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1.
《Applied Geochemistry》2005,20(9):1714-1736
The characteristics of ribbed moraines, with an emphasis on till geochemical prospecting, were studied in the area of Peräpohjola, southern Finnish Lapland. Percussion drilling, test pits and trenches were used for till sampling. The samples were partially extracted and then analysed using ICP-AES or GFAAS methods in the geolaboratory of the Geological Survey of Finland. The distribution of Au and Cu in fine fraction (<0.06 mm) of till in both horizontal and vertical dimensions showed that the uppermost part of the moraine ridges contained the highest metal contents. In the coarser fractions of till (0.06–0.5 and >2 mm), the anomalies were located on the distal side of the ribbed moraine ridges, down-ice from mineralized bedrock. Geochemical patterns together with fresh pyrite grains in till heavy mineral concentrates indicate a short glacial transport distance of the mineralized debris. This conclusion is also supported by the presence of a large proportion of local rock fragments and boulders in the uppermost till unit and at the surface, which is a result of glacial quarrying during the ribbed moraine formation.  相似文献   

2.
Although the term ‘geochemical baseline’ appears in the international geochemical mapping programmes IGCP 259 and 360, it has never been well defined. Several considerations relevant to such a definition are discussed. A geochemical baseline for an element refers to its natural variations in concentration in the surficial environment. Geochemical baselines were studied in Finland by comparing results from regional geochemical mapping programmes based on samples of till, clay and organic stream sediment. The geochemical background changes regionally with the basic geology and locally with the type and genesis of the overburden. Baseline concentrations depend on sample material collected, grain size and extraction method. In Finland, concentrations of potentially harmful elements tend to be higher in fine-grained marine and lacustrine sediments than in glacial till. Concentrations are also systematically higher in the < 0.06 mm fraction than in the < 2 mm size fraction of till samples. Only small proportions of the total heavy metal concentrations in Finnish marine clays are bioavailable. Geochemical baselines are needed for environmental legislation and political decision-making, especially in the assessment of contaminated soil. In many areas of Finland, natural concentrations of several heavy metals exceed the guide or limit values designated for contaminated soils. Thus baselines must always be verified in any assessment of sites for contamination.  相似文献   

3.
Rocks of the Archaean Kuhmo greenstone belt and the fine fraction (− 63 μm) of 236 till samples were analysed for Co, Cr, Cu, Mn, Ni, Pb, Zn and Fe mainly with the AAS method. The influence of bedrock upon the lithology and geochemistry of till is clear, and can be seen immediately at the proximal contact. Expected trace-metal contents for the fine fraction of till were calculated using the average metal contents of the rock types and the proportions of these rock types in the till (2–6 cm pebbles). The expected values were then compared with the measured values in the fine fraction of till at the same locality. This approach seems effective for lithological comparison between various grain-size fractions of till.In general, the measured metal contents are clearly lower than expected. This indicates that the proportion of the greenstone material in the fine fraction of the till is smaller than in the coarse fraction. Correlations between measured and expected values of individual trace metals as well as their correlation with the different rock types are low, in most cases indicating inconsistent variation in the lithology of the coarse and fine fraction of the till. Good positive correlations between the number of pebbles of mafic metavolcanics and contents of Cu, Co, Mn and Fe in the fine fraction of till show that the influence of these rocks on the trace-element geochemistry of the tills in the study area is most significant as might be expected.  相似文献   

4.
 In central Newfoundland (NTS 12A/10, 15, 16, 2H/1), As, Pb, and Zn concentrations in the clay-sized (<0.002 mm) and silt and clay-sized (<0.063 mm) fractions of till reflect compositional differences among and within rock terranes at scales of kilometers to tens of kilometers. In those fractions, till derived from volcanic bedrock of Victoria Lake Group (Tulks Hill) is notably enriched in As (50–>1000 ppm), exceeding levels commonly set for purposes of environmental protection. Near Pb-Zn mines at Buchans, geochemical variation with depth reflects the dispersal of detritus from mineralized bedrock, and differences in sediment type and provenance. There, surface sediments are rich in granitic debris derived from the Topsails igneous terrane 5 km north of Buchans and contain low concentrations of trace metals. These sediments are compositionally unrelated to either Buchans Group volcanic rock or an underlying, older till enriched in sulphide minerals and trace metals. Metal-rich till extending up to 10 km southwest of Buchans results from combined glacial and debris flow transport related to two distinct geological events. Trace metals are enriched (two- to fourfold) in the clay-sized fraction of till compared to the silt and clay-sized, and are associated with Al- and Mg-bearing minerals that preferentially concentrate in the clay fraction. The geochemistry of the silt and clay-sized fraction can approximate that of the <2-mm fraction. Background variations in till illustrate the important role of a geological framework to the interpretation of geochemical surveys and the origins of trace metals in the environment. Received: 31 October 1996 · Accepted: 27 May 1997  相似文献   

