Calculated mineral equilibria in the greenschist-blueschist-eclogite facies in Na2O−FeO−MgO−Al2O3−SiO2−H2O     

Michel Guiraud  Tim Holland  Roger Powell 《Contributions to Mineralogy and Petrology》1990,104(1):85-98

The new, greatly expanded internally-consistent dataset of Holland and Powell includes thermodynamic data for a wide range of mineral end-members in common rock-forming minerals, in particular, including FeMg_-1 substitutions in glaucophane, garnet, chloritoid and carpholite, and FeMg_-1 and MgSiAl_-1Al_-1 substitutions in talc and chlorite. Moreover, we have the uncertainties and correlations for these data. With the data, we have calculated the full pressure-temperature phase diagram for the system Na₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (NFMASH) for quartz (or coesite) and H₂O in excess, in the range 300°–800°C and 5–50 kbars. By solving the set of non-linear equations formed by the equilibrium relationships for an independent set of equations between the end-members in an assemblage in NFMASH, the compositions of the minerals (and PT) can be calculated. Thus the changes in MgSiAl_-1Al_-1 along NMASH reactions, and FeSiAl_-1Al_-1 along NFASH reactions, are calculated, and the changes in FeMg_-1 and MgSiAl_-1Al_-1 along NFMASH reactions are calculated. From this information it is straightforward to generate PT diagrams for specific rock compositions. Mineral assemblages and mineral compositional changes in the phase diagram are discussed in relation to greenschist, blueschist and eclogite facies assemblages in metapelitic rocks. It is found that the correspondence between the predictions of the phase diagrams and the observations on rocks is remarkably good. When semiquantitative extensions of the phase diagram to include Ca(MgFe)_-1, NaSiCa_-1Al_-1, Fe³⁺Al_-1 and KNa_-1 substitutions are taken into account the agreement is essentially complete.  相似文献   

Model for Predicting Virus Movement Through Soils   总被引：3，自引：0，他引：3  

Udoyara S. Tim  Saied Mostaghimi 《Ground water》1991,29(2):251-259

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Alluvial gold in Kalimantan, Indonesia: A colloidal origin?     

John B. Seeley  Tim J. Senden 《Journal of Geochemical Exploration》1994,50(1-3)

Extensive placer gold deposits occur in Quaternary palaeochannels in the Ampalit and Cempaga Buang drainage basins near Kasongan, Central Kalimantan, Indonesia. Broad interfluvial Pleistocene terraces surrounding the drainages also contain significant amounts of placer gold. The “channel” and “terrace” gold deposits together cover an area in excess of 180 km².Exploration for gold at Ampalit followed traditional alluvial methods, and assumed that mechanical and gravitational factors were the principal mechanisms affecting gold accumulation and concentration. Gold was considered to be physically reworked from terraces and redeposited in the present drainage channels or underlying, laterally displaced, palaeochannels. However, a comparison of gold grains from the Ampalit drainage channel and adjacent terraces indicates that gold grains in the drainage channels are possibly of colloidal origin and not mechanically transported to their present domain.The morphology of gold grains from the Ampalit drainage is compared with grains from adjacent terraces, using scanning electron microscopy and atomic force microscopy. The latter procedure allows for nondestructive analysis to give true three dimensional surface topography down to nanometre resolution. Evidence from force microscopy supports a colloidal origin for gold aggregates.The average purity of gold produced from the Ampalit dredging operation is 970 fine (970/1000) and the majority of examined gold grains extracted from beneath the drainage channel have a purity of 998 fine. This unusually high degree of purity suggests a selective dissolution/aggregation mechanism. We propose that gold is transported downward and laterally in groundwater percolating through the terrace sands and gravels to the current drainage as a humic acid-stabilised colloid. As the pregnant groundwater migrates toward the present drainage channels, it encounters a sleep chemical gradient and the colloid aggregates. The aggregation process occurs near clay zones within palaeochannel sands and gravels beneath the present river sediments. Here the colloidal aggregates form small grains (usually <1 mm) of extremely high purity.Today, the inherently unstable gold deposits at Ampalit are in a state of both aggregation and dissolution. Identification of low energy zones, location of point bar lags in palaeo and recent drainage channels, and the extrapolation of areas of low energy are not solely valid as exploration guides for gold accumulation in this environment. Exploration methods that also recognise ground water migration and composition, and the geochemical controls of dissolution and aggregation, will improve the ability to identify ore accumulations, thus contributing to improved economics of mining these deposits.  相似文献   

