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21.
GERNOT ARP CHRISTIAN OSTERTAG‐HENNING SELÇÜK YÜCEKENT JOACHIM REITNER VOLKER THIEL 《Sedimentology》2008,55(5):1227-1251
Fossil stromatolites may reveal information about their hydrochemical palaeoenvironment, provided that assignment to a specific microbial community and a corresponding biogeochemical mechanism of formation can be made. Tithonian stromatolites of the Münder Formation at Thüste, north Germany, have traditionally been considered as formed by intertidal cyanobacterial communities. However, thin sections of the stromatolites show elongated angular traces of former gypsum crystals in a dense arrangement, but no algal or cyanobacterial filament traces. Moreover, high Fe2+ and Mn2+ contents, oxygen‐isotope and sulphur‐isotope ratios of carbonate‐bound sulphates, and sulphurized hydrocarbon biomarkers of the stromatolitic carbonate indicate that CaCO3 precipitation occurred near the oxic–anoxic interface as a result of intensive bacterial sulphur cycling rather than photosynthetic activity. Furthermore, anaerobic oxidation of methane by Archaea may have driven CaCO3 precipitation in deeper parts of the biofilm community, as reflected by high concentrations of squalane with a strongly negative δ13C in conjunction with evaporite pseudomorphs showing extremely low δ13CCarb ratios. Consequently, the Thüste stromatolites are now interpreted as having initially formed by gypsum impregnation of biofilms. Subsequently, early Mg‐calcitic calcitization within the biofilms occurred because of combined bacterial iron, manganese and sulphate reduction, with an increasing contribution of anaerobic oxidation of methane with depth. This model plausibly explains the prominent preservation of signals derived from oxygen‐independent metabolic pathways, whereas virtually no geochemical record exists for an aerobic community that may, nevertheless, have prevailed at the stromatolite surface. Photic‐zone stromatolites with a prominent signal of anaerobic oxidation of methane may be common in, and indicative of, oxygen‐depleted sulphate‐bearing environments with high rates of methane production, conditions that possibly were fulfilled at the Archaean to Proterozoic transition. 相似文献
22.
Ultrahigh pressure macro diamonds from Copeton (New South Wales, Australia), based on Raman spectroscopy of inclusions 总被引:1,自引:0,他引:1
Mining of Cenozoic alluvial deposits at Copeton and Bingara (Eastern Australia) has produced two million macrodiamonds (0.25 ct median size). Raman spectroscopy is used to identify included minerals within uncut Copeton diamonds, with sealed chamber remnant pressures of 31.7 to 35.6 kbar for coesite, 13.6 and 22.7 kbar for clinopyroxene, and 7.6 kbar for grossular garnet. Assuming elastic behaviour, these values generate inclusion entrapment PT loci which intersect, restricting diamond formation conditions: from 250 °C, 43 kbar to 800 °C, 52 kbar. Larger than error (± 100 °C and ± 4 kbar), this range shows a systematic variation in inclusion composition with diamond zoning and N properties. Published research shows 1) Copeton and Bingara diamonds are unique, and 2) modern alluvium in the Bingara district carries mantle-formed garnet, captured by post-tectonic alkali basalt from an extensive diamondiferous ultrahigh pressure (UHP) terrane that stalled at depth because it is dominated by mafic eclogite. The combined Raman and geological results indicate two sets of subduction UHP diamond formation conditions/protolith are required, firstly cooler oceanic slab and secondly including higher temperature continental crust. The Copeton and Bingara stones are UHP macrodiamonds, and Carboniferous 40Ar/39Ar age dates on clinopyroxene inclusions should be interpreted as ages of crystallisation, representing the termination of subduction. The characteristic features of ruptured inclusions and etched percussion marks on Copeton and Bingara diamond indicate volcanic delivery to the earth's surface. Alluvial deposits elsewhere in Eastern Australia may carry similar diamond along with diamond of different origin. 相似文献
23.
