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41.
The Long Valley Exploratory Well, at the center of the Resurgent Dome of Long Valley caldera, penetrated pre-caldera basement rocks at a depth of 2101.72–2313.0 m, beneath the caldera-forming Bishop Tuff and post-caldera Early Rhyolite. The basement rocks contain prominent quartzites, with ubiquitous milky white quartz veins (with minor calcite and pyrite) and fractures of varied orientation and geometry. The other members of the basement sequence are very fine-grained quartz-rich graphitic pelites with calcite veins, spotted hornfels, and shallow intrusive rocks. Previous studies established the presence of a post-caldera, paleohydrothermal system (500–100 ka) to a depth of 2000 m that affected the Bishop Tuff and a recent (40 ka to present) hydrothermal system at shallow depth (<1 km). The deeper extent of these hydrothermal activities is established in this paper by a detailed oxygen isotope analysis of the drill core samples. 238 analyses of δ18O in 50 quartz veins within the 163.57 m depth interval of basement rocks reveal extreme heterogeneity in δ18O values (8–19.5‰). Majorities of the 84 bulk analyses of quartzites show variation of δ18O within a narrow range of 14–16‰. However, certain samples of these quartzites near the contacts with veins and fractures exhibit sharp drops in δ18O. The interbedded pelitic rocks and spotted hornfels have whole-rock δ18O ranging from 2.2 to 11.8‰. Clear, euhedral vuggy quartz that partially fills earlier open fractures in both the quartzites and quartz veins, has distinctive δ18O, ranging between −3.2 and +8.4‰. Low values of δ18O are also found in the hydrothermal minerals and whole rocks adjacent to the thin veins, clearly indicating infiltration of meteoric water. Three distinct observed patterns of fractionation in δ18O between veins and host quartzites are analyzed with the principles of mass balance, equilibrium oxygen isotope fractionation in closed system, and kinetically controlled oxygen isotope exchange in an open system. This analysis suggests that the early quartz veins formed due to a magmatic-hydrothermal activity with no influx of external water once the system comprising the sedimentary envelope and a magmatic-hydrothermal fluid phase became closed. Two-stage isotopic exchange processes caused fractionation in the δ values that originally formed arrays with slope 1 in a δvein quartz–δhost quartzite space. Another array in the same space, with near zero slope was also formed due to variation in temperature, initial isotopic compositions of the quartzite sequence and the fluid phase. Variation in temperature was mostly in the range of 300–400°C giving Δ (=δvein quartz–δhost quartzite)≈−2.8 to +2.8. The δ18O of the fluid could range from −5 to +10; however a narrower range of +5 to +10 can explain the data. This episode of hydrothermal activity could take place either as a single pulse or in multiple pulses but each as a closed system. A later, fracture-controlled, meteoric water (δ18O−0.46 to −12.13) flow and interaction (at 250°C) is interpreted from the analysis of δ18O values of the coexisting quartz and calcite pairs and existence of markedly 18O-depleted pelitic horizons interbedded with 18O-enriched quartzite layers. Thus, the interpreted earlier magmatic-hydrothermal activity was overprinted by a later meteoric-hydrothermal activity that resulted in steep arrays of δ18O values in the δvein quartz–δhost quartzite space. Calculations show that the likely life span of the post-caldera, hydrothermal activity in the depth range of 2.1–2.3 km beneath Long Valley was 0.08–0.12 Ma. Diffusive ±advective transport of oxygen isotopes from fracture-channelized meteoric water to nearly impermeable wall rocks caused a lowering of δ18O values in the quartz over short distances and in calcites over greater distances. Thus, the hydrothermal activity appears pervasive even though the meteoric water flow was primarily controlled by fractures. 相似文献
42.
Gold mineralization of the Hutti mine, southern India, is situated in closely spaced laminated quartz veins and associated alteration haloes along steeply dipping shear zones within a sequence of rather uniform amphibolites. Intense shearing has resulted in large-scale mylonitization of the wall rocks. Anastomosing shear zones, with intervening lensoid bodies of unsheared amphibolites, are characteristic features of the deposit. The general pattern of symmetrical alteration comprises a distal zone of chlorite-rich rock, with a proximal biotite-rich zone adjacent to laminated quartz veins. Arsenopyrite thermometry yielded a temperature range of 350-477 °C for the biotite alteration zone, which preceded the formation of the laminated quartz veins. Mass balance calculations on the alteration zones indicate a gradual mass and volume loss during alteration. The alteration is accompanied by intense potash metasomatism and addition of sulfur, which resulted in the formation of arsenopyrite, pyrrhotite, and pyrite. Results of fluid inclusion studies suggest that low salinity (3.9-13.5 wt% NaCl equivalent) H2O-CO2 rich fluids were responsible for gold-rich laminated quartz vein formation in the Hutti deposit. These fluids constituted a later counterpart of the protracted fluid activity that first formed the biotite alteration zone. The estimated P-T values range from 1.0 to 1.7 kbar at 280-320 °C. These data, along with the alteration assemblages and the characteristic gold-sulfide association, both in the altered wall rock and laminated quartz veins, suggest that gold, transported as reduced bisulfide complexes, was deposited in response to sulfidation reactions in the wall rocks. Comparison of P-T conditions of formation of gold-quartz veins at Hutti with two other large gold deposits in the eastern Dharwar Craton, namely Kolar (1.8 kbar/280 °C) and western Ramagiri (1.45-1.7 kbar/240-270 °C), indicates broadly similar lode-gold forming conditions in the Dharwar Craton. 相似文献
43.
