Experimental constraints on major and trace element partitioning during partial melting of eclogite   总被引：6，自引：0，他引：6  

Stephan Klemme  Jonathan D. BlundyBernard J. Wood 《Geochimica et cosmochimica acta》2002,66(17):3109-3123

Isobaric partial melting experiments were performed on an Fe-free synthetic composition to simulate partial melting of subducted oceanic crust. Nominally anhydrous experiments at 3.0 GPa yielded melts in equilibrium with garnet (13 to 16 mol.% grossular) and aluminous clinopyroxene (14 to 16 wt.% Al₂O₃). Melt compositions show decreasing Si and alkalis and increasing Ca, Mg, and Ti contents with increasing temperatures. Experiments at 1200 and 1300°C were rutile saturated, whereas experiments at 1400°C contained no residual rutile. We argue that during the initial stages of subduction, accessory rutile is likely to be stable in subsolidus eclogites of average midocean ridge basalt composition and that only large degrees of partial melting will eradicate rutile from an eclogitic source. At 3 GPa, any eclogites with a bulk TiO₂ content of ≥1.5 wt.% rutile will produce rutile-saturated partial melts, except at very high degrees of melting. At higher pressures, all bulk Ti may dissolve in clinopyroxene and garnet, leaving no accessory rutile.Trace element partition coefficients for 24 trace elements between clinopyroxene, garnet, and melt were determined by secondary-ion mass spectrometry analysis of experimental run products at 1400°C and 3 GPa. Partition coefficients for the rare earth elements agree well with previous studies and have been evaluated using the lattice strain model. Partitioning data for high-field strength elements indicate complementary D_Zr/D_Hf for clinopyroxene and garnet. Partial melting of an eclogitic component of different modal compositions may therefore explain both subchondritic and superchondritic Zr/Hf ratios. Superchondritic Zr/Hf has recently been observed in some ocean island basalts (OIB), and this may be taken as further evidence for components of recycled oceanic crust in OIB. The data also indicate slight Nb/Ta fractionation during partial melting of bimineralic eclogite, which is not, however, sufficient to explain some recently observed Nb/Ta fractionation in island arc rocks. Accessory rutile, however, can explain such fractionation.  相似文献   

Unsaturated zones as archives of past climates: toward a new proxy for continental regions   总被引：3，自引：1，他引：3  

W. Edmunds  S. Tyler 《Hydrogeology Journal》2002,10(1):216-228

Natural chemical and isotopic tracers contained in unsaturated-zone moisture profiles are being developed as potential new archives for reconstructing recharge history, as well as palaeoclimatic or palaeobotanical conditions over time scales ranging from 20–120,000 years. Results worldwide to date are reviewed, and examples from northern Africa and the western USA are discussed in detail. Encouraging results are obtained from relatively homogeneous deposits such as Quaternary dune sands, where Cl profiles are compared both with the instrumental record, such as rainfall and river-gauging records, and ³H profiles. Model studies have helped to define the persistence time of unsaturated-zone signals, where evidence of a 20-year event such as the Sahel drought may persist for 1,000 years. Significant questions remain regarding the assumptions used in interpreting profiles, particularly the extent to which preferential flow may occur, transient flow phenomena, and stability of tracer-input function. Unsaturated zones that exhibit strong preferential flow are probably unsuitable as archives. Questions remain also on the assumption that flow remains downward, especially in deep unsaturated zones where non-isothermal vapour transport may occur. Estimation of depositional flux for Cl and other parameters is probably the greatest source of uncertainty, both at the spatial scale and also in the long term. Advances are being made in all areas, however, and the use of multiple tracers (chemical, especially Cl and NO₃) and isotopic signals (δ¹⁸O, δ²H, ³⁶Cl), together with soil hydraulic properties, is promising for palaeohydrological reconstruction. Electronic Publication  相似文献   

Os isotopes in mantle xenoliths from the Eifel volcanic field and the Vogelsberg (Germany): age constraints on the lithospheric mantle     

Gerhard Schmidt  Jonathan Snow 《Contributions to Mineralogy and Petrology》2002,143(6):694-705