5.
Geochemical investigations of till is a widely used method in metal exploration as the till commonly inherits the geochemical signature (including the metal contents) of the parent bedrock. In this investigation, over 2000 till samples were collected in the Sarvlaxviken area, southern Finland, where several polymetallic (Cu, Zn, Pb, As, Sn, W and In) veins recently have been discovered in Proterozoic crust along the border between Late Svecofennian granites and the Wiborg Batholith. The bedrock is commonly covered by compact and poorly sorted basal till, formed during the Late Weichselian glaciation event. Several glacial-transported boulders, with high contents of Cu, Zn, As, Sn, Mo and Bi and derived from the local bedrock, have also been discovered on top of the till and provide evidence for concealed mineralisation in the local bedrock under the till cover. The frequent distribution of till in the Sarvlaxviken area provides excellent conditions for the search of such hidden mineralisation by means of systematic till sampling, even if large farm field areas, composed of clay-rich sediments, and seawater-covered areas (Sarvlaxviken bay), had to be avoided in the sampling program. The till samples were collected during university courses and training programs led by the authors and were analysed in a cost-efficient and certified laboratory. Obtained geochemical data were statistically processed by using K-means clustering algorithms which can be used to treat large sets of geochemical data. The results provided anomalies that mainly occur in till with a thickness of <1 m and are considered to be derived from a local bedrock source. The discovered anomalies provide strong evidence for numerous undiscovered veins beneath the till cover.  相似文献   

6.
Contamination of gold ring and natural gold grains into plastic capsules used in INAA analysis and the fine fraction of till (0.064 mm) have been studied. An artificial contamination of till samples with a gold ring caused Au contents of 100–600 ppb in the fine fraction. A few rubbings of plastic capsules with a gold ring gave Au amounts of 15–170 ng per capsule. Natural gold grains of sizes 0.1–1.0 mm added into “gold-free” till samples before drying and sieving caused Au contents of 1–27 ppb into the fine fraction. In a regional geochemical survey an anomaly of a few hundreds of square kilometers with concentrations of 100–600 ppb Au in the fine fraction of till was observed. Later studies showed that this area was very low in Au, concentrations being generally below one ppb. The anomaly was interpreted as a contamination caused by the gold rings of the samplers.  相似文献   

7.
The physical partitioning of Hg into different grain size fractions of till is predominantly controlled by the primary bedrock mineralogy, the distance of glacial transport, and the relative stability of cinnabar (HgS) in the soil weathering environment. At sites located short distances down-ice from bedrock cinnabar mineralization, the highest Hg concentrations in unoxidized till were measured in the sand- and granule-sized fractions reflecting the abundance of cinnabar in those size ranges. Similar partitioning was measured in oxidized till as cinnabar was found to be relatively resistant to postglacial weathering. Discrete clay-sized cinnabar grains obtained from the unoxidized till were viewed under the scanning electron microscope and suggest that the terminal grade of cinnabar is in the clay-sized range. In till collected from areas barren of cinnabar mineralization, the highest Hg levels were found in the clay-sized fraction which is attributed to the high adsorption of Hg by clay minerals.  相似文献   

