourada are more akin to the low-CaO granites of the Yilgarn craton. The geochemical characteristics of the Mesoarchean studied granites approach those of the biotite granite group of Dharwar but the latter are enriched in HFSE and HREE compared to the Canaã dos Carajás granites. The Neoarchean Planalto suite granite has no counterpart in the Mesoarchean Rio Maria domain of the Carajás Province, neither in the Yilgarn and Dharwar cratons. Geochemical modeling suggests that partial melting of a source similar in composition to an Archean basaltic andesite of the Carajás Province could give origin to the Bom Jesus and Cruzadão granites. In the case of the Bom Jesus granite the residue of melting contained, in similar proportions, plagioclase, clinopyroxene, amphibole, and garnet, with subordinate ilmenite. The variations in (La/Yb)N and Sr/Y of the Cruzadão granite are controlled by changes in the residual melting phases. Garnet and amphibole are abundant in the residue of the variety with higher (La/Yb)N, whereas in the rocks with moderate to lower (La/Yb)N garnet is absent, magnetite appears in the residue and amphibole initially increases but is replaced by orthopyroxene in the rocks with the lowest (La/Yb)N. In the Canaã dos Carajás and Serra Dourada granites garnet was probably an absent phase in the residue of melting and the influence of amphibole was also apparently limited. A crustal environment and a variable pressure from 10 to 7–8kbar are estimated for the generation of the granite magmas. The Bom Jesus and Cruzadão granites of Carajás derived from basaltic andesite sources and not of TTG and their geochemical contrasts can be explained by variation in the pressure of melting. The sources of Archean granite magmas are more diversified and could be more mafic than generally admitted.

In-situ interaction of cement paste and shotcrete with claystones in a deep disposal context

<i>In-situ</i> sampling was performed in the Andra Meuse/Haute Marne (France) Underground Research Laboratory (URL) allowing the study of two cement based materials/claystone interfaces that have undergone 4 to 5 years of interaction. The first interface concerned a shotcrete that was sprayed on the wall of an access drift at the laboratory level and the second one, a class G cement paste that was injected in a borehole filled from the surface and was intercepted during the excavation of a new gallery. In the first case, the hydrodynamic conditions were controlled by the ventilation of the drift; while in the second, the cement paste and claystone materials were considered saturated and far from any mechanical perturbation. A multi scale investigation was carried out to identify any evidence of alkaline perturbation in the cement based materials and the claystone. Chemical, mineralogical, and textural measurements were thus performed on the different materials in contact at the nanometer to a centimeter scale. Results showed that all the perturbations resulting from the geochemical contrast between the cement materials and the claystone were limited to a μm scale on each side of the interfaces. Carbonation was observed in the cement materials leading to an opening or a clogging of the porosity according to the hydrodynamic conditions and the formulation of the cement material. The distribution of the cation exchange population was also modified in the claystone in contact with the cement paste where a potassium saturation of the exchangeable fraction was identified. The originality of the present work is that realistic field controlled samples from the Andra URL were studied, hereby allowing the evaluation of the impact of natural heterogeneities of the <i>in-situ</i> experimental conditions (that is, hydrodynamic conditions, engineered damage zone, mineralogical variations) on the perturbations at the cement paste/claystone interfaces. An important result is that the clogging of porosity was not homogenous along the interfaces.

15 years of in situ cement–argillite interaction from Tournemire URL: Characterisation of the multi-scale spatial heterogeneities of pore space evolution

The solution selected by some countries to isolate radioactive wastes from the biosphere for up to one million years in deep geological repositories includes a multi-barrier disposal design, with steel canister, bentonite and cement materials. The geochemical contrast between such materials and the host rock formation creates perturbations potentially altering the confinement properties of the formation. In this context, the French Institute for the Radiological protection and Nuclear Safety (IRSN) have developed an in situ experimental programme based on the study of cement/argillaceous formation interfaces in their Underground Research Laboratory at Tournemire (Aveyron, France). An in situ engineered analogue of a cement/clay-rock interface which has undergone 15 years of interaction has been characterised. Such important interaction time for an in situ engineered analogue provides a bridge between laboratory-derived data and the long time scale of safety assessment modelling. As the mineralogical and petrological investigations have already been published, this work presents for the first time a quantitative characterisation of the spatial distribution of the porosity in the cement and the clay-rock in terms of time scale and design. Interfaces have been characterised using an autoradiography technique in addition to petrophysical measurements. This technique enables visualisation and quantification of the spatial distribution of the porosity using 2D mapping of decimetric-scale specimens. Thus autoradiographs allow highlighting the relationship between the field heterogeneities and the pore space evolution in each material in contact. Moreover, the porosity measurements show a clogging of the porosity in the clay-rock while the porosity increases in the cement. The extension of the porosity evolution extends to a centimetre on both sides of the interface but is heterogeneously distributed in space as a function of the fissure network and interface geometries. The connected fissure network visualised using autoradiography in the clogged area could permit solute (e.g. radionuclide) transport and may also be interpreted as an evolution of the mechanical properties of the clay-rock formation upon alkaline perturbation. This set of data, with the spatial quantification of the porosity in both cement and clay materials will be useful to constrain reactive transport modelling and thus to predict long term evolution of an engineered barrier.

