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41.
Acceleration of buoyancy-driven fractures and magmatic dikes beneath the free surface 总被引:3,自引:0,他引:3
By performing buoyancy-driven fracture experiments in brittle gelatin we observe that the ascent velocity of a fracture containing a finite volume of fluid increases when approaching the free surface. We theoretically describe this free-surface effect and quantify it by introducing an effective depth-dependent fracture toughness and developing an ascent model on the basis of linear fracture mechanics. We develop a successful inversion approach and resolve the actual and critical fracture length and the ascent velocity far away from the free surface from the observation of the fracture tip migration alone. Other parameters, as the fluid volume included in the fracture and the in situ fracture toughness, can be derived. Applying the model and inversion to the 1998 eruption at Piton de la Fournaise, Reunion Island, reveals estimates of the length and critical length of the feeding magma batch, the magma batch volume and the in situ fracture toughness. It further indicates that the ascent velocity of the magma batch was probably much smaller at greater depths and that the batch might have been initiated several months or years before the eruption. 相似文献
42.
S. N. Feldstein Rebecca A. Lange Torsten Vennemann James R. O'Neil 《Contributions to Mineralogy and Petrology》1996,126(1-2):51-66
Complete chemical analyses, including ferric and ferrous iron, H2O contents and δD values for 16 phlogopite and biotite and 2 hornblende separates are presented. Samples were obtained from
volcanic rocks from four localities: (1) phlogopite phenocrysts from minette lavas from the western Mexico continental arc,
(2) biotite and hornblende phenocrysts from andesite lavas from Mono Basin, California, (3) phlogopite and biotite from clinopyroxenite
nodules entrained in potassic lavas from the East African Rift, Uganda, and (4) phlogopite phenocrysts from a wyomingite lava
in the Leucite Hills, Wyoming. The Fe2O3 contents in the micas range from 0.8 to 10.5 wt%, corresponding to 0.09 to 1.15 Fe3+ per formula unit (pfu). Water contents vary from 1.6 to 3.0 wt%, corresponding to 1.58 to 3.04 OH pfu, significantly less
than would be expected for a site fully occupied by hydroxyl. Cation- and anion-based normalization procedures provide accurate
mineral formulae with respect to most cations and anions, but are unable to generate accurate estimates of Fe3+/FeT, and overestimate OH at the expense of O on the hydroxyl site. These inaccuracies are present despite acceptable adjusted
totals and stoichiometric calculated site occupancies. The phlogopite and biotite phenocrysts in arc-related lavas from western
Mexico and eastern California have the highest Fe3+/FeT ratios (56–87%), reflecting high magmatic oxygen fugacities (ΔNNO = +2 to +5), in contrast to those from Uganda (25–40%)
and the Leucite Hills (23%). There is no correlation between the OH content and the Fe3+/FeT ratio in the micas. Values of KMg/Fe2+D (± 2σ errors) were calculated for three phlogopite-olivine pairs (0.12 ± 0.12, 0.26 ± 0.14, 0.09 ± 0.12), two biotite-hornblende
pairs (0.73 ± 0.08 and 1.22 ± 0.10) and a single phlogopite-augite pair (1.15 ± 0.12). Values of KF/OHD for two biotite and
hornblende pairs could not be determined without significant error because of the extremely low F contents (< 0.2 wt%) of
the four phases. The δD values obtained in this study encompass a large range (−137 to −43‰). The phlogopite and biotite separates
from Uganda have δD values of −70 to −49‰, which overlap those believed to represent “primary” mantle. There is a larger range
in δD values (−137 to −43‰) for phlogopite phenocrysts from western Mexico minette lavas, although their range in δ18O values (5.2–6.2‰) is consistent with “normal” mantle. It is unlikely, therefore, that the variable δD values reflect heterogeneity
in the mantle source region of the minette magmas. Nor can the extremely low δD values reflect degassing of H2 or H2O since almost 100% loss of dissolved water in the magma is required, an unrealistic scenario given the stability of the hydrous
phenocrysts. The very low δD values of the Mascota minette phlogopites require that the hydrogen be introduced from an external
source (e.g., meteoric water). Whatever the process responsible for the observed hydrogen isotope composition, it had no effect
on the δ18O value, f
O
2, a
H
2O or bulk composition of the host magmas.
Received: 5 January 1995 / Accepted: 19 March 1996 相似文献
43.