8.
Quantifying long-term rates of chemical weathering and physical erosion is important for understanding the long-term evolution of soils, landscapes, and Earth's climate. Here we describe how long-term chemical weathering rates can be measured for actively eroding landscapes using cosmogenic nuclides together with a geochemical mass balance of weathered soil and parent rock. We tested this approach in the Rio Icacos watershed, Puerto Rico, where independent studies have estimated weathering rates over both short and long timescales. Results from the cosmogenic/mass balance method are consistent with three independent sets of weathering rate estimates, thus confirming that this approach yields realistic measurements of long-term weathering rates. This approach can separately quantify weathering rates from saprolite and from overlying soil as components of the total. At Rio Icacos, nearly 50% of Si weathering occurs as rock is converted to saprolite; in contrast, nearly 100% of Al weathering occurs in the soil. Physical erosion rates are measured as part of our mass balance approach, making it particularly useful for studying interrelationships between chemical weathering and physical erosion. Our data show that chemical weathering rates are tightly coupled with physical erosion rates, such that the relationship between climate and chemical weathering rates may be obscured by site-to-site differences in the rate that minerals are supplied to soil by physical erosion of rock. One can normalize for variations in physical erosion rates using the “chemical depletion fraction,” which measures the fraction of total denudation that is accounted for by chemical weathering. This measure of chemical weathering intensity increases with increasing average temperature and precipitation in data from climatically diverse granitic sites, including tropical Rio Icacos and six temperate sites in the Sierra Nevada, California. Hence, across a wide range of climate regimes, analysis of chemical depletion fractions appears to effectively account for site-to-site differences in physical erosion rates, which would otherwise obscure climatic effects on chemical weathering rates. Our results show that by quantifying rates of physical erosion and chemical weathering together, our mass balance approach can be used to determine the relative importance of climatic and nonclimatic factors in regulating long-term chemical weathering rates.  相似文献   

9.
Located in the uplands of the Valley and Ridge physiographic province of Pennsylvania, the Susquehanna/Shale Hills Critical Zone Observatory (SSHO) is a tectonically quiescent, first-order catchment developed on shales of the Silurian Rose Hill Formation. We used soil cores augered at the highest point of the watershed and along a subsurface water flowline on a planar hillslope to investigate mineral transformations and physical/chemical weathering fluxes. About 25 m of bedrock was also drilled to estimate parent composition. Depletion of carbonate at tens of meters of depth in bedrock may delineate a deep carbonate-weathering front. Overlying this, extending from ∼6 m below the bedrock-soil interface up into the soil, is the feldspar dissolution front. In the soils, depletion profiles for K, Mg, Si, Fe, and Al relative to the bedrock define the illite and chlorite reaction fronts. When combined with a cosmogenic nuclide-derived erosion rate on watershed sediments, these depletion profiles are consistent with dissolution rates that are several orders of magnitudes slower for chlorite (1-5 × 10−17 mol m−2 s−1) and illite (2-9 × 10−17 mol m−2 s−1) than observed in the laboratory. Mineral reactions result in formation of vermiculite, hydroxy-interlayered vermiculite, and minor kaolinite. During weathering, exchangeable divalent cations are replaced by Al as soil pH decreases.The losses of Mg and K in the soils occur largely as solute fluxes; in contrast, losses of Al and Fe are mostly as downslope transport of fine particles. Physical erosion of bulk soils also occurs: results from a steady-state model demonstrate that physical erosion accounts for about half of the total denudation at the ridgetop and midslope positions. Chemical weathering losses of Mg, Na, and K are higher in the upslope positions likely because of the higher degree of chemical undersaturation in porewaters. Chemical weathering slows down in the valley floor and Al and Si even show net accumulation. The simplest model for the hillslope that is consistent with all observations is a steady-state, clay weathering-limited system where soil production rates decrease with increasing soil thickness.  相似文献   

10.
Chemical weathering of silicate minerals has long been known as a sink for atmospheric CO2, and feedbacks between weathering and climate are believed to affect global climate. While warmer temperatures are believed to increase rates of weathering, weathering in cool climates can be accelerated by increased mineral exposure due to mechanical weathering by ice. In this study, chemical weathering of silicate minerals is investigated in a small temperate watershed. The Jamieson Creek watershed is covered by mature coniferous forest and receives high annual precipitation (4000 mm), mostly in the form of rainfall, and is underlain by quartz diorite bedrock and glacial till. Analysis of pore water concentration gradients indicates that weathering in hydraulically unsaturated ablation till is dominated by dissolution of plagioclase and hornblende. However, a watershed scale solute mass balance indicates high relative fluxes of K and Ca, indicating preferential leaching of these solutes possibly from the relatively unweathered lodgement till. Weathering rates for plagioclase and hornblende calculated from a watershed scale solute mass balance are similar in magnitude to rates determined using pore water concentration gradients.When compared to the Rio Icacos basin in Puerto Rico, a pristine tropical watershed with similar annual precipitation and bedrock, but with dissimilar regolith properties, fluxes of weathering products in stream discharge from the warmer site are 1.8 to 16.2-fold higher, respectively, and regolith profile-averaged plagioclase weathering rates are 3.8 to 9.0-fold higher. This suggests that the Arrhenius effect, which predicts a 3.5- to 9-fold increase in the dissolution rate of plagioclase as temperature is increased from 3.4° to 22 °C, may explain the greater weathering fluxes and rates at the Rio Icacos site. However, more modest differences in K and Ca fluxes between the two sites are attributed to accelerated leaching of those solutes from glacial till at Jamieson Creek. Our findings suggest that under conditions of high rainfall and favorable topography, weathering rates of silicate minerals in warm tropical systems will tend to be higher than in cool temperate systems, even if the temperate system is has been perturbed by an episode of glaciation that deposits regolith high in fresh mineral surface area.  相似文献   

