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The oceanic serpentinization of peridotites and the influenceof such an alteration on element cycling during their subductiondewatering are here investigated in a mantle slice (Erro–Tobbioperidotite), first exposed to oceanic serpentinization and laterinvolved in alpine subduction, partial dewatering and formationof a high-pressure olivine + titanian-clinohumite + diopside+ antigorite assemblage in the peridotites and in veins. Previouswork indicates that high-pressure veins include primary brines,representing a residue after crystallization of the vein assemblageand containing recycled oceanic Cl and alkalis. To reconstructthe main changes during oceanic peridotite serpentinizationand subsequent subduction, we analysed samples in profiles fromserpentinized oceanic peridotites to high-pressure serpentinites,and from high-pressure ultramafites to veins. Here we presentresults indicating that the main features of the oceanic serpentinizationare immobility of rare earth elements (REE), considerable waterincrease, local CaO decrease and uptake of trace amounts ofSr, probably as a function of the intensity of alteration. Srentered fine-grained Ca phases associated with serpentine andchlorite. Trace-element analyses of mantle clinopyroxenes andhigh-pressure diopsides (in country ultramafites and veins),highlight the close similarity in the REE compositions of thevarious clinopyroxenes, thereby indicating rock control on thevein fluids and lack of exotic components carried by externallyderived fluids. Presence of appreciable Sr contents in vein-formingdiopside indicates cycling of oceanic Sr in the high-pressurefluid. This, together with the recognition of pre-subductionCl and alkalis in the vein fluid, indicates closed-system behaviourduring eclogitization and internal cycling of exogenic components.Diopside and Ti-clinohumite are the high-pressure minerals actingas repositories for REE and Sr, and for high field strengthelements (HFSE), respectively. The aqueous fluid equilibratedwith such an assemblage is enriched in Cl and alkaline elementsbut strongly depleted in REE and HFSE (less than chondrite abundances).Sr is low [(0·2–1·6) x chondrites], althoughselectively enriched relative to light REE. KEY WORDS: eclogite facies; fluid; trace elements; serpentinite; subduction 相似文献
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依据1960-2005年长江流域147个气象站逐日降水,ECHAM5/MPI-OM气候模式模拟的长江流域79个格点20世纪实验期(1941-2000年)以及未来3种排放情景(SRES-B1,A1B,A2)下21世纪前50年逐日降水数据,建立年最大强降水和汛期<1.27 mm/d的最长干旱持续天数序列。运用广义极值分布,广义帕雷托分布,广义逻辑分布与韦克比分布等4种分布函数定量拟合了长江流域降水极值的概率分布。研究表明:韦克比分布函数能够较好地拟合长江流域降水极值的概率分布。在3种排放情景下,未来降水极值的重现期呈现不同的空间分布特征。长江流域,尤其是中下游大部地区,1951-2000年间的50年一遇强降水和干旱事件,在2001-2050年间发展成为25年一遇降水极值事件。未来气候变暖条件下,降水极值重现期出现的这种变化趋势,将会对水资源趋势产生重大的影响。 相似文献
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By FEDERICO MAGNANI RODERICK C. DEWAR † MARCO BORGHETTI 《Tellus. Series B, Chemical and physical meteorology》2009,61(2):385-393
Leakage (spillover) refers to the unintended negative (positive) consequences of forest carbon (C) management in one area on C storage elsewhere. For example, the local C storage benefit of less intensive harvesting in one area may be offset, partly or completely, by intensified harvesting elsewhere in order to meet global timber demand. We present the results of a theoretical study aimed at identifying the key factors determining leakage and spillover, as a prerequisite for more realistic numerical studies. We use a simple model of C storage in managed forest ecosystems and their wood products to derive approximate analytical expressions for the leakage induced by decreasing the harvesting frequency of existing forest, and the spillover induced by establishing new plantations, assuming a fixed total wood production from local and remote (non-local) forests combined. We find that leakage and spillover depend crucially on the growth rates, wood product lifetimes and woody litter decomposition rates of local and remote forests. In particular, our results reveal critical thresholds for leakage and spillover, beyond which effects of forest management on remote C storage exceed local effects. Order of magnitude estimates of leakage indicate its potential importance at global scales. 相似文献
14.
