Mantle contribution to the evolution of Middle Tertiary silicic magmatism during early stages of extension: the Egan Range volcanic complex,east-central Nevada     

Todd C. Feeley  Anita L. Grunder 《Contributions to Mineralogy and Petrology》1991,106(2):154-169

The Egan Range volcanic complex lies 30 km northwest of Ely, on the edge of a highly extended domain in east-central Nevada. It consists mainly of lavas with subordinate tuffs and sedimentary rocks. The rocks are divided into three stratigraphic and lithologic groups that correlated with widespread middle Tertiary volcanic rocks associated with early stages of extension in the region. Volcanic rocks of the early group are predominantly two-pyroxene dacite and andesite lavas, all of which contain quenched, mafic inclusions and have compositions indicating they were derived by mixing between a contaminated mantle melt and a rhyodacitic crustal component. Rocks of the middle group are relatively homogeneous biotite, hornblende dacite and rhyodacite lavas. Elevated compatible and incompatible element concentrations and straight-line correlations of compositional data in the early and middle groups support a simple mixing model. Minor fractionation of clinopyroxene is required to explain some low Cr concentrations. Major element variations of the late group can be successfully modeled by crystal fractionation of observed phenocrysts accompanied by moderate assimilation of a crustal component to account for elevated Rb, Th, U, and light rare earth element concentrations. Rocks of all three groups appear to be related to a common primary magma type, the composition of which can be modeled from the mafic inclusions in the early group. Low Ni and Mg contents in the inclusions indicate that olivine was fractionated prior to their participation in mixing of early group magmas. Based on estimated volumes of volcanic rocks in the Egan Range volcanic complex and in the region, and on the petrologic models for each group, a significant amount of basalt must have been added to the crust during this middle Tertiary magmatic episode.  相似文献   

Relative depletion of niobium in some arc magmas and the continental crust: partitioning of K, Nb, La and Ce during melt/rock reaction in the upper mantle   总被引：4，自引：0，他引：4  

Peter B. Kelemen  Nobumichi Shimizu  Todd Dunn 《Earth and Planetary Science Letters》1993,120(3-4):111-134

Depletion of Nb relative to K and La is characteristic of lavas in subduction-related magmatic arcs, as distinct from mid-ocean ridge basalts. Nb depletion is also characteristic of the continental crust. This and other geochemical similarities between the continental crust and high-Mg# andesite magmas found in arcs suggests that the continental crust may have formed by accretion of andesites. Previous studies have shown that the major element characteristics of high-Mg# andesites may be produced by melt/rock reaction in the upper mantle. In this paper, new data on partitioning of K, Nb, La and Ce between garnet, orthopyroxene and clinopyroxene in mantle xenoliths, and on partitioning of Nb and La between orthopyroxene and liquid, show that garnet and orthopyroxene have Nb crystal/liquid distribution coefficients which are much larger than those of K and La. Similar fractionations of Nb from K and La are expected in spinel and olivine. For this reason, reactions between migrating melt and large masses of mantle peridotite can produce substantial depletion of Nb in derivative liquids. Modeling shows that reaction between ascending, mantle-derived melts and mantle peridotite is a viable mechanism for producing the trace element characteristics of high-Mg# andesite magmas and the continental crust.

Alternatively, small-degree melts of metabasalt and/or metasediment in the subducting slab may leave rutile in their residue, and will thus have large Nb depletions relative to K and La [1]. Slab melts are too rich in light rare earth elements and other incompatible elements, and too poor in compatible elements, to be parental to arc magmas. However, ascending slab melts may be modified by reaction with the mantle. Our new data permit modeling of the trace element effects of reaction between small-degree melts of the slab and mantle peridotite. Modeling shows that this type of reaction is also a viable mechanism for producing the trace element characteristics of high-Mg# andesites and the continental crust. These findings, in combination with previous results, suggest that melt/rock reaction in the upper mantle has been an important process in forming the continental crust and mantle lithosphere.  相似文献   

A bioclimatic design methodology for urban outdoor spaces     

H. Swaid  M. Bar-El  M. E. Hoffman 《Theoretical and Applied Climatology》1993,48(1):49-61

Summary The development of a bioclimatic urban design methodology is described. The cluster thermal time constant (CTTC) model for predicting street-level urban air temperature variations is coupled with the wind-profile power law and the index of thermal stress (ITS.) for human comfort. TheCTTC model and the power law produce the diurnal air temperature and wind speed variations in various canyonlike urban forms. The thermal comfort requirements for lightly-dressed, moderately-walking/seated persons in the outdoor space in summer are then obtained using the ITS. model. The proposed methodology enables a first-order assessment of the climatic implications of different features of the physical structure of the city such as street orientation, canyon height-to-width ratio, building density, and street shading. The application of the proposed methodology is demonstrated for Tel Aviv.With 9 Figures  相似文献   

A carbon balance model for the great dismal swamp ecosystem     

Rachel Sleeter  Benjamin M. Sleeter  Brianna Williams  Dianna Hogan  Todd Hawbaker  Zhiliang Zhu 《Carbon balance and management》2017,12(1):2