高、低煤阶煤层气藏地质特征及控气作用差异性研究 总被引:4,自引:0,他引:4
高、低煤阶煤层气地质特征及控气作用差异性是研究煤层气富集成藏的重要组成部分,是煤层气勘探开发理论研究过程中重要的基础性研究领域之一。本文以中国沁水、阜新盆地和美国粉河盆地等典型的含气盆地为例,探讨了高、低煤阶煤层气的储层物性差异,分析了构造控气和水文地质控气作用的差异性。研究表明,高煤阶气藏含气量高,CH4百分含量高,δ13C1值大于-38.75‰,储层渗透率变化小,储层改造难,构造热事件对煤层气的生成、富集贡献大,持续的水动力使气藏遭到破坏,且破坏幅度大,现今地下水格局对气藏的形成具有一定的影响;低煤阶气藏含气量低,CH4百分含量低,δ13C1值大于-49.11‰,储层渗透率变化大,储层易改造,煤热演化史及煤阶影响着煤层气的生成、富集,在煤层气生成过程中活跃的水动力是甲烷生成的主要的水文地质条件之一,但持续的水动力使气藏遭到破坏,且破坏幅度小,而合适的地层水矿化度则是低煤阶煤层气生成的重要条件,地下水格局对气藏的调整和改造起到决定性的影响。 相似文献
24.
四川汶川MS 8.0大地震地表破裂带的遥感影像解析 总被引:21,自引:1,他引:20
2008年5月12日发生于四川盆地西部龙门山断裂带的汶川MS 8.0级大地震造成巨大的人员伤亡和财产损失,并形成了空间上基本连续分布的地表破裂带(地震断层)。根据地表破裂带的解译标志及影像特征,我们充分利用震后中国科学院航空遥感飞机所获取的高分辨率航空遥感图像以及我国台湾福卫-2卫星遥感图像进行详细解译分析,并结合震后的多次野外科学考察与验证,初步查明了四川汶川MS 8.0级大地震所产生地表破裂带的空间分布特征。遥感解译分析表明汶川大地震产生的地表破裂带总计长约300 km,其几何学特征十分复杂,主要沿先存的NE走向活动断裂带呈不连续展布;变形特征以逆冲挤压为主兼具右旋走滑分量。按同震地表破裂带所在断裂带位置,可将其分为两条: 中央地表破裂带:沿映秀-北川断裂带分布,从西南开始呈北东向延伸至平武县水观乡石坎子北东一带,长约230 km,最大垂直位移量达6.0 m左右,最大右旋水平位移达5.8 m;山前地表破裂带:沿灌县-安县断裂带分布,由都江堰市向峨乡一带开始呈北东向延伸至安县雎水镇一带,长约70 km,以逆冲挤压为主,最大垂直位移量可达2.5 m。此外,遥感图像分析还表明上述地表破裂带与地质灾害分布在空间上具有十分密切的相关性,因此,挤压逆冲-走滑型地震断层的致灾效应研究是未来应该加以重视的研究课题。 相似文献
25.
L. Moreno J. J. Durán M. A. Casermeiro J. R. Quintana Ma A. Fernández 《Environmental Geology》2008,54(3):465-477
Over a period of 4 years and 4 months, the geopurification installations at Dehesas de Guadix (Granada, Spain) were monitored
to determine the impact on soil and groundwater of the controlled discharge of urban wastewater, and also to identify the
best indicators of the entry of the recharged water into the aquifer. The installations are located in an area where the climate
is Mediterranean sub-arid, with an average precipitation of less than 287 mm/year, and a rate of evapotranspiration that is
almost three times greater. The system was controlled by determining the balance of majority nutrients and boron in the soil
and in the groundwater, both at the points affected directly by the wastewater discharge and at others. The quantity of mass
discharged was relatively large (COD 14,656 g/m2, NO3 85 g/m2, NO2 4 g/m2, NH4 2,425 g/m2, PO4 1,143 g/m2, K 1,531 g/m2, B 63 g/m2). It was observed that the elimination of nutrients within the soil (COD 97.5%, PO4 94.4%, K 59.17%, N
total 18.8%, B 12.69%) was very efficient except for the nitrogen, which nevertheless did not reach the groundwater, as it was
eliminated at deep levels of the unsaturated zone. Only 12.69% of the boron was removed, and appreciable, increasing amounts
of this element did reach the groundwater. Unexpectedly, none of the majority nutrients behaved as a reliable indicator of
the impact on groundwater; despite this, the boron and the bicarbonate did clearly reflect the arrival of the recharged water,
and are proposed as the best indicators. 相似文献
26.
