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991.
On post-glacial sea level: I. General theory 总被引:3,自引:0,他引:3
992.
Previous models of hydrodynamics in contact metamorphic aureoles assumed flow of aqueous fluids, whereas CO2 and other species are also common fluid components in contact metamorphic aureoles. We investigated flow of mixed CO2–H2O fluid and kinetically controlled progress of calc‐silicate reactions using a two‐dimensional, finite‐element model constrained by the geological relations in the Notch Peak aureole, Utah. Results show that CO2 strongly affects fluid‐flow patterns in contact aureoles. Infiltration of magmatic water into a homogeneous aureole containing CO2–H2O sedimentary fluid facilitates upward, thermally driven flow in the inner aureole and causes downward flow of the relatively dense CO2‐poor fluid in the outer aureole. Metamorphic CO2‐rich fluid tends to promote upward flow in the inner aureole and the progress of devolatilization reactions causes local fluid expulsion at reacting fronts. We also tracked the temporal evolution of P‐T‐XCO2conditions of calc‐silicate reactions. The progress of low‐ to medium‐grade (phlogopite‐ to diopside‐forming) reactions is mainly driven by heat as the CO2 concentration and fluid pressure and temperature increase simultaneously. In contrast, the progress of the high‐grade wollastonite‐forming reaction is mainly driven by infiltration of chemically out‐of‐equilibrium, CO2‐poor fluid during late‐stage heating and early cooling of the inner aureole and thus it is significantly enhanced when magmatic water is involved. CO2‐rich fluid dominates in the inner aureole during early heating, whereas CO2‐poor fluid prevails at or after peak temperature is reached. Low‐grade metamorphic rocks are predicted to record the presence of CO2‐rich fluid, and high‐grade rocks reflect the presence of CO2‐poor fluid, consistent with geological observations in many calc‐silicate aureoles. The distribution of mineral assemblages predicted by our model matches those observed in the Notch Peak aureole. 相似文献
993.
Development of shear bands in saturated soils is a multi-stage process based on the theoretical and numerical investigations in this paper. The soil is initially in homogenous shear strain state, and the instability can be characterized by a dimensionless number D. The inhomogenous distribution of shear strains appears when D>1, and the shear band will initiate and develop gradually. Numerical solutions show that only single shear band that is finally formed in the central region of the specimen even several disturbances (distributed along the specimen) appear in the beginning. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
994.
995.
Spacecraft data from the OMNI database were used to calculate the value of the polytropic index of the solar wind by fitting the logarithms of proton density and temperature in selected time intervals from 1 January 1995 to 30 June 2012. Bernoulli’s integral and the correlation coefficient were used to filter the results. An alternative method based on the maximization of the correlation coefficient was employed to confirm our results. The long-term behavior of the polytropic index we obtained is found to be virtually identical for both methods. We noticed a characteristic behavior of the estimated polytropic index values, particularly from 1995 to 2006, which tends to have a periodicity of about one year. The distribution of the polytropic index is best described by a κ-Gaussian distribution with mean ≈?1.8 and standard deviation ≈?2.4. We finally examined the possible correlation between the polytropic index values and solar activity. 相似文献
996.
STEREO A and B observations of the radial magnetic field between 1 January 2007 and 31 October 2008 show significant evidence
that in the heliosphere, the ambient radial magnetic field component with any dynamic effects removed is uniformly distributed.
Based on this monopolar nature of the ambient heliospheric field we find that the surface beyond which the magnetic fields
are in the monopolar configuration must be spherical, and this spherical surface can be defined as the inner boundary of the
heliosphere that separates the monopole-dominated heliospheric magnetic field from the multipole-dominated coronal magnetic
field. By using the radial variation of the coronal helmet streamers belts and the horizontal current – current sheet – source
surface model we find that the spherical inner boundary of the heliosphere should be located around 14 solar radii near solar
minimum phase. 相似文献
997.
998.
Dynamic Characteristics of Granite Subjected to Intermediate Loading Rate 总被引:18,自引:11,他引:18
Summary A large diameter split Hopkinson pressure bar (SHPB) has been developed. This equipment is briefly described, together with a shaped striker that initiates a half-sine incident waveform to obtain the complete stress-strain relationship of the Bukit Timah granite at medium strain rate. Good constant strain rate was derived, and the dynamic complete stress-strain curves and energy absorption of the granite were measured at a strain rate between 20 and 60 per second. Repeated impact between 60–90% of the static strength of the granite was also conducted. Results from the tests show that the cumulative damage of the granite depends on the peak stress of the dynamic loads with a fixed duration. The dynamic fracture strength of the granite loaded at medium strain rate is directly proportional to the cube root of the strain rate. For the granite loaded at this strain rate, Youngs modulus is unchanged. Energy absorption of the samples loading to fragmentation determined its fragmented size distribution. At high strain rate, the rock possesses large energy absorption and the particle size of the fragments is much smaller. 相似文献
999.
