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1.
The potential for using groundwater helium surveys in exploration has been evaluated by conducting local and regional surveys in several mineralized and background areas in Australia. Dissolved helium contents decrease markedly upwards, particularly in the top 10–15m, due to degassing in attaining dynamic equilibrium with the atmosphere. Gradients of depth profiles are not uniform but because total concentration variations to 50m or more are commonly less than 10, whereas regional variations are 103–105, samples from the greatest constant depth possible below water-table are adequate for survey purposes. Helium concentrations in excess of the atmosphere equilibrium value of 0.044 μl He/l H2O were found to be associated with uranium mineralization at Honeymoon (6.9–44.4 μl/l), Manyingee (0.08–1.68 μl/l), Bennett Well (9.9–29.5 μ/l) and the Stuart Shelf—Roxby Downs (910–2495 μl/l), with a carbonatite at Mt. Weld (0.18–13.6 μl/l), and with kimberlites at Wandagee (0.2–3.25 μl/l). No anomaly was associated with the shallow calcrete uranium deposit at Yeelirrie. However, equivalent or higher concentrations were found in groundwaters from unmineralized areas around these deposits, e.g. up to 85 μl/l at Manyingee—Bennett Well, to 215 μl/l at Yeelirrie and to 1525 μl/l, Stuart Shelf—Mt. Gunson. Similarly, helium in waters from stock wells near Mt. Weld appears to indicate the presence of the carbonatite but in a survey of a geologically similar area near Malcolm, equivalent anomalies were not related to carbonatite.Although the uranium (and thorium) mineralization is a significant source, high helium concentrations can arise by accumulation in groundwaters with long residence times and by leakage from deep sources, particularly granitoid basement. The concentration reached is also a function of the porosity and configuration of the aquifer and the permeability of the aquiclude to helium. These factors are more important in determining dissolved helium concentrations than the uranium and thorium contents of the rocks. However, the effects are not readily quantifiable, particularly in exploration areas where geology and hydrology are relatively unknown, hence helium groundwater data can be difficult if not impossible to interpret.  相似文献   

2.
The nature of the petrogenetic links between carbonatites and associated silicate rocks is still under discussion (i.e., [Gittins J., Harmer R.E., 2003. Myth and reality of the carbonatite–silicate rock “association”. Period di Mineral. 72, 19–26.]). In the Paleozoic Kola alkaline province (NW Russia), the carbonatites are spatially and temporally associated to ultramafic cumulates (clinopyroxenite, wehrlite and dunite) and alkaline silicate rocks of the ijolite–melteigite series [(Kogarko, 1987), (Kogarko et al., 1995), (Verhulst et al., 2000), (Dunworth and Bell, 2001) and (Woolley, 2003)]. In the small (≈ 20 km2) Vuoriyarvi massif, apatite is typically a liquidus phase during the magmatic evolution and so it can be used to test genetic relationships. Trace elements contents have been obtained for both whole rocks and apatite (by LA-ICP-MS). The apatites define a single continuous chemical evolution marked by an increase in REE and Na (belovite-type of substitution, i.e., 2Ca2+ = Na+ + REE3+). This evolution possibly reflects a fractional crystallisation process of a single batch of isotopically homogeneous, mantle-derived magma.The distribution of REE between apatite and their host carbonatite have been estimated from the apatite composition of a carbonatite vein, belonging to the Neskevara conical-ring-like vein system. This carbonatite vein is tentatively interpreted as a melt. So, the calculated distribution coefficients are close to partition coefficients. Rare earth elements are compatible in apatite (D > 1) with a higher compatibility for the middle REE (DSm : 6.1) than for the light (DLa : 4.1) and the heavy (DYb : 1) REE.  相似文献   