Urbanization produces substantial land use changes by causing the construction of different urban infrastructures in the city region for habitation, transportation, industry, and other reasons. As a result, it has a significant impact on Land Surface Temperature (LST) by disrupting the surface energy balance. The objective of this paper is to assess the impact of land-use/land-cover (LU/LC) dynamics on urban land surface temperature (LST) of Bhubaneswar City in Eastern India during 30 years (1991–2021) using Landsat data (TM, ETM + , and OLI/TIRS) and machine learning algorithms (MLA). The finding reveals that the mean LST over the entire study domain grows significantly between 1991 and, 2021due to urbanization (β coefficient 0.400, 0.195, 0.07, and 0.06 in 1991, 2001, 2011, and 2021 respectively) and loss of green space (β coefficient − 0.295, − 0.025, − 0.125 and − 0.065 in 1991, 2001, 2011 and 2021 respectively). The highest class recorded for agricultural land (49.60 km2, accounting for 33.94% of the total land area) was in 1991 followed by vegetation (41.27 km2, 28.19% of the total land area), and built-up land (27.59 km2, 18.84% of the total land area). The sharp decline of vegetation cover will continue until 2021 due to increasing built-up areas (r = − 0.531, − 0.329, − 0.538, and − 0.063 in the 1991, 2001, 2011 and 2021 respectively). Built-up land (62.60 km2, accounting for 42.76% of the total land area, an increase of 35.01 km2 from 1991) as the highest class followed by water bodies (21.57%, 32.60 km2 of the land area), and agricultural land (31.57 km2, 21.57% of the land area) in 2021. Remote sensing techniques proved to be an important tool to urban planners and policymakers to take adequate steps to promote sustainable development and minimize urbanization influence on LST. Urban green space (UGS) can help improve the overall liveability and environmental sustainability of Bhubaneswar city.
相似文献44.
A wavelet‐based stochastic formulation has been presented in this paper for the seismic analysis of a base‐isolated structural system which is modelled as a two‐degree‐of‐freedom (2‐DOF) system. The ground motion has been modelled as a non‐stationary process (both in amplitude and frequency) by using modified Littlewood–Paley basis wavelets. The proposed formulation is based on replacing the non‐linear system by an equivalent linear system with time‐dependent damping properties. The expressions of the instantaneous damping and the power spectral density function (PSDF) of the superstructure response have been obtained in terms of the functionals of input wavelet coefficients. The proposed formulation has been validated by simulating a ground motion process. The effect of the frequency non‐stationarity on the non‐linear response has also been studied in detail, and it has been clearly shown how ignoring the frequency non‐stationarity in the ground motion leads to inaccurate non‐linear response calculations. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
45.
The nonlinearity of the soil affects soil–structure interaction to a considerable extent. For a reliable and safe analysis of soil interaction effects on the dynamic response of structures, a more realistic and relatively straightforward method incorporating the nonlinear hysteretic nature of the underlying soil–foundation system needs to be developed. The present paper models the soil–foundation system as a single degree of freedom spring–dashpot system with nonlinear hysteresis in form of elasto-perfectly plastic behavior. Analytical results for the lateral dynamic stiffness on footing have been presented. An example study has been carried out in case of circular footings. It is shown how the analytical results can be used to get a preliminary idea of the lateral dynamic stiffness of footings on a soil medium prior to a detailed computational geo-mechanics analysis provided the static nonlinear load–deformation characteristic of the soil medium is known and can be modeled by a hysteretic elasto-plastic behavior. The corresponding results are presented in a graphical form. The results have been computed showing parametric variations with the change in the amplitude and dimensionless frequency of the non-dimensional excitation force. Analytical results are also presented for the asymptotic cases at low and very high values of dimensionless frequency parameter. 相似文献
46.
