High-precision Nd/Nd measurements in terrestrial rocks: Constraints on the early differentiation of the Earth’s mantle     

Guillaume Caro  Bernard Bourdon  Stephen Moorbath 《Geochimica et cosmochimica acta》2006,70(1):164-191

We present new ultra-high precision ¹⁴²Nd/¹⁴⁴Nd measurements of early Archaean rocks using the new generation thermal ionization mass spectrometer Triton. Repeated measurements of the Ames Nd standard demonstrate that the ¹⁴²Nd/¹⁴⁴Nd ratio can be determined with external precision of 2 ppm (2σ), allowing confident resolution of anomalies as small as 5 ppm. A major analytical improvement lies in the elimination of the double normalization procedure required to correct our former measurements from a secondary mass fractionation effect. Our new results indicate that metasediments, metabasalts, and orthogneisses from the 3.6 to 3.8 Ga West Greenland craton display positive ¹⁴²Nd anomalies ranging from 8 to 15 ppm. Using a simple two-stage model with an initial ε¹⁴³Nd value of 1.9 ± 0.6 ε-units, coupled ¹⁴⁷Sm-¹⁴³Nd and ¹⁴⁶Sm-¹⁴²Nd chronometry constrains mantle differentiation to 50-200 Ma after formation of the solar system. This chronological constraint is consistent with differentiation of the Earth’s mantle during the late stage of crystallization of a magma ocean. We have developed a two-box model describing ¹⁴²Nd and ¹⁴³Nd isotopic evolution of depleted mantle during the subsequent evolution of the crust-mantle system. Our results indicate that early terrestrial protocrust had a lifetime of ca. 0.7-1 Ga in order to produce the observed Nd isotope signature of Archaean rocks. In the context of this two box mantle-crust system, we model the evolution of isotopic and chemical heterogeneity of depleted mantle as a function of the mantle stirring time. Using the dispersion of ¹⁴²Nd/¹⁴⁴Nd and ¹⁴³Nd/¹⁴⁴Nd ratios observed in early Archaean rocks, we constrain the stirring time of early Earth’s mantle to 100-250 Ma, a factor of 5 shorter than the stirring time inferred from modern oceanic basalts.  相似文献   

Chemical and isotopic evidence for widespread Eoarchean metasedimentary enclaves in southern West Greenland     

Nicole L. Cates  Stephen J. Mojzsis 《Geochimica et cosmochimica acta》2006,70(16):4229-4257

New mapping, geochemistry and zircon U-Pb ion microprobe geochronology of pre-3750 Ma rocks from West Greenland was used to identify sedimentary protoliths in a problematic high-grade metamorphic terrane. Samples were collected from southernmost part of the Itsaq Gneiss Complex where Akilia association supracrustal rocks have previously been noted. Supracrustal lithologies include laterally continuous and variably deformed units of amphibolite, ultramafics and ferruginous quartz-pyroxene rocks. Oxygen isotope and mass-independently fractionated sulfur isotopes, immobile trace elements and rare earth element patterns are consistent with origin of quartz-pyroxene rocks as chemical sediments deposited in a marine hydrothermal setting. We describe a further supracrustal lithology: Garnet-bearing quartz-biotite schists with elevated oxygen isotope values (δ¹⁸O_SMOW ? +16‰) and mass-independently fractionated S isotopes consistent with a low-temperature aqueous sedimentary origin. In several enclaves, granitoid gneisses within low-strain limbs transect lithologic contacts and contain inclusions of surrounding rocks. This supports the interpretation that some orthogneisses were originally emplaced as igneous veins that cut supracrustal lithologies. Zircon geochronology on orthogneisses that preserve intrusive relationships confirms minimum ages of ca. 3750 Ma for the supracrustals and pooled [Th/U]_zircon and δ¹⁸O_zircon values of older zircon populations are consonant with igneous growth in the bulk composition of the host rocks. Low [Zr]_WR and high Zr saturation temperatures further minimize the possibility of zircon inheritance. A >3750 Ma age and chemical sedimentary origin for various Akilia association lithologies underscores the widespread occurrence of rocks of this kind beyond the type locality on Akilia (island) at the southern limit of the Itsaq Gneiss Complex.  相似文献   

Erratum to “Regional atmospheric circulation change in the North Pacific during the Holocene inferred from lacustrine carbonate oxygen isotopes, Yukon Territory, Canada” [Quaternary Research 64 (2005) 21-35]     

Lesleigh Anderson  Mark B. Abbott  Stephen J. Burns 《Quaternary Research》2006,65(2):350-351

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Galvanic interaction between grinding media and arsenopyrite and its effect on flotation: Part I. Quantifying galvanic interaction during grinding     

Guozhi Huang  Stephen Grano 《International Journal of Mineral Processing》2006

The Magotteaux Mill® system was used to quantify the galvanic interaction between four types of grinding media (mild steel, 15% chromium, 21% chromium, and 30% chromium media) and arsenopyrite, in situ of the mill. The galvanic current was determined from the polarization curves of individual electrodes, in situ of the mill. The high chromium media had a significantly weaker galvanic interaction with arsenopyrite, and produced a very much lower amount of oxidized iron species in the mill discharge than mild steel medium. Increasing the dissolved oxygen in the mill slurry enhanced galvanic interaction. A theoretical model was developed to describe the relationship between galvanic current and oxidized iron species in the slurry, as measured by EDTA extraction. The electrochemical data were correlated with the measurement of oxidation products after grinding under different conditions. The stronger the galvanic interaction, the higher the amount of hydrophilic oxidized iron species in the mill discharge.  相似文献   

Slurry flow in mills with TCPL — An efficient pulp lifter for ag/sag mills     

Sanjeeva Latchireddi  Stephen Morrell 《International Journal of Mineral Processing》2006

