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41.
J. A. Walker M. J. Carr L. C. Patino C. M. Johnson M. D. Feigenson R. L. Ward 《Contributions to Mineralogy and Petrology》1995,120(3-4):378-390
Lavas erupted behind the volcanic front in southeastern Guatemala have many important distinctions from lavas erupted on the volcanic front. These include: generally higher MgO, Nb, Sr, TiO2, and rare earth element concentrations; higher La/Yb and Nb/Y ratios; and lower Ba/La, La/Nb, Ba/Zr and Zr/Nb ratios. These major and trace element distinctions are caused by reduced fractionation during ascent and storage in the crust, lower degrees of melting in the source, and greatly reduced contributions from the subducted Cocos plate in the source. In addition, because all of these important distinctions are even borne in lavas erupted within 20 km of the front, there is little apparent petrogenetic continuity between front and behind-the-front magmas. What little geochemical continuity exists is in radiogenic isotopes: 143Nd/144Nd falls across the arc, Pb isotopic ratios (except 206Pb/204Pb) rise across the arc, and 87Sr/86Sr rise across the arc after an initial discontinuity within 20 km of the front. These continuous across-arc changes in radiogenic isotopes are caused by increased contamination with older, more isotopically disparate rocks, away from the front. Once the effects of crustal contamination are removed, the remaining isotopic variability behind the front is non-systematic and reflects the inherent isotopic heterogeneity of the source, the mantle wedge. Geochemical disconnection in southeastern Guatemala suggests that behind-the-front magmas are produced by decompression melting near the top of the wedge, not by flux-dominated melting near the base of the wedge. 相似文献
42.
Paul G. LuceyJohn Hinrichs Mary KellyDennis Wellnitz Noam IzenbergScott Murchie Mark RobinsonBeth E. Clark James F. Bell III 《Icarus》2002,155(1):181-188
Data obtained by the near-infrared spectrometer carried by the NEAR-Shoemaker spacecraft show that the spectral properties of the asteroid Eros vary with temperature. The manner in which they vary demonstrates that the mineral olivine is a major constituent of the surface. The near-IR temperature-dependent spectral properties of Eros in the northern hemisphere, and for two individual regions on the surface, show clear evidence of the presence of the mineral olivine and are a close match to the temperature-spectral behavior of LL-type ordinary chondrite meteorites. While the presence of other olivine-rich meteorites cannot be excluded, H-type ordinary chondrites are clearly too pyroxene-rich to be permitted as a major surface component of Eros. The results of the thermal-spectral analysis are consistent with results from analysis of conventional reflectance spectra of the asteroid and contribute unambiguous detection of olivine to the understanding of the surface composition of Eros. 相似文献
43.
44.
In this paper, we formulate a finite element procedure for approximating the coupled fluid and mechanics in Biot’s consolidation
model of poroelasticity. Here, we approximate the pressure by a mixed finite element method and the displacements by a Galerkin
method. Theoretical convergence error estimates are derived in a discrete-in-time setting. Of particular interest is the case
when the lowest-order Raviart–Thomas approximating space or cell-centered finite differences are used in the mixed formulation
and continuous piecewise linear approximations are used for displacements. This approach appears to be the one most frequently
applied to existing reservoir engineering simulators. 相似文献
45.
In this paper, we formulate a finite element procedure for approximating the coupled fluid and mechanics in Biot’s consolidation
model of poroelasticity. Here, we approximate the pressure by a mixed finite element method and the displacements by a Galerkin
method. Theoretical convergence error estimates are derived in a continuous in-time setting for a strictly positive constrained
specific storage coefficient. Of particular interest is the case when the lowest-order Raviart–Thomas approximating space
or cell-centered finite differences are used in the mixed formulation, and continuous piecewise linear approximations are
used for displacements. This approach appears to be the one most frequently applied to existing reservoir engineering simulators. 相似文献
46.
Use of annular flumes to determine the influence of current velocity and bivalves on material flux at the sediment-water interface 总被引:3,自引:0,他引:3
John Widdows Mary D. Brinsley Peter N. Salkeld Mike Elliott 《Estuaries and Coasts》1998,21(4):552-559
A benthic annular flume for both laboratory and in situ deployment on intertidal mudflats is described. The flume provides a means of quantifying material flux (i.e., biodeposition of suspended particulates, sediment resuspension, nutrients, oxygen, and contaminants) across the sediment-water interface in relation to changes in current velocity and benthic community structure and/or population density of key macrofauna species. Flume experiments have investigated the impact of the infaunal bivalveMacoma balthica and the epifaunal bivalveMytilus edulis on seston and sediment flux at the sediment-water interface. The bioturbatorMacoma was found to increase the sediment resuspension and/or erodability by 4-fold, at densities similar to those recorded at the Skeffling mudflat (Humber estuary) (i.e., >1000 individuals m?2). There was a significant correlation between sediment resuspension andMacoma density (r=0.99; p<0.001), which supported previous in situ field observations indicating bioturbation byMacoma enhanced sediment erodability. Biodeposition rates (g m?2 h1) ofMytilus edulis andCerastoderma edule were quantified and related to changes in population density in a mussel bed (Cleethorpes, Humber estuary). Biodeposition rates were up to 40-times the natural sedimentation rates. At the highest mussel bed densities (i.e., 50–100% cover or >1400 mussels m?2) the physical presence of this epifaunal bivalve on the sediment surface reduced erosion by 10-fold. The shift from net biodeposition to net erosion occurred at current velocities of 20–25 cm s?1. These results demonstrate that infaunal and epifaunal bivalves can have a significant impact on seston flux or sediment deposition and on sediment resuspension or erodability in estuaries where there are extensive mudflats. 相似文献
47.
Michael Joseph Carr Pedro Pina Avelino Paulo Mauricio de Carvalho 《Astrophysics and Space Science》1998,261(1-4):335-336
As an alternative to computationally expensive N-body simulations for gravitional clustering, the Zel'dovich approximation
(ZA) was studied in 3D, 2D and 1D. Plots of the density contrast were compared against linear theory and the exact solution.
The ZA was found to perform very well in the linear regime, better than linear theory, and to give a good approximation well
into the non-linear regime.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
48.
49.
Several isoprenoid compounds were isolated from a heat-treated marine sediment in the southern California continental shelf (from Tanner Basin) which were not present in the original sediment. Among the compounds identified were: phytol, dihydrophytol, C18-isoprenoid ketone, phytanic and pristanic acids, C19- and C20-monoolefins, and the alkanes 2,6,10-trimethylpentadecane, pristane and phytane. The significance and possible routes leading to these compounds is discussed. 相似文献
50.
Recent sediments from Tanner Basin (off southern California) and Bandaras Bay (in Gulf of California) have been analyzed for normal, isoprenoid and steroidal alcohols using chromatographic (column, GLC, TLC), and spectroscopic (u.v., i.r., MS) methods, prior to and after heat-treatment (from 65 to 150°C). Normal saturated alcohols (C14-C24) and monounsaturated alcohols (C22-C24) were identified, as well as the isoprenoid alcohols, phytol and dihydrophytol.Two series of sterols (Δ5 and Δ7) were found in Tanner Basin, and Δ5-sterols and triterpenes, in Bandaras Bay sediment. Sterols from both sediments contained the corresponding stanols. 相似文献