Phase transformations and inherited lattice preferred orientations: implications for seismic properties     

David Mainprice  Michel Humbert  Francis Wagner 《Tectonophysics》1990,180(2-4):213-228

In phase transitions via either the martensitic (diffusionless shear) or nucleation and growth mechanism a specific orientation relationship may exist between the two phases. In cases where the orientation relationship is known, the lattice preferred orientation (LPO) inherited by the new phase may be calculated from the LPO of the old phase. The method of calculation is presented in a form suitable for the spherical harmonic method of texture analysis using the orientation distribution function (ODF). Examples are presented for the -β-quartz, calcite-aragonite, orthopyroxene-clinopyroxene and olivine-spinel transformations.

The seismic properties of the transformed and untransformed phases are calculated from the ODF and the single crystal elastic constants. In particular the -β quartz transformation is considered in detail. The quartz polycrystal is very anisotropic in the alpha field (V_p anisotropy coefficient, A = 8.1%) and almost isotropic in the β-field (A = 2.1%). The transition is accompanied by V_p velocity increase of 0.6 km/s. In the other example discussed, olivine-β-spinel, there is also a decrease in V_p anisotropy coefficient from 11.1% (olivine) to 4.0% (β-spinel). The estimate of the volume fraction of olivine at the 400 km discontinuity (associated with this phase transition) is shown to depend on the direction of wave propagation.  相似文献   

River basin management and food crisis in Nigeria     

Francis C. Okafor   《Geoforum》1985,16(4):413-421

River Basin Development is one of the key agricultural programmes embarked upon by Nigeria to meet the challenges of escalating import bills on food. This paper takes a critical look at the various efforts made by the management of the River Basin Authorities in responding to the current food crisis. The rationale for the huge financial allocation made to the Authorities is questioned, and the management lapses in their key areas of operation, such as irrigation, mechanization and extension services, are discussed. It is found that despite the huge financial investments made in the river basins, their impact is hardly felt, not only in terms of food production capacity but also in the production performance of the peasant farmers. The paper argues that the problem of food crisis in Nigeria could be solved by strengthening the ability of the peasant farmers and farmers co-operatives through financial aid and extension services, and not by huge financial investments on high technology agricultural projects, characterized by centralized administration and expatriáte management.  相似文献   

A reduced-rank prediction model of stream-channel size response to traditional agricultural land-use practices in southwestern Nigeria     

Francis O. Odemerho 《Journal of Hydrology》1984,70(1-4):85-100

Stream-channel morphologic responses are found to be related to different parameters measuring traditional agricultural land-use patterns and practices in 50 small headwater basins in southwest Nigeria. The problem of intercorrelations among these parameters made it initially difficult to establish their precise channel enlargement effects and to calibrate an impact prediction model. Through factor analysis of the 22 land-use and morphometric parameters, six factors identified as measures of traditional land-use practice, farm size, planting activities, shortened fallow, relief and overland flow, were found to account for 86% of the variance in the data. The factor-defining variables are length of cropping period, areas in short fallow, farm-plot size, length of farm preparation, relief ratio and overland flow. In a multiple regression analysis, only the first three variables were found to be statistically significant in explaining stream-channel morphologic responses. Thus, areas in short fallows, average farm size and length of cropping period adequately described those aspects of the traditional farming practices that affect basin hydrologic and channel responses. Since these variables were orthogonally derived, they formed the basis for the evaluation of the channel impact status of traditional land-use activities. The duplication of information and effects in the original 22-variable full-rank model were removed while utilizing the three-factor reduced model.  相似文献   

Paragenetic relationship of thermal maturation (coalification) and tectonic framework of some Canadian Rocky Mountain coals     

Francis T.C. Ting 《Organic Geochemistry》1984,5(4):279-281

Use of the attitude of the optical axis of vitrinite as an indicator of paragenetic relationship of thermal maturation/coalification and tectonics is discussed. A constant angular relationship between the optical axis and bedding plane of coals is observed in seven coal mines in the Canadian Rocky Mountain coal fields. This constant relationship suggests that thermal maturation predated tectonic structures.  相似文献   

Evidence for Enhanced Mineral Dissolution in Organic Acid-Rich Shallow Ground Water   总被引：3，自引：0，他引：3  

Peter B. McMahon  Don A. Vroblesky  Paul M. Bradley  Francis H. Chapelle  Cheryl D. Gullett 《Ground water》1995,33(2):207-216