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The U.P. Mammoth Site,Carbon County,Wyoming, USA: More Questions than Answers     

C. Vance Haynes  Todd A. Surovell  Gregory W. L. Hodgins 《Geoarchaeology》2013,28(2):99-111

In the summer of 1960, mammoth bones were discovered by a dragline operator in southern Wyoming at the Union Pacific (U.P.) Mammoth site. Although subsequent archaeological work during 1960 and 1961 identified artifacts in association with the mammoth remains, many authors have since questioned the nature of that association. Also, little has been published about the site other than a brief article in National Geographic Magazine in 1962. In this paper, we present additional information on the U.P. Mammoth site including stratigraphic profiles from the first author's geoarchaeological work in 1961, stratigraphic and spatial location of bones and artifacts derived from the original field notes, and new radiocarbon dates. Although the precise stratigraphic provenance for many artifacts and skeletal elements remains unclear, a compelling argument can be made for spatial and stratigraphic association of the mammoth remains with the artifact assemblage suggesting some kind of human interaction with the animal.  相似文献   

Spatial differences in denitrification and bacterial community structure of streams: relationships with environmental conditions     

Alyssa M. Baxter  Laura Johnson  Todd Royer  Laura G. Leff 《Aquatic Sciences - Research Across Boundaries》2013,75(2):275-284

In streams, benthic bacterial communities are integral to multiple aspects of ecosystem function, including carbon and nitrogen cycles. Variation both in terms of bacterial community structure (based on taxonomic and/or functional genes) and function can reveal potential drivers of spatiotemporal patterns in stream processes. In this study, the abundance and diversity of 16S rRNA genes and abundance of nosZ genes, encoding for nitrous oxide reductase, were related to denitrification and environmental conditions. Denitrification rates varied among the three streams examined, and within a given stream there were significant longitudinal differences. Likewise, bacterial community structure based on analysis of the 16S rRNA gene also differed significantly among streams. However, variation in denitrification rate was not well correlated with environmental or biological variables measured. In addition, relatively large numbers of denitrifiers occurred when denitrification rates were low. In conclusion, although the streams differed in environmental conditions as well as bacterial community structure, these differences did not explain much of the spatial variation in denitrification rates.  相似文献   

Secondary infall and dark matter haloes     

Yago Ascasibar  Yehuda Hoffman  Stefan Gottlöber 《Monthly notices of the Royal Astronomical Society》2007,376(1):393-404

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Non-parametric reconstruction of the primordial power spectrum at horizon scales from WMAP data     

Domenico Tocchini-Valentini  Yehuda Hoffman  Joseph Silk 《Monthly notices of the Royal Astronomical Society》2006,367(3):1095-1102

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Testing tidal-torque theory – II. Alignment of inertia and shear and the characteristics of protohaloes     

Cristiano Porciani  Avishai Dekel  Yehuda Hoffman 《Monthly notices of the Royal Astronomical Society》2002,332(2):339-351

We investigate the cross-talk between the two key components of tidal-torque theory, the inertia ( I ) and shear ( T ) tensors, using a cosmological N -body simulation with thousands of well-resolved haloes. We find that the principal axes of I and T are strongly aligned , even though I characterizes the protohalo locally while T is determined by the large-scale structure. Thus, the resultant galactic spin, which plays a key role in galaxy formation, is only a residual due to ∼10 per cent deviations from the perfect alignment of T and I . The   T – I   correlation induces a weak tendency for the protohalo spin to be perpendicular to the major axes of T and I , but this correlation is erased by non-linear effects at late times, making the observed spins poor indicators of the initial shear field.
However, the   T – I   correlation implies that the shear tensor can be used for identifying the positions and boundaries of protohaloes in cosmological initial conditions – a missing piece in galaxy formation theory. The typical configuration is of a prolate protohalo lying perpendicular to a large-scale high-density ridge, with the surrounding voids inducing compression along the major and intermediate inertia axes of the protohalo. This leads to a transient sub-halo filament along the large-scale ridge, whose subclumps then flow along the filament and merge into the final halo.
The centres of protohaloes tend to lie in ∼1 σ overdensity regions, but their association with linear density maxima smoothed on galactic scales is vague: only ∼40 per cent of the protohaloes contain peaks. Several other characteristics distinguish protohaloes from density peaks, e.g. they tend to compress along two principal axes while many peaks compress along three axes.  相似文献   

The Ogle-Tr-56 Star–Planet System     

Todd Vaccaro  Walter Van Hamme 《Astrophysics and Space Science》2005,296(1-4):231-234

We simultaneously fitted light and velocity data for the star–planet system OGLE-TR-56 with the Wilson–Devinney (WD) binary star program. We solved for orbital and planet parameters, along with the ephemeris using all currently available observational data. Parameters for the star (OGLE-TR-56a) were kept fixed at values derived from spectral characteristics and stellar evolutionary tracks. Our results are in good agreement with parameters obtained by other authors and have slightly smaller errors. We found no significant change in orbital period that may be due to orbital decay.  相似文献