Pressure dependence of melt viscosities on the join diopside-albite     

Mark Brearley  James E. Dickinson  Jr.  Christopher M. Scarfe 《Geochimica et cosmochimica acta》1986,50(12):2563-2570

The pressure dependence of melt viscosities on the join diopside-albite has been studied using falling-sphere viscometry. The five melt compositions investigated are: diopside, Ab₂₅Di₇₅, Ab₅₀Di₅₀, Ab₇₅Di₂₅ and albite. Experiments were performed at 1500° and 1600°C and at pressures of 5, 10, 15, 20 and 25 kbar. The positive and negative pressure dependence of the viscosity of diopside and albite, respectively, were confirmed. All intermediate compositions show an initial decrease in viscosity with increasing pressure; however, melt of Ab₂₅Di₇₅ composition passes through a minimum viscosity at approximately 12 kbar and 1600°C. This behavior is analogous to the variation in the viscosity of water with pressure at low temperature.

It is suggested that the three-dimensional, fully polymerized, albite structure dominates flow at low pressures. With increasing pressure, disruption of this structure and decrease in the average size of the flow units leads to domination by the diopside structure. The variation in viscosity with composition along the join at one atmosphere can be adequately modelled using the and (1965) configurational entropy model with an additional two-lattice configurational entropy of mixing term. The pressure dependence of viscosity in the diopside-albite system, however, cannot be predicted by the model, because there is an absence of information on the pressure dependence of the model parameters.

It is probable that relatively polymerized magmas (e.g. rhyolites to SiO₂-saturated basalts) show a negative pressure dependence of viscosity to depths where they originate in the lower crust or upper mantle. In contrast, the most depolymerized, naturally-occurring melts, such as strongly SiO₂-undersaturated basalts and picrites, may exhibit a viscosity minimum. The viscosity of these melts may be sufficiently high at depths within the upper mantle to inhibit their segregation, rise and eventual eruption at the surface.  相似文献   

Finite-difference elastic wave propagation in 2D heterogeneous transversely isotropic media1     

Christopher Juhlin 《Geophysical Prospecting》1995,43(6):843-858

The velocity-stress formulation for propagation of elastic seismic waves through 2D heterogeneous transversely isotropic media of arbitrary orientation is presented. The equations are recast into a finite-difference scheme and solved numerically using fourth-order spatial operators and a second-order temporal operator on a staggered grid. Absorbing, free-surface and symmetry boundary conditions have been implemented. Test cases compare well with other published solutions. Synthetic seismograms are calculated over two idealized models: (i) vertical fractures in granite with a dolerite sill reflector and (ii) a dipping anisotropic shale. Comparisons with the isotropic counterparts show significant differences which may have to be accounted for in seismic processing in the future.  相似文献   

The non-independence of rainfall erosivity and soil erodibility     

Christopher Morgan 《地球表面变化过程与地形》1983,8(4):323-338

Three high erosivity conditions (50 mm hr^?1, 100 mm hr^?1, and 200 mm hr^?1) were generated in a laboratory using a rainfall simulator and coherent soil block samples from fourteen different soil erodibility conditions. The data acquired supports the theoretical contention that soil loss should not increase as a simple linear function of storm intensity. Rather, a variable relationship is caused by the rupturing of surface seals and the changing relative significance of splash, wash and rainwash processes. Slope angle appears to influence soil loss at the higher erosivity conditions of 100 mm hr^?1 and 200 mm hr^?1 on slopes that were either very steep (> 20°) or very shallow (< 3°), but on moderate slopes the relationship is unclear. Examination of the variation of soil loss with erosivity when soil loss for a specific high erosivity condition is known revealed that conversion and power factors are of doubtful value and little generality. A satisfactory predictive equation, a power curve, is seen to be of value only when comparing rainwash soil loss between the higher erosivity conditions. The relationship is most safely considered as soil and site specific. Where the influence of slope and soil erodibility are disregarded, a strong association between soil loss and rainfall intensity is found. That soil loss, and hence, soil erodibility varies non-uniformly with erosivity is clear. The findings indicate caution is required when comparing conclusions drawn from studies based upon different erosivity conditions.  相似文献   

Lead-210 sediment geochronology in a changing coastal environment     

Jeffrey P. Chanton  Christopher S. Martens  George W. Kipphut 《Geochimica et cosmochimica acta》1983,47(10):1791-1804

