New trace-element data of rutile in kimberlite-borne ~1.85 Ga eclogite and pyroxenite xenoliths from the central Slave craton,
as well as ~110 Ma MARID xenoliths from the Kaapvaal craton, provide constraints on the origins of lithospheric and sublithospheric
mantle variability in high field strength element ratios. Rutiles in eclogites and pyroxenites have Zr/Hf ranging from 20
to 62 and Nb/Ta ranging from 10 to 40. Rutiles in MARID xenoliths have Zr/Hf from 24 to 33 and Nb/Ta from 10 to 41. Calculated
whole-rock Zr/Hf is suprachondritic for eclogites with suggested gabbroic protoliths and subchondritic for boninite-like eclogites;
the latter is consistent with cpx-controlled depletion in the protolith source. Within each eclogite type, positive correlations
of Zr/Hf with La/Lu and negative correlations with Lu/Hf likely reflect fractionation of cpx and/or plagioclase during crystallisation
of the protoliths. Zr/Hf–Nb/Ta relationships of some MARID-type rocks, which are products of lithospheric mantle metasomatism,
and eclogite xenoliths plot on a silicate differentiation trend, whereas other samples have higher Nb/Ta at a given Zr/Hf.
Fractionation of a few percent rutile from an HFSE-rich mafic melt can generate a trend towards strongly increased Nb/Ta at
minimally changed Zr/Hf in the residual melt. Superposition of rutile fractionation on the effects of silicate differentiation,
which fractionates Zr/Hf more strongly than Nb/Ta, can explain the Zr/Hf–Nb/Ta relationships of most eclogites from the central
Slave craton as well as those of MARID rocks, metasomatised peridotites and group II kimberlites. By contrast, Zr/Hf–Nb/Ta
relationships suggest that Group I kimberlites are mixtures between depleted peridotite and carbonatite. Thus, high Nb/Ta
is a signature of lithospheric processes and may not be important in deeply subducted eclogites that bypass extended residence
in the lithosphere. Conversely, considerable primary Zr/Hf variability was inherited by the eclogites, which is indicative
of the compositional diversity of ancient subducted oceanic crust, which is expected to have generated substantial heterogeneity
in sublithospheric basalt sources. 相似文献
Measurements were made of sulphur and oxygen isotope ratios of sulphate in some Slovenian rivers, lakes and tap waters. δ34S ranged from −0.2 to + 13.3‰, δ18O ranged from +4.9 to + 13.6‰, and the sulphate content varied from 0.8 to 41.4 mg/L. Rivers flowing from the Julian Alps
contain a very low amount of sulphate that is leached from a thin horizon of soil by rain. As confirmed by their low δ18O values, these sulphates do not enter the rivers directly in rain, but arise from biochemical cycling in the soil. The low
δ34S of this sulphate indicates that it originates from the oxidation of sedimentary sulphides. The evolution of sulphates along
the river course was investigated for the Sava and Ljubljanica rivers. The variations observed in sulphate from the waters
studied result from variations in the contribution of sulphates of different origin. Downstream the Sava River sulphate is
depleted in the heavy isotopes of both sulphur and oxygen, with δ-values gradually tending toward the δ-values of groundwater
sulphates in the watershed. In contrast, the δ-values of sulphate in the Ljubljanica River are almost constant and similar
to those of sulphate in local groundwater. Introduction of water from Italian and Slovenian mines was recorded in the Soča
River, where the lowest δ34S value of sulphate sulphur (−0.2‰) was observed. In addition, the influence of sulphate from the oxidation of sedimentary
sulphides was recorded in the Sotla River. No evidence was found for introduction of sulphate from factories. 相似文献
On a global scale, peridotitic garnet inclusions in diamonds from the subcratonic lithosphere indicate an evolution from strongly sinusoidal REEN, typical for harzburgitic garnets, to mildly sinusoidal or “normal” patterns (positive slope from LREEN to MREEN, fairly flat MREEN–HREEN), typical for lherzolitic garnets. Using the Cr-number of garnet as a proxy for the bulk rock major element composition it becomes apparent that strong LREE enrichment in garnet is restricted to highly depleted lithologies, whereas flat or positive LREE–MREE slopes are limited to less depleted rocks. For lherzolitic garnet inclusions, there is a positive relation between equilibration temperature, enrichment in MREE, HREE and other HFSE (Ti, Zr, Y), and decreasing depletion in major elements. For harzburgitic garnets, relations are not linear, but it appears that lherzolite style enrichment in MREE–HREE only occurs at temperatures above 1150–1200 °C, whereas strong enrichment in Sr is absent at these high temperatures. These observations suggest a transition from melt metasomatism (typical for the lherzolitic sources) characterized by fairly unfractionated trace and major element compositions to metasomatism by CHO fluids carrying primarily incompatible trace elements. Melt and fluid metasomatism are viewed as a compositional continuum, with residual CHO fluids resulting from primary silicate or carbonate melts in the course of fractional crystallization and equilibration with lithospheric host rocks.
Eclogitic garnet inclusions show “normal” REEN patterns, with LREE at about 1× and HREE at about 30× chondritic abundance. Clinopyroxenes approximately mirror the garnet patterns, being enriched in LREE and having chondritic HREE abundances. Positive and negative Eu anomalies are observed for both garnet and clinopyroxene inclusions. Such anomalies are strong evidence for crustal precursors for the eclogitic diamond sources. The trace element composition of an “average eclogitic diamond source” based on garnet and clinopyroxene inclusions is consistent with derivation from former oceanic crust that lost about 10% of a partial melt in the garnet stability field and that subsequently experienced only minor reenrichment in the most incompatible trace elements. Based on individual diamonds, this simplistic picture becomes more complex, with evidence for both strong enrichment and depletion in LREE.
