Northwest Africa 032: Product of lunar volcanism     

T. J. Fagan  G. J. Taylor  K. Keil  T. E. Bunch  J. H. Wittke  R. L. Korotev  B. L. Jolliff  J. J. Gillis  L. A. Haskin  E. Jarosewich  R. N. Clayton  T. K. Mayeda  V. A. Fernandes  R. Burgess  G. Turner  O. Eugster  S. Lorenzetti 《Meteoritics & planetary science》2002,37(3):371-394

Abstract— Mineralogy, major element compositions of minerals, and elemental and oxygen isotopic compositions of the whole rock attest to a lunar origin of the meteorite Northwest Africa (NWA) 032, an unbrecciated basalt found in October 1999. The rock consists predominantly of olivine, pyroxene and chromite phenocrysts, set in a crystalline groundmass of feldspar, pyroxene, ilmenite, troilite and trace metal. Whole‐rock shock veins comprise a minor, but ubiquitous portion of the rock. Undulatory to mosaic extinction in olivine and pyroxene phenocrysts and micro‐faults in groundmass and phenocrysts also are attributed to shock. Several geochemical signatures taken together indicate unambiguously that NWA 032 originated from the Moon. The most diagnostic criteria include whole‐rock oxygen isotopic composition and ratios of Fe/Mn in the whole rock, olivine, and pyroxene. A lunar origin is documented further by the presence of Fe‐metal, troilite, and ilmenite; zoning to extremely Fe‐rich compositions in pyroxene; the ferrous oxidation state of all Fe in pyroxene; and the rare earth element (REE) pattern with a well‐defined negative europium anomaly. This rock is similar in major element chemistry to basalts from Apollo 12 and 15, but is enriched in light REE and has an unusually high Th/Sm ratio. Some Apollo 14 basalts yield a closer match to NWA 032 in REE patterns, but have higher concentrations of Al₂O₃. Ar‐Ar step release results are complex, but yield a whole‐rock age of ?2.8 Ga, suggesting that NWA 032 was extruded at 2.8 Ga or earlier. This rock may be the youngest sample of mare basalt collected to date. Noble gas concentrations combined with previously collected radionuclide data indicate that the meteorite exposure history is distinct from currently recognized lunar meteorites. In short, the geochemical and petrographic features of NWA 032 are not matched by Apollo or Luna samples, nor by previously identified lunar meteorites, indicating that it originates from a previously unsampled mare deposit. Detailed assessment of petrographic features, olivine zoning, and thermodynamic modelling indicate a relatively simple cooling and crystallization history for NWA 032. Chromite‐spinel, olivine, and pyroxene crystallized as phenocrysts while the magma cooled no faster than 2 °C/h based on the polyhedral morphology of olivine. Comparison of olivine size with crystal growth rates and preserved Fe‐Mg diffusion profiles in olivine phenocrysts suggest that olivine was immersed in the melt for no more than 40 days. Plumose textures in groundmass pyroxene, feldspar, and ilmenite, and Fe‐rich rims on the phenocrysts formed during rapid crystallization (cooling rates ?20 to 60 °C/h) after eruption.  相似文献   

Yamato 792947, 793408 and 82038: The most primitive H chondrites,with abundant refractory inclusions     

M. Kimura  H. Hiyagon  H. Palme  B. Spettel  D. Wolf  R. N. Clayton  T. K. Mayeda  T. Sato  A. Suzuki  H. Kojima 《Meteoritics & planetary science》2002,37(10):1417-1434

Abstract— In this paper we report petrological and chemical data of the unusual chondritic meteorites Yamato (Y)‐792947, Y‐93408 and Y‐82038. The three meteorites are very similar in texture and chemical composition, suggesting that they are pieces of a single fall. The whole‐rock oxygen isotopes and the chemical compositions are indicative of H chondrites. In addition, the mineralogy, and the abundances of chondrule types, opaque minerals and matrices suggest that these meteorites are H3 chondrites. They were hardly affected by thermal and shock metamorphism. The degree of weathering is very low. We conclude that these are the most primitive H chondrites, H3.2–3.4 (S1), known to date. On the other hand, these chondrites contain extraordinarily high amounts of refractory inclusions, intermediate between those of ordinary and carbonaceous chondrites. The distribution of the inclusions may have been highly heterogeneous in the primitive solar nebula. The mineralogy, chemistry and oxygen isotopic compositions of inclusions studied here are similar to those in CO and E chondrites.  相似文献   

