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1.
Some 65 years ago, C. E. Tilley, in his 1950 Presidential Address to the Geological Society of London, brought into sharp focus the then perhaps-not-fully-recognised problem of the origin of what he termed the ‘orogenic andesites’—and, in particular, their commonly constituent calc-alkaline lineage of lavas and pyroclastic materials. In spite of much investigation and progress since, the problem remains not fully resolved. The two most conspicuous features of such orogenic—i.e. island arc—volcanism are its copious, explosive, degassing and, as Tilley noted, its copious development of andesites and their more felsic associates. If these rock types were derived from basalt, the principal relevant changes in major element composition are reductions in Fe, Ca and Mg, and increases in Si, Al and (Na + K). Four principal mechanisms for inducing these changes have been proposed: (1) magmatic differentiation induced by fractional crystallisation, (2) magma mixing, (3) partial melting of an ultramafic-derived basaltic parent, and (4) crustal contamination. A fifth postulate—long recognised but only occasionally seriously considered—is that loss in the volatile phase during eruption and later degassing may make a significant contribution to the overall process. On the basis that Tilley's (and others’) orogenic basalt–andesite–dacite–rhyolite series constitutes a linear arrangement on the conventional FeOt–MgO–(Na2O + K2O) diagram it is shown on numerical grounds that fractional crystallisation alone cannot account for the observed compositional trend—that, while accumulates are neither uncommon nor insubstantial in amount in modern arcs, they are olivine-rich, not magnetite-rich, and that numerical, petrological and field evidence indicates that the high oxidation/magnetite subtraction hypothesis of orogenic andesite genesis can no longer be seriously entertained. The partial melting hypothesis encounters contrary numerical evidence, closely analogous with that adduced for fractional crystallisation. Magma mixing eventually encounters the difficulty of the nature of the felsic material required for rhyolite formation, and assimilation of the preservation of primitive isotope ratios as lava evolution progresses. A fifth mechanism—volatile loss—may make a significant contribution to the relevant compositional change, perhaps in combination with one or more of the other proposed processes. Geochemical, petrological and field evidence pertaining to this is reviewed and examined, and it is postulated that indeed a combination of fractional crystallisation as the principal mechanism, volatile loss as a contributing factor, provides a plausible process. A case is made for an exploratory experimental investigation of the possibility of significant loss in a gas/vapour phase.  相似文献   

2.
Analysis of modern paleontological, isotope-geochemical, and paleotectonic data on Vendian rocks in the Patom Basin (Siberia) is presented. The paper discusses depositional settings and features of the specifics of sedimentogenesis of the Barakun carbonate–terrigenous formation, which matches the lower strata of postglacial deposits of the Dal’nyaya Taiga Group. It is shown that formation of the transgressive Barakun Formation was governed by relatively deep-water (below the storm wave base) distal settings of a low-angle (about 1°) homoclinal ramp. Its formation was related to a more intense sagging of the northeastern part of the Ura Uplift. The formation of breccias and intrusions was fostered by high seismic activity of the paleobasin recorded in plastic deformations along with liquefaction and fluidization of sedimentary beds. The constant seismic destabilization of bottom beds provoked small but frequent landslides without the formation of turbidite flows. The continuous dislocation of sediments toward the depocenter actively governed specifics of the ramp architecture. The unusual behavior of sediments during their long-term thixotropic residence could be related to a high content of organic matter, processes of methanogenesis and, probably, dissemination of gas hydrate compounds.  相似文献   

3.
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4.
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5.
Simulation results of the equilibrium state of systems water-carbonaceous chondrite material, water-primary mantle material, water-ultramafic rock material, and water-mafic rock material open with respect to carbon dioxide and methane at 25°C, 1 bar indicate that highly alkaline reduced aqueous solutions with K/Na > 1 can be formed only if water is in equilibrium with compositions close to those of continental crust and primitive mantle. Yu.V. Natochin’s hypothesis that the living cell can be formed only in an aqueous environment with K/Na > 1 leads to the conclusion that terrestrial life could arise and further evolve on the Earth during the differentiation of primary chondritic material into the Earth’s core and mantle (during the first few million years of the planet’s lifetime) in an alkaline (pH 9–10) reduced (Eh = −400–500 mV) aqueous solution at a temperature of 50–60°C, in equilibrium with an N2-bearing atmosphere, which also contained CH4 (partial pressure from 10−2 to 10−8 bar), CO2 (partial pressure from 10−5 to 10−8 bar), NH3, H2, H2S, CO, and other gases.  相似文献   

