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1.
Microtektites: a chemical comparison of bottle-green microtektites,normal microtektites and tektites
F.A. Frey 《Earth and Planetary Science Letters》1977,35(1):43-48
Trace element abundances in Ivory Coast normal microtektites and Australasian bottle-green microtektites confirm that microtektites are genetically related to tektites in the associated strewn field. Although major and compatible trace element abundances imply that bottle-green microtektites are members of a fractional crystallization sequence, the similar rare earth element distributions in Australasian normal and bottle-green microtektites and tektites cannot be explained by a simple fractionation model. The similar REE abundances in tektites and microtektites of widely different major element composition also preclude simple models calling for sedimentary rock precursors. 相似文献
2.
B.P. Glass R.N. Baker D. Storzer G.A. Wagner 《Earth and Planetary Science Letters》1973,19(2):184-192
Over 6000 microscopic glass spherules between 125 μm and 1 mm in diameter were found in a sediment core (RC9-58) from the Caribbean Sea. These glassy objects are mostly confined to a zone ～ 40 cm thick at a depth of ～ 250 cm. We believe that the microscopic glass objects are microtektites belonging to the North American strewnfield, based on their geographical location, appearance, physical properties, stratigraphic age (middle Upper Eocene), fission track age (～34.6 my) and major element compositions. The occurrence of North American microtektites in the Caribbean Sea indicates that the North American strewnfield is two to three times larger than previously indicated. An estimate on the abundance of microtektites in core RC9-58 indicates that the North American strewnfield may contain greater than 1017 g of tektite material. 相似文献
3.
Sm-Nd and Rb-Sr analyses of tektites and other impactites can be used to place constraints on the age and provenance of target materials which were impact melted to form these objects. Tektites have large negative εNd(0) values and are uniform within each tektite group while the εSr(0) are large positive values and show considerable variation within each group. Chemical, trace element, and isotopic compositions of tektites are consistent with production by melting of sediments derived from old terrestrial continental crust. Each tektite group is characterized by a uniform Nd model age,TCHURNd, interpreted as the time of formation of the crustal segment which weathered to form the parent sediment for the tektites: (1) ～1.15 AE for Australasian tektites; (2) ～1.91 AE for Ivory Coast tektites; (3) ～0.9 AE for moldavites; (4) ～0.65 AE for North American tektites, and (5) ～0.9 AE for high-Si irghizites. Sr model ages,TURSr, are variable within each group reflecting Rb-Sr fractionation and in the favorable limit of very high Rb/Sr ratios, approach the time of sedimentation of the parent material which melted to form the tektites. Australasian tektites are derived from ～0.25 AE sediments, moldavites from ～0.0 AE sediments, Ivory Coast tektites from ～0.95 AE sediments. Possible parent sediments of other tektite groups have poorly constrained ages. Our data on moldavites and Ivory Coast tektites are consistent with derivation from the Ries and Bosumtwi craters, respectively. Irghizites are isotopically distinct from Australasian tektites and are probably not related. Sanidine spherules from a Cretaceous-Tertiary boundary clay have initial εNd ～ +2; εSr ～ +5 and are not derived from old continental crust or meteoritic feldspar. They may represent a mixture of basaltic oceanic crust and sediments, implying an oceanic impact. These isotopic results are also consistent with a volcanic origin for the spherules. 相似文献
4.
5.
Abstract The runoffs at four Ivory Coast hydrometric stations (monitoring flows from an area covering between 5930 and 66500 km2) were analysed with a set of statistical methods for the detection of breaks in the time series. The variables studied were the annual mean discharge and some characteristic discharges. From a quantitative standpoint, the existence of a clear break in the series of annual mean discharges at the beginning of the decade from 1970, the date from which the runoffs decrease significantly, was noted. A more qualitative study of the results showed that low flows were more affected than high flows by this variability of the regime. This fluctuation appears to be in accord with the drought phenomena observed during the same period in the Sahel, to the north of Ivory Coast. 相似文献
6.
B.P. Glass 《Earth and Planetary Science Letters》1986,77(3-4)
Fission-track and40Ar—39Ar dating of Australasian tektites indicates that the australites are older than the rest of the Australasian tektites. Some authors have suggested, therefore, that there should be two microtektite layers in the Australasian region: a younger layer approximately synchronous with the Brunhes/Matuyama geomagnetic reversal boundary (previously described in the literature) and an older layer of Australian microtektites (micro-australites) below the Brunhes/Matuyama reversal boundary and perhaps associated with the end of the Jaramillo event. Fifteen cores already known to contain a layer of Australasian microtektites at or slightly above the Brunhes/Matuyama reversal boundary were searched for an 0.8 to 0.9 m.y. old micro-australite layer. No evidence for this layer was found. Thus, the data do not support the conclusion that the Australasian tektites were produced by two impact events at different times. 相似文献
7.
