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41.
Both hypervelocity impact and dynamic spall experiments were carried out on a series of well-indurated samples of gabbro to examine the relation between spall strength and maximum spall ejecta thickness. The impact experiments carried out with 0.04- to 0.2-g, 5- to 6-km/sec projectiles produced decimeter- to centimeter-sized craters and demonstrated crater efficiencies of 6 × 10?9 g/erg, an order of magnitude greater than in metal and some two to three times that of previous experiments on less strong igneous rocks. Most of the crater volume (some 60 to 80%) is due to spall failure. Distribution of cumulative fragment number, as a function of mass of fragments with masses greater than 0.1 g yield values of b = d(log Nf)/d log(m) ?0.5 ?0.6, where N is the cumulative number of fragments and m is the mass of fragments. These values are in agreement or slightly higher than those obtained for less strong rocks and indicate that a large fraction of the ejecta resides in a few large fragments. The large fragments are plate-like with mean values of B/A and C/A 0.8 0.2, respectively (A = long, B = termediate, and C = short fragment axes). The small equant-dimensioned fragments (with mass < 0.1 g and B ~ 0.1 mm) represent material which has been subjected to shear failure. The dynamic tensile strenght of San Marcos gabbro was determined at strain rates of 104 to 105 sec?1 to be 147 ± 9 MPa. This is 3 to 10 times greater than inferred from quasi-static (strain rate 100 sec?1) loading experiments. Utilizing these parameters in a continuum fracture model predicts a tensile strenght of , where ε is strain rate. It is suggested that the high spall strenght of basic igneous rocks gives rise to enhanced cratering efficiencies due to spall in the <102-m crater diamter strength-dominated regime. Although the impact spall mechanism can enhance cratering efficiencies it is unclear that resulting spall fragments achieve sufficient velocities such that fragments of basic rocks can escape from the surfaces of planets such as the Moon or Mars. 相似文献
42.
V. Lykousis S. Alexandri J. Woodside P. Nomikou C. Perissoratis D. Sakellariou G. de Lange A. Dahlmann D. Casas G. Rousakis D. Ballas Chr. Ioakim 《Environmental Geology》2004,46(8):1030-1037
A 3000 km2 multibeam survey was carried out on the eastern Anaximander Mountains (Mts), (Eastern Mediterranean). The objective was to obtain detailed bathymetry of known mud volcanoes and identify new sites of active mud volcanism in the area. N-NW of the Amsterdam Mud-Volcano (MV) several mounds and cone-like morphological irregularities with a height of few tens of meters to about one hundred meters were detected and considered as potential MVs on the basis of their distinctive backscattered character. A group of two mounds was selected, the northern mound was sampled, documented as active, and named accordingly Athina MV. These new findings strongly support the presence of extensive active mud volcanism in the Anaximander Mountains especially in the area north of the Amsterdam MV.GEM 相似文献
43.
Differential aerobic and anaerobic oxidation of hydrocarbon gases discharged at mud volcanoes in the Nile deep-sea fan 总被引:1,自引:0,他引:1
Vincent Mastalerz Gert J. de Lange Anke Dählmann 《Geochimica et cosmochimica acta》2009,73(13):3849-2169
The present study investigates hydrocarbon oxidation processes at Isis and Amon mud volcanoes (MV’s), in the eastern Nile deep-sea fan. In the water column, molecular and carbon isotopic signatures of light hydrocarbons indicate that gases rapidly dissolve in seawater and are partially oxidized.In the upper sediments, anaerobic oxidation of the light hydrocarbons takes place, as clearly shown by their molecular and isotopic composition. These processes lead to the presence of a distinct Sulfate-Hydrocarbon Interface at 120-145 cm and 20-50 cm below the seafloor, for Isis and Amon MV’s, respectively. In contrast to processes occurring in the water column, a clear preferential oxidation of methane, propane and n-butane over ethane and i-butane is observed in the anoxic sediments. Furthermore, for the first time, fractionation factors have been determined for the anaerobic oxidation of propane and butane, being respectively −4.80‰ and −0.7‰ for δ13C, and −43.3‰ for δ2H of propane. 相似文献
44.
45.