Sediment accumulation rate studies utilizing excess ²¹⁰Pb and ¹³⁷Cs were conducted as part of recent investigations of biogeochemical cycling at a single site in Cape Lookout Bight, a rapidly changing coastal basin on the Outer Banks of North Carolina (U.S.A.). Cores three meters in length reveal a depositional history for the bight interior characterized by a gradual transition in texture from coarse-grained to fine-grained material over the period 1946–1979. This transition is controlled by progressive enclosure of the bight by an active northerly migrating recurved spit. The textural gradation is periodically interrupted by layers of well-sorted sand associated with major storm events. Lead-210 data indicate that the upper meter of the sediment has accumulated at a rate of 3.35 to 4.71 g · cm^?2 · yr^?1 or approximately 8.4 to 11.8 cm · yr^?1 (at ø = 0.84). Below 120 cm depth, dilution of clay and silt by low activity sand necessitates correction of the ²¹⁰Pb profile in order to establish a geochronology. Grain size ²¹⁰Pb distribution measurements at three depths reveal that the specific activity (dpm · g^?1) of clay is 3.2 times that of silt and 24.7 times that of sand. Corrections of bulk sediment excess ²¹⁰Pb activities based on these measurements lead to dates for textural changes which are consistent with charted changes in basin morphology and major storm events.Sixteen ¹³⁷Cs measurements between 33–241 cm depth reveal a peak activity at 105–115 cm and indicate a minimum sedimentation rate of approximately 2.7 g · cm^?2 · yr^?1.  相似文献   

Fate and transport of metals in H2S-rich waters at a treatment wetland     

Christopher H Gammons  Angela K Frandsen 《Geochemical transactions》2001,2(1):1-15

The aqueous geochemistry of Zn, Cu, Cd, Fe, Mn and As is discussed within the context of an anaerobic treatment wetland in Butte, Montana. The water being treated had a circum-neutral pH with high concentrations of trace metals and sulfate. Reducing conditions in the wetland substrate promoted bacterial sulfate reduction (BSR) and precipitation of dissolved metal as sulfide minerals. ZnS was the most common sulfide phase found, and consisted of framboidal clusters of individual spheres with diameters in the submicron range. Some of the ZnS particles passed through the subsurface flow, anaerobic cells in suspended form. The concentration of "dissolved" trace metals (passing through a 0.45 μm filter) was monitored as a function of H₂S concentration, and compared to predicted solubilities based on experimental studies of aqueous metal complexation with dissolved sulfide. Whereas the theoretical predictions produce "U-shaped" solubility curves as a function of H₂S, the field data show a flat dependence of metal concentration on H₂S. Observed metal concentrations for Zn, Cu and Cd were greater than the predicted values, particularly at low H₂S concentration, whereas Mn and As were undersaturated with their respective metal sulfides. Results from this study show that water treatment facilities employing BSR have the potential to mobilize arsenic out of mineral substrates at levels that may exceed regulatory criteria. Dissolved iron was close to equilibrium saturation with amorphous FeS at the higher range of sulfide concentrations observed (>0.1 mmol H₂S), but was more likely constrained by goethite at lower H₂S levels. Inconsistencies between our field results and theoretical predictions may be due to several problems, including: (i) a lack of understanding of the form, valence, and thermodynamic stability of poorly crystalline metal sulfide precipitates; (ii) the possible influence of metal sulfide colloids imparting an erroneously high "dissolved" metal concentration; (iii) inaccurate or incomplete thermodynamic data for aqueous metal complexes at the conditions of the treatment facility; and (iv) difficulties in accurately measuring low concentrations of dissolved sulfide in the field.  相似文献   

Seasonal variation in stable oxygen and carbon isotope values recovered from modern lacustrine freshwater mollusks: paleoclimatological implications for sub-weekly temperature records     

Christopher M. Wurster  William P. Patterson 《Journal of Paleolimnology》2001,26(2):205-218

A fingernail clam (Sphaerium simile, Sphaeriidae) from Science Lake, a small watershed located in Allegany State Park, New York, USA and a zebra mussel (Dreissena polymorpha, Dreissenidae) from Keuka Lake, New York, the third largest Finger Lake of central New York, were selected to evaluate the applicability of using ¹⁸O_(CaCO3) and ¹³O_(CaCO3) values for sub-weekly climate records. Seasonal variation in ¹⁸O_(CaCO3) values was compared with predicted equilibrium values to test the hypothesis that lacustrine molluscs produce shell aragonite according to environmental variables. For the purpose of comparison, aragonite temperature-fractionation equations determined by Grossman& Ku (1986) and Patterson et al. (1993) were used. Sphaerium simile appears to produce ¹⁸O_(CaCO3) values predicted by Patterson et al. (1993), while Dreissena polymorpha produces ¹⁸O_(CaCO3) values in agreement with Grossman & Ku (1986). We attribute the difference to family-specific temperature-fractionation relationships. Because both types of mollusc record climate variables with a high degree of integrity, they should each serve as excellent paleoclimate proxies.The fingernail clam collected from a small watershed exhibits higher variation about the seasonal pattern than did the zebra mussel collected from a large watershed. This is attributed to the increased sensitivity of the small watershed to storm perturbation. Analysis of fossil molluscs from such watersheds might be useful in discerning paleo-storminess.  相似文献   

Analysis of sustained long-period activity at Etna Volcano, Italy   总被引：1，自引：0，他引：1  

Gilberto Saccorotti  Ivan Lokmer  Christopher J. Bean  Giuseppe Di Grazia  Domenico Patan 《Journal of Volcanology and Geothermal Research》2007,160(3-4):340-354