6.
The premise of the Wilson et al. comment is that the Ti-in-quartz solubility calibration (Thomas et al. in Contrib Mineral Petrol 160:743–759, 2010) is fundamentally flawed. They reach this conclusion because P–T estimates using the Ti-in-quartz calibration differ from their previous interpretations for crystallization conditions of the Bishop and Oruanui rhyolites. If correct, this assertion has far-reaching implications, so a careful assessment of the Wilson et al. reasoning is warranted. Application of the Ti-in-quartz calibration as a thermobarometer in rutile-free rocks requires an estimation of TiO2 activity in the liquid ( (liquid–rutile); referenced to rutile saturation) and an independent constraint on either P or T to obtain the crystallization temperature or pressure, respectively. The foundation of Wilson et al.’s argument is that temperature estimates obtained from Fe–Ti oxide thermometry accurately reflect crystallization conditions of quartz in the two rhyolites discussed. We maintain that our experimental approach is sound, the thermodynamic basis of the Ti-in-quartz calibration is fundamentally correct, and our experimental results are robust and reproducible. We suggest that the reason Wilson et al. obtain implausible pressure estimates is because estimates for T and they used as input values for the Ti-in-quartz calibration are demonstrably too high. Numerous studies show that Fe–Ti oxide temperature estimates of some rhyolites are substantially higher than those predicted by well-constrained phase equilibria. In this reply, we show that when reasonable input values for T and (liquid–rutile) are used, pressure estimates obtained from the Ti-in-quartz calibration are well aligned with phase equilibria and essentially identical to melt inclusion volatile saturation pressures.  相似文献   

7.
Although the Kimberley Process Certificate Scheme has considerably reduced the flow of conflict diamonds from 4% to less than 1%, other forms of illicit diamonds are estimated at 20% of global production. While scholars and policy makers have given considerable attention to illicit mining and smuggling (with some success), illicit exploitation still hinders revenue generation that is needed for economic growth and development in Sub-Saharan African (SSA) countries heavily reliant on alluvial minerals. Based on surveys of 240 households in four diamondiferous chiefdoms in Kono District in Sierra Leone, key informant interviews, focus groups and secondary data, and drawing from the actor-oriented approach and Le Billon (Fuelling war: natural resources and armed conflict. London, International Institute for Strategic Studies, 2005) conceptualization of the political geography of the resource, this paper examines the problem of illicit diamond exploitation in post-conflict Sierra Leone. It investigates: (1) the causes of illicit diamond mining and smuggling; (2) the spatial and temporal aspects of illicit mining and smuggling, and (3) possible solutions to illicit diamond exploitation. The paper argues that the ineffectiveness of national mining laws/policies has created conditions that are exploited by local and international actors in the diamond industry. This is further exacerbated by the political geography of the resource. Study findings reveal that while better mining regulation could reduce illicit exploitation, bringing buyers closers to miners and offering them better prices for rough diamonds will minimize smuggling and thus increase government’s mineral revenue base.  相似文献   

8.
Angus Cameron 《Geoforum》2012,43(4):741-749
This paper argues that the metaphorical figure of the island plays an important but profoundly ambiguous role in the imagination of social space. The paper argues that ‘utopic’ islands have historically provided a fictional domain of experimentation that has informed the constitution of ‘real’ state spaces. From the 16th to 20th centuries this took the form of an increasingly consolidated and ‘global’ endotopia: a world, exemplified by the ‘political’ map, full of state spaces constituted as interiors. More recently, islands have served a very different metaphoric function, being used to create and legitimise spaces of exteriority – ‘xenospaces’ such as the online worlds of the ‘metaverse’ and the arcane legal/financial spaces of offshore – which in combination constitute an emergent xenotopia. The ‘philosopher’s island’ (Mackay, 2010), therefore, represents a complex and polyvalent spatial form that serves to continuously and expediently redefine the nature of social space.  相似文献   