The difficulties associated with slurry transportation in autogenous (ag) and semi-autogenous (sag) grinding mills have become more apparent in recent years with the increasing trend to build larger diameter mills for grinding high tonnages. This is particularly noticeable when ag/sag mills are run in closed circuit with classifiers such as fine screens/cyclones.  相似文献   

Naturally weathered feldspar surfaces in the Navajo Sandstone aquifer, Black Mesa, Arizona: Electron microscopic characterization   总被引：1，自引：0，他引：1  

Chen Zhu  David R. Veblen  Stephen J. Chipera 《Geochimica et cosmochimica acta》2006,70(18):4600-4616

Naturally weathered feldspar surfaces in the Jurassic Navajo Sandstone at Black Mesa, Arizona, was characterized with high-resolution transmission and analytical electron microscope (HRTEM-AEM) and field emission gun scanning electron microscope (FEG-SEM). Here, we report the first HRTEM observation of a 10-nm thick amorphous layer on naturally weathered K-feldspar in currently slightly alkaline groundwater. The amorphous layer is probably deficient in K and enriched in Si. In addition to the amorphous layer, the feldspar surfaces are also partially coated with tightly adhered kaolin platelets. Outside of the kaolin coatings, feldspar grains are covered with a continuous 3-5 μm thick layer of authigenic smectite, which also coats quartz and other sediment grains. Authigenic K-feldspar overgrowth and etch pits were also found on feldspar grains. These characteristics of the aged feldspar surfaces accentuate the differences in reactivity between the freshly ground feldspar powders used in laboratory experiments and feldspar grains in natural systems, and may partially contribute to the commonly observed apparent laboratory-field dissolution rate discrepancy. At Black Mesa, feldspars in the Navajo Sandstone are dissolving at ∼10⁵ times slower than laboratory rate at comparable temperature and pH under far from equilibrium condition. The tightly adhered kaolin platelets reduce the feldspar reactive surface area, and the authigenic K-feldspar overgrowth reduces the feldspar reactivity. However, the continuous smectite coating layer does not appear to constitute a diffusion barrier. The exact role of the amorphous layer on feldspar dissolution kinetics depends on the origin of the layer (leached layer versus re-precipitated silica), which is uncertain at present. However, the nanometer thin layer can be detected only with HRTEM, and thus our study raises the possibility of its wide occurrence in geological systems. Rate laws and proposed mechanisms should consider the possibility of this amorphous layer on feldspar surface.  相似文献   

AnnouncingEstuaries and Coasts, Journal of the Estuarine Research Federation     

Stephen T. Threlkeld 《Estuaries and Coasts》2006,29(1):1-3

This is the first issue ofEstuaries and Coasts (ISSN 1559-2723), which continues the Estuarine Research Federation's publication inEstuaries (1978–2005) (ISSN 0160-8347) of scientific reports, reviews, comments, and notes on estuarine and coastal scientific research from around the world. Numbering of volumes ofEstuaries and Coasts continues uninterrupted fromEstuaries since the expanded title was not intended to mark a change in the scope of the journal. It is our hope that the journal will continue to become more international in its coverage, and to include more coastal science than is sometimes associated with the single word nameEstuaries. In addition to the change of the journal's name there is also a new cover design that will change with each issue and highlight an estuary or coast associated with one or more articles in the issue. Another change to the journal is a new column entitled Perspectives in Estuarine and Coastal Science, which is intended to stimulate discussion about emerging topics in estuarine and coastal science. The H. T. Odum Synthesis Essay, initiated in 2005, will be continued on an annual basis.  相似文献   

The 2dF Galaxy Redshift Survey: the power spectrum and the matter content of the Universe     

Will J. Percival  Carlton M. Baugh  Joss Bland-Hawthorn  Terry Bridges  Russell Cannon  Shaun Cole  Matthew Colless  Chris Collins  Warrick Couch  Gavin Dalton  Roberto De Propris  Simon P. Driver  George Efstathiou  Richard S. Ellis  Carlos S. Frenk  Karl Glazebrook  Carole Jackson  Ofer Lahav  Ian Lewis  Stuart Lumsden  Steve Maddox  Stephen Moody  Peder Norberg  John A. Peacock  Bruce A. Peterson  Will Sutherland  Keith Taylor 《Monthly notices of the Royal Astronomical Society》2001,327(4):1297-1306

The 2dF Galaxy Redshift Survey has now measured in excess of 160 000 galaxy redshifts. This paper presents the power spectrum of the galaxy distribution, calculated using a direct Fourier transform based technique. We argue that, within the k -space region     , the shape of this spectrum should be close to that of the linear density perturbations convolved with the window function of the survey. This window function and its convolving effect on the power spectrum estimate are analysed in detail. By convolving model spectra, we are able to fit the power-spectrum data and provide a measure of the matter content of the Universe. Our results show that models containing baryon oscillations are mildly preferred over featureless power spectra. Analysis of the data yields 68 per cent confidence limits on the total matter density times the Hubble parameter     , and the baryon fraction     , assuming scale-invariant primordial fluctuations.  相似文献   

The Effect on Micropulsations of a Localized Earth Conductivity Anomaly     

W. J. Hughes 《Geophysical Journal International》1974,36(3):641-649

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Field observations of fronts and high-frequency temperature fluctuations in a thermal plume     

Theodore Green  Stephen Roffler 《Advances in water resources》1981,4(3):137-145

High-frequency temperature variations were measured at seven depths, at a point 130 m from the cooling-water outfall of a power plant. The data are studied using spectra, correlation functions and probability distributions. The sharp thermal fronts observed in the data are related to the surface thermal fronts seen using airborne infra-red scanning, and to laboratory studies of instabilities in mixing layers between uniformly flowing and quiescent fluids.  相似文献