11.
Contemporary ‘acid rain’ in the Hubbard Brook ecosystem has induced a series of geochemical responses. Neutralization is accomplished in essentially a 2-step process. Initially, hydrogen ion acidity is neutralized by the dissolution of reactive alumina primarilly found in the soil zone. In the Hubbard Brook area this reactive alumina has solution properties much like natural gibbsite. Aluminum-rich surface waters with a pH of 4.7 5.2 are typical of this neutralization stage. In a second step, both hydrogen ion acidity and aluminum acidity are neutralized by the chemical weathering of primary silicate minerals, i.e. by the alkali and alkaline earths contained in the bedrock and glacial till of the watershed. The chemical weathering reaction is much slower than the alumina dissolution reaction, so that the aluminum acidity stage (pH 4.7 5.2) may persist for substantial periods. Typically, however, in the Hubbard Brook area the aluminum acidity has been neutralized and a pH > 5.2 is obtained before surface waters reach a third-over stream channel. Because of the relatively low pH's throughout the soil zone and in the streamwater, carbonic acid reactions are essentially absent at the present time in the Hubbard Brook system. Water pathlength (or residence time) in the soil zone is the crucial factor in the state of acid rain neutralization, aluminum chemistry and chemical weathering. As measured by the losses of alkali and alkaline earths from the ecosystem, chemical weathering rate in the Hubhard Brook area at the present time is not especially high relative to other areas.  相似文献   

12.
Gold grains were studied in 102 till and gravel samples from an area with Au anomalies in the fine fraction of till. The form and size of gold nuggets were studied under the microscope, and the composition of selected grains was determined on a microprobe. The number of visible nuggets per sample was less than 10 per 81. In size, the nuggets were between 0.07 and 1 mm. Most of them were indefinite but flat in form, although idiomorphic gains were also observed. In composition, the gold grains in bedrock differed somewhat from those in till and gravel. Some of the nuggets found in gravel and in till were probably secondary in origin. The possibility that all the grains are not primary is emphasized in the interpretation of geochemical results.  相似文献   

13.
We calculate the chemical depletion fraction of the granitic bedrock by analysing the rock-soil enrichment of zirconium. In Vendée (France), chemical weathering rates account for 26% of the denudation rates. Such a chemical depletion fraction characterizes temperate regimes. It is three times lower than that of humid tropical regimes. To cite this article: J.-C. Maurin et al., C. R. Geoscience 337 (2005).  相似文献   

14.
《Applied Geochemistry》2001,16(1):123-136
Concentrations of several elements extractable with BaCl2 and acid ammonium acetate (pH 4.65) were measured in the organic and 4 mineral soil layers on the national forest inventory plots of the Finnish Forest Research Institute. The soil data also includes total concentrations of elements in the organic layer and site and soil physical characteristics. Data were compared with the aqua regia extractable element concentrations measured in the nationwide regional till geochemical mapping carried out by the Geological Survey of Finland.Correlations between concentrations in surface soil and underlying basal till were generally highest for K, Mg, Mn, P and Zn; in the organic layer and till they were highest for Cr and Cu. The strength of these correlations did not increase regularly from surface to deeper soil layers. All soil base cations with the exception of Ca, which is of relatively low solubility, were well correlated. The elements Zn, K, P, Al and Mn in till were the most reliable indicators of surface soil chemistry.Fuzzy clustering showed that the correlation between element concentrations in basal till and the two uppermost layers of mineral soil was better within areas of distinct till geochemistry, such as the schist belts in southwestern Finland, the Lake Ladoga–Bothnian Bay zone and the Kuusamo schist belt.Surface soil chemical variables were clearly better in discriminating fertility classes of forest sites than were element concentrations in basal till. The independent ability of till geochemistry to distinguish these productivity classes and to explain surface soil fertility was nevertheless demonstrated.  相似文献   