The effects of mantle metasomatism on the sulfide phase in mantle xenoliths in general, and on the Os isotopic system in particular, have received increased attention in recent years. Here, we report on Os isotopic systematics of metasomatized mantle xenoliths from the late Quaternary Eifel (Dreiser Weiher and Meerfelder Maar) and neighboring Vogelsberg volcanic fields, which provide insight into the effects of melt extraction and metasomatism on Os isotopes and place constraints on the evolution of the lithospheric mantle component beneath central Europe. Sixteen harzburgite, lherzolite, and pyroxenite xenoliths from the Eifel and two lherzolite xenoliths from the Vogelsberg were analyzed for Os isotopes. Samples from the anhydrous peridotite suite (Ib) are highly variable in their Os isotopes, ranging from subchondritic values (¹⁸⁷Os/¹⁸⁸Os=0.1236) to suprachondritic values (¹⁸⁷Os/¹⁸⁸Os=0.1420), indicating that some of these samples have been overprinted by the addition of radiogenic Os and have lost the primary mantle Os that was presumably present. The suprachondritic values suggest a source for this Os in a reservoir with a time-integrated Re/Os ratio greater than that of the bulk Earth. Eifel samples with Os contents >1.5 ng/g from the hydrous suite (Ia) have relatively unradiogenic Os isotope compositions (¹⁸⁷Os/¹⁸⁸Os=0.1208-0.1237) and Al₂O₃-Os isotopic systematics consistent with ancient melt depletion and isolation from the convecting asthenospheric mantle for time periods similar to the age of the overlying crust (~1.5 Ga) as well with results from peridotite massifs in the European region. The LREE-metasomatism and the enrichment of Os (up to 6.47 ng/g) and As (sulfide metasomatism?) in the hydrous suite is strongly inversely correlated with the Os isotope ratios, demonstrating that mantle processes such as metasomatism can significantly modify the Os isotope chemistry of mantle xenoliths.  相似文献   

Partitioning of fluorine and chlorine between biotite and granitic melt: experimental calibration at 200 MPa H2O     

Jonathan P. Icenhower  D. London 《Contributions to Mineralogy and Petrology》1997,127(1-2):17-29

Experiments from 640 to 680?°C, 200 MPa H₂O at?f _O2?≈?NNO, employing a natural?F-rich?vitrophyric rhyolite from Spor Mountain, Utah, assessed the effect of variable Mg′ [100Mg/(Mg?+?Mn?+?Fe)] on the partitioning of fluorine and chlorine between biotite (Bt) and melt. Over this temperature interval, Bt (?±?fluorite, ?±?quartz) is the sole liquidus phase. Partition coefficients for fluorine between biotite and glass (D_F ^Bt/melt) show a strong dependence on the Mg′ of Bt.?D_F ^Bt/melt varies from???1.5 to 7.2 over the range of Mg′ from 21 to 76. A strong linear correlation between?D_F ^Bt/melt?and Mg′ has a slope of 9.4 and extrapolates through the origin (i.e., D_F ^Bt/melt?≈?0 at Mg′?=?0, an annite-siderophyllite solid solution in these experiments). D_Cl ^Bt/melt values (???1 to 6) in the same experiments vary inversely with Mg′. The Al-content of biotite does not vary with the aluminum saturation index (ASI?=?molar Al₂O₃/Σ alkali and alkaline earth oxides) of melt, but two exchange mechanisms involving Al appear to operate in these micas: (1) Al^vi?+?Al^iv?? Si^iv?+?Mg^iv, and Mg^iv?+?2Al^iv? 2Si^iv?+?□^iv. The effects of other components such as Li or other intensive parameters including f _O2 have yet to be evaluated?systematically. At comparable Mg′ of Bt, however, the Spor Mountain rhyolite yields higher D_F ^Bt/melt values than an Li-rich, strongly peraluminous melt previously investigated. The results indicate that the Mg′ of Bt exerts the principal control on halogen partitioning, with ASI and T as second-order variables. The experimental partition coefficients compare well with other experimental results but not with most volcanic rocks. Magmatic Bt from most rhyolites records higher D_F ^Bt/melt due to reequilibration with degassed (H₂O-depleted) magma and perhaps with F₂O_?1 exchange that may accompany oxidation ([Fe³⁺O] [Fe²⁺OH]_?1). This behavior is evident in magmatic biotite from a zoned peraluminous rhyolite complex near Morococala, Bolivia: Bt is sharply zoned with F-rich rims, but Bt(core)-melt inclusion pairs fall on our experimental curve for D_F ^Bt/melt. These experimental data can be used in part to assess the preservation of magmatic volatile contents in plutonic or volcanic silicic rocks. For plutonic rocks, the actual F-content of melt, not a relative activity ratio involving HF species, can be reasonably estimated if the mica has not undergone subsolidus reequilibration. This information is potentially useful for some shallow-level Ca-poor magmas that are thought to be rich in F (e.g., A- and S-type granites) but do not conserve F well as rocks.  相似文献   