Zircon U–Pb and Hf isotopic study of Mesozoic felsic rocks from eastern Zhejiang, South China: Geochemical contrast between the Yangtze and Cathaysia blocks

The Jiangshan–Shaoxing Fault Zone (JSFZ) in Zhejiang Province has been proposed to represent a suture between the Yangtze and Cathaysia blocks in South China. In this study, in-situ zircon U–Pb and Hf isotopic analysis and whole-rock major- and trace-element measurement of early to middle Cretaceous felsic rocks across the fault zone were conducted to constrain the nature of the fault zone. Twelve Cretaceous granitoid bodies were sampled from the NW and SE sides of the fault zone, respectively, with composition ranging from diorite to granite (SiO 2 = 56.2–76.6 wt.%). These granitoids yielded U–Pb ages ranging from 135–100 Ma, with a systematic variation in zircon Hf isotopic compositions ( ε Hf( t) = + 6.9 to –7.0 in the NW side vs. + 1.9 to − 12.9 in the SE side). The T DM2 values for the granitoids from the NW side are 0.34 to 1.33 Ga, with two peaks at ca. 876 and 1170 Ma respectively, whereas those from the SE side are 0.70 to 1.62 Ga, with a single peak at ca. 1126 Ma. The Hf isotopic disparity for the two sides may indicate a fundamental difference in the lower crustal compositions of the Yangtze and Cathaysia blocks, supporting that the JSFZ is possibly a suture zone between the two blocks. Our results together with the available geological data suggest that the Mesoproterozoic materials are important for both the Yangtze and Cathaysia basement and the Neoproterozoic magmatic activities were important in the Yangtze Block, possibly related to the break-up of the Rodinia supercontinent, but less significant in the Cathaysia Block. This may imply that the two blocks have not completely juxtaposed in the Neoproterozoic.

Eocene break-off of the Neo-Tethyan slab as inferred from intraplate-type mafic dykes in the Gaoligong orogenic belt, eastern Tibet

Eocene (40–42 Ma) basaltic dykes in the Gaoligong–Tengliang belt, eastern Tibet, are characterized by high Na 2O (2–4%), in contrast with the widespread post-collisional potassic and ultrapotassic rocks in the Tibetan plateau. Despite the ubiquitous negative Nb anomalies, these dykes have relatively high Nb and Zr contents, making them distinct significantly from the Gangdese arc magmas. All these, together with the positive Nb anomaly in some samples, indicate an intraplate affinity for the Gaoligong–Tengliang dykes. Specifically, the Gaoligong dykes represent the asthenosphere-derived melts which has been contaminated to various degrees by the lithosphere mantle-derived melts, whereas the Tengliang samples were directly derived from an enriched lithosphere mantle. The thin lithosphere (< 80 km) inferred from basalt geochemistry is unusual in Tibet, thus demanding a peculiar mechanism to thin lithosphere. A slab break-off model is preferred given the geochemical contrast between pre-40 Ma and post-40 Ma magmas in Tibet, and the temporal correlation among this intraplate magmatism, the termination of Gangdese arc magmatism and regional thermally-driven metamorphism. Therefore, the occurrence of intraplate-type magmas in the Gaoligong orogenic belt likely represents magmatic expression of the detachment of subducting Neo-Tethyan slab from the Indian continental plate during the Eocene. In the light of the slab break-off concept and thermo-mechanical modeling, the Eocene slab break-off furthermore suggests the onset of the India–Asia collision between 52 and 57 Ma.

Quantitative assessment of the success of geochemical exploration techniques using minimum probability methods

Hypergeometric statistics have been used to establish a quantitative measure of performance for geochemical exploration techniques over known mineral showings. An alternative and complementary measure of exploration performance is geochemical contrast, which determines how convincing or compelling a geochemical result is. Using the identical philosophy employed to assess exploration ‘accuracy’, the Student's t distribution is used to create a quantitative measure of ‘geochemical contrast’. First, thresholds are selected to separate anomalous and background populations. Then, Student's t test statistics for each of these sets of anomalous and background samples are calculated, and the Student's t probability is determined for the highest test statistic. This probability describes the chance that the anomalous and background populations were derived from the same underlying distribution. If this probability is low, then the concentrations of the anomalous and background samples are very different, and high geochemical contrast exists. If this probability is high, the alternative is true. As a result, this approach can be used to quantitatively compare the geochemical contrast of competing exploration techniques. Furthermore, because both of these ‘accuracy’ and ‘geochemical contrast’ measures are probabilities that vary inversely with exploration performance, their joint probability (their product) can be used to collectively rate the performance of exploration techniques.

Petrogeochemical characteristics of the kimberlites from the Middle Markha region with application to the problem of the geochemical heterogeneity of kimberlites

A comparative geochemical investigation of kimberlites from the Middle Markha region and traditional diamond-bearing areas of Yakutia supported the division of kimberlites into two geochemical types. One of them includes kimberlites from the traditional diamond-bearing areas of Yakutia, and the other is represented by the kimberlites of the Middle Markha region and the rocks of the Zolotitsa field and V. Grib pipe of the Arkhangelsk province. The obtained representative parameters of the two kimberlite types indicate a sharp geochemical contrast between them, and the individual features of correlation relationships between the elements in the rocks of the two types suggest that these differences are related to fundamental genetic factors, which concern primarily the group of highly charged incompatible trace and radioactive elements. The presence of geochemically contrasting rocks within a rather uniform petrochemical association and the geochemically specialized occurrences of kimberlites and related rocks are consequences of repeated metasomatic transformations of mantle rocks under the influence of deep-derived fluids or volatiles released during the recycling of subneous source and subsequent derivation of geochemically specialized types of deep magmas showing signatures of individual mantle sources.