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121.
Alan E. Rubin 《Geochimica et cosmochimica acta》1984,48(9):1779-1789
Relatively coarse-grained rims occur around all types of chondrules in type 3 carbonaceous and ordinary chondrites. Those in H-L-LL3 chondrites are composed primarily of olivine and low-Ca pyroxene; those in CV3 chondrites contain much less low-Ca pyroxene. Average grain sizes range from ~4 μm in H-L-LL3 chondrites to ~10 μm in CV3 chondrites. Such rims surround ~50%, ~10% and ≤ 1% of chondrules in CV3, H-L-LL3 and CO3 chondrites, respectively, but are rare (≤1%) around CV3 Ca,Al-rich inclusions. Rim thicknesses average ~150 μm in H-L-LL3 chondrites and ~400 μm in CV3 chondrites.The rims in H-L-LL3 chondrites are composed of material very similar to that which comprises darkzoned chondrules and recrysiallized matrix. Dark-zoned chondrules and coarse-grained rims probably formed in the solar nebula from clumps of opaque matrix material heated to sub-solidus to sub-liquidus temperatures during chondrule formation. Mechanisms capable of completely melting some material while only sintering other material require steep thermal gradients; suitable processes are lightning, reconnecting magnetic field lines and, possibly, aerodynamic drag heating.CV chondrites may have formed in a region where the chondrule formation mechanism was less efficient, probably at greater solar distances than the ordinary chondrites. The lesser efficiency of heating could be responsible for the greater abundance of coarse-grained rims around CV chondrules. Alternatively, CV chondrules may have suffered fewer particle collisions prior to agglomeration. 相似文献
122.
Todd P. Trooien Alan R. Bender John H. Bischoff 《Ground Water Monitoring & Remediation》1986,6(2):99-105
The union of a piezoresistive pressure transducer and a porous ceramic cup was termed "transiometer." The transiometer was constructed from economical and readily available materials. It could be used to measure soil water potentials in both saturated and unsaturated conditions, and was well suited to continuous monitoring with data acquisition equipment.
Transiometer testing was conducted at two sites, one of moderate permeability and the other of slow permeability. The slowly permeable site was instrumented with four replications of the following: (1) transiometers installed at four depths, (2) a transi-ometerwithout the ceramic cup, (3) apiezometer, and (4) access tubes for monitoring soil moisture with a neutron probe. The moderately permeable site was instrumented with a transiometer, two piezometers, and an access tube for monitoring with a neutron probe.
In saturated conditions the transiometer had a faster response time after installation than the piezometer. Faster response makes the transiometer more desirable for use in slowly permeable soils, especially when monitoring dynamic soil water.
Calculated random error of the transiometer measuring system, including a digital voltmeter and a scanner, was typically 0.09 feet (2.8cm), with a maximum calculated to be 0.38 feet (11.5cm). The two most significant components were imprecision of the scanner card and calibration shift. The transiometer was sensitive to atmospheric pressure fluctuations, with sensitivity to atmospheric pressure change increasing with installation depth. 相似文献
Transiometer testing was conducted at two sites, one of moderate permeability and the other of slow permeability. The slowly permeable site was instrumented with four replications of the following: (1) transiometers installed at four depths, (2) a transi-ometerwithout the ceramic cup, (3) apiezometer, and (4) access tubes for monitoring soil moisture with a neutron probe. The moderately permeable site was instrumented with a transiometer, two piezometers, and an access tube for monitoring with a neutron probe.
In saturated conditions the transiometer had a faster response time after installation than the piezometer. Faster response makes the transiometer more desirable for use in slowly permeable soils, especially when monitoring dynamic soil water.