Probabilistic Assessment of Ground-Water Contamination: 2. Results of Case Study     

Jonathan D. Istok  Christopher A. Rautman 《Ground water》1996,34(6):1050-1064

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Trace-element and stable-isotope geochemistry of non-marine ostracod shells in Quaternary palaeoenvironmental reconstruction   总被引：5，自引：0，他引：5  

Jonathan A. Holmes 《Journal of Paleolimnology》1996,15(3):223-235

Ostracods are small bivalved aquatic crustancean. They secrete shells of low-Mg calcite that are often preserved in lake sediments. Recent work has shown that the uptake of trace elements (especially Mg and Sr) into the shell may be a function of the salinity and temperature of the host water. Furthermore, ostracod shells are a source of carbonate for stable-isotope analysis. This paper reviews the application of ostracod shell chemistry to Quaternary palaeolimnology. Although such work has revealed the excellent potential of these techniques to provide quantitative palaeolimnological reconstructions, a number of problems have also emerged. These problems relate to (1) methods used for extraction of ostracod shells from sediment and their subsequent cleaning (2) post-mortem diagenesis and alteration of the shell (3) complications with the calcification mechanism (4) spatial and temporal variability in shell composition (5) the ecological tolerances of individual species and (6) the relationships between shell chemistry and palaeohydrology. To some extent, these problems are an inevitable outcome of the diversity of lacustrine systems: they may be overcome by developing a thorough understanding of the physiology, life-cycle and ecology of the species concerned, together with the modern limnology of the study site. Overall, these techniques have excellent potential in Quaternary palaeolimnology, especially when used with other palaeoenvironmental indicators.  相似文献   

Intraplate uplift: new constraints on the Hoggar dome from the Illizi basin (Algeria)          下载免费PDF全文

Kara L. English  Jonathan Redfern  Giovanni Bertotti  Joseph M. English  Rachida Yahia Cherif 《Basin Research》2017,29(3):377-393

Zones of anomalously high topography within continental interiors, distant from active plate boundaries, are interpreted as being either dynamically supported by viscous flow in the underlying mantle or influenced by plate tectonics. Constraining the models of their genesis requires accurate data on the timing and dimensions of such features. New apatite fission‐track and thermal maturity data from the Illizi Basin in Algeria quantify the magnitude and timing of kilometre–scale uplift and exhumation of the northern flank of the Hoggar swell in North Africa. The findings of this study, integrated with previously published thermochronological data, confirm that long‐wavelength regional uplift occurred during the Cenozoic extending over a distance in excess of 1500 km from north to south. The uplift, centred on the Hoggar Massif, significantly impacted the flanking Illizi and Tim Mersoï basins. The combination of thermal history modelling and regional stratigraphic observations indicates that the onset of exhumation of the Illizi Basin likely occurred during the Eocene, broadly coincident with magmatism on the Hoggar Massif to the south and the onset of tectonic shortening in the Atlasic belt to the north.  相似文献   

Unbundling Stockholm: The networks, planning and social welfare nexus beyond the unitary city     