3.
Regional exploration for pegmatite hosted Sn-mineralization in Sweden by LKAB Prospektering has, since the early 1980s, generally been based on the use of heavy-mineral concentrates from till collected by roadside sampling. The average sampling density has been 0.5–1.0 sample/km2, but with an uneven distribution of sample points within areas investigated.Planning of the regional sampling grid in this study has been based on a statistical analysis to answer the following question: What sample density and grid-spacing should be used to hit an anomalous target with the highest possible probability.The statistical analysis indicates that using a rectangular grid, 750 × 1,500 m (0.88 sample/km2), the probability of intersecting an anomalous target is 100%. Heavy-mineral till concentrate sampling using this grid design in northern central Sweden in 1981 revealed one single, extremely high sample with 4,622 ppm Sn indicating mineralization in the area north of the village of Järkvissle.In 1982–1983, while boulder tracing up-ice from the anomalous regional sample point, cassiterite and spodumene-bearing boulders and outcrops were found. Two parallel mineralized pegmatites within an area of approximately 5,000 m2 with grades of 0.04–0.07% Sn and 0.40–0.92% Li2O have recently been located NW of Järkvissle.Semi-regional (250 × 500 m) and local (20 × 20–160 m) sampling of till gave some anomalous values of Sn and Li in concentrates, which to some extent could be correlated with mineralized boulders and outcrops, and also indicated new targets for drilling.The results from a simplified sample preparation technique for heavy-mineratl till concentrates show that a new device, named the Goldhound, can be used in future geochemical prospecting for Sn and Li in Sweden.  相似文献   

4.
Concentrations of Au, Ag, As, Cd and Sb in aquatic bryophytes collected from the Dolgellau Mineral Belt, North Wales, U.K. are reported. One aquatic liverwort, Scapania undulata (L) Dum. and two mosses, Fontinalis squamosa Hedw. and Racomitrium aciculare (Hedw.) Brid. were collected from sites upstream and downstream of the recently reopened Gwynfyndd Au mine. There was little inter-species variation in metal contents for these three bryophytes, but Scapania undulata appeared the most sensitive to changes in water concentrations of Ag, As and Sb. Gold concentrations varied little between the contaminated and control sites. Concentrations in the range < 4–18 ng Aug g−1 D.W. were typical background levels, while bryophytes collected immediately below the mine contained 6–45 ng Au g−1. Silver and Sb both showed more pronounced ( 5–10 fold) elevations above control concentrations in samples collected downstream of the mine. Background concentrations for these elements were 5–85 ng Ag g−1 and 0.15–1.3 μg Sb g−1.Arsenic concentrations downstream of the mine (160–1080 μg g−1) greatly exceeded the background range of 9–32 μg g−1. It is suggested, therefore, that As may be an ideal ‘pathfinder’ element when prospecting for auriferous deposits using aquatic bryophytes.  相似文献   

5.
The geochemistry of dissolved and suspended loads in river catchments of two low mountain ranges in Central Europe allows comparison of pertinent chemical weathering rates. Distinct differences in lithology, i.e. granites prevailing in the Black Forest compared to Palaeozoic sediments in the Rhenish Massif, provide the possibility to examine the influence of lithology on weathering. Here we determine the origin of river water using the stable isotope ratio δ18OH2O and we quantify the geogenic proportions of sulphate from stable isotope ratios δ34SSO4 and δ18OSO4. Particularly in catchments with abundant pyrite, determination of the geogenic amount of sulphate is important, since oxidation of pyrite leads to acidity, which increases weathering. Our results show that spatially averaged silicate weathering rates are higher for the river catchments Acher and Gutach in the Black Forest (10–12 t/km2/yr) compared to the river catchments of the Möhne dam and the Aabach dam in the Rhenish Massif (2–6 t/km2/yr). Correspondingly, the CO2 consumption by silicate weathering in the Black Forest (334–395 × 103 mol/km2/yr) is more than twice as high as in the Rhenish Massif (28–151 × 103 mol/km2/yr). These higher rates for watersheds of the Black Forest are likely due to steeper slopes leading to higher mechanical erosion with respective higher amounts of fresh unweathered rock particulates and due to the fact that the sediments in the Rhenish Massif have already passed through at least one erosion cycle. Carbonate weathering rates vary between 12 and 38 t/km2/yr in the catchments of the Rhenish Massif. The contribution of sulphuric acid to the silicate weathering is higher in the catchments of the Rhenish Massif (9–16%) than in the catchments of the Black Forest (5–7%) due to abundant pyrite in the sediments of the Rhenish Massif. Three times higher long-term erosion rates derived from cosmogenic nuclides compared to short-term erosion rates derived from river loads in Central Europe point to three times higher CO2 consumption during the past 103 to 104 years.  相似文献   

