A detailed electron microprobe study has been conducted on natural and experimentally grown zoned plagioclase feldspars. Discontinuous, sector, and oscillatory chemical zoning are observed superimposed on continuous normal or reverse zoning trends. The relative accuracy of 3 element (Na, Ca, K) microprobe traverses was found statistically to be 2 mole percent. Comparison of microprobe data on natural zoned plagioclase with zoned plagioclase grown in controlled experiments has shown that it may be possible to distinguish zonal development resulting from physio-chemical changes to the bulk magma from zoning related to local kinetic control on the growth of individual crystals. 相似文献
Industrial plants that refine bauxite to alumina using the high temperature process have always held the belief that anatase was detrimental to the extraction of boehmite while rutile was not. This study shows that this effect is real and that it is observable at temperatures as low as 90 °C. The extraction of gibbsite is shown to be unaffected which leads us to believe that the kinetics of both the Ti-bearing mineral and the Al-bearing mineral is important in this phenomenon. In addition, it is shown that not only is the presence of anatase an issue in boehmite extraction but so too is the presence of sodium titanate. Rutile was found to have the least impact of the three mineral phases. 相似文献
Titanite growth at the expense of rutile during retrograde hydration of eclogite into amphibolite is a common phenomenon. We investigated an amphibolite sample from the Tromsø eclogite facies terrain in Northern Norway to gain insight into the trace element distribution between rutile and titanite during incomplete resorption of the rutile by titanite. Patchy compositional zoning of Al, Ti, and F in titanite relates to the presence of a fluid with variable Ti/Al and/or F during its growth. Laser ablation ICP–MS and electron microprobe data for high field strength elements (HFSE: Nb, Zr, Ta, and Hf) of rutile resorbed by titanite indicate a pronounced enrichment of these elements in the rim of a large single rutile crystal (~8 mm) and a systematic decrease towards uniform HFSE contents in the large core. HFSE contents of smaller rutile grains (~0.5 mm) and rutile inclusions (<100 μm) in the titanite overgrowth are similar or higher than in the rims of large rutile crystals. Element profiles from the rim inward demonstrate that HFSE enrichment in rutile is controlled by diffusion. HFSE ratios in diffusion-altered rutile show systematic variations compared with the uniform core composition of the large rutile. Modelling of Zr and Nb diffusion in rutile indicates that diffusion coefficients in rutile in fluid-dominated natural systems must be considerably higher than those determined experimentally at 1 bar in dry systems. Variations of HFSE contents in the newly formed titanite show no systematic spatial distribution. HFSE ratios in titanite and the rims of rutile are different, indicating different solid/fluid distribution coefficients in these minerals. Element fractionation by diffusion into the relict rutile and during fluid-mediated growth of new titanite could substantially change the HFSE budget of these minerals and could affect their use for geochemical tracing and other applications, such as Zr-based geothermobarometry. 相似文献
The pipe shapes, infill and emplacement processes of the Attawapiskat kimberlites, including Victor, contrast with most of the southern African kimberlite pipes. The Attawapiskat kimberlite pipes are formed by an overall two-stage process of (1) pipe excavation without the development of a diatreme (sensu stricto) and (2) subsequent pipe infilling. The Victor kimberlite comprises two adjacent but separate pipes, Victor South and Victor North. The pipes are infilled with two contrasting textural types of kimberlite: pyroclastic and hypabyssal-like kimberlite. Victor South and much of Victor North are composed of pyroclastic spinel carbonate kimberlites, the main features of which are similar: clast-supported, discrete macrocrystal and phenocrystal olivine grains, pyroclastic juvenile lapilli, mantle-derived xenocrysts and minor country rock xenoliths are set in serpentine and carbonate matrices. These partly bedded, juvenile lapilli-bearing olivine tuffs appear to have been formed by subaerial fire-fountaining airfall processes.