Trace element data for sublithospheric inclusions in diamonds are less abundant. REE in majoritic garnets indicate source compositions that range from being similar to lithospheric eclogitic sources to strongly LREE enriched. Lower mantle sources, assessed based on CaSi–perovskite as the principal host for REE, are not primitive in composition but show moderate to strong LREE enrichment. The bulk rock LREEN–HREEN slope cannot be determined from CaSi–perovskites alone, as garnet may be present in these shallow lower mantle sources and then would act as an important host for HREE. Positive and negative Eu anomalies are widespread in CaSi–perovskites and negative anomalies have also been observed for a majoritic garnet and a coexisting clinopyroxene inclusion. This suggests that sublithospheric diamond sources may be linked to old oceanic slabs, possibly because only former crustal rocks can provide the redox gradients necessary for diamond precipitation in an otherwise reduced sublithospheric mantle. 相似文献
Measurements of seismic anisotropy in fractured rock are used at present to deduce information about the fracture orientation and the spatial distribution of fracture intensity. Analysis of the data is based upon equivalent-medium theories that describe the elastic response of a rock containing cracks or fractures in the long-wavelength limit. Conventional models assume frequency independence and cannot distinguish between microcracks and macrofractures. The latter, however, control the fluid flow in many subsurface reservoirs. Therefore, the fracture size is essential information for reservoir engineers. In this study we apply a new equivalent-medium theory that models frequency-dependent anisotropy and is sensitive to the length scale of fractures. The model considers velocity dispersion and attenuation due to a squirt-flow mechanism at two different scales: the grain scale (microcracks and equant matrix porosity) and formation-scale fractures. The theory is first tested and calibrated against published laboratory data. Then we present the analysis and modelling of frequency-dependent shear-wave splitting in multicomponent VSP data from a tight gas reservoir. We invert for fracture density and fracture size from the frequency dependence of the time delay between split shear waves. The derived fracture length matches independent observations from borehole data. 相似文献
Debates about implementing `sustainable development', `sustainability' or even `sustainable futures and resource use' are now engaging most intergovernmental and national bureaucracies. The notion itself appears to have replaced or become confused with efforts specifically directed at environmental protection, also a vague and contested notion. The origin, academic approaches and use of the idea is documented and discussed with reference to selected bureaucracies engaged in environmental `governance'. I ask why and with what impact these bureaucracies have adopted the term in the post cold-war era. The methodology used is political interest analysis: why have bureaucracies adopted the notion so eagerly since the later 1980s? I argue that the sustainability debate served to strengthen bureaucracy during an era when its powers were under attack. The debate promised to extend the role of bureaucracy from that of agent of policy implementation to that of significant political actor, required to select experts as well as futures. But to what purpose and in whose interest? In seeking `sustainability' have governments found more than a new tool for governing? I wish to stimulate theoretical and empirical investigations that replace the ubiquitous `policy-maker' of much academic research with an `actor' whose spatial nature, political roles and ambitions, as well as legal and resource limitations, are not ignored. 相似文献
Journal of Paleolimnology - The Tibetan Plateau, also known as the “Water Tower of Asia” because of its function as a water storage and supply region, responds dramatically to modern... 相似文献
Coastal wetlands present particular challenges for coastal governance and for the implementation of the Ramsar Convention, not least because coastal areas are focal points of human activity and of governance ambiguity. Through the evaluation of Ramsar delivery at both national and local levels in Japan and England, the relationship between Ramsar implementation and coastal governance was examined. In England, Ramsar status is primarily treated as a nature conservation designation which limits the wider opportunities inherent in the designation. In contrast, in Japan, the Ramsar Convention is used as a policy driver at the national level and as a leverage to encourage citizen engagement, economic benefit, and wetland conservation at the local level. It was concluded that through the implementation of the Ramsar Convention in important coastal wetland areas, significant steps can be taken towards delivering integrated approaches to coastal governance. 相似文献
Eutrophication is known to affect the community structure of macroalgae by e.g. decreasing the depth penetration of species and by shifting dominance from perennial to annual species. However, there is substantial lack of knowledge in the Baltic Sea regarding the distribution of many of the macroalgal species, how natural environmental factors affect their occurrence and how they respond to eutrophication. As macroalgae are used as indicators of the quality of the sea areas in the EU legislation (Water Framework Directive, Marine Strategy Framework Directive), this kind of knowledge is essential. The aim of this study was to determine which variables were related to variation in species occurrence and their lower limit of occurrence in the Finnish marine area. The study was carried out on data from five study areas along the Finnish coastline and included about 30 taxa. Our results showed that both the macroalgal communities and the occurrence (presence/absence) of most of the species differed between the study areas and that the differences were mainly related to salinity and exposure, although also eutrophication related factors played a role. Of the perennial species, eutrophied conditions seemed to favour only the occurrence of Sphacelaria arctica and Polysiphonia fucoides. Secchi depth was important in determining the lower limit of occurrence of brown and red algal species. However, Secchi depth was rarely the only factor causing variation in the lower limit of occurrence as also exposure, salinity and slope of the shore affected it. We conclude that in the northern Baltic Sea, the taxonomic composition of the macroalgal communities is not a very useful indicator of eutrophication as perennial species seem to tolerate rather eutrophied conditions, when suitable substrate is available. The lower limit of occurrence of many of the brown and red algal species is a good indicator of eutrophication but due to lack of suitable substrate in more eutrophied areas, especially in the depths where light becomes limiting, it is only applicable in the middle and outer archipelago areas. Furthermore, when planning monitoring programmes or setting thresholds for evaluating the ecological status of the sea, the natural variation in the lower limit of occurrence of macroalgae across sea areas is problematic and should carefully be taken into account. 相似文献