The Perturbations of Alternating Geomagnetic Fields by Conductivity Anomalies     

F. Walter Jones  Albert T. Price 《Geophysical Journal International》1970,20(3):317-334

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BIOGEOGRAPHY: RESEARCH OPPORTUNITIES FOR GEOGRAPHERS     

Thomas R. Vale  Albert J. Parker 《The Professional geographer》1980,32(2):149-157

The authors of this paper suggest several themes in biogeography worthy of increased research attention by geographers: human impacts on biota, disturbance ecology, and certain aspects of biota-environment interactions. Researchers must gain a firmer command of biological as well as geographical concepts and approaches if biogeography is to join geomorphology and climatology in developing a true geography of environments.  相似文献   

THE CLOVIS (no. 1), NEW MEXICO,METEORITE AND Ca,Al AND Ti-RICH INCLUSIONS IN ORDINARY CHONDRITES     

Albert F. Noonan 《Meteoritics & planetary science》1975,10(1):51-59

Microprobe analyses of the major silicates in Clovis (no. 1), New Mexico, establish it as an H3 chondrite. Inclusions identified in Clovis are: breccia fragments; angular and vesicular chondrule or rock fragments composed almost entirely of glass and olivine (Fa_12–22); chondrules, composed principally of pyroxene (Fs_2–33) and olivine (Fa_1–28); and Ca, Al and Ti-rich inclusions. These refractory enriched inclusions, similar in composition to those found in some carbonaceous chondrites, are rare in ordinary chondrites but in this study were observed in Sharps, Virginia (H3), Gobabeb, South Africa (H4), Dimmitt, Texas (H4), Weston, Connecticut (H4–6) and Clovis. Sodium, known to rim similar inclusions in carbonaceous chondrites, also occurs in the interiors of inclusions observed in this study, sometimes in moderate amounts. The Na distribution is regarded as primary and is not attributable, at least in total, to secondary Na extraction from the host matrix.  相似文献   

Rotational distortion of polytropes by analytic approximation,II     

Albert P. Linnell 《Astrophysics and Space Science》1977,48(1):165-183

Chandrasekhar's (1933) paper on rotational distortion of polytropes contained a perturbation term in the potential which was linear inv, the rotation parameter. The same paper, and subsequent papers by various authors, developed an analytic expression for the boundary also linear inv. The latter expression is equivalent to a two term Taylor series about the unperturbed boundary, and is in error by 12% near critical rotation, for a polytropic index 3.0. The boundary can be located directly from the functions representing density, potential, and the potential gradient. The boundary error by this procedure is 0.2% near critical rotation.  相似文献   

Conduction models of the temperature distribution in the East Rift Zone of Kilauea Volcano     

Albert J. Rudman  David Epp   《Journal of Volcanology and Geothermal Research》1983,16(3-4)

Temperature variations in the 1966-meter Hawaii Geothermal Project well HGP-A are simulated by model studies using a finite element code for conductive heat flow. Three models were generated: a constant temperature source from a vertical dike; a constant heat-generating magma chamber; and a transient heat source from a tapered vertical dike. Fair correlation is obtained between the HGP-A well temperature and the tapered dike 125 years after it is injected with an initial (transient) 1200° C temperature. Results provide background information from which to evaluate the importance of water convection in maintaining the temperature distribution in the East Rift Zone.  相似文献   

Marble-hosted sulfide ores in the Angouran Zn-(Pb–Ag) deposit, NW Iran: interaction of sedimentary brines with a metamorphic core complex     

H. Albert Gilg  Maria Boni  Giuseppina Balassone  Cameron R. Allen  David Banks  Farid Moore 《Mineralium Deposita》2006,41(1):1-16