Total concentrations of formate, acetate, and isobutyrate varied from less than 5 to greater than 9,000 μmol/l over distances of < 3 m in ground water from a shallow hydrocarbon contaminated aquifer. Laboratory incubations of aquifer material indicate that organic acid concentrations were dependent on the amount of hydrocarbon loading in the sediment and the relative rates of microbial organic acid production and consumption. In heavily contaminated sediments, production greatly exceeded consumption and organic acid concentrations increased. In lightly contaminated sediments rates were essentially equal and organic acid concentrations remained low. Concentrations of dissolved calcium, magnesium, and iron generally were one to two orders of magnitude higher in organic acid-rich ground water than in ground water having low organic acid concentrations. Carbonate and Fe(III)-oxyhydroxide minerals were the likely sources of these elements. Similarly, concentrations of dissolved silica, derived from quartz and k-feldspar, were higher in organic acid-rich ground water than in other waters. The positive relation (r = 0.60, p < .05, n = 16) between concentrations of silica and organic acids suggests that the microbially mediated buildup of organic acids in ground water enhanced quartz/k-feldspar dissolution in the aquifer, although it was not the only factor influencing their dissolution. A model that included organic acid microequivalents normalized by cation microequivalents significantly strengthened the correlation (r = 0.79, p < .001, n = 16) between dissolved silica and organic acid concentrations, indicating that competition between silica and cations for complexation sites on organic acids also influenced quartz/k-feldspar dissolution. Physical evidence for enhanced mineral dissolution in organic acid-rich waters included scanning electron microscopy images of highly corroded quartz and k-feldspar grains from portions of the aquifer containing organic acid-rich ground water. Microporosity generated in hydrocarbon contaminated sediments may adversely affect remediation efforts that depend on the efficient injection of electron acceptors into an aquifer or on the recovery of solutes from an aquifer.  相似文献   

USGS Reference Samples Attapulgite ATT-1 and Bentonite CSB-1     

John W. Hosterman  Francis J. Flanagan 《Geostandards and Geoanalytical Research》1987,11(1):1-9

The U.S. Geological Survey has prepared an attapulgite and a bentonite as reference samples because of their economic importance. The attapulgite (ATT-1) is from the Meigs-Attapulgite-Quincy fuller's earth district of Georgia and Florida, and the bentonite (CSB-1) is from the Northern Black Hills bentonite district of Wyoming, Montana, and South Dakota. The chemical and mineralogical properties of these two samples are quite different. Provisional values calculated from the few analytical data and the X-ray diffraction mineralogy are presented for those who wish to use the clays as reference samples.  相似文献   

A porous flow model for the geometrical form of volcanoes—critical comments     

G. Wadge  P. Francis 《Earth and Planetary Science Letters》1982,57(2):453-455

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Mössbauer studies of synthetic and natural micas on the polylithionite-siderophyllite join: reply     

Christian Levillain  Pierre Maurel  Francis Menil 《Physics and Chemistry of Minerals》1982,8(2):99-100

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Estimation of uranium endowment by subjective geological analysis—a comparison of methods and estimates for the San Juan Basin,New Mexico     

DeVerle P. Harris  Francis J. Carrigan 《Mathematical Geosciences》1983,15(1):227-227

This study investigates how estimates of uranium endowment made by a geologist using an appraisal system that is based upon a formalization of geoscience and decision rules compare with estimates made by informal and unconstrained intuitive processes. The motivation for this study derives from the premise that formalization of decisions would mitigate the heuristic biases and hedging that may result from the use of unconstrained intuitive processes. Estimates of the uranium endowment of the San Juan Basin of New Mexico by four methodologies are compared in this study. These methods, ranked from top to bottom by degree of decomposition (mitigating of heuristic bias)and control on hedging, are as follows Implicit 2 1.5 × 10⁶ s.t. of U₃O₈ Implicit 1 1.6 × 10⁶ s.t. of U₃O₈ NURE (1980) 2.4 × 10⁶ s.t. of U₃O₈ Appraisal system 3.9 × 10⁶ s.t. of U₃O₈ The magnitude of expected uranium endowment estimated by these methods, ranked from smallest to largest, is in this same order. With the exception of the NURE estimates, the magnitude of the variance (uncertainty)of uranium endowment, ranked from smallest to largest, also is in this same order. These results prompt the suggestion that the more decomposed and formalized the estimation procedure, the greater the expected value and the variance of uranium endowment. Equivalently, predicating U ₃ O ₈ endowment estimation strictly upon that part of the geologist's geoscience that is useful in making U ₃ O ₈ endowment estimates and upon his understanding of the region's history produced larger estimates than have previously been reported. However, this method of estimation also shows that uncertainty about the actual state of U ₃ O ₈ endowment is much greater than previously described.  相似文献   

Investigation of Reductive Dechlorination Supported by Natural Organic Carbon   总被引：1，自引：0，他引：1  

Heather V. Rectanus  Mark A. Widdowson  Francis H. Chapelle  Catherine A. Kelly  John T. Novak 《Ground Water Monitoring & Remediation》2007,27(4):53-62

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