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1.
The main landforms within the glacially scoured Precambrian rocks of the Swedish west coast are closely connected to the principal structural pattern and have lately been explained as mainly a result of etch processes, probably during the Mesozoic and with a possible second period of etching during the Neogene. To explore the effect of multiple glacial erosion on the rock surfaces, an island with two different lithologies and with striae from different directions was selected for a detailed study, focusing on the shape of roches moutonnées. Air‐photo interpretation of bedrock lineaments and roches moutonnées combined with detailed field mapping and striae measurements are used to interpret the structural and lithological control on the resulting shape. The study reveals a significant difference in shape between roches moutonnées in augen‐granite and orthogneiss. Low elongated and streamlined roches moutonnées occur in the gneiss area, striated by a Late Weichselian ice flow from the NE. This ice flow is subparallel with both the local dominant trend of topographically well‐expressed joints and the schistosity of the gneiss. Frequently, there are no signs of quarrying on the lee‐sides of the gneiss roches moutonnées and hence they resemble the shape of whalebacks, or ruwares, as typically associated with the exposed basal weathering surface found in tropical areas. The granite roches moutonnées were formed by an older ice flow from the ESE, which closely followed the etched WNW–ESE joint system of the granite. Late Weichselian ice flow from the NE caused only minor changes of the landforms. On the contrary, marks of the early ESE ice flow are poorly preserved in the gneiss area, where it probably never had any large effect as the flow was perpendicular to both schistosity and structures and, accordingly, also to the pre‐glacial relief. The study demonstrates that coincidence between ice flow direction and pre‐glacially etched structures is most likely to determine the effects of glacial erosion. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
2.
Michel Mbina Mounguengui François Baudin Jacques Lang Olivier Jocktane 《Comptes Rendus Geoscience》2003,335(3):327-334
The Interior Basin of Gabon, created during the break-up between South America and Africa, displays thick Neoproterozoic to Aptian p.p. fluvio-lacustrine deposits overlain by Aptian to Albian marine facies. Rock–Eval analyses from outcrop and drillhole samples show high content in organic matter (up to 25%) related to types I and II. These intervals are encountered within Permian, Neocomian–Barremian as well as Aptian siliciclastic succession. They constitute fairly good to excellent potential petroleum source rocks, which are most probably at the origin of oil indices recognized both in drillholes and in surface. 相似文献
3.
A precise, accurate and rapid method for the sequential determination of FeO and Fe2O3 in rocks, soils and some non‐refractory minerals by 1,10‐phenanthroline spectrophotometry is described. Fe(II) and Fe(III) were leached from the sample (?200 mesh) using a mixture of NH4HF2 and H2SO4 at 40–80 °C for 10 min on a hot plate. Both Fe(II) and Fe(III) could be conveniently estimated sequentially from the same reaction mixture at the μg g?1 to percentage level. The method is better than the existing wet chemical methods, including the commonly used Pratt's titrimetric redox method, for Fe(II) and Fe(III) determinations in rock and soil samples in terms of precision, accuracy and rapidity. The throughput of the method was very high; at least forty to fifty samples could be estimated easily in a day. The results obtained compare favourably with those obtained by Pratt's method, as well as for certified/recommended values of a set of eleven certified reference materials having FeO and Fe2O3 contents in the range 0.21–14.63% and 0.58–8.48%, respectively. The optimised 1,10 phenanthroline method was found to be accurate to within 0.21% m/m FeO and 0.30% m/m Fe2O3 compared with the literature values of the certified reference materials studied. 相似文献
4.