Calculated random error of the transiometer measuring system, including a digital voltmeter and a scanner, was typically 0.09 feet (2.8cm), with a maximum calculated to be 0.38 feet (11.5cm). The two most significant components were imprecision of the scanner card and calibration shift. The transiometer was sensitive to atmospheric pressure fluctuations, with sensitivity to atmospheric pressure change increasing with installation depth. 相似文献
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The problem of current channelling: A critical review 总被引:1,自引:0,他引:1
Alan G. Jones 《Surveys in Geophysics》1983,6(1-2):79-122
The notion that currents induced ‘elsewhere’, by external source fields, could wend their way in a frequency-independent ohmic-like manner through a region of interest has been the cause of many recent disputes within the geomagnetic induction community. In particular, two-dimensional (2D) models of the Rhinegraben, and of the region known as the ‘Eskdalemuir anomaly’ in southern Scotland, have been dismissed as erroneous by those who believe that the observations are more correctly interpreted as due to the effects of ‘channelled’ currents rather than ‘induced’ currents. In this review, attention is paid primarily to consider under what circumstances any perturbation of current flow, which may manifest itself as a ‘DC-like’ channelled current, could cause a ‘problem’ for those wishing to interpret their observations. Various concepts are introduced, particularly the ratio of 3D/2D current channelling numbers for the induction problem, which is shown to be the ratio of the length of the 3D body to the skin depth in the host medium. It is stressed that the worker must analyse his data by adequate statistical techniques, and that the simplest physical models possible, that describes the observations, must be sought. Finally, suggestions are made for further work to be undertaken. 相似文献
128.
The Abee E4 enstatite chondrite breccia consists of clasts (many rimmed by metallic Fe, Ni), dark inclusions and matrix. The clasts and matrix were well equilibrated by thermal metamorphism, as evidenced by uniform mineral compositions, recrystallized chondrules, low MnO content of enstatite and high abundance of orthoenstatite. The clasts acquired their metal-rich rims prior to this metamorphic episode. The occurrence in Abee of relatively unmetamorphosed dark inclusions, clasts with nearly random magnetic orientations and a matrix with a uniform magnetic orientation [18,19] indicates that clast and matrix metamorphism occurred prior to the agglomeration of the breccia.The dark inclusions are an unusual kind of enstatite chondritic material, distinguished from the clasts and matrix by their relative enrichments in REE [21–23], low relative abundances of kamacite, total metallic Fe, Ni and silica, lower niningerite/(total sulfide) ratios, high relative abundances of oldhamite and martensite, smaller euhedral enstatite, more heterogeneous enstatite and metallic Fe, Ni, more calcic enstatite and more nickeliferous schreibersite.We propose the following model for the petrogenesis of the Abee breccia: The maximum metamorphic temperature of breccia parent material was?- 840°C (the minimum temperature of formation of Abee niningerite) and perhaps near 950–1000°C (the Fe-Ni-S eutectic temperature). Euhedral enstatite crystals in metallic Fe, Ni- and sulfide-rich areas grew at these metamorphic temperatures into pliable metal and sulfide. Breccia parent material was impact-excavated from depth, admixed with dark inclusions and rapidly cooled (700 to 200°C in about 2 hours) [15]. During this cooling, clast and matrix material acquired thermal remanent magnetization. Random conglomeration of clasts and unconsolidated matrix materials caused the clasts to have random magnetic orientations and the matrix areas to have net magnetic intensities of zero (due to the cancellation of numerous randomly oriented magnetic vectors of equal intensity in the matrix). A subsequent ambient magnetic field imparted a uniform net magnetic orientation to the matrix and caused the magnetic orientations of the clasts to be somewhat less random. The Abee breccia was later consolidated, possibly by shock or by shallow burial and very long-period/low-temperature (< 215°C) metamorphism. 相似文献
129.
Alan P. Carr 《地球表面变化过程与地形》1983,8(3):293-296
The method described involves the use of an electro-optical distance measurer mounted on a theodolite. Observations are taken onto a series of acrylic reflectors, aligned at right angles to the line of sight, and slope distances and vertical and horizontal angles are recorded. From these data it is possible to obtain surface heights to an accuracy of ±0·5 cm at 200 m and to avoid any direct disturbance of the site under investigation. 相似文献
130.
Alan C. McKinnon 《Geoforum》1983,14(4):389-399
Over the past decade there has been a growing recognition by the business community, central government and local planners of the economic importance of warehousing. This importance can be measured in terms of the value of stock held in warehouses, their very rapid growth relative to other types of commercial property and the employment they create. This article reviews changes in business and planning attitudes to warehousing, accounts for the recent growth of warehouse floorspace in England and examines the spatial distribution of this growth. 相似文献