European palaeoclimate and man     

BJÖRN E. BERGLUND 《Boreas: An International Journal of Quaternary Research》1993,22(4):328-328

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A 2-D synthetic study of global traveltime tomography     

Olafur Gudmundsson  Robert W. Clayton 《Geophysical Journal International》1991,106(1):53-65

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Radon as an indicator of limited cross-shelf mixing of submarine groundwater discharge along an open ocean beach in the South Atlantic Bight during observed hypoxia     

Clayton McCoy  Richard Viso 《Continental Shelf Research》2011,31(12):1306-1317

Hypoxic conditions (dissolved oxygen (DO)<2 mg l⁻¹) have been documented in the nearshore coastal waters of Long Bay, South Carolina, United States of America, during summer months over the past several years. Hypoxia was documented in August 2009 in the nearshore (<500 m offshore) for ten consecutive days and four days in September 2009 corresponding with spring tides. This study measured radon activities of shallow beachface groundwater and nearshore bottom waters to estimate mixing rates and submarine groundwater discharge (SGD) in the nearshore waters of central Long Bay. Statistical analyses demonstrate significant correlations between high bottom water radon activities, low DO, and cooler bottom water temperatures during hypoxic conditions. Elevated radon activities during hypoxia were significantly influenced by upwelling favorable conditions which severely limited cross-shelf mixing. Model results indicate mixing of nearshore and offshore waters was limited by up to 93% (range: 43-100%) relative to non-hypoxic conditions. Data suggests previously overlooked natural phenomena including limited cross-shelf mixing and SGD can significantly influence nearshore water quality.  相似文献   

Northwest Africa 773: Lunar origin and iron‐enrichment trend     

T. J. Fagan  G. J. Taylor  K. Keil  T. L. Hicks  M. Killgore  T. E. Bunch  J. H. Wittke  D. W. Mittlefehldt  R. N. Clayton  T. K. Mayeda  O. Eugster  S. Lorenzetti  M. D. Norman 《Meteoritics & planetary science》2003,38(4):529-554

Abstract— The meteorite Northwest Africa 773 (NWA 773) is a lunar sample with implications for the evolution of mafic magmas on the moon. A combination of key parameters including whole‐rock oxygen isotopic composition, Fe/Mn ratios in mafic silicates, noble gas concentrations, a KREEP‐like rare earth element pattern, and the presence of regolith agglutinate fragments indicate a lunar origin for NWA 773. Partial maskelynitization of feldspar and occasional twinning of pyroxene are attributed to shock deformation. Terrestrial weathering has caused fracturing and precipitation of Carich carbonates and sulfates in the fractures, but lunar minerals appear fresh and unoxidized. The meteorite is composed of two distinct lithologies: a two‐pyroxene olivine gabbro with cumulate texture, and a polymict, fragmental regolith breccia. The olivine gabbro is dominated by cumulate olivine with pigeonite, augite, and interstitial plagioclase feldspar. The breccia consists of several types of clasts but is dominated by clasts from the gabbro and more FeO‐rich derivatives. Variations in clast mineral assemblage and pyroxene Mg/(Mg + Fe) and Ti/(Ti + Cr) record an igneous Fe‐enrichment trend that culminated in crystallization of fayalite + silica + hedenbergite‐bearing symplectites. The Fe‐enrichment trend and cumulate textures observed in NWA 773 are similar to features of terrestrial ponded lava flows and shallow‐level mafic intrusives, indicating that NWA 773 may be from a layered mafic intrusion or a thick, differentiated lava flow. NWA 773 and several other mafic lunar meteorites have LREE‐enriched patters distinct from Apollo and Luna mare basalts, which tend to be LREE‐depleted. This is somewhat surprising in light of remote sensing data that indicates that the Apollo and Luna missions sampled a portion of the moon that was enriched in incompatible heatproducing elements.  相似文献   

An approximate method for joint sequential simulation of multiple spatial variables     

