Petrogenesis of acapulcoites and lodranites: A shock-melting model     

Alan E. Rubin 《Geochimica et cosmochimica acta》2007,71(9):2383-2401

Acapulcoites are modeled as having formed by shock melting CR-like carbonaceous chondrite precursors; the degree of melting of some acapulcoites was low enough to allow the preservation of 3-6 vol % relict chondrules. Shock effects in acapulcoites include veins of metallic Fe-Ni and troilite, polycrystalline kamacite, fine-grained metal-troilite assemblages, metallic Cu, and irregularly shaped troilite grains within metallic Fe-Ni. While at elevated temperatures, acapulcoites experienced appreciable reduction. Because graphite is present in some acapulcoites and lodranites, it seems likely that carbon was the principal reducing agent. Reduction is responsible for the low contents of olivine Fa (4-14 mol %) and low-Ca pyroxene Fs (3-13 mol %) in the acapulcoites, the observation that, in more than two-thirds of the acapulcoites, the Fa value is lower than the Fs value (in contrast to the case for equilibrated ordinary chondrites), the low FeO/MnO ratios in acapulcoite olivine (16-18, compared to 32-38 in equilibrated H chondrites), the relatively high modal orthopyroxene/olivine ratios (e.g., 1.7 in Monument Draw compared to 0.74 in H chondrites), and reverse zoning in some mafic silicate grains. Lodranites formed in a similar manner to acapulcoites but suffered more extensive heating, loss of plagioclase, and loss of an Fe-Ni-S melt.Acapulcoites and lodranites experienced moderate post-shock annealing, presumably resulting from burial beneath material of low thermal diffusivity. The annealing process repaired damaged olivine crystal lattices, lending acapulcoites and lodranites the appearance of unshocked (i.e., shock-stage S1) rocks. Any high-pressure phases that may have formed during initial shock reverted to their low-pressure polymorphs during annealing. Some samples were subsequently shocked again; several acapulcoites reached shock-stage S2 levels, ALH 84190 reached S3, and the lodranite MAC 88177 reached S5.  相似文献   

Determination of the distribution of size of irregularly shaped particles from laser diffractometer measurements     

Claudio L. Schneider  Reiner Neumann  Alan S. Souza 《International Journal of Mineral Processing》2007

The formal stereological transformation equation for particle sieve size distribution from measurements in lower dimensional spaces is applied to laser diffractometer measurements. The transformation function for iron ore particles is measured experimentally, and modeled. The solution is tested against the measured transformation function data as well as synthetic composite distributions of the original sample. The natural size distribution of a sample taken from a grinding circuit stream was measured by a combination of standard sieving and cyclosizer, and the result is compared to the transformed size distribution calculated from laser diffractometer measurements. The stereological transformation technique performed well in all cases.  相似文献   

A three-dimensional finite-element model of wind effects upon higher harmonics of the internal tide     

Philip Hall  Alan M. Davies 《Ocean Dynamics》2007,57(4-5):305-323

A non-linear three-dimensional unstructured grid model of the M₂ tide in the shelf edge area off the west coast of Scotland is used to examine the spatial distribution of the M₂ internal tide and its higher harmonics in the region. In addition, the spatial variability of the tidally induced turbulent kinetic energy and associated mixing in the area are considered. Initial calculations involve only tidal forcing, although subsequent calculations are performed with up-welling and down-welling favourable winds to examine how these influence the tidal distribution (particularly the higher harmonics) and mixing in the region. Both short- and long-duration winds are used in these calculations. Tidal calculations show that there is significant small-scale spatial variability particularly in the higher harmonics of the internal tide in the region. In addition, turbulence energy and mixing exhibit appreciable spatial variability in regions of rapidly changing topography, with increased mixing occurring above seamounts. Wind effects significantly change the distribution of the M₂ internal tide and its higher harmonics, with appreciable differences found between up- and down-welling winds and long- and short-duration winds because of differences in mixing and the presence of wind-induced flows. The implications for model validation, particularly in terms of energy transfer to higher harmonics, and mixing are briefly discussed.  相似文献   