15.
An example of a confined geochemical cycle of minor elements, initiated and maintained almost entirely by vegetation, in a relatively rare landscape type of an isolated arid flat plateau blanketed by a fairly homogeneous mixture of autochthonous and of seemingly allochthonous poorly consolidated clastic materials. The plant-induced migration and redistribution of chemical elements involves the entire overburden of the bedrock. There is a definite enrichment of Ti, V, Cu, Ni, Co, Ga, and possibly of Zn, Cr, Sc, Sn in the <0.001 mm fraction of the soils and an enrichment or Zr and possibly of the rare earths and Pb in the 0.2 mm to 0.001 mm fraction, while the metals content of the >0.2 mm fraction is poorer in such constituents than the bedrock itself.--V. P. Sokoloff.  相似文献   

16.
 The geochemical and mineralogical characteristics of bulk samples and various size fractions of sulfide-bearing fine-grained sediments in the Petalax area, western Finland, were studied with ICP-AES (aqua regia digestion and "total" digestion) and X-ray diffraction. The sediments, which are dominated by particles <0.06 mm (clay and silt size), are composed of phyllosilicates (micas, kaolinite, chlorite, vermiculite), quartz, feldspars, amphiboles, organic matter, and secondary S minerals. The chemical analyses show that Na, Ca, and Sr are enriched in the silt and sand fractions, that Zr is enriched in the silt fraction, and that the concentrations of all other studied metals (Al, Ba, Co, Cr, Cu, Fe, K, La, Mg, Mn, Nb, Ni, Sc, Th, Ti, V, Y, and Zn) increase from the sand to the clay fraction. Sodium, Ca, and Sr occur largely in poorly dissolved feldspars, which explains the distribution of these metals in the sediments. The increase in the concentrations of most metals in the clay fraction is interpreted to be the result mainly of an increase in phyllosilicates in this size fraction. A geochemical comparison between the sulfide-bearing fine-grained sediments and glacial till shows that the former are enriched in several potentially toxic metals. It is therefore argued that the hydrological and ecological problems associated with the sulfide-bearing sediments are related not only to the production of acidity in oxidized layers, but also to mobilization and dispersion of toxic metals. Strategies to minimize damage of freshwater systems in areas covered with sulfide-bearing fine-grained sediments are suggested. Received: 20 February 1997 · Accepted: 25 August 1997  相似文献   

17.
The silt and clay-size (minus 0.063 mm) fractions of 766 samples of glacial tills from the Kuhmo-Suomussalmi area in eastern Finland were analyzed for 8 trace and minor elements (Co, Cr, Cu, Mn, Ni, Pb, Zn, Fe) by AAS after hot 7 mol/I HNO, dissolution treatment. The results were treated statistically. Threshold values were calculated separately for those tills overlying gneissic basement on the one hand and greenstone belt on the other. The results show that the overall element levels for the metals in the area are lower than elsewhere in Finland by a factor which varies between 1 and 46. The reason for the differences may be the absence of preglacial weathering material in the till fines which elsewhere enhance element levels. Marked differences in the element levels over each bedrock subunit show up in the overlying tills. Correlation, regression and factor analyses show that the elements have high mutual correlations suggestive of hydromorphic alteration, even though the macroscopically hydromorphic samples were omitted from the statistical analyses. The effect is weak, however, and does not suppress the primary correlation, which results in the emergence of two factors in factor analysis of the geochemical data of the greenstone belt tills, the Co-Cr-Cu-Mn-Ni-Fe and Pb-Zn factors, the latter corresponding to one of the two known types of mineralization encountered in the underlying bedrock.  相似文献   

