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891.
Ben?E.?MandlerEmail author Julie?M.?Donnelly-Nolan Timothy?L.?Grove 《Contributions to Mineralogy and Petrology》2014,168(4):1066
Melting experiments have been performed at 1 bar (anhydrous) and 1- and 2-kbar H2O-saturated conditions to study the effect of water on the differentiation of a basaltic andesite. The starting material was a mafic pumice from the compositionally zoned tuff deposited during the ~75 ka caldera-forming eruption of Newberry Volcano, a rear-arc volcanic center in the central Oregon Cascades. Pumices in the tuff of Newberry caldera (TNC) span a continuous silica range from 53 to 74 wt% and feature an unusually high-Na2O content of 6.5 wt% at 67 wt% SiO2. This wide range of magmatic compositions erupted in a single event makes the TNC an excellent natural laboratory in which to study the conditions of magmatic differentiation. Our experimental results and mineral–melt hygrometers/thermometers yield similar estimates of pre-eruptive H2O contents and temperatures of the TNC liquids. The most primitive (mafic) basaltic andesites record a pre-eruptive H2O content of 1.5 wt% and a liquidus temperature of 1,060–1,070 °C at upper crustal pressure. This modest H2O content produces a distinctive fractionation trend that is much more enriched in Na, Fe, and Ti than the calc-alkaline trend typical of wetter arc magmas, but slightly less enriched in Fe and Ti than the tholeiitic trend of dry magmas. Modest H2O contents might be expected at Newberry Volcano given its location in the Cascade rear arc, and the same fractionation trend is also observed in the rim andesites of the rear-arc Medicine Lake volcano in the southern Cascades. However, the Na–Fe–Ti enrichment characteristic of modest H2O (1–2 wt%) is also observed to the west of Newberry in magmas erupted from the arc axis, such as the Shevlin Park Tuff and several lava flows from the Three Sisters. This shows that modest-H2O magmas are being generated directly beneath the arc axis as well as in the rear arc. Because liquid lines of descent are particularly sensitive to water content in the range of 0–3 wt% H2O, they provide a quantitative and reliable tool for precisely determining pre-eruptive H2O content using major-element data from pumices or lava flows. Coupled enrichment in Na, Fe, and Ti relative to the calc-alkaline trend is a general feature of fractional crystallization in the presence of modest amounts of H2O, which may be used to look for “damp” fractionation sequences elsewhere. 相似文献
892.
Hydrogeological and mixing process of waters in deep aquifers in arid regions: south east Tunisia 总被引:1,自引:0,他引:1
Jalila Makni Fatma Ben Brahim Saoussan Hassine Salem Bouri Hamed Ben Dhia 《Arabian Journal of Geosciences》2014,7(2):799-809
This study addresses the hydrogeochemistry of thermal and cold waters from south east Tunisia. Temperature intervals are 38.5–68 °C and 22–27.8 °C for thermal water and cold water, respectively. Three distinct hydrogeological systems supply water either for irrigation or for drinking; they are: (1) the Continental Intercalaire geothermal aquifer (CI), (2) the Turonian aquifer and (3) the Senonian aquifer. A synthetic study including hydrochemical, hydrogeological and geothermal approaches have been applied in order to evaluate the inter-aquifers water transfer in south east of Tunisia. By using silica geothermometers and saturation indices for different solid phases, estimated thermal reservoir temperature varies between 52 and 87 °C and between 75 and 110 °C, respectively. Based on chemical and thermal data, mixing, which occurs between the ascending deep geothermal water and shallow cold water, is about 57 % cold water. 相似文献
893.
Mohsen Layeb Moez Ben Fadhel Yosra Layeb-Tounsi Mohamed Ben Youssef 《Arabian Journal of Geosciences》2014,7(8):3349-3363
From northern Tunisia, small-scale well-preserved microbialites, contemporaneous to the global oceanic anoxic event 2 (OAE 2) are first reported on the southern Tethyan Margin. These microbialites are encased within the pelagic organic-rich black shales of the Bahloul Formation (Cenomanian–Turonian transition). Biostratigraphic, petrographic, and geochemical investigations carried out to constrain their biogenicity and genesis character led to consider them as thrombolites and stromatolites occurring in lenticular bioherms/biostromes and columnar bodies co-relatable to the global ‘filament event’ of the authors, close to the base of the Watinoceras ammonite zone. Abundant clotted micrite, cyanobacterial filaments, and algal tissues point to the key process of microbial carbonate precipitation and to a major role played by microbes in the stabilisation and subsequent lithification, which in turn favoured the preservation of the original structure of the microbialites. These microbially induced carbonate formations are considered as favoured by chemosynthetic fauna of bivalve molluscs and lithistid sponges which were able to host symbiotic microbial communities. The latter contributed to the precipitation of authigenic calcite and non-carbonate minerals (e.g. pyrite) fuelled by microbial activity under sulphate-reducing conditions. The carbonate body onset is considered to be initiated by seafloor instability due to syndepositional fault acting that induced the appraisal of uplifted tilted blocks within oxygenated waters but near the anoxic water masses. Generated depressions allowed the development of chemosynthetic-based communities. Deep faults related to Triassic salt domes acted as conducts for hydrocarbon and salt brine expulsion to the seafloor and the microbialite growth was enhanced by an abrupt uprising sea level under warmer conditions. 相似文献
894.