6.
A carbon and oxygen isotope survey based on 42 samples from the Amba Dongar carbonatite complex of Gujarat, India, indicates that the magmatic differentiation series sövite → alvikite → ankeritic carbonatite is beset with a distinct isotope trend characterized by a moderate rise in 13C coupled with a sizeable increase in 18O. From an average of −4.6 ± 0.4 ‰ [PDB] for the least differentiated (coarse) sövite member, δ13C values slowly increase in the alvikite (−3.7 ± 0.6 ‰) and ankeritic fractions (−3.0 ± 1.1 ‰), whereas δ18O rises from 10.3 ± 1.7 ‰ [SMOW] to 17.5 ± 5.8 ‰ over the same sequence, reaching extremes between 20 and 28 ‰ in the latest generation of ankeritic carbonatite. While an apparent correlation between δ13C and δ18O over the δ18O range of 7–13 ‰ conforms with similar findings from other carbonatite complexes and probably reflects a Rayleigh fractionation process, the observed upsurge of 18O notably in the ankeritic member is demonstrably related to a late phase of low-temperature hydrothermal activity involving large-scale participation of 18O-depleted groundwaters. As a whole, the Amba Dongar carbonatite province displays the characteristic 13C/12C label of deep-seated (primordial) carbon, reflecting the carbon isotope composition of the subcontinental upper mantle below the Narmada Rift Zone of the Indian subcontinent.  相似文献   

7.
Summary The Speewah fluorite deposit (>2.28Mt at 25.5% CaF2) is sited adjacent to the crustal-scale Greenvale Fault on the western side of the Halls Creek Orogen, in the East Kimberley region of Western Australia. Host rocks are Palaeoproterozoic sedimentary rocks, dolerite and granophyre, Early Cambrian basalt, and the Yungul carbonatite. The deposit comprises mainly fluorite–quartz veins associated with lesser barite, sulfides and calcite, controlled by NNE–SSW and N–S brittle faults and fractures. Cross-cutting field relationships indicate that the fluorite veins were deposited post Early Cambrian.Fluorite–quartz vein textures, including colloform banding and comb texture, combined with microthermometric data from primary fluid inclusions, indicate that fluorite was deposited by the incremental infill of open-space structures in an epizonal, and probably epithermal, environment (<160°C) from complex, Li–Ca–Mg-rich, highly saline ore-fluids.The Yungul carbonatite and intimately-associated replacement-type fluorite have similar levels of REE enrichment and identical chondrite-normalised HREE patterns. Samarium and neodymium isotopic analyses of the fluorite indicate extreme differentiation of the 147Sm/144Nd ratio, from 0.0709 to 0.6918. These Sm–Nd isotopic data correspond to a mineral isochron with an age of 122±24Ma, interpreted to represent the age of fluorite deposition.Based on the potentially magmatic fluid composition, the replacement-type fluorite within the carbonatite, the similar HREE patterns of fluorite and carbonatite, and direct, if imprecise, isotopic dating of the fluorite, which confirms that fluorite mineralization is younger than the Early Cambrian basalts, the Speewah fluorite deposit is interpreted to be genetically related to the Yungul carbonatite. The large fluorite resource cannot have been derived from the exposed, low-volume carbonatite dyke. Rather, it must have been sourced from a larger carbonatite body at depth, whose presence is implied from basement-derived xenocrystic zircons in the Yungul carbonatite.  相似文献   