Abstract In the context of rainfall-runoff modelling carried out on the Sudanese savannah area in the northwest of the Ivory Coast, attempts are being made to reconstitute the flow at the outlets of catchments in 10 day time steps. By using algorithms with automatic fitting procedures for the parameters, it appeared necessary to make a choice concerning the calibration objective functions to be used. The paper presents the algorithms, data and objective functions that have been used. The results obtained from the calibrations made have been analysed. That analysis was done principally with the help of a comparative evaluation modulus which takes into account elements other than the value of the objective function alone and which enables the quality of the results to be picked out from a hydrological point of view. At the conclusion of the analysis, the objective function defined by Nash seems to stand out quite clearly in relation to the other formulae examined. 相似文献
8.
On 28th July, 1984 at about 12 hours 13 m UT an earthquake with an epicentral intensity of about VI occurred in the southwestern part of Nigeria, an area that has always been believed to be seismically inactive. This event was recorded at the LAMTO seismic observatory in Ivory Coast and its epicentre was located at around Ijebu-Ode. In Nigeria, the event was recorded only by a temporary monitoring station at Ahmadu Bello University (ABU) in Zaria, a distance of about 640 km from the epicentre. Two subsequent tremors that were reported in Ijebu-Ode in early August, 1984 were not recorded at Ahmadu Bello University.Geological, geophysical and geodynamic studies indicate the possibility of the existence of large fracture zones trending generally in the NE-SW direction across the country. The recent earthquakes in southwestern Nigeria could therefore be evidence to support the existence of the Pelusium megashear system or similar fracture zones that penetrate deep into the continental crust of West Africa. There is therefore a need for more adequate coverage of the area by seismic stations to permit a precise location of future events and detailed data analysis which would help in identifying area where large scale crustal adjustments might be taking place. 相似文献
9.
New experimental data depicting equatorial spread-F were taken during an HF radar sounding campaign in Korhogo (Ivory Coast, 9°24N, 5°37W, dip 4°S). Range-time-intensity maps of the radar echoes have been analyzed to identify the signatures of density depletions and bottomside spread-F. Density depletions are well known features of equatorial spread-F, and are believed to emerge after the development of a Rayleigh-Taylor instability on the bottomside F-layer. A simple model is developed and used to simulate the flow of density depletions over the radar field of view. The simulation permits an interpretation of the data that yields the zonal flow velocity as a function of local time. Comparisons with previous measurements are undertaken to assess the consistency of the computational results, and qualitative arguments are presented to identify bottomside spread-F. Using the computational results as reference, a morphological study of ionograms showing spread-F is undertaken which reveals the specific signature of bottomside spread-F on ionograms recorded just after sunset. 相似文献
10.
Olivine crystals in mare basalts 12004,8 and 12022,12 are normally zoned with Cr-poor rims. The Ni content of rare 2–10-μm metal inclusions in olivine decreases markedly as Fe/Mg in their immediate olivine hosts increases. Each metal grain appears to have been enclosed by late olivine almost immediately after it crystallized. The fractionation trend for the olivine and metal contrasts with the subsolidus equilibration trend for pallasites. For the basalts, not even local equilibration of Fe, Ni and Co at metal/olivine interfaces can be detected by microprobe. Ni and Co concentrations range from about 300 ppm in olivine cores to about 70 ppm in rims. The limits of detection, at 95% confidence, are 36 ppm (Ni) and 25 ppm (Co). The distribution of Ni and Co in olivine, like that of Mg and Cr, records the depletion of these elements in the melt.Fractional solidification models, using the Ni and Co concentrations of the whole rock, and Ni and Co concentrations of the earliest formed olivine, metal and “opaques” as initial compositions, allow metal and olivine compositions to be predicted if the order of crystallization is known. Conversely the order of crystallization can be established if known olivine and metal compositions are reproduced. Calculated Ni and Co contents for metal and olivine in these basalts correspond to observed concentrations only where metal precipitation is delayed until the liquid has crystallized 4–5 wt.% olivine. 相似文献
11.
