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181.
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
182.
Co-existing fluid and silicate inclusions in mantle diamond 总被引:1,自引:0,他引:1
We document the compositions of co-existing silicate macro-inclusions and fluid micro-inclusions in the fibrous coats of eight coated diamonds from the Panda kimberlite (Canada). The mineral inclusions in the diamond coats come from either the peridotite suite (Cr-pyrope, orthopyroxene, olivine and Cr-diopside) or the eclogite suite (omphacite). Therefore, fibrous diamonds grow in the same paragenetic environments as octahedral diamonds. The inclusions document a more fertile source composition (lower Mg# and higher CaO) than for equivalent phases in octahedral diamonds from Panda and worldwide. However, moderate to high Cr2O3 contents in garnet and clinopyroxene inclusions suggest that this apparent fertility is due to a secondary process. Geothermometry of the silicate inclusions yields low equilibration temperatures of 930 to 1010 °C. The co-existing fluid micro-inclusions are dominated by H2O, carbonate and KCl. Fluid inclusions in both the peridotitic and eclogitic samples fall along linear arrays between Fe–Ca–Mg carbonate and KCl. Inclusions in the one eclogitic sample also contain quartz. We suggest that the diamonds have trapped both metasomatised minerals and the metasomatic fluid, and so provide a snap shot of a metasomatic event in the mantle. 相似文献
183.
184.
Fluid inclusion and stable isotope (O, H, C, and S) constraints on the genesis of the Serrinha gold deposit, Gurupi Belt, northern Brazil 总被引:1,自引:0,他引:1
Evandro L. Klein Chris Harris Christophe Renac André Giret Candido A. V. Moura Kazuo Fuzikawa 《Mineralium Deposita》2006,41(2):160-178
The Serrinha gold deposit of the Gurupi Belt, northern Brazil, belongs to the class of orogenic gold deposits. The deposit is hosted in highly strained graphitic schist belonging to a Paleoproterozoic (∼2,160 Ma) metavolcano-sedimentary sequence. The ore-zones are up to 11 m thick, parallel to the regional NW–SE schistosity, and characterized by quartz-carbonate-sulfide veinlets and minor disseminations. Textural and structural data indicate that mineralization was syn- to late-tectonic and postmetamorphic. Fluid inclusion studies identified early CO2 (CH4-N2) and CO2 (CH4-N2)-H2O-NaCl inclusions that show highly variable phase ratios, CO2 homogenization, and total homogenization temperatures both to liquid and vapor, interpreted as the product of fluid immiscibility under fluctuating pressure conditions, more or less associated with postentrapment modifications. The ore-bearing fluid typically has 18–33mol% of CO2, up to 4mol% of N2, and less than 2mol% of CH4 and displays moderate to high densities with salinity around 4.5wt% NaCl equiv. Mineralization occurred around 310 to 335°C and 1.3 to 3.0 kbar, based on fluid inclusion homogenization temperatures and oxygen isotope thermometry with estimated oxygen fugacity indicating relatively reduced conditions. Stable isotope data on quartz, carbonate, and fluid inclusions suggest that veins formed from fluids with δ18OH2O and δDH2O (310–335°C) values of +6.2 to +8.4‰ and −19 to −80‰, respectively, which might be metamorphic and/or magmatic and/or mantle-derived. The carbon isotope composition (δ13C) varies from −14.2 to −15.7‰ in carbonates; it is −17.6‰ in fluid inclusion CO2 and −23.6‰ in graphite from the host rock. The δ34S values of pyrite are −2.6 to −7.9‰. The strongly to moderately negative carbon isotope composition of the carbonates and inclusion fluid CO2 reflects variable contribution of organic carbon to an originally heavier fluid (magmatic, metamorphic, or mantle-derived) at the site of deposition and sulfur isotopes indicate some oxidation of the originally reduced fluid. The deposition of gold is interpreted to have occurred mainly in response to phase separation and fluid-rock interactions such as CO2 removal and desulfidation reactions that provoked variations in the fluid pH and redox conditions. 相似文献
185.
Calculation of lava effusion rates from Landsat TM data 总被引:1,自引:0,他引:1
Andrew J. L. Harris Luke P. Flynn Laszlo Keszthelyi Peter J. Mouginis-Mark Scott K. Rowland Joseph A. Resing 《Bulletin of Volcanology》1998,60(1):52-71
We present a thermal model to calculate the total thermal flux for lava flowing in tubes, on the surface, or under shallow
water. Once defined, we use the total thermal flux to estimate effusion rates for active flows at Kilauea, Hawaii, on two
dates. Input parameters were derived from Landsat Thematic Mapper (TM), field and laboratory measurements. Using these parameters
we obtain effusion rates of 1.76±0.57 and 0.78±0.27 m3 s–1 on 23 July and 11 October 1991, respectively. These rates are corroborated by field measurements of 1.36±0.14 and 0.89±0.09 m3 s–1 for the same dates (Kauahikaua et al. 1996). Using weather satellite (AVHRR) data of lower spatial resolution, we obtain
similar effusion rates for an additional 26 dates between the two TM-derived measurements. We assume that, although total
effusion rates at the source declined over the period, the shut down of the ocean entry meant that effusion rates for the
surface flows alone remained stable. Such synergetic use of remotely sensed data provides measurements that can (a) contribute
to monitoring flow-field evolution, and (b) provide reliable numerical data for input into rheological and thermal models.
We look forward to being able to produce estimates for effusion rates using data from high-spatial-resolution sensors in the
earth observing system (EOS) era, such as Landsat 7, the hyperspectral imager, the advanced spaceborne thermal emission spectrometer,
and the advanced land imager.
