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1.
Mantle hydrocarbons: abiotic or biotic? 总被引：15，自引：0，他引：15
Analyses of 227 rocks from fifty localities throughout the world showed that mantle derived rocks such as tectonized peridotites in ophiolite sequences (tectonites) arid peridotite xenoliths in alkali basalts contain heavier hydrocarbons (n-alkanes), whereas igneous rocks produced by magmas such as gabbro arid granite lack them. The occurrence of hydrocarbons indicates that they were not derived either from laboratory contamination or from held contamination; these compounds found in the mantle-derived rocks are called here "mantle hydrocarbons." The existence of hydrocarbons correlates with petrogenesis. For example, peridotite cumulates produced by magmatic differentiation lack hydrocarbons whereas peridotite xenoliths derived from the mantle contain them. Gas chromatographic-mass spectrometric records of the mantle hydrocarbons resemble those of aliphatics in meteorites and in petroleum. Features of the hydrocarbons are that (a) the mantle hydrocarbons reside mainly along grain boundaries and in fluid inclusions of minerals; (b) heavier isoprenoids such as pristane and phytane are present; and (c) delta 13C of the mantle hydrocarbons is uniform (about -27%). Possible origins for the mantle hydrocarbons are as follows. (1) They were in organically synthesized by Fischer-Tropsch type reaction in the mantle. (2) They were delivered by meteorites and comets to the early Earth. (3) They were recycled by subduction. The mantle hydrocarbons in the cases of (1) and (2) are abiogenic and those in (3) are mainly biogenic. It appears that hydrocarbons may survive high pressures and temperatures in the mantle, but they are decomposed into lighter hydrocarbon gases such as CH4 at lower pressures when magmas intrude into the crust; consequently, peridotite cumulates do not contain heavier hydrocarbons but possess hydrocarbon gases up to C4H10. 相似文献
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The Yehlangian of Ting and Graban includes the Paoan shale,the Yülungshanlimestone,the Feihsienkuan shale and the Sanchiao limestone.In their opinion theseformations would represent the Upper Permian of south-western China.The presentpaper tries to show that both by their stratigraphical relation and their faunal contentsthe Yülungshan limestone and the Feihsienkuan shale belong undoubtedly to the LowerTriassic and that the Sanchiao limestone is much younger,probably of Upper Triassicage.The Paoan shale... 相似文献
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Holger Stünitz 《Contributions to Mineralogy and Petrology》1998,131(2-3):219-236
Dynamic recrystallization in the strict sense of the term is the reconstitution of crystalline material without a change
in chemical composition, driven by strain energy in the form of dislocations. Driving potentials additional to internal strain
energy may contribute to the recrystallization of naturally deformed minerals, which form solid solutions such as feldspar,
amphiboles and pyroxenes, if they change their composition during recrystallization. To estimate the relative importance of
these driving potentials, the chemical composition of porphyroclasts and recrystallized grains of plagioclase, clinopyroxene
and hornblende have been investigated in samples from a high grade shear zone of the Ivrea Zone, Italy. The plagioclases show
two different recrystallization microstructures: bulging recrystallization at grain boundaries and discrete zones of recrystallized
grains across porphyroclasts probably involving fracturing. Deformation took place under amphibolite facies conditions on
a retrograde P,T-path. Porphyroclast and recrystallized compositions from bulging recrystallization microstructures differ only in their Or-content
and yield a ΔG between mean host grain and mean recrystallized grain composition at fixed P,T-conditions of approximately 5 Joules/10−4 m3. Extreme compositional variations yield approximately 60 J/10−4 m3. The increase of free energy due to dislocations calculated for common glide systems in plagioclase are on the order of 100
Joules/10−4 m3 for high values of dislocation densities of 1014 m−2. Thus, the effect of chemically induced driving energies on grain boundary velocity appears small for mean compositions but
may be as great as that of deformational energies for larger chemical differences. In the other type of microstructure, porphyroclasts
and recrystallized grains in discrete zones differ in their anorthite content. The maximum ΔG induced by the compositional disequilibrium is on the order of 100 J/10−4 m3. This maximum value is of the same magnitude as the ΔG derived from high dislocation densities of 1014 m−2. The resulting combined ΔG is approximately twice as high as for deformational ΔG alone, and heterogeneous nucleation may become a feasible recrystallization mechanism which is evident from the microstructures.