Volcanic glass compositions and tephra layer age are critical for anchoring their sources and correlating among different sites; however, such work may be imprecise when the tephra has varied compositions. The ash from Changbaishan Millennium eruption (940s AD), a widely distributed tephra layer, has been detected in the far-east areas of Russia, the Korean Peninsula, Japan, and in Greenland ice cores. There are some debates on the presence of this tephra from sedimentary archives to the west of Changbaishan volcano, such as lake and peat sediments in the Longgang volcanic field. In this paper, major element compositions for clinopyroxene and Fe-Ti oxides were performed on proximal tephra from Changbaishan and the Millennium eruption ash record in Lake Sihailongwan. Clinopyroxene and Fe-Ti oxides microlites from Sihailongwan show augite- ferroaugite and titanmagnetite compositions, similar to those from dark pumice in Changbaishan proximal tephra, but different from the light grey pumice, which has ferrohedenbergite and ilmenite microlite compositions. This result implies that the tephra recorded in Sihailongwan was mainly from the trachytic eruptive phase of the Millennium eruption, and the rhyolitic eruptive phase made a relatively small contribution to this area. Analyzing clinopyroxene and Fe-Ti oxides microlites is a new method for correlating tephra layers from Changbaishan Millennium eruption. 相似文献
12.
Widespread Plio-Pleistocene (2.43-0.06 Ma) tephra zones recognised in deep-sea cores from high latitudes (>60°) in the Southern Pacific Ocean were thought to have originated from calc-alkaline rhyolitic eruptions in New Zealand, some 5000 km distant. Electron microprobe analyses of the glasses reveal a wide diversity of alkalic felsic compositions, as well as minor components of basic and intermediate glasses, incompatible with a New Zealand Neogene source but similar to contemporaneous eruptives from the Antarctic region. Most tephra zones are trachytic; seven zones are peralkaline rhyolite. The rhyolitic zones represent a deep-sea record of widespread silicic eruptions from continental Antarctica, possibly Marie Byrd Land. The extent of these rhyolitic zones suggest a greater frequency of large explosive eruptions in Antarctica than previously documented. The coarse grain size of some of the shards (up to 3 mm), their great distance from the closest sources (>1600 km for some cores), and the presence of nonvolcanic ice-rafted debris indicate some of the glasses, especially the more basic compositions, may have been ice-rafted, contrary to previous suggestions of a fallout origin. 相似文献
13.
Temporal evolution of the Mariana arc during rifting of the Mariana Trough traced through the volcaniclastic record 总被引：1，自引：0，他引：1
The sedimentary sequences that accumulate around volcanic arcs may be used to reconstruct the history of volcanism provided the degree of along-margin sediment transport is modest, and that reworking of old sedimentary or volcanic sequences does not contribute substantially to the sediment record. In the Mariana arc, the rare earth and trace element compositions of ash layers sampled by Deep Sea Drilling Project (DSDP) site 451 on the West Mariana Ridge, and sites 458 and 459 on the Mariana Forearc, were used to reconstruct the evolution of the arc volcanic front during rifting of the Mariana Trough. Ion microprobe analysis of individual glass shards from the sediments shows that the glasses have slightly light rare earth element (LREE)-enriched compositions, and trace element compositions typical of arc tholeiites. The B/Be ratio is a measure of the involvement of subducted sediment in petrogenesis, and is unaffected by fractional crystallization. This ratio is variable over the period of rifting, increasing up-section at site 451 and reaching a maximum in sediments dated at 3–4 Ma, ∼ 3–4 million years after rifting began. This may reflect increased sediment subduction during early rifting and roll-back of the Pacific lithosphere. Parallel trends are not seen in the enrichment of incompatible high field strength (HFSE), large ion lithophile (LILE) or rare earth elements (REE), suggesting that flux from the subducting slab alone does not control the degree of melting. Re-establishment of arc volcanism on the trench side of the basin at ca 3 Ma resulted in volcanism with relative enrichment in incompatible REE, HFSE and LILE, although these became more depleted with time, possibly due to melt extraction from the mantle source as it passed under the developing back-arc spreading axis, prior to melting under the volcanic front. 相似文献
14.