9.
The composition of clay minerals in the subcolloid fraction from the uppermost layer of bottom sediments in the northern part of Amur Bay was determined by X-ray powder diffraction analysis, and the enrichment of 33 elements in the subcolloid and pelite fractions of the surface deposits from a number of sites at the marginal filter of the Razdol’naya River was studied by ICP-MS.Fe, U, and chalcophile elements are contained in the highest concentrations in sediments from all sampling sites within the filter. The bottom sediments are not enriched in trace, alkali, and alkali-earth elements. The maximum concentrations of chemical elements were found in deposits from the brackish part of the marginal filter, perhaps, because of the formation of Fe and Mn (Al) hydroxides. Bottom sediment at the boundary between the brackish and marine parts of the filter contain the lowest concentrations of the examined elements.  相似文献   

10.
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11.
《International Geology Review》2012,54(17):2143-2163
ABSTRACT

The ~100,000 km2 granitoid belt in the Lesser Xing’an-Zhangguangcai Ranges (LXZR) is an important component of the Phanerozoic granitic provinces (known as the ‘granitoid ocean’) in NE China. To reveal the mechanisms of Mesozoic magmatism and crustal growth in the LXZR, we present systematic geologic data, lithofacies information, zircon U-Pb ages, Hf isotopic, and element geochemical data for the Dong’an granites in the northern LXZR. The results show the following: (1) the Dong’an granites can be subdivided into three lithofacies, namely, medium- to coarse-grained alkali-feldspar granite, fine-grained alkali-feldspar granite, and granitic aplite, which were emplaced during the Early Jurassic (187–178 Ma); (2) the Dong’an granites are characterized by high SiO2 and total alkali contents and low Fe2O3t contents and Mg# values, and they are enriched in LILEs (e.g., Rb and K) and depleted in HFSEs (e.g., Nb and Ta); and (3) the zircons from these granites yield εHf(t) values of ?2.4 to +5.7 and two-stage model ages of 772–1542 Ma. These results indicate that the Dong’an granites were emplaced during the Early Jurassic, are highly fractionated I-type (HFI) granites and were likely derived from partial melting of the thickened lower crust. Regionally, the Dong’an granites and the Early-Middle Jurassic intrusive rocks (201–163 Ma; 90,000 km2) in the LXZR show a trend of comagmatic evolution controlled by fractional crystallization, and they were successively emplaced during magma evolution. The HFI granites that are geochemically similar to the Dong’an granites formed in the later stage of magma evolution. Combined with regional geological observations, we conclude that the granitic rocks in the LXZR formed in an active continental margin setting related to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent. This process may record an important episode of crustal accretion in the LXZR.  相似文献   

12.
Accumulation of manganiferous rocks in the history of the Earth’s lithosphere evolution began not later than the end of the Middle Archean. Primary manganese sediments were accumulated at that time in shallow-water sedimentation basins with the active participation of organic matter. The concentration of Mn in the primary sediments usually did not reach economic values. The formation of genuine manganese ores is related to later processes of the transformation of primary ores—diagenesis, catagenesis, metamorphism, and retrograde diagenesis. Types of basins of manganese ore sedimentation and character of processes of the formation of manganese sediments during the Earth’s shell evolution changed appreciably and correlated with the general evolution of paleocontinents. Major periods, epochs, and phases of manganese ore genesis are defined. At the early stages of lithosphere formation (Archean-Proterozoic), manganese was deposited in basins commonly confined to the central part of Western Gondwana and western part of Eastern Gondwana, as well as the western part of the Ur paleocontinent. Basins of manganese ore sedimentation were characterized by the ferruginous-siliceous, carbonaceous-clayey, and carbonaceous-carbonate-clayey composition. The Early-Middle Paleozoic epoch of manganiferous sediment accumulation was characterized by the presence of several small sedimentation basins with active manifestation of volcanic and hydrothermal activity. Since the formation of Pangea in the Late Paleozoic until its breakup, accumulation of Mn was closely associated with processes of diagenesis and active participation of the oxidized organic matter.  相似文献   