The recrystallization mechanism depends on the nature of the driving potential. Grain boundary migration (GBM) and heterogeneous
nucleation can release Gibbs free energy induced by compositional disequilibrium, whereas this is not likely for subgrain
rotation. Therefore, only GBM and heterogeneous nucleation may link metamorphism and deformation, so that syndeformational
recrystallization may represent a transitional process ranging from dynamic recrystallization to metamorphic reaction.
Received: 8 July 1996 / Accepted: 17 November 1997 相似文献
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Estimation or simulation? That is the question 总被引：1，自引：0，他引：1
Jorge Kazuo Yamamoto 《Computational Geosciences》2008,12(4):573-591
The issue of smoothing in kriging has been addressed either by estimation or simulation. The solution via estimation calls
for postprocessing kriging estimates in order to correct the smoothing effect. Stochastic simulation provides equiprobable
images presenting no smoothing and reproducing the covariance model. Consequently, these images reproduce both the sample
histogram and the sample semivariogram. However, there is still a problem, which is the lack of local accuracy of simulated
images. In this paper, a postprocessing algorithm for correcting the smoothing effect of ordinary kriging estimates is compared
with sequential Gaussian simulation realizations. Based on samples drawn from exhaustive data sets, the postprocessing algorithm
is shown to be superior to any individual simulation realization yet, at the expense of providing one deterministic estimate
of the random function. 相似文献
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The Variance of Mango‘s Light—Hydrocarbon Parameter K 总被引：1，自引：0，他引：1
The analyses of light hydrocarbons in oils from the Tarim Basin show that the Mango‘s parameter K is about unity except those oils trapped in the eastern part of the Tazhong (Central Tarim)Fault Uplift,The regular variance of K may indicates the accumulation and admixture of the oil populations in the eastern part of the Tazhong Fault Uplift. 相似文献
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How Alpine or Himalayan are the Central Andes? 总被引：2，自引：0，他引：2
Although non-collisional mountain belts, such as the Andes, and collisional mountain belts, such as the Alps and the Himalayas–Tibet,
have been regarded as fundamentally different, the Central Andes share several features with the Himalayas–Tibet. The most
important of these are extremely thickened (≥70 km) continental crustal roots supporting high plateaus and mountain fronts
characterized by large basement thrusts. The main prerequisite for very thick crustal roots and extreme mountainous topography
appears to be large-scale underthrusting of continental crust of normal thickness, irrespective of whether the crustal thrusts
are antithetic with respect to subduction as in the Andes, or synthetic with respect to preceding subduction of oceanic lithosphere
as in the Himalayas. In both cases sole thrusts near the base of the continental crust nucleated in thermally anomalous zones
of the hinterland and then propagated across ramps into shallower detachments located within thick sedimentary or metasedimentary
cover rocks. In contrast to the Central Andes and the Himalayas, the Alps are characterized by intracrustal detachment which
allowed both the subduction of lower crust and a stacking of relatively thin upper crustal slivers, which make up a narrow
mountain chain with a more subdued topography.