Following the installation of a broadband network on Mt. Etna, sustained Long-Period (LP) activity was recorded accompanying a period of total quiescence and the subsequent onset of the 2004–2005 effusive episode. From about 56000 events detected by an automatic classification procedure, we analyse a subset of about 3000 signals spanning the December 17th, 2003–September 25th, 2004, time interval. LP spectra are characterised by several, unevenly-spaced narrow peaks spanning the 0.5–10 Hz frequency band. These peaks are common to all the recording sites of the network, and different from those associated with tremor signals. Throughout the analysed time interval, LP spectra and waveforms maintain significant similarity, thus indicating the involvement of a non-destructive source process that we interpret in terms of the resonance of a fluid-filled buried cavity. Polarisation analysis indicates radiation from a non-isotropic source involving large amounts of shear. Concurrently with LP signals, recordings from the summit station also depict Very-Long-Period (VLP) pulses whose rectilinear motion points to a region located beneath the summit craters at depths ranging between 800 and 1100 m beneath the surface. Based on a refined repicking of similar waveforms, we obtain robust locations for a selected subset of the most energetic LP events from probabilistic inversion of travel-times calculated for a 3D heterogenous structure. LP sources cluster in a narrow volume located beneath the summit craters, and extending to a maximum depth of ≈ 800 m beneath the surface. No causal relationships are observed between LP, VLP and tremor activities and the onset of the 2004–2005 lava effusions, thus indicating that magmatic overpressure played a limited role in triggering this eruption. These data represent the very first observation of LP and VLP activity at Etna during non-eruptive periods, and open the way to the quantitative modelling of the geometry and dynamics of the shallow plumbing system.  相似文献   

Non-stationary Approaches for Mapping Terrain and Assessing Prediction Uncertainty   总被引：3，自引：0，他引：3  

Christopher D. Lloyd  & Peter M. Atkinson 《Transactions in GIS》2002,6(1):17-30

It is well known that terrain may vary markedly over small areas and that statistics used to characterise spatial variation in terrain may be valid only over small areas. In geostatistical terminology, a non-stationary approach may be considered more appropriate than a stationary approach. In many applications, local variation is not accounted for sufficiently. This paper assesses potential benefits in using non-stationary geostatistical approaches for interpolation and for the assessment of uncertainty in predictions with implications for sampling design. Two main non-stationary approaches are employed in this paper dealing with (1) change in the mean and (2) change in the variogram across the region of interest. The relevant approaches are (1) kriging with a trend model (KT) using the variogram of residuals from local drift and (2) locally-adaptive variogram KT, both applied to a sampled photogrammetrically derived digital terrain model (DTM). The fractal dimension estimated locally from the double-log variogram is also mapped to illustrate how spatial variation changes across the data set. It is demonstrated that estimation of the variogram of residuals from local drift is worthwhile in this case for the characterisation of spatial variation. In addition, KT is shown to be useful for the assessment of uncertainty in predictions. This is shown to be true even when the sample grid is dense as is usually the case for remotely-sensed data. In addition, both ordinary kriging (OK) and KT are shown to provide more accurate predictions than inverse distance weighted (IDW) interpolation, used for comparative purposes.  相似文献   

Seismic image quality beneath strongly scattering structures and implications for lower crustal imaging: numerical simulations     

Francesca Martini  Christopher J. Bean  Sean Dolan  David Marsan† 《Geophysical Journal International》2001,145(2):423-435

  相似文献   

Formation and failure of volcanic debris dams in the Chakachatna River valley associated with eruptions of the Spurr volcanic complex, Alaska   总被引：1，自引：0，他引：1  

Christopher F. Waythomas 《Geomorphology》2001,39(3-4)

The formation of lahars and a debris avalanche during Holocene eruptions of the Spurr volcanic complex in south-central Alaska have led to the development of volcanic debris dams in the Chakachatna River valley. Debris dams composed of lahar and debris-avalanche deposits formed at least five times in the last 8000–10,000 years and most recently during eruptions of Crater Peak vent in 1953 and 1992. Water impounded by a large debris avalanche of early Holocene (?) age may have destabilized an upstream glacier-dammed lake causing a catastrophic flood on the Chakachatna River. A large alluvial fan just downstream of the debris-avalanche deposit is strewn with boulders and blocks and is probably the deposit generated by this flood. Application of a physically based dam-break model yields estimates of peak discharge (Q_p) attained during failure of the debris-avalanche dam in the range 10⁴<Q_p<10⁶ m³ s⁻¹ for plausible breach erosion rates of 10–100 m h⁻¹. Smaller, short-lived, lahar dams that formed during historical eruptions in 1953, and 1992, impounded smaller lakes in the upper Chakachatna River valley and peak flows attained during failure of these volcanic debris dams were in the range 10³<Q_p<10⁴ m³ s⁻¹ for plausible breach erosion rates.Volcanic debris dams have formed at other volcanoes in the Cook Inlet region, Aleutian arc, and Wrangell Mountains but apparently did not fail rapidly or result in large or catastrophic outflows. Steep valley topography and frequent eruptions at volcanoes in this region make for significant hazards associated with the formation and failure of volcanic debris dams.  相似文献