A 10-yr record of stable isotope ratios of hydrogen and oxygen in precipitation at Calgary, Alberta, Canada   总被引：2，自引：0，他引：2  

HAIDONG PENG  BERNHARD MAYER  STUART HARRIS  H. ROY KROUSE 《Tellus. Series B, Chemical and physical meteorology》2004,56(2):147-159

  相似文献   

Magnetohydrodynamics of stellar interiors     

David Hughes  Robert Rosner  Nigel Weiss 《Astronomy& Geophysics》2005,46(4):4.35-4.36

David Hughes, Robert Rosner and Nigel Weiss describe what was achieved during a programme on stellar magnetic fields at the Isaac Newton Institute in Cambridge. Over a four-month period more than 90 participants visited the Institute for a mixture of structured workshops and informal collaboration.  相似文献   

Relative contributions of silicate and carbonate rocks to riverine Sr fluxes in the headwaters of the Ganges   总被引：1，自引：0，他引：1  

Mike J. Bickle  Hazel J. Chapman  Nigel B.W. Harris  Talat Ahmad 《Geochimica et cosmochimica acta》2005,69(9):2221-2240

Exhumation of the Himalayan-Tibetan orogen is implicated in the marked rise in seawater ⁸⁷Sr/⁸⁶Sr ratios since 40 Ma. However both silicate and carbonate rocks in the Himalaya have elevated ⁸⁷Sr/⁸⁶Sr ratios and there is disagreement as to how much of the ⁸⁷Sr flux is derived from silicate weathering. Most previous studies have used element ratios from bedrock to constrain the proportions of silicate- and carbonate-derived Sr in river waters. Here we use arrays of water compositions sampled from the head waters of the Ganges in the Indian and Nepalese Himalaya to constrain the end-member element ratios. The compositions of tributaries draining catchments restricted to a limited range of geological units can be described by two-component mixing of silicate and carbonate-derived components and lie on a plane in multicomponent composition space. Key elemental ratios of the carbonate and silicate components are determined by the intersection of the tributary mixing plane with the planes Na = 0 for carbonate and constant Ca/Na for silicate. The fractions of Sr derived from silicate and carbonate sources are then calculated by mass-balance in Sr-Ca-Mg-Na composition space. Comparison of end-member compositions with bedrock implies that secondary calcite deposition may be important in some catchments and that dissolution of low-Mg trace calcite in silicate rocks may explain discrepancies in Sr-Ca-Na-Mg covariation. Alternatively, composition-dependent precipitation or incongruent dissolution reactions may rotate mixing trends on cation-ratio diagrams. However the calculations are not sensitive to transformations of the compositions by incongruent dissolution or precipitation processes provided that the transformed silicate and carbonate component vectors are constrained. Silicates are calculated to provide ∼50% of the dissolved Sr flux from the head waters of the Ganges assuming that discrepancies between Ca-Mg-Na covariation and the silicate rock compositions arise from addition of trace calcite. If the Ca-Mg-Na mixing plane is rotated by composition-dependent secondary calcite deposition, this estimate would be increased. Moreover, when ⁸⁷Sr/⁸⁶Sr ratios of the Sr inputs are considered, silicate Sr is responsible for 70 ± 16% (1σ) of the ⁸⁷Sr flux forcing changes in seawater Sr-isotopic composition. Since earlier studies predict that silicate weathering generates as little as 20% of the total Sr flux in Himalayan river systems, this study demonstrates that the significance of silicate weathering can be greatly underestimated if the processes that decouple the water cation ratios from those of the source rocks are not properly evaluated.  相似文献   

The 21st Himalayan-Karakoram-Tibet workshop: Clarification     

Nigel Harris 《Gondwana Research》2006,10(3-4):398

  相似文献   

A Quantitative Study of Magmatic Inclusions in the Plutonic Ejecta of Ascension Island   总被引：1，自引：2，他引：1  