Effects of speciation on equilibrium fractionations and rates of oxygen isotope exchange between (PO4)aq and H2O 总被引:1,自引:0,他引:1
The effects of phosphate speciation on both rates of isotopic exchange and oxygen isotope equilibrium fractionation factors between aqueous phosphate and water were examined over the temperature range 70 to 180°C. Exchange between phosphate and water is much faster at low pH than at high pH, an observation that is similar to what has been observed in the analogous sulfate-water system. Oxygen isotope fractionations between protonated species like H3PO4 and H2PO4− that are dominant at relatively low pH and species like PO43− and ion pairs like KHPO4− that are dominant at relatively high pH, range between 5 and 8‰ at the temperatures of the experiments. In aqueous phosphate systems at equilibrium, 18O/16O ratios increase with increasing degree of protonation of phosphate. This effect can be explained in part by the relative magnitudes of the dissociation constants of the protonated species. Under equilibrium conditions, carbonate in solution or in solid phases concentrates 18O relative to orthophosphate in solution or in solid phases at all temperatures, supporting the traditional view that biogenic phosphate is precipitated in near oxygen isotope equilibrium with body/ambient aqueous fluids with no attendant vital effects. 相似文献
44.
Laug Andreas Haberzettl Torsten Pannes Andre Schwarz Anja Turner Falko Wang Junbo Engels Stefan Rigterink Sonja Börner Nicole Ahlborn Marieke Ju Jianting Schwalb Antje 《Journal of Paleolimnology》2021,66(3):171-186
Journal of Paleolimnology - The Tibetan Plateau, also known as the “Water Tower of Asia” because of its function as a water storage and supply region, responds dramatically to modern... 相似文献
45.
Torsten Henkel Thomas Stephan Elmar K. Jessberger Peter Hoppe Roger Strebel Sachiko Amari Roy S. Lewis 《Meteoritics & planetary science》2007,42(7-8):1121-1134
Abstract— Thirteen presolar silicon carbide grains—three of supernova (SN) origin and ten of asymptotic giant branch (AGB) star origin—were examined with time‐of‐flight‐secondary ion mass spectrometry (TOF‐SIMS). The grains had been extracted from two different meteorites—Murchison and Tieschitz—using different acid residue methods. At high lateral resolution of ~300 nm, isotopic and elemental heterogeneities within the micrometer‐sized grains were detected. The trace elemental abundances, when displayed in two‐element correlation plots, of Li, Mg, K, and Ca show a clear distinction between the two different meteoritic sources. The different concentrations might be attributed to differences of the host meteorites and/or of extraction methods whereas the stellar source seems to be less decisive. In one SN grain with 26Mg‐enrichment from extinct 26Al, the acid treatment, as part of the grain separation procedure, affected the Mg/Al ratio in the outer rim and therefore the inferred initial 26Al/27Al ratio. A second SN grain exhibits a lateral heterogeneity in 26Al/27Al, which either is due to residual Al‐rich contamination on the grain surface or to the condensation chemistry in the SN ejecta. 相似文献
46.
Comparison of Absolute and Relative Moment Tensor Solutions For The January 1997 West Bohemia Earthquake Swarm 总被引:2,自引:0,他引:2
Moment tensor solutions for 70 clustered events of the 1997 West Bohemia microearthquake swarm, as calculated by two different methods, are given. The first method is a single-event, absolute moment tensor inversion which inverts body-wave peak amplitudes using synthetic Green functions. The second method is a multiple-event, relative method for which Green functions are reduced to 2 geometrical angles of rays at the sources. Both methods yield similar moment tensors, which can be divided into at least two or three different classes of focal solutions, indicating that, during the swarm activity, different planes of weakness were active. The major source component of most events is a double couple. However, the deviations from the double-couple mechanisms seem to be systematic for some classes of solutions. Error analysis was based on transforming the estimate of the standard deviation of amplitudes extracting from the seismograms into confidence regions of the absolute moment tensor. They show that the non-DC components are significant at a fairly high confidence level. 相似文献
47.
Numerical simulations of the propagation path and the arrest of fluid-filled fractures in the Earth 总被引:3,自引:0,他引:3
T. Dahm 《Geophysical Journal International》2000,141(3):623-638
48.