Jonathan Rutherford 《Geoforum》2008,39(6):1871-1883

This paper focuses on the extent to which recent infrastructure-oriented urban developments in Stockholm concord with various aspects of the ‘splintering urbanism’ thesis of Graham and Marvin. This contextualisation allows us to extend their work empirically and conceptually. In the first instance, we study a particular case of the decline of a unitary networked city (in an urban context largely absent from their book). In the second instance, we develop their notion of ‘unbundling’ to capture not just core changes in the organisation of infrastructure provision, but an overarching disjunction of the established nexus between networks, planning and social welfare in the city. This disjunction operates through interlinked transformations concerning, for example, privatisation and outsourcing in network services, separation of infrastructure planning from broader urban planning, contradictions between the environmental and social mandates of infrastructure, and a (prospective) curtailment of the redistributive, social role of essential network service provision. We conclude nonetheless that this ‘destructive’ moment of unbundling has not so far been pursued by more explicitly ‘creative’ urban fragmentation strategies, due largely to the vestiges of a socio-political consensus based around redistribution and equality. In this respect, the Stockholm case pleads for a conception of ‘splintering’ as a dynamic and multi-stage process which is not only always ongoing, unstable and incomplete, but also non-linear and open to resistance/regulation.  相似文献   

Characterization of the shallow groundwater system in an alpine watershed: Handcart Gulch,Colorado, USA   总被引：2，自引：2，他引：0  

Katherine Gurley Kahn  Shemin Ge  Jonathan Saul Caine  Andrew Manning 《Hydrogeology Journal》2008,16(1):103-121

Water-table elevation measurements and aquifer parameter estimates are rare in alpine settings because few wells exist in these environments. Alpine groundwater systems may be a primary source of recharge to regional groundwater flow systems. Handcart Gulch is an alpine watershed in Colorado, USA comprised of highly fractured Proterozoic metamorphic and igneous rocks with wells completed to various depths. Primary study objectives include determining hydrologic properties of shallow bedrock and surficial materials, developing a watershed water budget, and testing the consistency of measured hydrologic properties and water budget by constructing a simple model incorporating groundwater and surface water for water year 2005. Water enters the study area as precipitation and exits as discharge in the trunk stream or potential recharge for the deeper aquifer. Surficial infiltration rates ranged from 0.1–6.2×10^?5 m/s. Discharge was estimated at 1.28×10^?3 km³. Numerical modeling analysis of single-well aquifer tests predicted lower specific storage in crystalline bedrock than in ferricrete and colluvial material (6.7×10^?5–2.0×10^?3 l/m). Hydraulic conductivity in crystalline bedrock was significantly lower than in colluvial and alluvial material (4.3×10^?9–2.0×10^?4 m/s). Water budget results suggest that during normal precipitation and temperatures water is available to recharge the deeper groundwater flow system.  相似文献   

Seasonal Variation in Guild Structure of the Puget Sound Demersal Fish Community   总被引：2，自引：0，他引：2  

Jonathan C. P. Reum  Timothy E. Essington 《Estuaries and Coasts》2008,31(4):790-801

Identification of food web linkages is a major aim in ecology because it provides basic information on trophic flows and the potential for interspecific interactions. In addition, policy and restoration measures mandated to conform to ecosystem-based management principals can benefit from information on temporal and spatial variability in community-level interactions. Here, we analyzed guild structure of the demersal fish assemblage in Puget Sound, WA, a temperate estuarine system on the US west coast. Using diet information from 2,401 stomachs collected across three seasons (fall, winter, and summer), we identified guild membership for 21 fish species, examined seasonal guild switching, and tested for seasonal shifts in predation and for differences in the degree of diet overlap at the assemblage level. We accounted for ontogenetic variation in diet by dividing species into large (L) and small (S) size classes when appropriate. Using cluster analysis and a permutation approach, we identified seven significant guilds that were typified by predation on benthic invertebrates, pelagic invertebrates, and piscivory. Of the 18 species with more than one season of diet information, six switched guilds (Pacific sanddab L, sturgeon poacher, Pacific tomcod S, speckled sanddab, rex sole, and rock sole S). At the assemblage level, we tested for seasonal differences in prey use between seasons by performing an analysis of similarities based on Bray–Curtis diet similarities and found no significant difference. However, diet overlap was significantly higher in the summer than the fall and winter (with summer > fall > winter) indicating that diets within the assemblage converged in the summer. These results indicate that analyses of guild structure and diet overlap can reveal seasonal variation in community trophic structure and highlight intra-annual food web variation in the Puget Sound demersal fish community.  相似文献