HARRIS  CHRIS 《Journal of Petrology》1986,27(1):251-276

Plutonic blocks in the volcanic and pyroclastic rocks of AscensionIsland contain a variety of silicate melt and fluid inclusions,mainly in the intermediate (monzonite-syenite) and granite types.The silicate melt inclusions consist of trapped granitic meltwith a small contraction bubble. The fluid inclusions are: (i)highly saline (40 to > 67 wt. per cent NaCI) aqueous inclusions;(ii) low density CO₂ + H₂O inclusions, predominantly vapourand (iii) high density approximately pure H₂O inclusions. Mixedsilicate melt-saline aqueous fluid inclusions also occur showingthat the original magma became saturated with respect to salineaqueous fluid and a magmatic origin is thus indicated for thesaline fluid inclusions. Microprobe analysis of the silicatemelt inclusions suggests that the original magma may have containedgreater than 5 wt. per cent H₂O before saturation was reachedand homogenization temperatures show the trapping of the silicatemelt took place between 715-790?C. Separated amphiboles fromthe granites have D values which are significantly differentfrom the whole rock D values of the dry but otherwise chemicallysimilar obsidians. Sheppard & Harris (1985) interpret thisdifference as reflecting a sea water source for the water inthe gramtes. Qualitative and quantitative study of the fluidinclusions (extending the earlier work of Roedder & Coombs,1967) suggests that at least one episode of fluid immiscibility(into aqueous liquid+vapour, i.e. ‘boiling’) occurred.This resulted in a loss of H₂O via the vapour and increasedthe salinity of the remaining liquid. The initial fluids belongedto the system H₂O-CO₂-NaCl-KCl and the CO₂ was strongly partitionedinto the vapour phase during fluid unmixing, hence X_co2 decreasedas the fluid evolved. A suite of gabbroic blocks from Dark SlopeCrater contain pure CO fluid inclusions of just higher thancritical density. Assuming a trapping temperature of 1000–1200?Cthe pressure must have been > 18 kb–28 kb or 7-11 kmin depth, i.e. well below the base of the Ascension Cone. Thepure CO₂ inclusions in this suite of gabbros and the fluidsin the H₂O-CO₂NaCl-KCl system would seem to require entirelydifferent origins and it may be significant that these gabbrosshow a different isotopic Pb signature than do the volcanicsand other blocks.  相似文献   

Bentonites in the Chalk of central eastern England and their relation to the opening of the Northeast Atlantic     

Nigel Robert Pacey 《Earth and Planetary Science Letters》1984,67(1):48-60

Within the post-Cenomanian Chalk sequence of central eastern England about 20 thin persistent marl horizons occur. These marls contain from 30 to 65% non-carbonate material the bulk of which is composed of a well crystallised Mg-rich smectite with a notable paucity of illite and quartz. The Chalk matrix above and below the marls contains up to 2.0% of non-carbonate matter which again is dominated by the presence of an Mg-rich smectite but which contains a larger proportion of illite and quartz than occurs in the marls. For both sets of non-carbonate materials the excellent crystallinity and Mg-rich nature of the smectite strongly suggest that it was formed by the in situ alteration of an aeolian transported volcanic ash, i.e. it is a true bentonite. Quartz and illite in the Chalk matrix are probably detrital components.Relative to a standard shale, non-carbonate materials from the Chalk marl horizons exhibit a highly anomalous trace element pattern with accentuated levels of Zr, Nb and Th but depleted levels of Ba, Rb and K. This trace element chemistry corroborates a volcanogenic origin for the smectite insofar as it is not explicable in terms of any realistic sedimentary process. The chemistry is best interpreted in terms of the magnetic processes responsible for the formation of the original ash precursor of the marls. It appears that this precursor had a pantelleritic composition and therefore originated from a source overlying an area where incipient continental rifting was taking place.Micromorphological examinations prove that residues from the Chalk marls contain small amounts of pyroclastic debris, including pumice and fragments of sideromelane glass. The morphology of the fragments suggests that the volcanic eruptions responsible for the ejection of the original ash into the air were of the shallow-water phreatomagmatic (Surtseyan) type.The distribution of marls in the Chalk of eastern England suggests that the ashfalls responsible for their origin were of the tropospheric type and were drawn by westerly winds from a centre to the west or northwest of mainland Britain. Of the numerous volcanic centres that lie in offshore areas to the west, the Anton Dohrn Seamount in the Rockall Trough appears to a likely centre of origin for the ashfalls. However, ashfalls responsible for the formation of the traces of smectite in the Chalk matrix were probably of the general background (stratospheric) type and may have been drawn from more distant centres.Volcanism at the Anton Dohrn Seamount was related to tensional effects in the continental crust that immediately preceded the formation of the Rockall Trough and the separation of Greenland from northwestern Europe during Tertiary times. Activity at the centre, and Cretaceous bentonites derived therefrom, therefore represent the earliest stages in the development of the Thulean igneous province.  相似文献   