We report results from analysis performed on an eclipsing supergiant high mass X-ray binary pulsar XTE J1855-026 observed with the X-ray Imaging Spectrometer (XIS) on-board Suzaku Observatory in April 2015. Suzaku observed this source for a total effective exposure of \(\sim \)87 ks just before an eclipse. Pulsations are clearly observed and the pulse profiles of XTE J1855-026 did not show significant energy dependence during this observation consistent with previous reports. The time averaged energy spectrum of XTE J1855-026 in the 1.0–10.5 keV energy range can be well fitted with a partial covering power law model modified with interstellar absorption along with a black-body component for soft excess and a gaussian for iron fluorescence line emision. The hardness ratio evolution during this observation indicated significant absorption of soft X-rays in some segments of the observation. For better understanding of the reason behind this, we performed time-resolved spectroscopy in the 2.5–10.5 keV energy band which revealed significant variations in the spectral parameters, especially the hydrogen column density and iron line equivalent width with flux. The correlated variations in the spectral parameters indicate towards the presence of clumps in the stellar wind of the companion star accounting for the absorption of low energy X-rays in some time segments. 相似文献
47.
Rahul Sharma Abdul Jaleel Chetana Jain Biswajit Paul Anjan Dutta 《Journal of Astrophysics and Astronomy》2018,39(1):16
MXB 1658-298 is a transient Low-Mass X-ray Binary (LMXB), which shows eclipses, dips and bursts in its light curve. This source has undergone three active periods separated by long quiescent phases. The latest phase of enhanced X-ray emission was observed during 2015–2016. We have analysed broadband data from Swift/XRT and NuSTAR observations carried out in 2015. During NuSTAR observation, one thermonuclear X-ray burst took place. The X-ray emission during the burst was brighter by a factor of \(\sim 200\), compared to the pre-burst emission. This work focuses on the spectral analysis of MXB 1658-298 during the persistent and the burst phases using NuSTAR observation of 2015. We have also determined the temperature and radius evolution during the burst using the time-resolved spectroscopy. The burst phase shows mild Photospheric Radius Expansion (PRE). 相似文献
48.
Biswajit Paul 《Journal of Astrophysics and Astronomy》2017,38(3):39
Neutron stars in X-ray binary systems are fascinating objects that display a wide range of timing and spectral phenomena in the X-rays. Not only parameters of the neutron stars, like magnetic field strength and spin period evolve in their active binary phase, the neutron stars also affect the binary systems and their immediate surroundings in many ways. Here we discuss some aspects of the interactions of the neutron stars with their environments that are revelaed from their X-ray emission. We discuss some recent developments involving the process of accretion onto high magnetic field neutron stars: accretion stream structure and formation, shape of pulse profile and its changes with accretion torque. Various recent studies of reprocessing of X-rays in the accretion disk surface, vertical structures of the accretion disk and wind of companion star are also discussed here. The X-ray pulsars among the binary neutron stars provide excellent handle to make accurate measurement of the orbital parameters and thus also evolution of the binray orbits that take place over time scale of a fraction of a million years to tens of millions of years. The orbital period evolution of X-ray binaries have shown them to be rather complex systems. Orbital evolution of X-ray binaries can also be carried out from timing of the X-ray eclipses and there have been some surprising results in that direction, including orbital period glitches in two X-ray binaries and possible detection of the most massive circum-binary planet around a Low Mass X-ray Binary. 相似文献
49.
Ramanpreet Kaur Biswajit Paul Brijesh Kumar Ram Sagar 《Monthly notices of the Royal Astronomical Society》2008,386(4):2253-2261
We have investigated multiband optical photometric variability and stability of the Hα line profile of the transient X-ray binary IGR J01583+6713. We set an upper limit of 0.05 mag on photometric variations in the V band over a time-scale of three months. The Hα line is found to consist of non-Gaussian profile and quite stable for a duration of two months. We have identified the spectral type of the companion star to be B2 IVe while the distance to the source is estimated to be ∼4.0 kpc. Along with the optical observations, we have also carried out analysis of X-ray data from three short observations of the source, two with the Swift –XRT and one with the RXTE –PCA. We have detected a variation in the absorption column density, from a value of 22.0 × 1022 cm−2 immediately after the outburst down to 2.6 × 1022 cm−2 four months afterwards. In the quiescent state, the X-ray absorption is consistent with the optical reddening measurement of E ( B − V ) = 1.46 mag. From one of the Swift observations, during which the X-ray intensity was higher, we have a possible pulse detection with a period of 469.2 s. For a Be X-ray binary, this indicates an orbital period in the range of 216–561 d for this binary system. 相似文献
50.