8.
A deformed ca. 570 Ma syenite–carbonatite body is reported from a Grenville-age (1.0–1.2 Ga) terrane in the Sierra de Maz, one of the Western Sierras Pampeanas of Argentina. This is the first recognition of such a rock assemblage in the basement of the Central Andes. The two main lithologies are coarse-grained syenite (often nepheline-bearing) and enclave-rich fine-grained foliated biotite–calcite carbonatite. Samples of carbonatite and syenite yield an imprecise whole rock Rb–Sr isochron age of 582 ± 60 Ma (MSWD = 1.8; Sri = 0.7029); SHRIMP U–Pb spot analysis of syenite zircons shows a total range of 206Pb–238U ages between 433 and 612 Ma, with a prominent peak at 560–580 Ma defined by homogeneous zircon areas. Textural interpretation of the zircon data, combined with the constraint of the Rb–Sr data suggest that the carbonatite complex formed at ca. 570 Ma. Further disturbance of the U–Pb system took place at 525 ± 7 Ma (Pampean orogeny) and at ca. 430–440 Ma (Famatinian orogeny) and it is concluded that the Western Sierras Pampeanas basement was joined to Gondwana during both events. Highly unradiogenic 87Sr/86Sr values in calcites (0.70275–0.70305) provide a close estimate for the initial Sr isotope composition of the carbonatite magma. Sm–Nd data yield Nd570 values of +3.3 to +4.8. The complex was probably formed during early opening of the Clymene Ocean from depleted mantle with a component from Meso/Neo-proterozoic lower continental crust.  相似文献   

9.
The unbridled development and use of water resources in the Tarim River basin over the last 50 years have led to a serious degradation of soil (>12×103 km2 of land desertified between the 1960s and 1990s) and water quality (rise in maximum salinity was from 1.3 g l–1 in 1960, to 4.0 g l–1 in 1981–1984, and to 7.8 g l–1 in 1998). Approximately 300 km of the lower reaches of the river course dried up between the 1950s and 1970s, seriously altering the downstream hydrological processes (a 4–6 m drop in groundwater levels from the 1960s to the 1980s, and 0.2 m yr–1 thereafter) and ecosystems (67% and 50% decrease (1958–1978) in Populus euphratica forest acreage and biomass, respectively). Between the 1950s and 1990s, 3820 km2 of P. euphratica forest and 200 km2 of shrub- and grassland were lost in the lower reaches of the Tarim River. In this study, based on estimates of soil organic carbon in desertified lands, it has been found that in the last 30 years (1970–2000), approximately 112 Tg of organic carbon (28% originating in the 0–1.0 m soil profile) has been released into the atmosphere as a result of soil degradation in the Tarim River basin. Intensive anthropogenic disturbance has been one of the foremost factors leading to the deterioration of water resources in this region. The key to solving these problems is to enhance the scientific and technical level of monitoring, management, and restoration of water resources and associated water and soil components of local ecosystems.  相似文献   

10.
Deposits of very large rock avalanches were identified at the southern foot of the Rocky Range of the Northern Caucasus. Cliffs facing the Ardon River are 1–1.5 km high and composed of Cretaceous and upper Jurassic, hard, crystaline limestone, underlain by softer, middle Jurassic shale, siltstone and sandstone flysh. The largest rock avalanche, at Karivhoh, is ~2×109 m3 in volume, travelled more than 7 km, and covered about 18 km2 with deposits up to 200–300 m thick. All rock-avalanche bodies are composed of intensively crushed debris overlain by a blocky carapace. Numerous subsequent landslides develop within these deposits, and pose a threat to villages built on them.  相似文献   