P.S. Pauw S.E.A.T.M. Van der Zee A. Leijnse J.R. Delsman P.G.B. De Louw W.J. De Lange G.H.P. Oude Essink 《Ground water》2015,53(5):771-781
Groundwater‐surface water (GW‐SW) interaction in numerical groundwater flow models is generally simulated using a Cauchy boundary condition, which relates the flow between the surface water and the groundwater to the product of the head difference between the node and the surface water level, and a coefficient, often referred to as the “conductance.” Previous studies have shown that in models with a low grid resolution, the resistance to GW‐SW interaction below the surface water bed should often be accounted for in the parameterization of the conductance, in addition to the resistance across the surface water bed. Three conductance expressions that take this resistance into account were investigated: two that were presented by Mehl and Hill (2010) and the one that was presented by De Lange (1999). Their accuracy in low‐resolution models regarding salt and water fluxes to a dense drainage network in a confined aquifer system was determined. For a wide range of hydrogeological conditions, the influence of (1) variable groundwater density; (2) vertical grid discretization; and (3) simulation of both ditches and tile drains in a single model cell was investigated. The results indicate that the conductance expression of De Lange (1999) should be used in similar hydrogeological conditions as considered in this paper, as it is better taking into account the resistance to flow below the surface water bed. For the cases that were considered, the influence of variable groundwater density and vertical grid discretization on the accuracy of the conductance expression of De Lange (1999) is small. 相似文献
46.
R. Dietrich E.R. Ivins G. Casassa H. Lange J. Wendt M. Fritsche 《Earth and Planetary Science Letters》2010,289(1-2):22-29
A vertical crustal uplift rate of 39 mm yr? 1 is measured between 2003 and 2006 using Global Positioning System (GPS) measurements at the northeastern edge of the Southern Patagonia Icefield (SPI). This is the largest present-day glacial isostatic rate ever recorded. The combination of SPI's rapid melting and the unique regional slab-window tectonics that promotes a relatively low viscosity, is central to our interpretation of the observations. The two effects lead to a strong interaction of short relaxation times with ice loads that change on a comparable time scale. The profile of GPS observations link ice loss to the soft viscoelastic isostatic flow response over the time scale of the Little Ice Age (LIA), including ice loss in the period of observation. The agreement of the results with our model predictions strongly suggests the large crustal uplift in Patagonia is due an accelerated glacier wasting since the termination of the LIA and that the effective regional mantle viscosity is near 4.0–8.0 × 1018 Pa s. A century-long diminution of the icefields, at rates that are about 1/4 – 1/2 the contemporary loss rates, is consistent with multidecadal-scale temperature trends estimated for the past 50–100 years and is, in fact, a key feature in any model capable of explaining the uplift data. 相似文献
47.
The petrology of five phenocryst-poor (2–5%) andesites and dacites, all of which were erupted from different short-lived,
monogenetic vents, is compared to that of phenocryst-rich (10–25%) andesites erupted from the adjacent stratovolcano, Volcán
Tequila, in the Mexican arc. Despite differences in phenocryst abundances, these magmas have comparable phase assemblages
(plagioclase + orthopyroxene + titanomagnetite + ilmenite + apatite ± augite ± hornblende), and similarly wide variations
in phenocryst compositions, coupled to complex zoning patterns. For the phenocryst-poor lavas, equilibrium pairs of two Fe–Ti
oxides lead to a narrow range of calculated temperatures for each sample that range from 934 (±24) to 1,073 (±6)°C and oxygen
fugacities that range from +0.1 to +0.7 log units relative to the Ni–NiO buffer. Application of the plagioclase-liquid hygrometer
to each sample at these calculated temperatures leads to maximum melt water concentrations of 4.6–3.1 wt% during plagioclase
crystallization, indicating that the magmas were fluid saturated at depths ≥6.4–4.5 km. There is a wide, continuous range
in the composition of plagioclase (≤44 mol% An) and orthopyroxene (≤16% Mg#) phenocrysts in each sample, which is consistent
with a loss of dissolved water (≤2.8 wt%) from the melt phase during degassing as the magmas ascended rapidly to the surface.
Evidence is presented that shows the effect of dissolved water is to reduce the activity of MgO relative to FeO in the melt
phase, which indicates that degassing will also affect the Mg# of pyroxene phenocrysts, with higher melt water concentrations
favoring Fe-rich pyroxene. Both plagioclase and orthopyroxene commonly display diffusion-limited growth textures (e.g., skeletal
and hopper crystals, large interior melt hollows, and swallow tails), which are consistent with large undercoolings produced
by degassing-induced crystallization. Therefore, degassing is proposed as a possible cause for the phenocryst compositional
diversity documented in the phenocryst-poor andesite and dacite lavas erupted from peripheral vents, including the coexistence
of normally zoned plagioclase and reversely zoned orthopyroxene. Degassing-induced crystallization may also explain some of
the phenocryst complexity in crystal-rich andesites erupted from large stratovolcanoes, including Volcán Tequila. 相似文献
48.