Received: 10 August 1998 / Accepted: 1 March 1999 相似文献
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H. J. Harrington 《Australian Journal of Earth Sciences》2013,60(3-4):501-502
The Broken River Embayment is a major transverse structure in the Tasman Orogen in northern Queensland. Bell (1980) suggested that it is a hinge zone of an orocline, which he named the Big Bend Megafold. An alternative explanation, presented in this paper, involves a Silurian‐Devonian triple plate junction. 相似文献
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《Journal of Structural Geology》1999,21(8-9):939-948
Strains in rocks can be observed but ancient stresses can only be inferred. We should re-examine the potential of strain geometry as the key to understanding and interpreting common shear structures ranging from faults to plastic shear zones. The concept of failure along zero extension directions can be applied to natural structures in rocks and is predicated on strain compatibility between differently strained volumes. Zero extension directions are considered for two strain configurations, plane strain (k=1) and uniaxial shortening (k=0). The crucial difference between shear fractures, or faults, and plastic yield zones is that the former are preceded by dilatation while the latter are isovolumetric. Volume changes during deformation affect the orientations of zero extension directions and hence of the resulting structures. With isovolumetric strain, yield occurs on planes at 45° to the principal shortening direction in plane strain and at 54.7° to this axis in uniaxial shortening. Uniaxial shortening experiments on rock samples allow estimation of the relative volumetric strains when yield zones initiate. When this volumetric strain is used to estimate the orientation of shear fractures in plane strain, ca 70° dips are predicted for normal faults at high crustal levels, decreasing downwards to 45°. 相似文献
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Natural Hazards - Evidence shows the global climate will continue to change over this century and beyond. A clear understanding of the climate change risk is suggested to be the foundation of the... 相似文献
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This paper addresses the use of fossil species of the horse Equus as stratigraphic markers in the Pliocene and Pleistocene. Current research shows that the species recently so used do not succeed one another in orderly fashion, but occur and co-occur at ages other than presupposed by the stratigraphic schemes (usually earlier). Horse species should be identified on their morphology, not on their stratigraphic age. ‘Subspecies’ of fossil horses do not conform to subspecies in zoology, as they lack a geographic range. They thus lack taxonomic status and stratigraphic significance and should be avoided. 相似文献
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Matthew R. Bennett Richard I. Waller Neil F. Glasser Michael J. Hambrey David Huddart 《第四纪科学杂志》1999,14(2):125-135
The interpretation of glacigenic diamictons is a subjective process, for which quantitative support is frequently sought from parameters such as clast shape and fabric. It has been widely suggested that different glacigenic diamicton facies possess distinct clast‐fabric signatures. This paper examines this concept using a data set of 111 clast fabrics, and a synthesis of published results. Eigenvalues are calculated and compared for a variety sedimentary facies. It is concluded that clast fabric alone is not able to discriminate between different glacigenic facies, and it is argued that clast fabric offers little quantitative support in the interpretation of glacigenic sediments. It is suggested, therefore, that although clast fabric may continue to have a role as an indicator of relative strain at specific sites, its use in the discrimination of glacigenic facies is limited. Consequently, we should be much more selective in undertaking such analyses in the future. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
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Attila Demény Ramón Casillas Julio de La Nuez Géza Nagy 《Chemie der Erde / Geochemistry》2008,68(4):369-381
Carbonate xenoliths containing olivine and rimmed by kaersutitic amphibole were collected in basaltic rocks of the Basal Complex of La Palma. The mineralogical composition and microscopic appearance may suggest a relationship with carbonatites in general, thus a major element, trace element and stable isotope study was conducted to investigate the origin of the carbonate formation. Based on electron microprobe analyses, the carbonate is calcite with up to 6.3 wt% MgO and 7.2 wt% SiO2. The elevated SiO2 content may suggest a melt origin for the carbonate. However, the C and O isotope compositions of the carbonate xenoliths (δ13C and δ18O around −1‰ and 13‰, respectively) are similar to those of calcite veins and amygdales in basaltic rocks of the Basal Complexes of La Palma and Fuerteventura and are interpreted as produced by fluid degassing and metasomatism by CO2-H2O fluid derived from mobilization of sedimentary material. Trace element contents determined by laser-ablation ICP-MS analyses support the assumed origin, thus, the relationship with carbonatitic melts can be excluded. Based on trace element compositions, the amphibole surrounding the xenoliths is not related genetically to the carbonate. The elevated SiO2 content of the calcite can be attributed to submicron relics of pyroxene, thus, the use of this feature as an evidence for melt origin is questionable. 相似文献