Received: 25 July 1997 / Accepted: 26 February 1998 相似文献
186.
Thomas Stachel Jeff W. Harris Gerhard P. Brey 《Contributions to Mineralogy and Petrology》1998,132(1):34-47
Syngenetic diamond inclusions from the Mwadui kimberlite reveal that an unusually fertile section of lithospheric mantle
beneath the Central African Craton was sampled. This is shown by a very high ratio of lherzolitic to harzburgitic garnet inclusions
(1:2) and low Mg/Fe-ratios in olivine and orthopyroxene. Geothermometry applied to the peridotitic inclusions indicates disequilibrium
between non-touching inclusion pairs to be common. Disequilibrium between garnet-olivine and garnet-orthopyroxene pairs suggests
successive iron enrichment during diamond formation, e.g. leading to the presence of harzburgitic garnet and lherzolitic olivine
in the same diamond. Apart from the dominant peridotitic inclusion suite (88%), rare eclogitic inclusions occur (2%) and a
number of uncertain paragenesis. Two diamonds, one with eclogitic garnets with moderate pyroxene solid solution and the other
with a single ferro-periclase inclusion, suggest the contribution of a small sub-lithospheric component. The finding of the
association Fe-FeO-Fe3O4 in one single diamond indicates diamond formation over a large range of f
O2 conditions, possibly along redox fronts. Steep compositional gradients may also be reflected by the joint occurrence of harzburgitic
garnet and a SiO2-phase in the same diamond. Alternatively the formation of the SiO2-phase may be due to extreme carbonation of the peridotitic source. Further unusual findings include the exsolution of a silicate
phase from magnetite inclusions, (i.e. primary solution of γ-olivine) and an ilmenite inclusion with an eskolaite (Cr2O3) component of 14.5 mol%, the latter together with harzburgitic paragenesis silicate inclusions.
Received: 23 August 1997 / Accepted: 7 January 1998 相似文献
187.
Fault-valve behaviour in optimally oriented shear zones: an example at the Revenge gold mine, Kambalda, Western Australia 总被引:1,自引:0,他引:1
Phung T Nguyen Lyal B Harris Chris McA Powell Stephen F Cox 《Journal of Structural Geology》1998,20(12):1625-1640
Quartz vein systems developed in and adjacent to shear zones host major gold deposits in the Kambalda region of the Norseman–Wiluna greenstone belt. At the Revenge Mine, two groups of mineralised reverse shear zones formed as conjugate, near-optimally oriented sets during ESE subhorizontal shortening adjacent to a major transpressional shear system. The shear zones developed at temperatures of about 400°C in a transitional brittle–ductile regime. Deformation was associated with high fluid fluxes and involved fault-valve behaviour at transiently near-lithostatic fluid pressures. During progressive evolution of the shear system, early brittle and ductile deformation was overprinted by predominantly brittle deformation. Brittle shear failure was associated with fault dilation and the formation of fault-fill veins, particularly at fault bends and jogs. A transition from predominantly brittle shear failure to combined shear along faults and extension failure adjacent to faults occurred late during shear zone evolution and is interpreted as a response to a progressive decrease in maximum shear stress and a decrease in effective stresses. The formation of subhorizontal stylolites, locally subvertical extension veins and minor normal faults in association with thrust faulting, indicates episodic or transient reorientation of the near-field maximum principal stress from a subhorizontal to a near-vertical attitude during some fault-valve cycles. Local stress re-orientation is interpreted as resulting from near-total shear stress release and overshoot during some rupture events. Previously described fault-valve systems have formed predominantly in severely misoriented faults. The shear systems at Revenge Mine indicate that fault-valve action, and associated fluctuations in shear stress and fluid pressure, can influence the mechanical behaviour of optimally-oriented faults. 相似文献
188.
189.
N. W. Harris & M. E. Bailey 《Monthly notices of the Royal Astronomical Society》1998,297(4):1227-1236
We present results from long-term numerical integrations of hypothetical Jupiter-family comets (JFCs) over time-scales in excess of the estimated cometary active lifetime. During inactive periods these bodies could be considered as 'cometary' near-Earth objects (NEOs) or 'cometary asteroids'. The contribution of cometary asteroids to the NEO population has important implications not only for understanding the origin of inner Solar system bodies but also for a correct assessment of the impact hazard presented to the Earth by small bodies throughout the Solar system. We investigate the transfer probabilities on to 'decoupled' subJovian orbits by both gravitational and non-gravitational mechanisms, and estimate the overall inactive cometary contribution to the NEO population. Considering gravitational mechanisms alone, more than 90 per cent of decoupled NEOs are likely to have their origin in the main asteroid belt. When non-gravitational forces are included, in a simple model, the rate of production of decoupled NEOs from JFC orbits becomes comparable to the estimated injection rate of fragments from the main belt. The Jupiter-family (non-decoupled) cometary asteroid population is estimated to be of the order of a few hundred to a few thousand bodies, depending on the assumed cometary active lifetime and the adopted source region. 相似文献
190.
Summary This paper addresses the digital dissemination of geographically referenced census information in the UK. A number of important weaknesses in the 1991 model of data access are identified, and the possibility of future access to census information via the World Wide Web is then addressed in detail. Two case studies demonstrate the potential to overcome some fundamental weaknesses in earlier access models, including the provision of integrated data and metadata, graphical interfaces to geographical datasets, and an integrated interface and analysis environment. 相似文献