Deep-water clastic evaporites deposition in the Messinian Adriatic foredeep (northern Apennines,Italy): did the Mediterranean ever dry out? 总被引：1，自引：0，他引：1
A new genetic facies model for deep-water clastic evaporites is presented, based on work carried out on the Messinian Gessoso-solfifera Formation of the northern Apennines during the last 15 years. This model is derived from the most recent siliciclastic turbidite models and describes the downcurrent transformations of a parent flow mainly composed of gypsum clasts. The model allows clearer comprehension of processes controlling the production and deposition of clastic evaporites, representing the most common evaporite facies of the northern Apennines, and the definition of the genetic and stratigraphic relationship with primary shallow-water evaporites formed and preserved in marginal settings. Due to the severe recrystallization processes usually affecting these deposits, petrographic and geochemical analyses are needed for a more accurate interpretation of the large spectrum of recognized gravity-driven deposits ranging from debrisflow to low-density turbidites. Almost all the laminar ‘balatino’ gypsum, previously considered a deep-water primary deposit, is here reinterpreted as the fine-grained product of high to low-density gravity flows. Facies associations permit the framing of the distribution of clastic evaporites into the complex tectonically controlled depositional settings of the Apennine foredeep basin. The Messinian Salinity Crisis occurred during an intense phase of geodynamic reorganization of the Mediterranean area that also produced the fragmentation of the former Miocene Apennine foredeep basin. In this area, primary shallow-water evaporites equivalent to the Mediterranean Lower Evaporites, apparently only formed in semi-closed thrust-top basins like the Vena del Gesso Basin. The subsequent uplift and subaerial exposure of such basins ended the evaporite precipitation and promoted a widespread phase of collapse leading to the resedimentation of the evaporites into deeper basins. Vertical facies sequences of clastic evaporites can be interpreted in terms of the complex interplay between the Messinian tectonic evolution of the Apennine thrust belt and related exhumation–erosional processes. The facies model here proposed could be helpful also for better comprehension of other different depositional and geodynamic contexts; the importance of clastic evaporites deposits has been overlooked in the study of other Mediterranean areas. Based on the Apennine basins experience, it is suggested here that evaporites diffused into the deeper portions of the Mediterranean basin may consist mainly of deep-water resedimented deposits rather than shallow-water to supratidal primary evaporites indicative of a complete basin desiccation. 相似文献
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ANDREOLI MARCO A. G.; HART RODGER J.; ASHWAL LEWIS D.; COETZEE HENK 《Journal of Petrology》2006,47(6):1095-1118
The digital image of airborne radiometric data across SouthAfrica reveals that the largest anomaly, 100 nGy/h, is causedby the granulite-facies rocks of the Namaquan metamorphic complex,whereas most of the country is <60 nGy/h. This observationis consistent with geochemical data that show that the 1900± 100 Ma greenschist-facies Richtersveld Terrane nearNamibia (max. U = 3·4 ppm; Th = 20·1 ppm) andthe adjacent, 1100 ± 100 Ma, amphibolite-facies Aggeneys/SteinkopfTerranes (max. U 10 ppm; Th 52 ppm) are the least enrichedin U, Th and K. In contrast, the lower-T granulite-facies OkiepTerrane near Springbok hosts more enriched granites (max. U 17 ppm; Th 66 ppm) and noritic intrusions (max. U = 14 ppm;Th = 83 ppm). The most enriched rocks are found in the 1030Ma higher-T granulite-facies core of the Namaquan belt and includequartzo-feldspathic gneisses (max. U = 46 ppm; Th = 90 ppm)and charnockites (max. U = 52 ppm; Th = 400 ppm). Our findingscontradict the notion that granulite-facies terrains are characteristicallydepleted in U and Th. In this study we modeled the heat productionin the core of the Namaquan complex, where the granulites havehad a very unusual metamorphic history, and show that ultra-high-T(1000°C, P 10 kbar) metamorphic conditions could have beenachieved by radiogenic heating without invoking external heatsources. However, monazite-rich veins of charnockite and patchesof granulites mark the passage of CO2-dominated melts and fluidsderived from fractionated noritic intrusions. KEY WORDS: charnockite; granulite; Namaqualand; thorium; uranium; radioactive heating; metamorphism 相似文献
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A great deal of attention has been given to the production and spatial distribution of sodium in comets after the discovery
of the sodium tail, by Cremonese et al. (1997a), on Hale-Bopp. The sodium has been observed in several comets in the past,
but the Hale-Bopp represent the first time where it will be deeply analyzed considering the several data and scientists working
on that. The sodium tail stimulated different studies trying to explain the mechanism source and provided the new lifetime
for photoionization of the neutral sodium atom. We took into account other sodium observations performed in this century and
we focalized our attention to comet Hale-Bopp to understand the main sources responsible for the sodium features observed.
We analyzed the sodium tail observations performed by Cremonese et al. (1997b) and Wilson et al. (1998) finding that the Hale-Bopp
had four different tails. The wide field images and the high resolution spectroscopy performed along the sodium tail provided
very important clues to distinguish the two sodium tails observed and their possible sources. Considering most of the data
reported in several papers has been possible to draw a real sketch on what has occurred to the comet during March and April
1997. We are going to demonstrate that the sodium tail observed by Wilson et al. (1998) was not the same reportedby Cremonese
et al. (1997a) and in the images taken by the European Hale-Bopp Team there were two distinct sodium tails. The observations
allowed us to define “narrow sodium tail” the tail reported by Cremonese et al. (1997a), and “diffuse sodium tail” the tail
overimposed to the dust tail. We suggest that the narrow sodium tail was due to a molecular process instead of the diffuse
one due to the release of sodium atoms by the dust particles. Such a conclusion is supported by the spatial distribution of
sodium on the nucleus and in the coma as reported from other authors.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献