本文对产于贵州兴义乌沙地区上三叠统卡尼阶法郎组瓦窑段中的17件鱼龙标本进行研究,认为本次研究标本均为Qianichthyosaurus zhoui。通过对其肢骨的定性及定量分析认为,Qianichthyosaurus zhoui在后肢形态上存在性双形现象。暨雄性后肢外形窄长,雌性后肢则相对宽短;雄性的股骨比肱骨长度略长,而雌性的股骨与肱骨长度相当;雄性股骨近端和远端相较于雌性更为膨大股骨远端宽度为中轴宽度的253倍,股骨长与股骨中轴的比值较大,通常大于35;雌性股骨近端和远端相较于雄性较小股骨远端宽度一般为中轴宽度的2倍左右,股骨长与股骨中轴的比例较小,比值约为3左右雌性第一跖骨面积较小且具有凹缺,雄性第一跖骨面积较大不具凹缺。 相似文献
895.
城市化与人类生存环境之间的矛盾日益突出,准确、快速、定量的监测城市生态环境质量已成为客观了解城市生态状况的一个研究重点.利用遥感影像反演土地覆被指数、地表温度、地表裸露度和湿度等自然生态参数,构建城市生态环境质量评价的指数体系,并将之运用于德阳市城市建成区生态环境定量评价.在此基础上,利用均值-标准差对研究区生态环境进... 相似文献
896.
According to the norms adopted by the National Network of observation of the marine life quality (RNO 1997) and US National Oceanic and Atmospheric Administration (US NOAA 1999), the superficial sediments of the Bizerte lagoon (northern Tunisia) show a contamination by nickel, which is the probable
consequence of the closeness to the industrial zone, and by lead, the result of detrital material rich in lead of the geological
lands of Jbel Kchabta and Jbel Messeftine. In the lagoon southern and eastern sectors, the contamination of superficial sediments
is essentially by copper, zinc, nickel and iron. This pollution is related, on one hand, to the rejections of the steelworks
El Fouledh and the military arsenal, and on the other hand, to Menzel Bourguiba waste disposal. The rest of the lagoon is
exempted from any metallic contamination of the surface sediments. The enriched zones in manganese and zinc correspond to
the higher contents in kaolinite and in smectite. Positive correlations were recorded between the percentage in fine fraction
on the one hand, and the sedimentary contents in TOC, total nitrogen and heavy metals on the other hand. The majority of the
analysed elements are “argilophilous” except zinc and manganese, which are clayey organophile. These results show that this
lagoon receives concentrations in anthropogenic metals that risk provoking more or less important disruptions, which are harmful
and irreversible on the fauna and flora of this lagoon and on the whole ecobiological equilibrium. 相似文献
897.
Amateur decision-making in avalanche terrain with and without a decision aid: a stated choice survey
Avalanches pose a serious threat to recreational backcountry travelers in mountainous terrain. This study explores how the
three main amateur user groups of avalanche terrain in western Canada (backcountry skiers, out-of-bound skiers, and snowmobile
riders) balance recreational goals with safety concerns when choosing backcountry destinations under varying avalanche conditions.
Using a discrete choice experiment (DCE), a stated preference technique, the study first examines the strengths and weaknesses
in the decision process of the three amateur groups by comparing their responses with the choice patterns of professional
mountain guides. The results show that the decision-making strategies employed by the respective amateur groups vary considerably
in their level of complexity and the degree to which avalanche safety considerations are incorporated. Second, we examine
the effects of a decision aid that preprocesses the most crucial pieces of avalanche hazard information on the decision preferences
of the amateur groups in the DCE. The results show that a relatively simple decision aid can influence the decision-making
process considerably and steer users towards more avalanche hazard sensitive behaviour. 相似文献
898.
The intention of this study is to locate the probable sources of the Kafrein dam seepage problem. A methodology known as excitation-response analysis will be considered. Analytical expressions, depicting the groundwater level response in the underlying aquifer as a result of changes in the reservoir water level, were derived from a linearized groundwater flow equation for one-dimensional semi-infinite, isotropic and homogeneous porous media. The results revealed that a large proportion of the seepage water originates from a site at a distance about 1 km from the observation point M1 and streamlines along the existing faults. Emphasis in this work was also placed on controling and mitigating the seepage problem. 相似文献
899.
David Ward Ben T. Ngairorue Andr Apollus Hermanus Tjiveze 《Journal of Arid Environments》2000,45(4):337
We examined the perceptions and realities of land degradation in a communal ranching area, Otjimbingwe, in arid Namibia (in south-western Africa). It is commonly perceived that large-scale degradation of Otjimbingwe has occurred due to a mixture of improper pastoral practices and pressures induced by a high human population growth rate. We sought to determine whether the inhabitants perceived land degradation to have taken place and whether their perceptions were consistent with empirical data on environmental quality. Furthermore, we wished to determine whether these pastoralists had management strategies to help them withstand the harsh environmental conditions in which they live. All respondents in our surveys perceived that the environment had become degraded. The claimed source of this degradation, a decline in annual rainfall, is inconsistent with long-term rainfall records (there was neither change nor cyclicity in rainfall over time). There is also little evidence of a decline in plant cover and soil quality in spite of the very high stocking densities. No overall pastoral strategy exists in Otjimbingwe. Options for management are extremely limited due to a variety of external and internal pressures such as a high human population growth rate, high immigration into Otjimbingwe, restricted water availability due to dams constructed upstream, and limited movement opportunities for livestock in drought periods. 相似文献
900.