Olena Babak  David F. Machuca-Mory  Clayton V. Deutsch 《Stochastic Environmental Research and Risk Assessment (SERRA)》2010,24(3):327-336

Multivariate simulation is an important longstanding problem in geostatistics. Fitting a model of coregionalization to many variables is intractable and often not permitted; however, the matrix of collocated correlation coefficients is often well informed. Performing a matrix simulation with LU decomposition of the correlation matrix at each step of sequential simulation is implemented in some software. The target correlation matrix is not reproduced because of conditioning to local data and the particular variable ordering in the sequential/LU decomposition. A correction procedure is developed to calculate a modified correlation matrix that leads to reproduction of the target correlation matrix. The theoretical and practical aspects of this correction are developed.  相似文献   

Using overlapping sonobuoy data from the Ross Sea to construct a 2D deep crustal velocity model     

M. M. Selvans  R. W. Clayton  J. M. Stock  R. Granot 《Marine Geophysical Researches》2012,33(1):17-32

Sonobuoys provide an alternative to using long streamers while conducting multi-channel seismic (MCS) studies, in order to provide deeper velocity control. We present analysis and modeling techniques for interpreting the sonobuoy data and illustrate the method with ten overlapping sonobuoys collected in the Ross Sea, offshore from Antarctica. We demonstrate the importance of using the MCS data to correct for ocean currents and changes in ship navigation, which is required before using standard methods for obtaining a 1D velocity profile from each sonobuoy. We verify our 1D velocity models using acoustic finite-difference (FD) modeling and by performing depth migration on the data, and demonstrate the usefulness of FD modeling for tying interval velocities to the shallow crust imaged using MCS data. Finally, we show how overlapping sonobuoys along an MCS line can be used to construct a 2D velocity model of the crust. The velocity model reveals a thin crust (5.5 ± 0.4 km) at the boundary between the Adare and Northern Basins, and implies that the crustal structure of the Northern Basin may be more similar to that of the oceanic crust in the Adare Basin than to the stretched continental crust further south in the Ross Sea.  相似文献   

Effects of land use on water column bacterial activity and enzyme stoichiometry in stream ecosystems     

Clayton J. Williams  Andrew B. Scott  Henry F. Wilson  Marguerite A. Xenopoulos 《Aquatic Sciences - Research Across Boundaries》2012,74(3):483-494

Fifty streams, located in southern Ontario, Canada, were visited in September 2008 to investigate the effect of varying land use, land cover, and associated resource inputs on water column bacterial abundance (BACT), production (BP), and extracellular enzyme activity and stoichiometry. Principle components analysis was used to summarize landscape data, producing three components (PCs), which explained 79.2% of the variability in the data. The PCs grouped into the following gradients: (PC1) urban land use and continuous annual cropping to wetland-like cover, (PC2) rotational cropping to forest-like cover, and (PC3) increasing rural and agricultural land uses with increasing watershed size. These landscape gradients created imbalanced resource availability. Nutrient resources were more abundant in streams with more intensive anthropogenic land uses, but carbon availability was primarily controlled by the abundance of natural land covers (wetland and wooded areas). BACT, BP, and enzyme activities were positively related primarily to nutrient availability and/or anthropogenic land use (Stepwise R ² range: 0.33?C0.73). The ratio of ??-glucosidase to alkaline phosphatase activity approached a 1:1 balance with increasing anthropogenic land use, decreased wetland and forest cover, and increased total dissolved nitrogen. The ratio of leucine-aminopeptidase to alkaline phosphatase activity approached 1:1 with both increased dissolved organic carbon and nitrogen. Moreover, enzyme C:N:P ratios moved closer to 1:1:1 with faster water column bacterial turnover times. These results suggest that water column microbial communities are better able to balance resource availability with growth in streams receiving nutrient subsidies from anthropogenic sources and under these conditions when carbon resources increase.  相似文献   

Tropical Visions in an Age of Empire – Edited by Felix Driver and Luciana Martins
The Tropics and the Traveling Gaze: India, Landscape, and Science 1800–1856 – By David Arnold     

Daniel Clayton 《Singapore journal of tropical geography》2008,29(2):236-239

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