Mapping susceptibility of rainfall-triggered shallow landslides using a probabilistic approach 总被引:2,自引:0,他引:2
To prepare a landslide susceptibility map is essential to identify hazardous regions, construct appropriate mitigation facilities,
and plan emergency measures for a region prone to landslides triggered by rainfall. The conventional mapping methods require
much information about past landslides records and contributing terrace and rainfall. They also rely heavily on the quantity
and quality of accessible information and subjectively of the map builder. This paper contributes to a systematic and quantitative
assessment of mapping landslide hazards over a region. Geographical Information System is implemented to retrieve relevant
parameters from data layers, including the spatial distribution of transient fluid pressures, which is estimated using the
TRIGRS program. The factor of safety of each pixel in the study region is calculated analytically. Monte Carlo simulation
of random variables is conducted to process the estimation of fluid pressure and factor of safety for multiple times. The
failure probability of each pixel is thus estimated. These procedures of mapping landslide potential are demonstrated in a
case history. The analysis results reveal a positive correlation between landslide probability and accumulated rainfall. This
approach gives simulation results compared to field records. The location and size of actual landslide are well predicted.
An explanation for some of the inconsistencies is also provided to emphasize the importance of site information on the accuracy
of mapping results. 相似文献
27.
Almeev Renat; Holtz Francois; Koepke Jurgen; Haase Karsten; Devey Colin 《Journal of Petrology》2008,49(1):25-45
Phase equilibria simulations were performed on naturally quenchedbasaltic glasses to determine crystallization conditions priorto eruption of magmas at the Mid-Atlantic Ridge (MAR) east ofAscension Island (7–11°S). The results indicate thatmid-ocean ridge basalt (MORB) magmas beneath different segmentsof the MAR have crystallized over a wide range of pressures(100–900 MPa). However, each segment seems to have a specificcrystallization history. Nearly isobaric crystallization conditions(100–300 MPa) were obtained for the geochemically enrichedMORB magmas of the central segments, whereas normal (N)-MORBmagmas of the bounding segments are characterized by polybariccrystallization conditions (200–900 MPa). In addition,our results demonstrate close to anhydrous crystallization conditionsof N-MORBs, whereas geochemically enriched MORBs were successfullymodeled in the presence of 0·4–1 wt% H2O in theparental melts. These estimates are in agreement with direct(Fourier transform IR) measurements of H2O abundances in basalticglasses and melt inclusions for selected samples. Water contentsdetermined in the parental melts are in the range 0·04–0·09and 0·30–0·55 wt% H2O for depleted and enrichedMORBs, respectively. Our results are in general agreement (within±200 MPa) with previous approaches used to evaluate pressureestimates in MORB. However, the determination of pre-eruptiveconditions of MORBs, including temperature and water contentin addition to pressure, requires the improvement of magma crystallizationmodels to simulate liquid lines of descent in the presence ofsmall amounts of water. KEY WORDS: MORB; Mid-Atlantic Ridge; depth of crystallization; water abundances; phase equilibria calculations; cotectic crystallization; pressure estimates; polybaric fractionation 相似文献
28.