Ccile Quantin Vojt ch Ettler Jrmie Garnier Ond&#x;ej &#x;ebek 《Comptes Rendus Geoscience》2008,340(12):872-882
Soil profiles developed on serpentinites from several localities in the Czech Republic were investigated by a combination of geochemical and mineralogical methods (selective extractions, XRD, SEM/EDS) to determine the partitioning and mobility of chromium (Cr) and nickel (Ni). Whereas Cr is released from pyroxenes, amphiboles and Cr-spinels present in the bedrock, Ni is primarily mobilized from olivines and small Fe-Ni sulphide inclusions. The results of extraction methods indicated significant differences in the availability and mobility of Cr and Ni in soils. Chromium is almost not phytoavailable, as indicated by the DTPA extractions. The selective extractions showed that Cr is tightly bound to well-crystallised Fe-oxides and primary rock-derived phases. In contrast, Ni is substantially more mobile in soil, being mainly controlled by Mn-oxides and amorphous/less crystallised Fe-oxides. The DTPA extraction indicated that up to 10% of total Ni might be phytoavailable. 相似文献
5.
Juan Morales Avto Goguitchaichvili Luis M. Alva-Valdivia Jaime Urrutia-Fucugauchi 《Comptes Rendus Geoscience》2006,338(8):507-513
Twenty years after Tanaka and Kono's pioneering contribution (Tanaka and Kono, 1984), we give some new details on the effect of applied field strength during Thellier paleointensity experiments. Special attention is paid to the relation of magnitude of laboratory field and Coe's quality factors (Coe et al., 1978). Full thermoremanent magnetizations were imparted on natural samples containing low-Ti titanomagnetites of pseudo-single domain structure in a 40-μT magnetic field from 600?°C to room temperature. The samples were subjected to the routine Thellier procedure using a wide range of applied laboratory fields. Results indicate that values of laboratory fields may be accurately reproduced within 2% of standard error. The quality factors, however, decrease when the magnitude of ‘ancient’ field does not match to applied laboratory fields. To cite this article: J. Morales et al., C. R. Geoscience 338 (2006). 相似文献
6.
Yali Sun Shengling Sun Christina Yan Wang Peng Xu 《Geostandards and Geoanalytical Research》2013,37(1):65-76
A method was developed for the determination of low‐level rare earth elements (REEs) and thorium in ultramafic samples by inductively coupled plasma‐mass spectrometry. The conventional method for the digestion of ultramafic rocks using HNO3 and HF results in considerable amounts of insoluble fluorides because of the high contents of Mg (generally up to 24% m/m) in these rocks. In this study, we used H3BO3 as a complexing agent to break down the insoluble fluorides, and then separated the REEs from Fe and Mg major and Ba, Ca, Cr minor matrices by anion exchange and co‐precipitation, respectively. The whole procedural blanks estimated from sample‐free analyses ranged from 0.232 ng for Ce to 0.006 ng for Tm and Lu. Limits of detection for this method, defined as three times the standard deviation of these blank analyses, varied from 0.51 ng g?1 for Ce to 0.03 ng g?1 for Lu. The recovery of REEs using this technique, as determined using the standard addition method, ranged from 92.9% for Y to 102.0% for Er with 3% (RSD) variation. The method was validated using GAS (GeoPT‐12), JP‐1 and PCC‐1, and the results were comparable to literature values, elucidating the applicability to the determination of ultra trace REEs in ultramafic rocks. 相似文献
7.