Determining Paleosol Topography Using Seismic Refraction     

Tim C. Rolph  John Shaw  Edward Derbyshire  An Zhisheng 《Quaternary Research》1994,42(3)

The seismic refraction reversed profiling technique has been used to investigate the topography of the last interglacial soil (paleosol S1) within the central Chinese Loess Plateau near Xifeng. The results suggest an essentially flat-lying soil at a depth which varies by only a few meters over an area of more than 10 km². In addition, the results indicate a high-velocity layer at 50-60 m depth which is thought to coincide with a layer of carbonate concretions at the base of paleosol S5. The results agree well with the local loess-paleosol stratigraphy for this area and indicate that the seismic refraction method is a rapid technique for investigating paleotopography.  相似文献   

The tectonic and geologic evolution of Southeast Asian Seas and Islands : Part 2. Dennis E. Hayes (editor). Geophysical monograph, 27—American Geophysical Union, Washington, D.C., 1983, vi + 396 pp., US $42.00 (hardback)     

H. L. Davies 《Tectonophysics》1985,117(3-4)

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Lagging mantle convection,the geoid and mantle structure     

Geoffrey F. Davies 《Earth and Planetary Science Letters》1984,69(1):187-194

In numerical models of convection incorporating migration of a simulated subduction zone, the main descending flow lags far behind the migrating trench, and a geoid low is associated with the main descending flow. This provides physical plausibility for the suggestion by Chase and Sprowl that present very long-wavelength (degree 2–4) geoid lows are associated with Mesozoic trench locations, and suggests further that the present long-wavelength geoid, deep mantle structure and hotspot distribution may be straightforward consequences of plate evolution since the Upper Paleozoic.  相似文献   

The polarization of Pc5 ULF waves around dawn: A possible ionospheric conductivity gradient effect     

G. Chisham  D. Orr  T. K. Yeoman  D. K. Milling  M. Lester  J. A. Davies 《Annales Geophysicae》1995,13(2):159-167

In this study over 100 Pc5 events observed on the SAMNET magnetometer array have been catalogued. The ground horizontal polarization of these waves has been investigated and it has been shown that there are large differences between the polarization azimuth distributions on either side of noon. In the morning the azimuth distributions are spread evenly across all possible orientations whereas in the afternoon they are predominantly in a North-South orientation. The difference appears to be the result of a specific polarization pattern that occurred across the SAMNET array for 31 of the Pc5 events. These 31 Pc5 events all occurred in the morning sector, close to the dawn terminator. It is suggested that this special polarization pattern is a consequence of ionospheric conductivity gradients that occur at around dawn. This theory is reinforced by the comparison of the local times of these polarization variations with the local time of sunrise. By employing ionospheric conductivity data from the EISCAT radar for two of these Pc5 events, this connection has been studied in more detail.  相似文献   

Late Cretaceous genesis of the Kula plate     

Mark T. Woods  Geoffrey F. Davies 《Earth and Planetary Science Letters》1982,58(2):161-166

We propose that the Pacific-Kula ridge began spreading approximately 85 m.y. B.P., during Late Cretaceous time. Extrapolation of the Great Magnetic Bight backwards in time results in an implausible ridge configuration. This implies that plate velocity vectors for the Pacific, Kula, and Farallon plates were not constant during this interval. Evidence for splitting of the Kula plate from the Pacific plate along the Chinook trough is the relationship between the north-striking Amlia and Adak fracture zones, the Chinook and Emperor troughs, and the magnetic lineations south of the Aleutian trench. If this hypothesis is correct, it will require that Mesozoic reconstructions of the Pacific basin and their relation to Cenozoic reconstructions be re-examined. A previously unrecognized Mesozoic plate may be required. We propose calling this the Izanagi plate.  相似文献   

