The Uintjiesberg kimberlite diatreme occurs within the Proterozoic Namaqua–Natal Belt, South Africa, approximately 60 km to the southwest of the Kaapvaal craton boundary. It is a group I, calcite kimberlite that has an emplacement age of 100 Ma. Major and trace element data, in combination with petrography, are used to evaluate its petrogenesis and the nature of its source region. Macrocryst phases are predominantly olivine with lesser phlogopite, with very rare garnet and Cr-rich clinopyroxene. Geochemical variation amongst the macrocrystic samples (Mg# 0.85–0.87, SiO2=27.0–29.3%, MgO=26.1–30.5%, CaO=10.9–13.5%) is shown to result from 10% to 40% entrainment and partial assimilation of peridotite xenoliths, whereas that shown by the aphanitic samples (Mg# 0.80–0.83, SiO2=19.1–23.0%, MgO=17.9–23.9%, CaO=16.5–23.7%) is consistent with 7–25% crystal fractionation of olivine and minor phlogopite. Changing trajectories on chemical variation diagrams allow postulation of a primary magma composition with 25% SiO2, 26% MgO, 2.3% Al2O3, 5%H2O, 8.6% CO2 and Mg#=0.85.
Forward melting models, assuming 0.5% melting, indicate derivation of the primary Uintjiesberg kimberlite magma from a source enriched in light rare earth elements (LREE) by 10× chondrite and heavy REE (HREE) by 0.8–2× chondrite, the latter being dependent on the proportion of residual garnet. Significant negative Rb, K, Sr, Hf and Ti anomalies present in the inferred primary magma composition are superimposed on otherwise generally smooth primitive mantle-normalized trace element patterns, and are inferred to be a characteristic of the primary magma composition. The further requirement for a source with chondritic or lower HREE abundances, residual olivine with high Fo content (Fo94) suggests derivation from a mantle previously depleted in mafic melt but subsequently enriched in highly incompatible elements prior to kimberlite genesis. These requirements are interpreted in the context of melting of continental lithospheric mantle previously enriched by metasomatic fluids derived from a sublithospheric (plume?) source. 相似文献
Kokchetavite, a new polymorph of K-feldspar (KAlSi3O8), has been identified as micrometer-size inclusions in clinopyroxene and garnet in a garnet-pyroxene rock from the Kokchetav ultrahigh-pressure terrane, Kazakhstan. Kokchetavite has a hexagonal structure with a =5.27(1) Å, c=7.82(1) Å, V=188.09 Å3, Z=1, and is found to be associated with phengite + /-cristobalite (or quartz) + siliceous glass ± phlogopite/titanite/calcite/zircon, occurring as multi-phase inclusions in clinopyroxene and garnet. It is concluded that kokchetavite could not be an exsolution phase in host minerals. Instead, it might be metastably precipitated from an infiltrated K-rich melt during rock exhumation. Alternatively, although less likely, kokchetavite might be derived from dehydration of K-cymrite, which, in turn, was formed at high pressures. In either case, kokchetavite is a metastable polymorph of K-feldspar. 相似文献
Polarized infrared absorption spectra of thin single-crystal slabs parallel to (010) and (001) of a staurolite from Pizzo Forno, Ticino, with analyzed composition (Fe2.9Mg0.9Zn0.1Mn0.1)Al17.5Ti0.1(Si7.7Al0.3)O48H3 have been measured in the range of 3000–4000 cm?1. From the pleochroitic behaviour of the OH-vibrations three groups of bands can be distinguished: the bands of group I, a strong band at 3445 cm?1 plus a weak shoulder at 3358 cm?1, and the bands of group II, a weak band centered at 3677 cm?1 plus a shoulder at 3635 cm?1, are assigned to the H1 and H2 protons, respectively. The bands of group III, a weak band at 3577 cm?1 plus a shoulder, cannot be interpreted on the basis of the proton positions known so far. We assign them to an additional proton H3, which is bonded to O1 and shows a bifurcated hydrogen bridge to two O5 in a vacant T2 site. 相似文献
Waterfowl and American coot data collected from aerial censusing on Currituck Sound were compared during three periods: before Eurasian watermilfoil (Myriophyllum spicatum) colonization (1961–1965); during Eurasian watermilfoil infestation (1968–1977); and following the near absence of submersed macrophytic vegetation after Eurasian watermilfoil decline (1981–1990). Numbers of dabbling ducks (Anas rubripes, Anas platyrhynchos, Anas acuta, Anas americana, Anas crecca), ring-necked ducks (Aythya collaris), and American coot (Fulica americana) were significantly different in Currituck Sound (p<0.05) in at least two of these periods and were highest during Eurasian watermilfoil infestation. These species also constituted higher percentages of the Atlantic Flyway counts during Eurasian watermilfoil infestation. than in periods before or after. Numbers of snow geese (Chen caerulescens), Canada geese (Branta canadensis), and canvasbacks (Aythya valisneria) were significantly different between periods; however, these birds exhibited a consistent decline from the early to the late period. This study suggests that submersed macrophytes, specifically Eurasian watermilfoil, support increased numbers of dabbling ducks, ring-necked ducks, and American coot. 相似文献