11.
Summary At the northeastern flank of Gebel Yelleq, northern Sinai, pure limestones of Upper Cretaceous age were subjected to a thermal overprint, caused by a c. 80m thick Tertiary olivine dolerite sill. Metasomatic supply of Si, Al, Fe, Mg and Ti was greater to the c. 7m wide upper than to the c. 25m wide lower thermal aureole. The greater width of the lower aureole is possibly due to a longer duration of the thermal overprint at this contact. Mineral assemblages in both aureoles are (from the contact outward):(i) clinopyroxene + garnet ± wollastonite + calcite(ii) garnet ± wollastonite + calcite;(iii) wollastonite + calcite.In places, late stage xenoblasts of apophyllite and witherite overgrow these assemblages. Garnets are grandites to melanites with Grs56–86Adr14–42Sch0–2Sps0–0.2Prp0 in the lower, and Grs29–94Adr5–64Sch0–12Sps0–0.2Prp0–1.7 in the upper aureole. Close to the upper contact, clinopyroxene is virtually pure diopside with X Mg = Mg/(Mg + Fe2+) = 0.97–1.0, whereas clinopyroxenes farther away from the upper contact and in the lower aureole have X Mg-values of 0.49 and 0.53, respectively.The minimum temperatures reached during contact metamorphism in the upper and lower aureole are defined by the lower stability limit of wollastonite. The temperatures are inferred with a calculated T-X(CO2) projection in the system CMASCH and are estimated at c. 290 °C and 380 °C for X(CO2) values of 0.05 and 0.25, respectively. A pressure of roughly 100 bar is estimated for the lower dolerite-limestone contact. As indicated by one-dimensional thermal modelling, a maximum temperature of 695 °C was attained at this contact, assuming a magma temperature of 1150 °C. Further modelling results indicate (i) wollastonite, which occurs first 13 m away from the lower contact, formed at a maximum temperature of c. 575 °C, (ii) there, wollastonite formation lasted for approximately 170 years and, (iii) at the outer rim of the lower aureole, the maximum temperature reached was 480 °C, and temperatures sufficient for wollastonite formation lasted for about 140 years.  相似文献   

12.
The Archean bedrock of the Ilomantsi area consists of narrow greenstone belts between dome-like granitoids. The main tectonic phases are isoclinal folding, dragging of isoclinal folds around subvertical axes and regional faulting. Conspicuous anomalies of As, Au, Mo and W were revealed in the course of regional geochemical mapping of till, which led to more detailed studies in Ilomantsi. The data presented in this study are based mainly on the results of 2960 till samples collected by light percussion drill at local scale (16 samples/km2), but also on the results of regional (1 sample/4 km2) and detailed (100–400 samples/km2) sampling and on rock samples collected from outcrops. The analytical methods most commonly used were emission spectrometry, atomic absorption spectrophotometry and spectrometry.The results indicate that the lowest part of the basal till is dominated by local debris and that the geochemistry of the till reflects well the underlying bedrock. The highest Au values exist near the contacts of tonalite and supracrustal rocks. Arsenic follows intimately the directions of faulting. The anomalous contents of W and Mo are restricted to the southern part of the 15-km2 Kuittila tonalite. Potassium, Na and Ca form conspicious anomalies in till, cutting both the supracrustal rocks and the granitoids. Local studies revealed in the Kuittila tonalite a notable network of Mo- and W-bearing quartz veins restricted to tensional zones of main fault directions, which are cut by gold-bearing shear zones and quartz vein nets. These shear zones are of en echelon type and they are silicified, sericitized, tournalinized, Na-depleted and partly carbonitized. The hosting tonalite is intensely biotitized. These metasomatic features of the bedrock can be correlated to the wider anomalies of K, Na and Ca in till. The implication is that significant migration of mobile elements has taken place in the bedrock of the study area and that, in places, gold has participated in these processes.  相似文献   