The Angouran Zn-(Pb–Ag) deposit, Zanjan Province, NW Iran, is located within the central Sanandaj-Sirjan Zone of the Zagros orogenic belt. The deposit has proven and estimated resources of 4.7 Mt of sulfide ore at 27.7% Zn, 2.4% Pb, and 110 g/t Ag, and 14.6 Mt of oxidized carbonate ores at 22% Zn and 4.6% Pb. It is hosted by a metamorphic core complex that is unconformably overlain by a Neogene volcanic and evaporite-bearing marine to continental sedimentary sequence. The sulfide orebody, precursor to the significant nonsulfide ores, is located at the crest of an open anticline at the contact between Neoproterozoic to Cambrian footwall micaschists and hanging wall marbles. ⁴⁰Ar–³⁹Ar data on muscovite from mineralized and unaltered footwall micaschists suggest a rapid Mid-Miocene exhumation of the metamorphic basement (∼20 Ma) and yield an upper age constraint for mineralization. The fine-grained sulfide ore is massive, replacive, often brecciated, clearly postmetamorphic and dominated by Fe-poor sphalerite, with minor galena, pyrite, anhydrite, quartz, muscovite, dolomite, and rare calcite. Sphalerite contains Na–Ca–Cl brine inclusions (23–25 mass% total dissolved solids) with homogenization temperatures of 180–70°C. Fluid inclusion chemistry (Na–K–Li–Ca–Mg–Cl–Br), ore geochemistry, S, and Pb isotope data suggest that the Angouran sulfide ore formed by the interaction of modified, strongly evaporated Miocene seawater and the lithotypes of an exhumed metamorphic core complex. Minor contributions of metals from Miocene igneous rocks cannot be excluded. Mineralization occurred in a collisional intra-arc setting with high heat flow, probably during the transition from an extensional to a compressional regime. The Angouran deposit may represent a new type of low-temperature carbonate-hosted Zn–Pb ore that is distinct from Mississippi Valley type and sedimentary-exhalative deposits.Editorial handling: B. Lehmann  相似文献   

Three-dimensional finite element modelling of stress relaxation tests in anisotropic clayey medium: direct problem and back analysis     

Richard Giot  Albert Giraud  Francoise Homand 《Geotechnical and Geological Engineering》2006,24(4):919-947

This paper deals with a new strategy for initial stress identification by stress relaxation methods, coupled with finite element calculation, and applied to the overcoring test. The back-analysis of such a test uses an inversion method which consists in the minimisation, with a gradient-based algorithm, of a cost functional of least-squares type, which quantizes the difference between measured and computed strains. The computed strains are assessed by three-dimensional finite element modellings of the overcoring test. The inversion methodology is applied to a recent in situ overcoring test performed at Mont Terri laboratory, Switzerland. The inversion gave good results and allows us to validate the inversion methodology. The constitutive law considered for this application is transverse isotropic elasticity but the inversion method developed is applicable to most constitutive laws and every kind of in situ test.  相似文献   

Establishing the Database of Inundation Potential in Taiwan     

Albert S. Chen  Ming-Hsi Hsu  Wei-Hsien Teng  Chen-Jia Huang  Sen-Hae Yeh  Wan-Yu Lien 《Natural Hazards》2006,37(1-2):107-132

The structural measure was the major solution for flood defense in Taiwan. However, the measure is always limited to the design standard and cannot prevent the damages when floods exceed certain scale. Therefore, non-structural measures for flood mitigation are the indispensable complements to structural solutions. The study introduces the establishment of inundation potential database that provides required information for the non-structural measures in Taiwan. The database was built by numerical simulations, based on different rainfall scenarios, and has been applied by the local governments of Taiwan for land use managements, flood warning systems, emergency responses, and flood insurance programs to reduce the flood damages and impacts.  相似文献   

Geology and geochemistry of jasperoids from the Gold Bar district, Nevada     

Ozcan Yigit  Albert H. Hofstra  Murray W. Hitzman  Eric P. Nelson 《Mineralium Deposita》2006,41(6):527-547