18.
High-density regional geochemical data for surface soils in central England and East Anglia reveal that much of their geochemical character is inherited from the tills that they are developed upon. Multivariate statistical analysis highlighted three significant element associations of Al–Fe–Ga–K–La–Mg–Rb, Ca–Sr and K–Fe accounting for almost 93% of the geochemical variability of soils derived from tills. Provenancing the geochemical signatures of the latter elements enabled the construction of ice flow paths associated with two different Middle Pleistocene ‘chalky’ till sheets. A lower till sheet relating to ‘Pennine’ ice flowing from west to east across the region, and an upper till sheet deposited by North Sea ice moving into northern East Anglia, and to the west of the Fen Basin, before fanning-out across central England. Overall, geochemical signatures of different till units are largely derived from local bedrock sources, with dilution and a new geochemical signature acquired as the ice flows over and incorporates new bedrock lithologies. The results show that high resolution soil geochemical data provides a further proxy with which the flow paths of former ice sheets can be delineated.  相似文献   

19.
Rockshelter and cave-mouth deposits accumulate in specialized niches through the breakdown of the enclosing bedrock, the addition of windblown and fluvial sediment and, in the case of prehistoric sites, human habitation debris. Certain modifications of routine sediment analysis are necessary in order to realize the full potential of paleoenvironmental interpretation. These sediments are coarse, necessitating the collection of large samples, commonly 10–20 kg. The size and shape of the coarse fraction yields information on rockfalls, freeze-thaw activity, and weathering effects. Travertine and soil-carbonate coatings on the rock fragments indicate variations in ambient humidity and soil formation, respectively. Studies of the granulometry, mineralogy, and geochemistry of the fine fraction lead to conclusions concerning primarily the postdepositional modification of the sediments. Heavy minerals can be important in determining the provenience of the fine sediment. These methods of analysis are discussed in detail with the Abri Pataud as a specific example, and caution is urged in making regional generalizations from the study of a single site.  相似文献   

20.
In vast tropical rain forest areas, weathering profiles are commonly characterized by a “stone line” overlain by a brown-yellow loose-clay horizon. Concordant with the topographic surface, such a stone line may be traced continuously over considerable distances. It is typically composed of coarse fragments of lithorelics, debris of laterite as Fe-oxides nodules, corroded quartz, gibbsitic aggregates, …, embedded in a clayey matrix. These materials cover the saprolitic weathering profile which is typically a few tens of metres thick.The origin of stone lines has given rise to much controversy and are still widely misunderstood. A broad range of processes, allochthonist or autochthonist, have been put forward in the literature. The findings in this paper conclude that these weathering profiles result from chemical leaching and differential movement between the matrix and the coarse fragments which accumulate by downward migration. Accumulation takes place at the lower limit of rain water impregnation and forms the stone line, whereas leaching and homogenization of fine material occur throughout the upper water-impregnated horizon. Although the materials of the loose-clay horizon and of the stone are extensively altered, the relics are chemically rather well recognizable.According to the above hypothesis, stone line weathering profiles should thus be mostly residual. The main aspects of geochemical dispersion processes of some stone line profiles in Gabon are presented as examples. These show that:
1. (a) The vertical redistribution of some major elements in the profiles, accumulation (Fe2O3, Al2O3, SiO2) or leaching (K2O, MgO, CaO, SiO2,..) are different from the bedrock composition;
2. (b) In some situations, it is possible to characterize the bedrock by using groups of trace elements such as V, Ni,.. for basic rocks or Ba, Sr,.. for gneisses for instance; the contrasts obtained can be smoothed in comparison with results from deeper in the profile.
3. (c) The persistence of geochemical anomalies arising from mineralization, throughout the weathering profile, up to the main sampling media, the surface soil. A “mushroom” dispersion pattern can be recognized where the foot of the mushroom corresponds to the element dispersion pattern can be recognized where the foot of the mushroom corresponds to the element dispersion in the saprolite and the bedrock, with the top of the mushroom being partly in the stone line and partly in the loose clay horizon.
Such a dispersion pattern has two consequences on exploration: (1) the spreading out of the surficial signal favoring the identification of anomalies during follow-up on a relatively wide spaced grid; and (2) at the same time, a reduction of the extension of the signals by dilution and leaching according to the weathering process; therefore, relatively low anomaly contents must be taken into account in exploration.Thus, anomalies arising from stone line profiles tend to be well-dispersed, but of weak magnitude, and represent in situ transfer from the parent rock.  相似文献   

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