The Victor South pipe has a simple bowl-like shape that flares from just below the basal sandstone of the sediments that overlie the basement. The sandstone is a known aquifer, suggesting that the crater excavation process was possibly phreatomagmatic. In contrast, the pipe shape and internal geology of Victor North are more complex. The northwestern part of the pipe is dominated by dark competent rocks, which resemble fresh hypabyssal kimberlite, but have unusual textures and are closely associated with pyroclastic juvenile lapilli tuffs and country rock breccias±volcaniclastic kimberlite. Current evidence suggests that the hypabyssal-like kimberlite is, in fact, not intrusive and that the northwestern part of Victor North represents an early-formed crater infilled with contrasting extrusive kimberlites and associated breccias. The remaining, main part of Victor North consists of two macroscopically similar, but petrographically distinct, pyroclastic kimberlites that have contrasting macrodiamond sample grades. The juvenile lapilli of each pyroclastic kimberlite can be distinguished only microscopically. The nature and relative modal proportion of primary olivine phenocrysts in the juvenile lapilli are different, indicating that they derive from different magma pulses, or phases of kimberlite, and thus represent separate eruptions. The initial excavation of a crater cross-cutting the earlier northwestern crater was followed by emplacement of phase (i), a low-grade olivine phenocryst-rich pyroclastic kimberlite, and the subsequent eruption of phase (ii), a high-grade olivine phenocryst-poor pyroclastic kimberlite, as two separate vents nested within the original phase (i) crater. The second eruption was accompanied by the formation of an intermediate mixed zone with moderate grade. Thus, the final pyroclastic pipe infill of the main part of the Victor North pipe appears to consist of at least three geological/macrodiamond grade zones.
In conclusion, the Victor kimberlite was formed by several eruptive events resulting in adjacent and cross-cutting craters that were infilled with either pyroclastic kimberlite or hypabyssal-like kimberlite, which is now interpreted to be of probable extrusive origin. Within the pyroclastic kimberlites of Victor North, there are two nested vents, a feature seldom documented in kimberlites elsewhere. This study highlights the meaningful role of kimberlite petrography in the evaluation of diamond deposits and provides further insight into kimberlite emplacement and volcanism. 相似文献
Adaptation to climate variability and change is important both for impact assessment (to estimate adaptations which are likely to occur) and for policy development (to advise on or prescribe adaptations). This paper proposes an "anatomy of adaptation" to systematically specify and differentiate adaptations, based upon three questions: (i) adapt to what? (ii) who or what adapts? and (iii) how does adaptation occur? Climatic stimuli include changes in long-term mean conditions and variability about means, both current and future, and including extremes. Adaptation depends fundamentally on the characteristics of the system of interest, including its sensitivities and vulnerabilities. The nature of adaptation processes and forms can be distinguished by numerous attributes including timing, purposefulness, and effect. The paper notes the contribution of conceptual and numerical models and empirical studies to the understanding of adaptation, and outlines approaches to the normative evaluation of adaptation measures and strategies. 相似文献
This paper estimates changes in thepotential damage of flood events caused by increasesof CO2 concentration in the atmosphere. It ispresented in two parts: 1. the modelling of floodfrequency and magnitude under global warming andassociated rainfall intensities and 2. the use ofgreenhouse flood data to assess changes in thevulnerability of flood prone urban areas, expressingthese in terms of direct losses.Three case studies were selected: theHawkesbury–Nepean corridor, the Queanbeyan and UpperParramatta Rivers. All three catchments are located insoutheastern Australia, near Sydney and Canberra.These were chosen because each had detailed buildingdata bases available and the localities are situatedon rivers that vary in catchment size andcharacteristics. All fall within a region that willexperience similar climate change under the availablegreenhouse scenarios. The GCMs' slab model scenariosof climate change in 2030 and 2070 will cause onlyminor changes to urban flood damage but the doubleCO2 scenarios estimated using the StochasticWeather Generator technique will lead to significantincreases in building damage.For all the case studies, the hydrological modellingindicates that there will be increases in themagnitude and frequency of flood events under thedouble CO2 conditions although these vary fromplace to place. However, the overall pattern of changeis that for the Upper Parramatta River the 1 in 100-year flood under currentconditions becomes the 1 in44-year event, the 1 in 35-year flood for theHawkesbury–Nepean and the 1 in 10 for Queanbeyan andCanberra. This indicates the importance of usingrainfall-runoff modelling in order to estimate changesin flood frequencies in catchments with differentphysical characteristics. 相似文献
Most scenarios of greenhouse climate change are obtained from general circulation models. These provide poor information on changes to extreme events. It is therefore, difficult to convert changes of flood frequency into their impact on flood damages. The procedures for estimating urban flood losses are outlined. Australian case studies illustrate the possible effects of greenhouse-induced changes in comparison to the variability under current climate; changes in urban flood losses for small and large catchments; and the implications for dam design. In all cases, relatively small increases in flood frequency would cause significant increases in loss. The policy implications are outlined, it must not be assumed that the availability of more precise data on future flood frequencies will be matched by policy response in the field of flood plain management. 相似文献