Ralf Gertisser Sylvain J. Charbonnier J?rg Keller Xavier Quidelleur 《Bulletin of Volcanology》2012,74(5):1213-1233
Merapi is an almost persistently active basalt to basaltic andesite volcanic complex in Central Java (Indonesia) and often referred to as the type volcano for small-volume pyroclastic flows generated by gravitational lava dome failures (Merapi-type nuées ardentes). Stratigraphic field data, published and new radiocarbon ages in conjunction with a new set of 40K–40Ar and 40Ar–39Ar ages, and whole-rock geochemical data allow a reassessment of the geological and geochemical evolution of the volcanic complex. An adapted version of the published geological map of Merapi [(Wirakusumah et al. 1989), Peta Geologi Gunungapi Merapi, Jawa Tengah (Geologic map of Merapi volcano, Central Java), 1:50,000] is presented, in which eight main volcano stratigraphic units are distinguished, linked to three main evolutionary stages of the volcanic complex—Proto-Merapi, Old Merapi and New Merapi. Construction of the Merapi volcanic complex began after 170?ka. The two earliest (Proto-Merapi) volcanic edifices, Gunung Bibi (109?±?60?ka), a small basaltic andesite volcanic structure on Merapi’s north-east flank, and Gunung Turgo and Gunung Plawangan (138?±?3?ka; 135?±?3?ka), two basaltic hills in the southern sector of the volcano, predate the Merapi cone sensu stricto. Old Merapi started to grow at ~30?ka, building a stratovolcano of basaltic andesite lavas and intercalated pyroclastic rocks. This older Merapi edifice was destroyed by one or, possibly, several flank failures, the latest of which occurred after 4.8?±?1.5?ka and marks the end of the Old Merapi stage. The construction of the recent Merapi cone (New Merapi) began afterwards. Mostly basaltic andesite pyroclastic and epiclastic deposits of both Old and New Merapi (<11,792?±?90 14C years BP) cover the lower flanks of the edifice. A shift from medium-K to high-K character of the eruptive products occurred at ~1,900 14C years BP, with all younger products having high-K affinity. The radiocarbon record points towards an almost continuous activity of Merapi since this time, with periods of high eruption frequency interrupted by shorter intervals of apparently lower eruption rates, which is reflected in the geochemical composition of the eruptive products. The Holocene stratigraphic record reveals that fountain collapse pyroclastic flows are a common phenomenon at Merapi. The distribution and run-out distances of these flows have frequently exceeded those of the classic Merapi-type nuées ardentes of the recent activity. Widespread pumiceous fallout deposits testify the occurrence of moderate to large (subplinian) eruptions (VEI 3–4) during the mid to late Holocene. VEI 4 eruptions, as identified in the stratigraphic record, are an order of magnitude larger than any recorded historical eruption of Merapi, except for the 1872?AD and, possibly, the October–November 2010 events. Both types of eruptive and volcanic phenomena require careful consideration in long-term hazard assessment at Merapi. 相似文献
1000.
E. V. Levashova S. G. Skublov X. H. Li S. G. Krivdik D. K. Voznyak A. A. Kulchitskaya V. I. Alekseev 《Geology of Ore Deposits》2016,58(3):239-262
The distribution of rare and rare earth elements in zircon at the Yastrebets, Azov (Zr–REE–Y), and Perzhan (Be) rare metal deposits of the Ukrainian Shield was studied. Additional evidence for magmatic genesis of these deposits is obtained: unaltered zircon is characterized by a magmatic REE distribution spectrum with a somewhat higher δ18O value than that of the mantle (6.6‰ on average). The final formation stage of the deposit was marked by predominance of fluids enriched in Y, REE, Nb, and heavy oxygen, resulting in anomalous geochemical characteristics of zircon rims and alteration zones (up to 81500 Y ppm, over 10300 ppm Nb, and 13.9‰ δ18O). The age of zircon formed in ore-bearing Yastrebets and Azov nonnepheline syenite deposits was estimated at ~1770 Ma (U–Pb, SHRIMP-II). 相似文献