On the influence of stratification and tidal forcing upon mixing in sill regions     

Alan M. Davies  Jiuxing Xing 《Ocean Dynamics》2007,57(4-5):431-451

A cross-sectional non-hydrostatic model with idealized topography was used to examine the processes influencing tidal mixing in the region of sills. Initial calculations with appropriate parameters for the sill at the entrance to Loch Etive showed that the model could reproduce the main features of the observed mixing in the region. In particular, the hydraulic jump in the sill region was reproduced, as was an intense mid-water jet that was observed to separate from the lee side of the sill. Shear instabilities associated with the jet appeared to be a source of mixing within the thermocline. In addition, internal lee waves were generated on the lee side of the sill, with the observed amplification because of trapping during the flood stage. Their magnitude and hence the mixing increased with increasing Froude number (F _r). In the case of vertically varying buoyancy frequency, its value near the sill top determined the F _r number, with its value below influencing internal waves magnitude at depth. At high F _r values particularly with strong currents, short waves and overturning occurred.  相似文献   

What explains agricultural performance: climate normals or climate variance?     

Robert Mendelsohn  Alan Basist  Ariel Dinar  Pradeep Kurukulasuriya  Claude Williams 《Climatic change》2007,81(1):85-99

This paper measures the influence of climate normals (average long-term surface wetness and temperature) and interannual climate variance on farms in the United States and Brazil using satellite data. The paper finds that just climate normals or just climate variance variables can explain both net revenues and how much land is used for cropland. However, because they are correlated with each other, it is important to include both normals and variance in the same statistical model to get accurate measures of their individual contribution to farm outcomes. In general, higher climate variance increases the probability that land is used for cropland in both countries and higher temperatures reduce both cropland and land values. Other annual effects were not consistent across the two countries.  相似文献   

Mechanisms of orthopyroxene dissolution in silica-undersaturated melts at 1 atmosphere and implications for the origin of silica-rich glass in mantle xenoliths   总被引：2，自引：0，他引：2  

Shaw  Cliff S. J.  Thibault  Yves  Edgar  Alan D.  Lloyd  Felicity E. 《Contributions to Mineralogy and Petrology》1998,132(4):354-370

Experiments dissolving orthopyroxene (En₉₃) in a variety of Si-undersaturated alkaline melts at 1 atmosphere and variable f _O2 demonstrate that orthopyroxene dissolves to form olivine, Si-rich melt and clinopyroxene. These phases form a texturally and chemically distinct boundary layer around the partly dissolved orthopyroxene crystals. The occurrence of clinopyroxene in the boundary layer is due to inward diffusion of Ca from the solvent melt to the boundary layer causing clinopyroxene saturation. Compositional profiles through the solvent and the boundary layer for a number of experiments demonstrate rapid diffusion of cations across the boundary layer – solvent interface. SiO₂ diffuses outward from the boundary layer whereas CaO and Al₂O₃ diffuse toward the Si-enriched boundary layer melt. The rate of Al diffusion is slower under reducing conditions compared to the rates in experiments performed in air. Concentrations of FeO and MgO in the boundary layer and solvent are approximately equal indicating rapid diffusion and attainment of equilibrium despite ongoing crystallisation of clinopyroxene within the boundary layer. The behaviour of Na₂O and K₂O is strongly affected by f _O2. Under reducing conditions Na₂O and K₂O concentrations are approximately equal in the boundary layer and solvent indicating normal diffusion down the concentration gradient and attainment of equilibrium. Under oxidising conditions, K₂O and to a lesser extent Na₂O, have compositional profiles indicative of uphill diffusion likely due to their preference for more polymerised Si- and Al-rich melts. Under reduced conditions Al-enrichment in the boundary layer melt is not as extreme and uphill diffusion did not occur. The composition of the solvent melt after the experiments indicates that it was contaminated by the boundary layer by convective mixing due to the onset of hydrodynamic instabilities brought on by density and viscosity contrasts between the two melts. Despite using a wide variety of solvent melt compositions we find that the boundary layer melts converge toward a common composition at high SiO₂ contents. The composition of glass generated by orthopyroxene dissolution at 1 atmosphere is similar in many respects to Si-rich glass found in many orthopyroxene-rich mantle xenoliths that have been attributed to high pressure in situ processes including mantle metasomatism. The results of this study suggest that at least some Si-rich melts are likely to have formed by dissolution of xenolith orthopyroxene at low pressure possibly by their Si-undersaturated host magmas. Received: 30 August 1996 / Accepted: 15 April 1998  相似文献   