During the summer of 1990, 12 gravity cores were collected in Lake Coeur d'Alene, Idaho at various depths and in a variety of depositional environments. All core subsamples were analysed to determine the bulk sediment chemistry; selected subsamples were analysed for trace element partitioning and 137Cs activity. The purpose of these analyses was to determine the trace element concentrations and distributions in the sediment column and to try to establish a trace element geochemical history of the lake in relation to mining and mining-related discharge operations in the area. Substantial portions of the near-surface sediments in Lake Coeur d'Alene are markedly enriched in Ag, As, Cd, Hg, Pb, Sb and Zn, and slightly enriched in Cu, Fe and Mn. Variations in the thickness of the trace element-rich sediments, which range from more than 119 cm to as little as 17 cm, indicate that the source of much of this material is the Coeur d'Alene River. An estimated 75 million tonnes of trace element-rich sediments have been deposited on or in the lake bed. Estimated trace element masses in excess of those considered representative of background conditions range from a high of 468 000 tonnes of Pb to a low of 260 tonnes of Hg. The similarity between the trace element-rich surface and subsurface sediments with respect to their location, their bulk chemistry, their interelement relations and their trace element partitioning indicate that the sources and/or concentrating mechanisms causing the trace element enrichment in the lake sediments have probably been the same through-out their depositional history. Based on a Mt St Helens'ash layer from the 1980 eruption, ages estimated from 137Cs activity and the presence of 80 discernible and presumably annual layers in a core collected near the Coeur d'Alene River delta indicate that deposition rates for the trace element-rich sediments have ranges from 2.1 to 1.3 cm/year. These data also indicate that the deposition of trace element-rich sediments began, at least in the Coeur d'Alene River delta, some time between 1895 and 1910, dates consistent with the onset of mining and ore processing activities that began in the area in the 1880s. 相似文献
15.
This is a preliminary study on the microtektites that were found in large numbers from the interval between 7.80 and 8.10
m depth of core S095-17957-2 (10°53.9’N, 115°18.3’E, water depth 2 195 m), northern Nansha area of the South China Sea. The
microtektites vary in shape, with spherules predominating, and are commonly less than 1 mm in diameter, transparent or semitransparent,
brownish in color, with bubbles inside. Based on coarse fraction stratigraphy and foraminifera/nanofossil biostratigraphical
events the microtektite layer was assigned to nearly the Bm-hes/Matuyama magnetic reversal boundary (some 0.78 MaBP). Obviously,
the present microtektites, and those found from the middle Pleistocene of the Indian Ocean, Australia and loess of northern
China, were products of the same impact event and therefore, are useful as a reliable mark in Quaternary stratigraphy, as
well as in paleoclimatic studies.
Project supported by the National Natural Science Foundation of China (Grant Nos. 49676287, 49732060). 相似文献
16.
Vanessa J. C. Holden Annie T. Worsley Colin A. Booth Graham Lymbery 《Ocean Dynamics》2011,61(12):2157-2179
Sediment pathways and links to offshore processes are considered in the textural and magnetic characteristics of sediments
of the intertidal flats and salt marshes of the north Sefton Coast, UK. In addition, sediment from a range of intertidal,
marine and fluvial locations within the northwest region has similarly been characterised. Subsequently, the characteristics
of these regional sediments, using a multivariate statistical approach of R- and Q-mode factor analyses, have been used to
investigate the sediment transport pathways of the north Sefton Coast sediment. The benefits of fractionating sediment samples
have been observed, and by using combinations of textural and isothermal remanent magnetic measurements, specific environments
within the research have been successfully differentiated and characterised. Linkages between potential sediment source areas
have also been established on a particle size-related basis. 相似文献
17.
Marc D. Norman Vickie C. BennettGraham Ryder 《Earth and Planetary Science Letters》2002,202(2):217-228
Highly siderophile element compositions of lunar impact melt breccias provide a unique record of the asteroid population responsible for large cratering events in the inner Solar System. Melt breccias associated with the 3.89 Ga Serenitatis impact basin resolve at least two separate impact events. KREEP-rich melt breccias representing the Apollo 17 poikilitic suite are enriched in highly siderophile elements (3.6-15.8 ppb Ir) with CI-normalized patterns that are elevated in Re, Ru and Pd relative to Ir and Pt. The restricted range of lithophile element compositions combined with the coherent siderophile element signatures indicate formation of these breccias in a single impact event involving an EH chondrite asteroid, probably as melt sheet deposits from the Serenitatis Basin. One exceptional sample, a split from melt breccia 77035, has a distinctive lithophile element composition and a siderophile element signature more like that of ordinary chondrites, indicating a discrete impact event. The recognition of multiple impact events, and the clear signatures of specific types of meteoritic impactors in the Apollo 17 melt breccias, shows that the lunar crust was not comprehensively reworked by prior impacts from 3.9 to 4.5 Ga, an observation more consistent with a late cataclysm than a smoothly declining accretionary flux. Late accretion of enstatite chondrites during a 3.8-4.0 Ga cataclysm may have contributed to siderophile element heterogeneity on the Earth, but would not have made a significant contribution to the volatile budget of the Earth or oxidation of the terrestrial mantle. Siderophile element patterns of Apollo 17 poikilitic breccias become more fractionated with decreasing concentrations, trending away from known meteorite compositions to higher Re/Ir and Pd/Pt ratios. The compositions of these breccias may be explained by a two-stage impact melting process involving: (1) deep penetration of the Serenitatis impactor into meteorite-free lower crust, followed by (2) incorporation of upper crustal lithologies moderately contaminated by prior meteoritic infall into the melt sheet. Trends to higher Re/Ir with decreasing siderophile element concentrations may indicate an endogenous lunar crustal component, or a non-chondritic late accretionary veneer in the pre-Serenitatis upper crust. 相似文献
18.