Single-crystal elasticity of stishovite was examined using a new experimental technique and an empirical macroscopic model. Employing high-frequency resonant ultrasound spectroscopy, single-crystal elastic constants of stishovite were determined: C11 = 443(3), C33 = 781(4), C12 = 193(2), C23 = 199(2), C44 = 256(2), and C33 = 316(2) GPa. The frequency range of the resonant ultrasound spectroscopy techniques was 6–20 MHz, which is much lower than the ~10 GHz range of the Brillouin scattering technique. Of the elastic constants, the shear elastic constants C44 and C66 are consistent with the average value of the previously mentioned Brillouin scattering. Conversely, the four elastic constants, C11, C33, C12, and C23, slightly deviate outside the range of previous Brillouin scattering results. The present results, except those for C12, are consistent with recent lattice dynamic analysis of inelastic X-ray scattering data. The adiabatic bulk modulus was calculated as 298 GPa, which is smaller and more consistent with the result of compression experiments than any other Brillouin scattering results (301–312 GPa). The present result shows greater P-wave velocity anisotropy (24.7%) than any preceding work. To understand the unique elastic properties of stishovite, the Gingham check model was proposed and examined. The result shows that the octahedron of 6-coordinated Si in stishovite crystal has stiffness comparable to that of diamond. 相似文献
We introduce a new method for prestack depth migration of seismic common-shot gathers. The computational procedure follows
standard steps of the reverse-time migration, i.e., downward continuation of the source and the receiver wavefields, followed
by application of an imaging condition (e.g. zero-lag cross-correlation of these fields). In our method we first find a sparse
data representation with a small number of Gaussian wave packets. We then approximate the downward wavefield propagation (for
the source and the receiver fields) by a rigid flow of these wave packets along seismic rays. In this case, the wave packets
are simply translated and rotated according to the ray geometry. One advantage of using Gaussian wave packets is that analytic
formulas can be used for translation, rotation, and the application of the cross-correlation imaging condition. Moreover,
they allow more sparse representations than competing methods. Finally we formulate a computationally and memory efficient
migration procedure, as only few rays have to be traced, and since it is cheap to compute the cross-correlation for the intersecting
rays. 相似文献
This article deals with the quantification of saltwater upconing below horizontal wells in freshwater lenses using analytical solutions as a computationally fast alternative to numerical simulations. Comparisons between analytical calculations and numerical simulations are presented regarding three aspects: (1) cyclic pumping; (2) dispersion; and (3) finite horizontal wells in a finite domain (a freshwater lens). Various hydrogeological conditions and pumping regimes within a dry half year are considered. The results show that the influence of elastic and phreatic storage (which are not taken into account in the analytical solutions) on the upconing of the interface is minimal. Furthermore, the analytical calculations based on the interface approach compare well with numerical simulations as long as the dimensionless interface upconing is below 1/3, which is in line with previous studies on steady pumping. Superimposing an analytical solution for mixing by dispersion below the well over an analytical solution based on the interface approach is appropriate in case the vertical flow velocity around the interface is nearly constant but should not be used for estimating the salinity of the pumped groundwater. The analytical calculations of interface upconing below a finite horizontal well compare well with the numerical simulations in case the distance between the horizontal well and the initial interface does not vary significantly along the well and in case the natural fluctuation of the freshwater lens is small. In order to maintain a low level of salinity in the well during a dry half year, the dimensionless analytically calculated interface upconing should stay below 0.25. 相似文献