13.
The outer sector of the Neoproterozoic Katangan Orogen of Central Africa is characterised by nappes thrust northwards, toward the foreland region, the major part of which occurs in the Democratic Republic of Congo (DRC). The rocks called R.A.T. (‘Roches Argilo-Talqueuses’) are terrigenous clastics traditionally considered as the oldest stratigraphical interval of these allochthonous units. They are correlated with the terrigenous clastic sediments at the base of the autochthonous Katangan succession in Zambia to the south, which were deposited at the opening stage of the Katangan Rift Basin. The lower interval of the R.A.T. represents red beds, whereas the upper one was deposited in anoxic conditions. Therefore, they are called red and grey R.A.T., respectively. This paper presents stratigraphic, structural and geochemical arguments against the traditional stratigraphical view and demonstrates that the R.A.T. rocks are younger than previously considered. They are interpreted here as synorogenic sediments of the Katangan foreland basin.Olistostromes with R.A.T. olistoliths, which occur either interbedded within ‘normal’ R.A.T. sediments or overlie angular unconformities, testify to pronounced tectonic movements and palæotopography of the basin in which the R.A.T. sediments were deposited. The provenance of other olistoliths implies that, contrary to the previous views, the R.A.T. olistostromes are younger than the overlying rock complexes and the contact between the two is tectonic. Clastic dykes of the incompetent R.A.T. lithologies injected into the overlying competent units suggest that the former were partly unconsolidated sediments over-ridden by the Katangan nappes. Plots of the geochemical compositions point to two distinct tectonosedimentary cycles and two types of sources, each related to a different stage of orogen evolution. The terrigenous materials of the Katangan autochthonous strata (Roan and Kundelungu Groups) and correlative allochthonous units are derived from basement granitic and metamorphic rocks eroded during the opening of the Katagan rift basin. By contrast, the R.A.T. rocks are related to the closure of the basin. Their provenance is from the orogenic source-the Katangan nappes advancing towards the foreland region in the north.The autochthonous Roan Group rocks in Zambia and their allochthonous correlatives in DRC contain one of the richest Cu-Co deposits known. In accord with the previous correlation, the CuCo mineralisation in the grey R.A.T. rocks was considered of the same age as the Zambian deposits. However, the results presented in this paper imply that the grey R.A.T. deposits represent a second generation of mineralisation in the Katangan belt, younger than the Roan Group orebodies. The R.A.T. Cu-Co mineralisation is related to the anoxic stage of the foreland basin, and the advancing nappes containing Roan-correlative orebodies acted as the sources of the metals. In conclusion, points pertaining to the revision of stratigraphical classification of the Katangan Supergroup are proposed.  相似文献   

14.
The general evolution of planets in the Solar System is discussed with a focus on the structure and history of Venus compared with the Earth. The history of the planets of the terrestrial group has been similar and included at least six correlated stages. Many common features the terrestrial planets shared in their early and late evolution have been due to their common origin from the protoplanetary gas-and-dust nebula and plume magmatism widespread on all the planets of the terrestrial group. The characteristic features of the structure and evolution of Venus are most brightly manifested in the specific composition of its atmosphere and of plume magmatism. Venus, with its surface as hot as 450 °C and the near-surface pressure of 92-93 bars, has a hot and dense atmosphere 93 times that of the Earth in mass. Most of its atmospheric mass (99%) belongs to the 65-km thick troposphere consisting of CO2 (96.5%) and N2 (3.5%). The upper troposphere includes a 25-30 km thick cloud layer composed mainly of sulfuric acid droplets, water vapor, and SO2. At a height of 49.58 km, the clouds approach the conditions of the terrestrial surface and might be hospitable to bacterial life. Volcanism, the most active and widespread process of Venusian geology, maintains continuous SO2 emission. There are diverse volcanic edifices on Venus, which are most often large and are similar to the Earth’s plume-related volcanoes. The evolution before 1 Ga, as well as the share and the role of alkaline rocks and carbonatites among its volcanics, are among the most debatable issues about Venus. Being located closer to the Sun, Venus cooled down more slowly and less intensely than the Earth after the primary accretion. In the Proterozoic, it began heating and reached its present state at ~ 1 or 2 Ga. In the future, as the Sun becomes a red giant, the Earth is predicted to begin heating up in 500-600 Myr to reach the temperature of present Venus in about 1.5 Gyr.  相似文献   