On the Lu-Hf Isotope Geochemistry of Silicate Rocks 总被引：9，自引：0，他引：9
Janne Blichert-Toft 《Geostandards and Geoanalytical Research》2001,25(1):41-56
This paper reviews the history (TIMS, hot‐SIMS, MC‐ICP‐MS), significance, geochemical behaviour and current uncertainties (λ176 Lu, Hf‐Nd Bulk Silicate Earth) surrounding the Lu‐Hf isotope system, and thus marks two decades of its application to geochemical problems. An appendix further presents (a) improvements to the original chemistry protocol of Blichert‐Toft et al. (1997) for application to Mg‐rich samples and (b) a compilation of previously published and new Hf isotope determinations by MC‐ICP‐MS for a set of international rock reference materials. Prior to the advent of multiple‐collector plasma source mass spectrometry (MC‐ICP‐MS), routine analysis of the Lu‐Hf isotope system developed only slowly because of the extreme difficulty of measuring Hf isotope compositions with thermal ionisation mass spectrometry, caused by the very high first ionisation potential of Hf. However, Hf isotope compositions can be measured relatively easily using MC‐ICP‐MS and this new technique now provides reproducible measurements at high precision regardless of the matrix from which Hf is separated. Of the commonly used long‐lived radiogenic isotope systems, only the Sm‐Nd and Lu‐Hf isotope systems are unaffected by parent/daughter fractionations related to volatile nebular processes and core formation. While other systems (Rb‐Sr, U‐Th‐Pb, Re‐Os) may also be used to investigate the chemical evolution of the Earth, Moon, Mars and parent bodies of differentiated meteorites, the larger uncertainties in their bulk chemical and isotopic values limit their application to determine geochemical budgets and assess planetary mantle‐crust evolution. In the study of garnet‐bearing rocks, both for dating purposes and as an isotopic tracer for source provenance and mantle processes, the Lu‐Hf isotope system likewise is of major interest because of the high partition coefficient of Lu compared to Hf for garnet with respect to other minerals. Furthermore, the larger Lu/Hf fractionation compared to Sm/Nd during melting beneath ridges produces proportionally higher Lu/Hf in the residue and faster in‐growth of a radiogenic Hf isotopic signature (compared to Nd), which may help shed light on the dynamics of mantle melting. While the chemistry protocol and mass spectrometric technique for high‐precision Lu‐Hf isotope analysis have been resolved in satisfactory ways over the past five years, more accurate determination of the decay constant for 176 Lu, at present known with a precision of only about 4%, still needs to be completed and a consensus reached on which value to use for future Lu‐Hf isotope studies. Although the current combined Lu‐Hf and Sm‐Nd Bulk Silicate Earth parameters are plagued by possible incompatibilities in chondrite selection and potential interlaboratory biases, a more accurate set of values may not be readily established owing to heterogeneities in the isotopic composition of chondrites that far exceed present analytical accuracy. 相似文献
8.
Chromite from the north–south-trending schist belt of Nuggihalli (southern Karnataka), belonging to the greenstone belt of South India (3.0–3.5 Ga) and crystallising at 1178 °C under oxygen fugacity of ?6.67 shows komatiitic affinity. The chemical data of the Cr-spinels when plotted in the form of the discriminant function of Johan (1979) show a remarkable follow-up of the curve A, which delineates the boundary between the ‘barren’ and ‘fertile’ (with respect to the plausible presence of sulphide phases) Cr-spinel bodies. Only a few samples plot marginally within the barren region (Fig. 1), suggesting that the chromite bands should hardly hold the possibility of becoming host of sulphide mineralisation of commercial relevance. 相似文献
9.
The Kuhmo greenstone belt (Eastern Finland) is mainly formed by mafic volcanic rocks and metasediments. Although the geological observations indicate that this belt appears as a supracrustal synform overlying a gneissic basement, its deep structures are actually unknown. The present gravity study intends to provide a better understanding of these structures. Interpretation of gravity anomalies of the Kuhmo belt, using two-dimensional modelling suggests that the structures of the belt are controlled by granitoids diapirs. Most of the calculated models show that the belt displays in section pocket-like shapes (purses) terminated by a gully-shape close to the surface. This particular geometry corroborates the hypothesis of a gravitational formation with the heavy volcanic material sinking in a light sialic crust. To cite this article: M. Jaffal et al., C. R. Geoscience 335 (2003). 相似文献
10.
Carlos Villaseca David Orejana Christian Pin José-Angel López Garcı́a Pilar Andonaegui 《Comptes Rendus Geoscience》2004,336(10):877-888
Basic magmatism in central Spain occurred sporadically during a long period of ca. 140 Ma. The geochemical data on representative samples from the five igneous episodes show that different mantle reservoirs were tapped. Crustal involvement (via recycling in mantle sources or by hybridization at emplacement levels) was important in the basic magmas related to the Hercynian orogeny (Gb1 to Gb3 suites). Samples occurring as later dykes have signatures ranging from OIB-like values (Gb4 alkaline lamprophyres) to those of a formerly enriched subcontinental lithospheric mantle (tholeiites Gb5). To cite this article: C. Villaseca et al., C. R. Geoscience 336 (2004). 相似文献