Gelification of Victorian tertiary soft brown coal wood. I. Relationship between chemical composition and microscopic appearance and variation in the degree of gelification     

Nigel J. Russell 《International Journal of Coal Geology》1984,4(2):99-118

The relationship between chemical composition and microscopic appearance of present-day wood-derived materials and Victorian xylites and gelified soft brown coal woods, Latrobe Valley, Australia, is discussed. The present-day wood-derived material consists of milled wood and lignin and holocellulose fractions prepared from an angiosperm, Eucalyptus regnans and a gymnosperm, Pinus radiata. The xylites and least gelified soft brown coal woods resemble the P. radiata lignin fraction; the latter contains residual cellulose.The atomic H/C ratio was found to be a useful guide to the degree of gelification. Increase in the degree of gelification was accompanied not only by an increase in the total huminite reflectance, but also by an increase in the reflectance of the individual huminite submacerals. The loss of hymotelinite autofluorescence under ultraviolet and blue light excitation with increase in the degree of gelification of Victorian soft brown coal woods was ascribed primarily to the elimination of cellulose and, possibly, modification of the lignin structure of xylite.  相似文献   

Flood hazard management in the United States and the National Flood Insurance Program     

Nigel W. Arnell 《Geoforum》1984,15(4):525-542

The National Flood Insurance Program (NFIP) both provides flood insurance to floodplain occupants and encourages local communities to adopt floodplain land-use regulations. As well as providing the core of the federal non-structural flood hazard management effort, the NFIP is a central element in both state and local activities. A major issue influencing the success of the NFIP in curbing flood damages is its treatment of properties built before the adoption of floodplain regulations. The potential for using the NFIP to encourage floodproofing and influence post-flood hazard mitigation must be fully explored.  相似文献   

Post-collision magmatism and tectonics in northwest Anatolia   总被引：1，自引：0，他引：1  

Nigel B. W. Harris  Simon Kelley  Aral I. Okay 《Contributions to Mineralogy and Petrology》1994,117(3):241-252

A suite of biotite-hornblende granodiorite intrusions has been emplaced into blueschist-facies metasediments in northwest Anatolia, following collision between two continental margins, now represented by the Tavanli and Sakarya zones. The ⁴⁰Ar/³⁹Ar ages of phengites and glaucophanes from the blueschists, metamorphosed under unusually high P-low T conditions (P=20±2 kbar, T=430±30° C), suggest that metamorphism apparently occurred over a period spanning at least 20 Ma from 108 to 88 Ma. Post-tectonic granodiorites were emplaced during the Eocene (53 to 48 Ma) resulting in a cordierite and andalusite-bearing thermal aureole, indicative of pressures of 3 kbar. Trace-element systematics of the granodiorites are consistent with a derivation either from mantle-derived magmas by fractional crystallisation in shallow magma chambers, or from anatexis of crustal lithologies of internediate composition at pressures <10 kbar. The preservation of high P-low T assemblages in the blueschists together with the range of ages determined for blueschist-facies metamorphism are indicative of rapid exhumation of delaminated fragments from a subducted continental margin. However decompression melting of the crust is unlikely to have been a significant cause of magmatism, both because exhumation of the blueschists from deep crustal levels predated magmatism by at least 25 Ma, and because of the small melt fraction (<0.1) that may be generated in crustal lithologies by this process. Melting in the mantle wedge is required either to generate a primary melt for the derivation of magmas of intermediate composition or to provide an advective heat source for crustal melting. The cause of melt formation in the upper mantle may be related to the termination of subduction following collision during the Mid-Eocene.  相似文献   

Nature of the Source Regions for Post-collisional, Potassic Magmatism in Southern and Northern Tibet from Geochemical Variations and Inverse Trace Element Modelling   总被引：20，自引：0，他引：20  

WILLIAMS  H. M.; TURNER  S. P.; PEARCE  J. A.; KELLEY  S. P.; HARRIS  N. B. W. 《Journal of Petrology》2004,45(3):555-607

Neogene potassic lavas in northern and southern Tibet have differentisotopic (  相似文献