A quantitative reconstruction of organic matter and nutrient diagenesis in Mediterranean Sea sediments over the Holocene 总被引:2,自引:0,他引:2
Daniel C. Reed Caroline P. Slomp Gert J. de Lange 《Geochimica et cosmochimica acta》2011,75(19):5540-5558
A multicomponent diagenetic model was developed and applied to reconstruct the conditions under which the most recent sapropel, S1, was deposited in the eastern Mediterranean Sea. Simulations demonstrate that bottom waters must have been anoxic and sulphidic during the formation of S1 and that organic matter deposition was approximately three times higher than at present. Nevertheless, most present day sediment and pore water profiles — with the exception of pyrite, iron oxyhydroxides, iron-bound phosphorus and phosphate — can be reproduced under a wide range of redox conditions during formation of S1 by varying the depositional flux of organic carbon. As a result, paleoredox indicators (e.g., Corg:S ratio, Corg:Porg ratio, trace metals) are needed when assessing the contribution of oxygen-depletion and enhanced primary production to the formation of organic-rich layers in the geological record. Furthermore, simulations show that the organic carbon concentration in sediments is a direct proxy for export production under anoxic bottom waters.The model is also used to examine the post-depositional alteration of the organic-rich layer focussing on nitrogen, phosphorus, and organic carbon dynamics. After sapropel formation, remineralisation is dominated by aerobic respiration at a rate that is inversely proportional to the time since bottom waters became oxic once again. A sensitivity analysis was undertaken to identify the most pertinent parameters in regulating the oxidation of sapropels, demonstrating that variations in sedimentation rate, depositional flux of organic carbon during sapropel formation, bottom water oxygen concentration, and porosity have the largest impact. Simulations reveal that sedimentary nutrient cycling was markedly different during the formation of S1, as well as after reoxygenation of bottom waters. Accumulation of organic nitrogen in sediments doubled during sapropel deposition, representing a significant nitrogen sink. Following reventilation of deep waters, N2 production by denitrification was almost 12 times greater than present day values. Phosphorus cycling also exhibits a strong redox sensitivity. The benthic efflux of phosphate was up to 3.5 times higher during the formation of S1 than at present due to elevated depositional fluxes of organic matter coupled with enhanced remineralisation of organic phosphorus. Reoxygenation of bottom waters leads to a large phosphate pulse to the water column that declines rapidly with time due to rapid oxidation of organic material. The oxidation of pyrite at the redox front forms iron oxyhydroxides that bind phosphorus and, thus, attenuate the benthic phosphate efflux. These results underscore the contrasting effects of oxygen-depletion on sedimentary nitrogen and phosphorus cycling. The simulations also confirm that the current conceptual paradigm of sapropel formation and oxidation is valid and quantitatively coherent. 相似文献
49.
50.
I. Leya H‐J. Lange M. LÜPke U. Neupert R. Daunke O. Fanenbruck R. Michel R. R
Sel B. Meltzow T. Schiekel F. Sudbrock U. Herpers D. Filges G. Bonani B. Dittrich‐Hannen M. Suter P. w. Kubik H‐a. Synal 《Meteoritics & planetary science》2000,35(2):287-318
Abstract— Thick spherical targets made of gabbro (R = 25 cm) and of steel (R = 10 cm) were irradiated isotropically with 1.6 GeV protons at the Saturne synchrotron at Laboratoire National Saturne (LNS)/CEN Saclay in order to simulate the interaction in space of galactic cosmic‐ray (GCR) protons with stony and iron meteoroids. Proton fluences of 1.32 × 1014 cm?2 and 2.45 × 1014 cm?2 were received by the gabbro and iron sphere, respectively, which corresponds to cosmic‐ray exposure ages of about 1.6 and 3.0 Ma. Both artificial meteoroids contained large numbers of high‐purity target foils of up to 28 elements at different depths. In these individual target foils, elementary production rates of radionuclides and rare gas isotopes were measured by x‐ and γ‐spectrometry, by low‐level counting, accelerator mass spectrometry (AMS), and by conventional rare gas mass spectrometry. Also samples of the gabbro itself were analyzed. Up to now, for each of the experiments, ~500 target‐product combinations were investigated of which the results for radionuclides are presented here. The experimental production rates show a wide range of depth profiles reflecting the differences between low‐, medium‐, and high‐energy products. The influence of the stony and iron matrices on the production of secondary particles and on particle transport, in general, and consequently on the production rates is clearly exhibited by the phenomenology of the production rates as well as by a detailed theoretical analysis. Theoretical production rates were calculated in an a priori way by folding depth‐dependent spectra of primary and secondary protons and secondary neutrons calculated by Monte Carlo techniques with the excitation functions of the underlying nuclear reactions. Discrepancies of up to a factor of 2 between the experimental and a priori calculated depth profiles are attributed to the poor quality of the mostly theoretical neutron excitation functions. Improved neutron excitation functions were obtained by least‐squares deconvolution techniques from experimental thick‐target production rates of up to five thick‐target experiments in which isotropic irradiations were performed. A posteriori calculations using the adjusted neutron cross sections describe the measured depth profiles of all these simulation experiments within 9%. The thus validated model calculations provide a basis for reliable physical model calculations of the production rates of cosmogenic nuclides in stony and iron meteorites as well as in lunar samples and terrestrial materials. 相似文献