13.
In the contact aureole of the Lilesville granite and comagmatic Pee Dee gabbro, N.C., greenschist-facies phyllites of the Carolina slate belt have been overprinted by a series of metamorphic reactions producing opx-bearing hornfelses and migmatitic gneisses. In the exterior aureole the slate belt assemblage (chl+ms+ep+ab+qz) gives way to the continuous reaction assemblages (chl+bt+cd+ ms+ab±ep+qz), (bt+cd+ms+An8–29+qz), (bt+cd+ kf+ms+pl+qz), (bt+cd+als±ms+kf+pl+qz), and (bt+cd+ga+kf+pl+qz), from lowest to highest grade. The interior aureole, interpreted as part of the floor of the granite, bears the continuous and discontinuous reaction assemblages (bt+cd+als+kf+pl+qz),(bt+cd+kf+ pl+qz), and, near the gabbro, (bt+cd+ga+opx+kf+ pl+qz). The leucosomes of the migmatitic interior aureole are predominantly trondhjemites with the assemblage (An35–45+qz±bt±cd±kf). Restites in the migmatitic interior aureole contain the AFM assemblages (bt), (bt + cd), (bt+cd+als), and (bt+cd+ga), plus kf, An40–50, and qz. Contact metamorphism was isobaric at 4.0–5.1 or 2.0–3.5 kb depending on choice of aluminosilicate triple point; temperatures reached 650° C in the migmatitic interior aureole and approached 750° C near the gabbro; was less than 0.8 in the migmatites, and was lower in the interior aureole and in the high grade exterior aureole. Partial melting in the migmatitic interior aureole took place during dynamothermal metamorphism caused by the magmatic diapir. Incipient melting occurred by the reaction bt+cd+kf+pl+qz+w = liquid. The melt was H2O-undersaturated and coefficients of the reactants were weighted heavily toward the felsic minerals; the proportion of felsic minerals in the leucosomes was controlled in part by modal abundance of kf, pl, and qz available for melting. The incorporation of K into biotite by subsolidus reactions, coupled with the high thermal stability and low solubility of biotite in a felsic melt, are responsible for the trondhjemitic composition of the early anatectic liquids.Abbreviations als Al2SiO5 - ab albite - An8–29 plagioclase with anorthite contents in the range indicated - bt biotite - cd cordierite - chl chlorite - ep epidote - ga garnet - kf K feldspar - ms muscovite - opx orthopyroxene - pl plagioclase undefined - qz quartz - w water  相似文献   

14.
A dike–vein complex of potassic type of alkalinity recently discovered in the Baikal ledge, western Baikal area, southern Siberian craton, includes calcite and dolomite–ankerite carbonatites, silicate-bearing carbonatite, phlogopite metapicrite, and phoscorite. The most reliable 40Ar–39Ar dating of the rocks on magnesioriebeckite from alkaline metasomatite at contact with carbonatite yields a statistically significant plateau age of 1017.4 ± 3.2 Ma. The carbonatite is characterized by elevated SiO2 concentrations and is rich in K2O (K2O/Na2O ratio is 21 on average for the calcite carbonatite and 2.5 for the dolomite–ankerite carbonatite), TiO2, P2O5 (up to 9 wt %), REE (up to 3300 ppm), Nb (up to 400 ppm), Zr (up to 800 ppm), Fe, Cr, V, Ni, and Co at relatively low Sr concentrations. Both the metapicrite and the carbonatite are hundreds of times or even more enriched in Ta, Nb, K, and LREE relative to the mantle and are tens of times richer in Rb, Ba, Zr, Hf, and Ti. The high (Gd/Yb)CN ratios of the metapicrite (4.5–11) and carbonatite (4.5–17) testify that their source contained residual garnet, and the high K2O/Na2O ratios of the metapicrite (9–15) and carbonatite suggest that the source also contained phlogopite. The Nd isotopic ratios of the carbonatite suggest that the mantle source of the carbonatite was mildly depleted and similar to an average OIB source. The carbonatites of various mineral composition are believed to be formed via the crystallization differentiation of ferrocarbonatite melt, which segregated from ultramafic alkaline melt.  相似文献   