Gold Bar is one of several Carlin-type gold mining districts located in the Battle Mountain–Eureka trend, Nevada. It is composed of one main deposit, Gold Bar; five satellite deposits; and four resources that contain 1.6 Moz (50 t) of gold. All of the deposits and resources occur at the intersection of north-northwest- and northeast-trending high-angle faults in slope facies limestones of the Devonian Nevada Group exposed in windows through Ordovician basin facies siliciclastic rocks of the Roberts Mountains allochthon. Igneous intrusions and magnetic anomalies are notably absent. The Gold Bar district contains a variety of discordant and stratabound jasperoid bodies, especially along the Wall Fault zone, that were mapped and studied in some detail to identify the attributes of those most closely associated with gold ore and to constrain genetic models. Four types of jasperoids, J₀, J₁, J₂, and J₃, were distinguished on the basis of their geologic and structural settings and appearance. Field relations suggest that J₀ formed during an early event. Petrographic observations, geochemistry, and δ¹⁸O values of quartz suggest it was overprinted by the hydrothermal event that produced ore-related J₁, J₂, and J₃ jasperoids and associated gold deposits. The greater amount of siliciclastic detritus present in J₀ jasperoids caused them to have higher δ¹⁸O values than J_1,2,3 jasperoids hosted in underlying limestones. Ore-related jasperoids are composed of main-ore-stage replacements and late-ore-stage open-space filling quartz with variable geochemistry and an enormous range of δ¹⁸O values (24.5 and −3.7‰). Jasperoids hosted in limestones with the most anomalous Au, Ag, Hg, ±(As, Sb, Tl) concentrations and the highest δ¹⁸O values are associated with the largest deposits. The 28‰ range of jasperoid δ¹⁸O values is best explained by mixing between an ¹⁸O-enriched fluid and an ¹⁸O-depleted fluid. The positive correlation between the sizes of gold deposits and the δ¹⁸O composition of jasperoids indicates that gold was introduced by the ¹⁸O-enriched fluid. The lowest calculated δ¹⁸O value for water in equilibrium with late-ore-stage quartz at 200°C (−15‰) and the measured δD value of fluid inclusion water extracted from late-ore-stage orpiment and realgar (−116‰) indicate that the ¹⁸O-depleted fluid was composed of relatively unexchanged meteoric water. The source of the ¹⁸O-enriched ore fluid is not constrained. The δ³⁴S values of late-ore-stage realgar, orpiment, and stibnite (5.7–15.5‰) and barite (31.5–40.9‰) suggest that H₂S and sulfate were derived from sedimentary sources. Likewise, the δ¹³C and δ¹⁸O values of late-stage calcite (−4.8 to 1.5‰ and 11.5 to 17.4‰, respectively) suggest that CO₂ was derived from marine limestones. Based on these data and the apparent absence of any Eocene intrusions in the district, Gold Bar may be the product of a nonmagmatic hydrothermal system.  相似文献   

Germanium isotopic variations in igneous rocks and marine sediments   总被引：1，自引：0，他引：1  

Olivier Rouxel  Albert Galy 《Geochimica et cosmochimica acta》2006,70(13):3387-3400

A new technique for the precise and accurate determination of Ge stable isotope compositions has been developed and applied to silicate rocks and biogenic opal. The analyses were performed using a continuous flow hydride generation system coupled to a MC-ICPMS. Samples have been purified through anion- and cation-exchange resins to separate Ge from matrix elements and eliminate potential isobaric interferences. Variations of ⁷⁴Ge/⁷⁰Ge ratios are expressed as δ⁷⁴Ge values relative to our internal standard and the long-term external reproducibility of the data is better than 0.2‰ for sample size as low as 15 ng of Ge. Data are presented for igneous and sedimentary rocks, and the overall variation is 2.4‰ in δ⁷⁴Ge, representing 12 times the uncertainty of the measurements and demonstrating that the terrestrial isotopic composition of Ge is not unique. Co-variations of ⁷⁴Ge/⁷⁰Ge, ⁷³Ge/⁷⁰Ge and ⁷²Ge/⁷⁰Ge ratios follow a mass-dependent behaviour and imply natural isotopic fractionation of Ge by physicochemical processes. The range of δ⁷⁴Ge in igneous rocks is only 0.25‰ without systematic differences among continental crust, oceanic crust or mantle material. On this basis, a Bulk Silicate Earth reservoir with a δ⁷⁴Ge of 1.3 ± 0.2‰ can be defined. In contrast, modern biogenic opal such as marine sponges and authigenic glauconite displayed higher δ⁷⁴Ge values between 2.0‰ and 3.0‰. This suggests that biogenic opal may be significantly enriched in light isotopes with respect to seawater and places a lower bound on the δ⁷⁴Ge of the seawater to +3.0‰.This suggests that seawater is isotopically heavy relative to Bulk Silicate Earth and that biogenic opal may be significantly fractionated with respect to seawater. Deep-sea sediments are within the range of the Bulk Silicate Earth while Mesozoic deep-sea cherts (opal and quartz) have δ⁷⁴Ge values ranging from 0.7‰ to 2.0‰. The variable values of the cherts cannot be explained by binary mixing between a biogenic component and a detrital component and are suggestive of enrichment in the light isotope of diagenetic quartz. Further work is now required to determine Ge isotope fractionation by siliceous organisms and to investigate the effect of diagenetic processes during chert lithification.  相似文献