The reflection of wave energy by undulations on the seabed     

A. G. Davies 《Dynamics of Atmospheres and Oceans》1982,6(4):207-232

The problem of the reflection of incident surface water waves by undulations on the seabed is treated on the basis of linear perturbation theory. The flow is assumed to be two-dimensional, uniplanar and nonseparating, and the undulations are assumed to be of limited horizontal extent, enabling results for the reflected and transmitted waves to be obtained by Fourier transform methods. A steady state formulation of the problem is adopted, and this provides general results which are applied to the case of a patch of sinusoidal undulations on the bed. The principal conclusions are that the reflection coefficient is both oscillatory in the ratio of the length of the patch to the surface wavelength and also, as expected, critically dependent on the ratio of the surface to bed wavelengths. In addition, the transmitted wave may experience a small phase shift, even in cases of zero reflection. Examples are presented which indicate that relatively few bottom undulations, with wavenumber equal to approximately twice the surface wavenumber, may give rise to a very substantial reflected wave. A possible consequence of this is a coupling between ripple growth and wave reflection, which may be important in problems of coastal protection.  相似文献   

Superdeep diamonds from the Juina area, Mato Grosso State, Brazil   总被引：4，自引：1，他引：3  

F. Kaminsky  O. Zakharchenko  R. Davies  W. Griffin  G. Khachatryan-Blinova  A. Shiryaev 《Contributions to Mineralogy and Petrology》2001,140(6):734-753

Alluvial diamonds from the Juina area in Mato Grosso, Brazil, have been characterized in terms of their morphology, syngenetic mineral inclusions, carbon isotopes and nitrogen contents. Morphologically, they are similar to other Brazilian diamonds, showing a strong predominance of rounded dodecahedral crystals. However, other characteristics of the Juina diamonds make them unique. The inclusion parageneses of Juina diamonds are dominated by ultra-high-pressure ("superdeep") phases that differ both from "traditional" syngenetic minerals associated with diamonds and, in detail, from most other superdeep assemblages. Ferropericlase is the dominant inclusion in the Juina diamonds. It coexists with ilmenite, Cr-Ti spinel, a phase with the major-element composition of olivine, and SiO₂. CaSi-perovskite inclusions coexist with titanite (sphene), "olivine" and native Ni. MgSi-perovskite coexists with TAPP (tetragonal almandine-pyrope phase). Majoritic garnet occurs in one diamond, associated with CaTi-perovskite, Mn-ilmenite and an unidentified Si-Mg phase. Neither Cr-pyrope nor Mg-chromite was found as inclusions. The spinel inclusions are low in Cr and Mg, and high in Ti (Cr₂O₃<36.5 wt%, and TiO₂>10 wt%). Most ilmenite inclusions have low MgO contents, and some have very high (up to 11.5 wt%) MnO contents. The rare "olivine" inclusions coexisting with ferropericlase have low Mg# (87-89), and higher Ca, Cr and Zn contents than typical diamond-inclusion olivines. They are interpreted as inverted from spinel-structured (Mg, Fe)₂Si₂O₄. This suite of inclusions is consistent with derivation of most of the diamonds from depths near 670 km, and adds ilmenite and relatively low-Cr, high-Ti spinel to the known phases of the superdeep paragenesis. Diamonds from the Juina area are characterized by a narrow range of carbon isotopic composition ('¹³C=-7.8 to -2.5‰), except for the one majorite-bearing diamond ('¹³C=-11.4‰). There are high proportions of nitrogen-free and low-nitrogen diamonds, and the aggregated B center is predominant in nitrogen-containing diamonds. These observations have practical consequences for diamond exploration: Low-Mg olivine, low-Mg and high-Mn ilmenite, and low-Cr spinel should be included in the list of diamond indicator minerals, and the role of high-Cr, low-Ti spinel as the only spinel associated with diamond, and hence as a criterion of diamond grade in kimberlites, should be reconsidered.  相似文献