Yu-Chia Chung 《Earth and Planetary Science Letters》1976,32(2):249-257
Seven vertical profiles of226Ra have been measured along an east-west traverse at about 30°N from San Diego to northwest of Hawaii. These profiles show that there is a distinct core of Ra maximum spreading westward as a tongue in the northeast Pacific deep water. This core starts in the east with 21.1 Ra units (1Ra unit= 10?14g/kg) at 3.9 km depth at about 130°W, and deepens westward to 4.1 km with its Ra reduced to 18.3 units at 150°W. A similar core with some uncertainty due to possible sampling errors extends westward near the bottom at 5.2 km depth from 19.4 Ra units at 150°W to 15.9 units at about 180° longitude. In addition, these profiles appear to be correlated with each other in structure above the cores of Ra maximum. These cores indicate that the Ra input depends locally on the type and composition of sediments and so the flux varies over the ocean bottom. On the basis of a one-dimensional diffusion-decay model, a horizontal diffusion coefficient of 106 cm2/sec has been computed along these cores. Although this value appears to be slightly lower, it is not inconsistent with those derived from other physical methods. 相似文献
19.
How might an international plan for crustal-movement studies in Africa be implemented? This was the basic question underlying much of the discussion at the First International Symposium on Crustal Movements in Africa, convened in May 1981 in Addis Ababa at the United Nations Economic Commission for Africa. Three years later, in November/December 1984, the Second Symposium was held in Cairo, Egypt; and in April 1986, some outstanding questions were dealt with at the Third Symposium on Geodesy in Africa held in Yamoussoukro, Ivory Coast, and at the International Symposium on Global Changes in Africa during the Quaternary, held in Dakar, Senegal.The present state and possible development of recent crustal movement studies in Africa are discussed. Finally, the cooperation of all scientists interested in such studies is solicited. 相似文献
20.
Masaki Enami Jun‐Ichi Kimura Motohiro Tsuboi Yui Kouketsu Takayoshi Nagaya Shuaimin Huang 《Island Arc》2019,28(1)
Garnet grains in Sanbagawa quartz eclogites from the Besshi region, central Shikoku commonly show a zoning pattern consisting of core and mantle/rim that formed during two prograde stages of eclogite and subsequent epidote–amphibolite facies metamorphism, respectively. Garnet grains in the quartz eclogites are grouped into four types (I, II, III, and IV) according to the compositional trends of their cores. Type I garnet is most common and sometimes coexists with other types of garnet in a thin section. Type I core formed with epidote and kyanite during the prograde eclogite facies stage. The inner cores of types II and III crystallized within different whole‐rock compositions of epidote‐free and kyanite‐bearing eclogite and epidote‐ and kyanite‐free eclogite at the earlier prograde stage, respectively. The inner core of type IV probably formed during the pre‐eclogite facies stage. The inner cores of types II, III, and IV, which formed under different P–T conditions of prograde metamorphism and/or whole‐rock compositions, were juxtaposed with the core of type I, probably due to tectonic mixing of rocks at various points during the prograde eclogite facies stage. After these processes, they have shared the following same growth history: (i) successive crystal growth during the later stage of prograde eclogite facies metamorphism that formed the margin of the type I core and the outer cores of types II, III, and IV; (ii) partial resorption of the core during exhumation and hydration stage; and (iii) subsequent formation of mantle zones during prograde metamorphism of the epidote–amphibolite facies. The prograde metamorphic reactions may not have progressed under an isochemical condition in some Sanbagawa metamorphic rocks, at least at the hand specimen scale. This interpretation suggests that, in some cases, material interaction promoted by mechanical mixing and fluid‐assisted diffusive mass transfer probably influences mineral reactions and paragenesis of high‐pressure metamorphic rocks. 相似文献