Isotopic and geochemical evidence for magma mixing in the petrogenesis of the Coire Uaigneich Granophyre,Isle of Skye,N.W. Scotland     

Alan P. Dickin  Richard A. Exley 《Contributions to Mineralogy and Petrology》1981,76(1):98-108

Field relations, petrographic features, major and selected trace element compositions, and Sr- and Pb- isotope characteristics indicate that the Coire Uaigneich Granophyre (CUG) was formed by the mixing of two magmas. One of these, a liquid of high ⁸⁷Sr/⁸⁶Sr ratio (ca. 0.731) formed by the anatexis of late Precambrian Torridonian sediments, contained relict quartz and zircon. The other liquid was an acid differentiate of basaltic magma and was enriched in incompatible elements such as Zr and Y. The two magmas mixed in the proportions of approximately two parts anatectic melt to one part acid differentiate. Hydrothermal metamorphism had no significant effect on the bulk chemistry of the CUG.  相似文献   

A review of wave-dominated clastic shoreline deposits     

Alan P. Heward 《Earth》1981,17(3):223-276

Barrier islands, beaches, wave-dominated deltas and chenier plains probably form more than one-fifth of present coasts, and constitute an even larger proportion of the subsiding lowland coasts likely to be preserved in the stratigraphic record. The geometry, vertical sequences and composition of their deposits are the result of: (1) shoreline behaviour, whether transgressive, regressive or stable; (2) physical processes, particularly the relative intensities of waves and tides; (3) climate; (4) global tectonic setting; and (5) topography of underlying deposits.Published interpretations indicate that equivalent deposits are abundant in the geological record. Sandstones are mostly quartzose, cemented by carbonates and quartz, and have sheet or linear geometries. Ancient deposits can provide evidence of palaeogeography and some are economically significant, particularly as hydrocarbon reservoir rocks.Four end-member sedimentation models are proposed for the interpretation of ancient deposits: (1) transgressive sheet sands associated with deltas; (2) non-deltaic transgressive sheet sands; (3) regressive sheet sands; and (4) linear sand bodies. Probable depositional settings, geometries, characteristics of deposits and palaeocurrent patterns are discussed.  相似文献   

Model systems for anatexis of pelitic rocks     

Alan Bruce Thompson  Robert J. Tracy 《Contributions to Mineralogy and Petrology》1979,70(4):429-438

Subsolidus and melting reactions involving calcic plagioclase in pelitic assemblages in the K-Na-Ca model system occur at higher temperatures than their K-Na counterparts. For the most calcic plagioclase compositions observed in high-grade pelitic rocks (An₂₅-An₄₀) the equilibria are rarely extended by more than 30 ° C above those in KA1O₂-NaAlO₂-Al₂O₃-SiO₂-H₂O, although all discontinuities in facies inferred for the K-Na system are continuously displaced when they involve Ca-bearing plagioclase. The maximum pressure-temperature overlap between muscovite dehydration and initial melting reactions occurs in the pressure range of 4–6 kbar between about 640 ° and 720 ° C. This provides optimum conditions for anatectic melt generation in felsic rocks of the appropriate compositions progressively metamorphosed in kyanite-sillimanite facies series. Progressive regional metamorphism at pressures of 2–4 kbar, corresponding to andalusite-sillimanite facies series, shows little overlap between muscovite dehydration and initial melting reactions. Consequently anatectic melt generation in andalusite-sillimanite facies series would require the participation of biotite in dehydration-melting reactions. Felsic intrusive rock in andalusite-sillimanite terranes could have risen upward from their anatectic sites in high grade kyanite-sillimanite facies series at depth. Many andalusite-sillimanite facies series terranes culminating in migmatites could represent upward movement of kyanite-sillimanite facies series rocks to shallower depths with uplift rates faster than cooling rates.  相似文献   

Climatic changes,magnetic intensity variations and fluctuations of the eccentricity of the earth's orbit during the past 2,000,000 years and a mechanism which may be responsible for the relationship — A discussion     

Alan D. Chave  Charles R. Denham 《Earth and Planetary Science Letters》1979,44(1):150-152

  相似文献