15.
Lake sediment cores, dated by 210Pb, were collected from Spectacle Pond (SP), Massachusetts, and Side Pistol Lake (SPL) and Sargent Mountain Pond (SMP), Maine, USA. SP is a kettle seepage lake in granitic sand and gravel. SMP is a drainage pond on granite with little soil in the small watershed. SPL is a drainage lake in granitic till. The three cores were analyzed for total Cd. For SP and SMP, maximum concentrations of 1.7 and 3.9 mg/kg, four and eight times background concentrations, respectively, occur in the late 1960s. Accumulation rates reach maximum values concurrently with concentration and are 0.054 and 0.016 μg/cm2/a, more than 10 times background. Concentration and accumulation rate age relationships in SMP and SP are similar for background values, timing and magnitude of increase to peak values, and the decrease nearly to background values since about 1975. The chemical response to decreased atmospheric deposition lags in SPL sediment. Kettle-like lakes more clearly indicate changes in atmospheric deposition than drainage lakes.  相似文献   

16.
We investigated rates of chemical weathering of volcanic and ophiolitic rocks on Luzon Island, the Philippines. Luzon has a tropical climate and is volcanically and tectonically very active, all factors that should enhance chemical weathering. Seventy-five rivers and streams (10 draining ophiolites, 65 draining volcanic bedrock) and two volcanic hot springs were sampled and analyzed for major elements, alkalinity and 87Sr/86Sr. Cationic fluxes from the volcanic basins are dominated by Ca2+ and Mg2+ and dissolved silica concentrations are high (500-1900 μM). Silica concentrations in streams draining ophiolites are lower (400-900 μM), and the cationic charge is mostly Mg2+. The areally weighted average CO2 export flux from our study area is 3.89 ± 0.21 × 106 mol/km2/yr, or 5.99 ± 0.64 × 106 mol/km2/yr from ophiolites and 3.58 ± 0.23 × 106 mol/km2/yr from volcanic areas (uncertainty given as ±1 standard error, s.e.). This is ∼6-10 times higher than the current best estimate of areally averaged global CO2 export by basalt chemical weathering and ∼2-3 times higher than the current best estimate of CO2 export by basalt chemical weathering in the tropics. Extrapolating our findings to all tropical arcs, we estimate that around one tenth of all atmospheric carbon exported via silicate weathering to the oceans annually is processed in these environments, which amount to ∼1% of the global exorheic drainage area. Chemical weathering of volcanic terranes in the tropics appears to make a disproportionately large impact on the long-term carbon cycle.  相似文献   

17.
Overbank and medium-order stream sediment samples were collected in Belgium and Luxembourg from 66 sampling locations (area of about 33,000 km2) and analysed for major and trace elements among which Zn, Pb, Cu and As. At each sampling location large bulk samples were taken, namely in the lower (normally at ≥1.5 m depth, over an interval of about 20–40 cm) and upper (normally upper 5–25 cm) parts of the overbank profiles and from the stream sediments. Furthermore, at a number of these sites, a detailed geochemical analysis of vertical overbank sediment profiles (sampling intervals of 10–20 cm) was subsequently carried out to unravel element variations through time and to help in the overall evaluation. For most sampled sections evidences such as 14C-dating and the absence of anthropogenic particles point towards a pre-industrial and often pristine origin of the lower overbank sediment samples. From the latter bulk samples, mean background concentrations were deduced. They reveal the existence of significant differences between the northern and southern part of Belgium (incl. Luxembourg) which relate to the difference in geological substrate. In the north dominantly non-lithified Quaternary and Tertiary sands, marls and clays occur while in the south Palaeozoic sandstones, shales and carbonate rocks outcrop. Consequently separate mean background values were calculated for the two areas. In the southern study area, some anomalous metal concentrations have been recorded in pre-industrial sediments. They are derived from mineralised Palaeozoic rocks, a feature which could be of interest for base metal exploration. In the upper overbank and stream sediments, in general, higher heavy metal and As contents were recorded with highest values in areas with metal mining, metal melting and cokes treatment industries. By comparing the trace element concentrations of the upper overbank or stream sediment samples with the concentrations detected in the lower overbank samples at each of the sampling locations, and by evaluating the vertical distribution patterns where available, the degree of pollution of the alluvial plain and the present-day stream sediments can be assessed. From this exercise, it is clear that highest pollution occurs in the northern part of Belgium, which relates to its high population density and industrial development.  相似文献   