15.
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16.
Dunites, peridotites, olivine and spinel pyroxenites, and metagabbroids have been described in the tectonic blocks of the Pekul’ney complex of the central Chukchi Peninsula together with garnet-hornblende-clinopyroxene and zoisite (clinozoisite)-garnet-hornblende rocks, which are indicative of high-pressure complexes. However, the interpretations of previous researchers on the composition, structure, setting, and processes of formation of this rock association are highly controversial. The petrographic and mineralogical results reported in this paper indicate that the blocks of the complex host bodies of cumulate ultramafics among metamorphic rocks. These relationships were supported by the finding of xenoliths and xenocrysts of metamorphic rocks in the ultramafics. The metamorphic country rocks are lower crustal amphibolites and schists with peak metamorphic parameters corresponding to the high-pressure portion of the epidoteamphibolite facies (610–680°C and 9–14 kbar). All the varieties of ultramafic rocks studied in the blocks of the complex are assigned to a single cumulate series (from dunite to clinozoisite-garnet hornblendite), and the compositions of their primary minerals show regular correlations similar to crystallization differentiation trends. Specific features of the ultramafics of the Pekul’ney complex are the early crystallization of hornblende (which is present already in peridotites), wide range of garnet crystallization (associating with clinopyroxene, ceylonite, and hornblende), presence of magmatic clinozoisite in the most evolved assemblages (with garnet, hornblende, and clinopyroxene), and absence of evidence for plagioclase crystallization. Clinopyroxene from the most evolved ultramafic rocks contains more than 15 wt % Al2O3. The classification of the rocks of the complex provides a basis for the interpretation of geological relations between them and the elucidation of the characteristics of the internal structure of the blocks of the complex and bodies of cumulate ultramafic rocks in them.  相似文献   

17.
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18.
The trial and conviction of seven public officials in L’Aquila, central Italy, for having allegedly given out misleading and incorrect information to the public before the 6 April 2009 earthquake has proved to be one of the most momentous developments of recent times in science and disaster risk reduction—and also one of the most misunderstood. It highlights the difficulty in transforming the findings of the earth sciences, which are often characterised by uncertainty, into information that can be used to protect ordinary citizens. This paper describes those elements of the disaster at L’Aquila which are pertinent to the trial and the legal proceedings that followed them. It analyses the political, social and scientific context of the trial, and the reaction of scientists and their institutions in Italy and other countries. I conclude that the defendants were tried as responsible public officials, not as scientists. The mass media in Italy and abroad tended to insist that what was on trial was the ability to predict earthquakes, and this had an enormous influence on the opinion of scientists in many different places. However, the trial was actually about the apparently fatal consequences of misleading the public with “incomplete, imprecise and contradictory information”, as the prosecutors put it. I believe that much of the international reaction to the trial was misguided because it was based on incomplete, second-hand information about the proceedings. If scientists were to make judgements on their own work in such a superficial manner, the results would be highly unreliable and public faith in science would plummet.  相似文献   

19.
The equilibrium composition was modeled for the water-carbonaceous chondrite matter system open to CO2 and CH4. It was shown that at 25°C, total pressure of 1 bar, and definite proportions between partial pressures of CO2 (10−5–10−8 bars) and CH4 (10−4–10−8 bar), the aqueous phase is characterized by a K/Na ratio of 4–12, which corresponds to the ratios of these elements in the cellular liquid of organisms. Aqueous phase has pH = 8−9, Eh = −450 ± 50 mV, and ammonia nitrogen, and K, Na, and Mg concentrations close to those in the cellular liquid.  相似文献   

20.
Abstract

The third (Es3) member of the Eocene Shahejie Formation is the main hydrocarbon source rock interval in the Raoyang Sag of the Bohai Bay Basin. The lower Es3 (Es3L) and upper Es3 (Es3U) submembers display clear differences in source rock quality. The formation mechanisms of the source rocks are investigated via geochemical methods using 60 samples from three wells to reconstruct the paleo-environment during deposition. The major element parameters exhibit changes in paleoclimate from humid to arid. Indicators such as Sr/Ba, B/Ga, the gammacerane index and isotopic data suggest fresh–brackish and hydrological open lakes with unstable water column stratification likely occurred during deposition of the Es3L submember and saline and hydrologically closed lakes with stable water column stratification likely during deposition of the Es3U submember. Carbon isotope values of organic matter, trace elements and biomarker parameters suggest that the Es3L submember had moderate productivity, with a significant contribution from terrigenous organic matter whereas the Es3U submember had slightly enhanced productivity, with no or minor contributions from terrigenous organic matter. Furthermore, the pristane/phytane ratio and the enrichment of Mo and U indicate that the euxinic bottom-water conditions (sulfidic) of the lakes during deposition of the Es3U submember were best for preserving organic matter. Comparison of the models of source rock deposition of the Es3L and Es3U submembers indicates that the redox conditions play an important role in the formation of organic-rich source rocks in the Raoyang Sag.  相似文献   

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