Pressures of Crystallization of Icelandic Magmas 总被引:1,自引:0,他引:1
Iceland lies astride the Mid-Atlantic Ridge and was createdby seafloor spreading that began about 55 Ma. The crust is anomalouslythick (20–40 km), indicating higher melt productivityin the underlying mantle compared with normal ridge segmentsas a result of the presence of a mantle plume or upwelling centeredbeneath the northwestern edge of the Vatnajökull ice sheet.Seismic and volcanic activity is concentrated in 50 km wideneovolcanic or rift zones, which mark the subaerial Mid-AtlanticRidge, and in three flank zones. Geodetic and geophysical studiesprovide evidence for magma chambers located over a range ofdepths (1·5–21 km) in the crust, with shallow magmachambers beneath some volcanic centers (Katla, Grimsvötn,Eyjafjallajökull), and both shallow and deep chambers beneathothers (e.g. Krafla and Askja). We have compiled analyses ofbasalt glass with geochemical characteristics indicating crystallizationof ol–plag–cpx from 28 volcanic centers in the Western,Northern and Eastern rift zones as well as from the SouthernFlank Zone. Pressures of crystallization were calculated forthese glasses, and confirm that Icelandic magmas crystallizeover a wide range of pressures (0·001 to 1 GPa), equivalentto depths of 0–35 km. This range partly reflects crystallizationof melts en route to the surface, probably in dikes and conduits,after they leave intracrustal chambers. We find no evidencefor a shallow chamber beneath Katla, which probably indicatesthat the shallow chamber identified in other studies containssilica-rich magma rather than basalt. There is reasonably goodcorrelation between the depths of deep chambers (> 17 km)and geophysical estimates of Moho depth, indicating that magmaponds at the crust–mantle boundary. Shallow chambers (<7·1 km) are located in the upper crust, and probablyform at a level of neutral buoyancy. There are also discretechambers at intermediate depths (11 km beneath the rift zones),and there is strong evidence for cooling and crystallizing magmabodies or pockets throughout the middle and lower crust thatmight resemble a crystal mush. The results suggest that themiddle and lower crust is relatively hot and porous. It is suggestedthat crustal accretion occurs over a range of depths similarto those in recent models for accretionary processes at mid-oceanridges. The presence of multiple stacked chambers and hot, porouscrust suggests that magma evolution is complex and involvespolybaric crystallization, magma mixing, and assimilation. KEY WORDS: Iceland rift zones; cotectic crystallization; pressure; depth; magma chamber; volcanic glass 相似文献
29.
Serena C. Tarantino Michele Zema Tiziana Boffa Ballaran Paolo Ghigna 《Physics and Chemistry of Minerals》2008,35(2):71-76
High-pressure single-crystal X-ray diffraction measurements of lattice parameters of the compound Li2VOSiO4, which crystallises with a natisite-type structure, has been carried out to a pressure of 8.54(5) GPa at room temperature.
Unit-cell volume data were fitted with a second-order Birch-Murnaghan EoS (BM-EoS), simultaneously refining V
0 and K
0 using the data weighted by the uncertainties in V. The bulk modulus is K
0 = 99(1) GPa, with K′ fixed to 4. Refinements of third order equations-of-state yielded values of K′ that did not differ significantly from 4. The compressibility of the unit-cell is strongly anisotropic with the c axis (K
0(c) = 49.7 ± 0.5 GPa) approximately four times more compressible than the a axis (K
0(a) = 195 ± 3 GPa). 相似文献
30.
Landslides pose a serious physical and environmental threat to vulnerable communities living in areas of unplanned housing
on steep slopes in the Caribbean. Some of these communities have, in the past, had to be relocated, at costs of millions of
dollars, because of major slides triggered by tropical storm rainfall. Even so, evidence shows that: (1) risk reduction is
a marginal activity; (2) there has been minimal uptake of hazard maps and vulnerability assessments and (3) there is little
on-the-ground delivery of construction for risk reduction. This article directly addresses these issues by developing a low-cost
approach to the identification of the potential pore pressure changes that trigger such slides we seek to address these three
commentaries directly. A complex 45–60° slope site in St Lucia, West Indies was selected as a pilot for a modelling approach
that uses numerical models (FLAC and CHASM) to verify the need for surface water management to effectively reduce landslide
risk. Following the model confirmation, a series of drains were designed and constructed at the site. Post-construction evidence
indicates the methodology to be sound, in that the site was stable in subsequent 1-in-1 to 1-in-4 year rainfall events. A
critical feature of the approach is that it is community-based from data acquisition through to community members participating
in construction. 相似文献