18.
Airborne gamma-ray spectrometry data (uranium, potassium and thorium contents) reveal geochemical heterogeneities within the monolithological Hyrôme watershed (ca. 150 km2) in the Armorican massif (western France). Our observations and computations provide important constraints on the spatial distribution and the associated magnitudes of chemical erosion processes at the scale of a small watershed. Two distinct, partially preserved, weathering profiles exhibit a strong correlation between regolith evolution and airborne-derived K/Th ratios, suggesting that the variability is linked to supergene processes. Using both airborne data and laboratory measurements on rock samples, the total net export of potassium has been estimated at 422 ± 50 kg/m2 and the chemical weathering rate of potassium at 17 ± 2 kg/km2/a.  相似文献   

19.
The history of the recording and interpretation of the Fennoscandian uplift illustrates the main history of Earth sciences because the results obtained had (and still have) immediate impact of the interpretation of a large number of fundamental problems in Earth sciences. Thanks to a paper of De Geer in 1888, the glacial isostatic origin was established. Fennoscandia became the classic area of glacial isostasy, and its sea level records were used for geophysical calculations of the properties and dynamics of the mantle and crust. The varve dated sea level curve of Lidén (1938) from the center of uplift provided an exceptionally well dated record. With the radiocarbon method, the records of shorelines and shorelevel displacement curves were drastically improved providing a totally new basis for the understanding of the geodynamics of the Fennoscandian uplift and for the geophysical interpretation of the data obtained. This is especially true in combination with the repeated levelling data obtained during the last decades for Finland and Sweden.The Late Cenozoic long term movements of the Fennoscandian Shield are characterized by a considerable subsidence. The postglacial uplift of Fennoscandia is complex (an exponential and a linear factor) and caused by two different mechanisms. The total absolute movement in relation to the last glaciation is an elliptic uplift cone of 830 m height surrounded by a subsidence through of 170 m height. The mass in the uplift cone and in the subsidence through is as 1:1 with a volume 0.7 × 106km3. The disappearence/appearance of mass give evidence of a mass transfer in a low viscosity asthenosphere. The properties and conditions of the asthenosphere are found to be: 1–10 × 1020 Poises in viscosity, 3 × 10–14 – 3 × 10–16 sec–1 in strain rates, 0.7% of the melting temperature, 3 mm in grain size, and 5–0.4 bar in stress. The main isostatic uplift (the exponential factor) originates from an asthenospheric dislocation glide process which in early-mid Holocene time changed over into a diffusion creep process. The present linear uplift factor (identified through the last 8000 yrs) seems to originate from mesospheric motions under the following approximate conditions: 0.6% of the melting temperature, 2 × 1022 Poises in viscosity, 3 × 10–16 sec–1 in strain rates and 8 bars in stress. Uplift irregularities and neotectonism are frequently established and often reveal an old geodynamic inheritance (e.g. the Pre-Baikalian/Gothian bedrock seam of high geodynamic activity). The peak rates of glacial isostasy are associated with intensive fracturing, faulting and seismic activity.  相似文献   

20.
In this article, we model the volcanism near the proposed nuclear waste repository at Yucca Mountain, Nevada, U.S.A. by estimating the instantaneous recurrence rate using a nonhomogeneous Poisson process with Weibull intensity and by using a homogeneous Poisson process to predict future eruptions. We then quantify the probability that any single eruption is disruptive in terms of a (prior) probability distribution, since not every eruption would result in disruption of the repository. Bayesian analysis is performed to evaluate the volcanic risk. Based on the Quaternary data, a 90% confidence interval for the instantaneous recurrence rate near the Yucca Mountain site is (1.85×10–6/yr, 1.26×10–5/yr). Also, using-these confidence bounds, the corresponding 90% confidence interval for the risk (probability of at least one disruptive eruption) for an isolation time of 104 years is (1.0×10–3, 6.7×10–3), if it is assumed that the intensity remains constant during the projected time frame.  相似文献   

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