Stefan Doberschütz Peter Frenzel Torsten Haberzettl Thomas Kasper Junbo Wang Liping Zhu Gerhard Daut Antje Schwalb Roland Mäusbacher 《Journal of Paleolimnology》2014,51(2):253-266
This study focuses on Holocene monsoon dynamics on the central Tibetan Plateau (TP) inferred using a sediment record from Lake Nam Co. A high-resolution (decadal) multi-proxy approach, using geochemical, micropaleontological, and sedimentological methods was applied. Fifteen AMS-14C ages were used to establish the chronology, assuming a reservoir effect of 1,420 years. Our results point to a first strong monsoonal pulse at Lake Nam Co ~11.3 cal ka BP, followed by colder and drier conditions until ~10.8 cal ka BP. A warm and humid climate from ~10.8 to ~9.5 cal ka BP is related to a strong summer monsoon on the central TP, triggering a lake-level highstand at ~9.5 cal ka BP. Declining minerogenic input after ~9.5 cal ka BP indicates less moisture availability until ~7.7 cal ka BP. Following stable conditions between ~7.7 and ~6.6 cal ka BP, a warm and wet climate is inferred for the time span from ~6.6 to ~4.8 cal ka BP. A change towards drier conditions after ~4.8 cal ka BP points to a weakening of the Indian Ocean Summer Monsoon on the central TP, further diminished after ~2.0 cal ka BP. A short-term wet spell occurred from ~1.5 to ~1.3 cal ka BP. By comparing the results derived from Lake Nam Co with several lacustrine records from the central, northern, and eastern TP, a similar but not synchronous pattern of monsoon-driven paleoenvironmental change is observed. Although the general trend of lake and catchment evolution on the TP during the Holocene is also reproduced in this record, pronounced spatial and temporal offsets with respect to distinct climate events were detected, suggesting periods of non-uniform moisture and temperature evolution. 相似文献
49.
An early detection of the presence of rupture directivity plays a major role in the correct estimation of ground motions and
risks associated to the earthquake occurrence. We present here a simple method for a fast detection of rupture directivity,
which may be additionally used to discriminate fault and auxiliary planes and have first estimations of important kinematic
source parameters, such as rupture length and rupture time. Our method is based on the inversion of amplitude spectra from
P-wave seismograms to derive the apparent duration at each station and on the successive modelling of its azimuthal behaviour.
Synthetic waveforms are built assuming a spatial point source approximation, and the finite apparent duration of the spatial
point source is interpreted in terms of rupture directivity. Since synthetic seismograms for a point source are calculated
very quickly, the presence of directivity may be detected within few seconds, once a focal mechanism has been derived. The
method is here first tested using synthetic datasets, both for linear and planar sources, and then successfully applied to
recent Mw 6.2–6.8 shallow earthquakes in Peloponnese, Greece. The method is suitable for automated application and may be
used to improve kinematic waveform modelling approaches. 相似文献
50.
The variations of environmental conditions (T°, pH, δ13CDIC, [DIC], δ18O, Mg/Ca, and Sr/Ca) of ostracod habitats were examined to determine the controls of environmental parameters on the chemical and isotopic composition of ostracod valves. Results of a one-year monitoring of environmental parameters at five sites, with depths of between 2 and 70 m, in Lake Geneva indicate that in littoral to sub-littoral zones (2, 5, and 13 m), the chemical composition of bottom water varies seasonally in concert with changes in temperature and photosynthetic activity. An increase of temperature and photosynthetic activity leads to an increase in δ13C values of DIC and to precipitation of authigenic calcite, which results in a concomitant increase of Mg/Ca and Sr/Ca ratios of water. In deeper sites (33 and 70 m), the composition of bottom water remains constant throughout the year and isotopic values and trace element contents are similar to those of deep water within the lake. The chemical composition of interstitial pore water also does not reflect seasonal variations but is controlled by calcite dissolution, aerobic respiration, anaerobic respiration with reduction of sulphate and/or nitrate, and methanogenesis that may occur in the sediment pores. Relative influence of each of these factors on the pore water geochemistry depends on sediment thickness and texture, oxygen content in bottom as well as pore water. Variations of chemical compositions of the ostracod valves of this study vary according to the specific ecology of the ostracod species analysed, that is its life-cycle and its (micro-)habitat. Littoral species have compositions that are related to the seasonal variations of temperature, δ13C values of DIC, and of Mg/Ca and Sr/Ca ratios of water. In contrast, the compositions of profundal species are largely controlled by variations of pore fluids along sediment depth profiles according to the specific depth preference of the species. The control on the geochemistry of sub-littoral species is a combination of controls for the littoral and profundal species as well as the specific ecology of the species. 相似文献