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It is widely accepted that chemical weathering of Ca–silicate rocks could potentially control long-term climate change by providing feedback interaction with atmospheric CO2 drawdown by means of precipitation of carbonate, and that in contrast weathering of carbonate rocks has not an equivalent impact because all of the CO2 consumed in the weathering process is returned to the atmosphere by the comparatively rapid precipitation of carbonates in the oceans. Here, it is shown that the rapid kinetics of carbonate dissolution and the importance of small amounts of carbonate minerals in controlling the dissolved inorganic C (DIC) of silicate watersheds, coupled with aquatic photosynthetic uptake of the weathering-related DIC and burial of some of the resulting organic C, suggest that the atmospheric CO2 sink from carbonate weathering may previously have been underestimated by a factor of about 3, amounting to 0.477 Pg C/a. This indicates that the contribution of silicate weathering to the atmospheric CO2 sink may be only 6%, while the other 94% is by carbonate weathering. Therefore, the atmospheric CO2 sink by carbonate weathering might be significant in controlling both the short-term and long-term climate changes. This questions the traditional point of view that only chemical weathering of Ca–silicate rocks potentially controls long-term climate change. 相似文献
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GREGORY J. RETALLACK 《Sedimentology》2012,59(4):1208-1236
The Late Neoproterozoic Ediacara Member of the Rawnsley Quartzite in South Australia has been considered aeolian, fluvial, intertidal and deep marine by various authors. Palaeosols would not be expected for the deep marine interpretation, but some palaeosols should be evident for the aeolian–fluvial–intertidal interpretations, and this is the first study to examine the Ediacara Member at a petrographic and geochemical scale appropriate to recognize potential palaeosols. Recognition of palaeosols and floodplain facies in Neoproterozoic rocks is a challenge because such rocks are too ancient for diagnostic non‐marine fossils such as root traces. The varied thickness of Ediacara Member red siltstones and white sandstones is distinct from laterally persistent overlying and underlying grey shales and limestones with acritarchs, stromatolites and other marine fossils. The sandstones are trough cross‐bedded and fill palaeovalleys. The red siltstones have poorly sorted, highly angular, silt‐size grains characteristic of loess. Particular sandy and silty beds were sampled for detailed petrographic and geochemical studies, because they include desiccation cracks, sand crystals, ice cracks, carbonate nodules and soft‐sediment deformation like those of palaeosols. Chemical and grain‐size variations within these beds reveal surficial clay formation and oxidation from feldspar as in soils. Petrographic studies also revealed surficial disruption of these palaeosols by filamentous structures comparable with microbial ropes of biological soil crusts. This array of palaeosol features may be of use for recognizing palaeosols in other Neoproterozoic siliciclastic sequences. 相似文献
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Conditions for indentation and channelised flow are investigated with two-dimensional thermomechanical models of Alpine-type continental collision. The models mimic the development of an orogen at an initial central portion of weakened lithosphere 150 km wide, coherent with several geological reconstructions. We study in particular the role of lower crustal strength in developing peculiar geometries after 20 Ma of shortening at 1 cm/year. Crustal layers produce geometries of imbricate layers, which result from two contrasted mechanisms of either channelised ductile lateral flow or horizontal rigid-like indentation:
- – Channelised lateral flow develops when the lateral lower crust has a viscosity less than 1021 Pa s, exhibiting velocities opposite to the direction of convergence. This mechanism of deformation produces subhorizontal shear zones at the boundaries between the lower crust and the more competent upper crust and lithospheric mantle. It is also associated with a topographic plateau that equilibrates with a wide (about 200 km) but quasi-constant crustal root about 50 km deep.
- – In contrast, indentation occurs with lateral lower crust layers that have a viscosity greater than about 1023 Pa s, producing significant shortening and thickening of the central crust. In this case topography develops steep and narrow (around 100 km wide), associated with a thickened crust exceeding 60 km depth. A crustal-scale pop-up forms bounded by subvertical shear zones that root into the mantle lithosphere.
Keywords: Continental collision indentation; Channel flow; Lower crust; Elastic–viscous–plastic rheology 相似文献