首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 78 毫秒
1.
Four volcanic units have been distinguished on the islanf of Fayal. In order of decreasing age, these are:
  • the eastern rift, and products of the activity preceding the collapse of the caldera visible at the summit of the stratovolcano, characterized by an alkaline series: basalt-hawaiite-mugearite-trachyte;
  • the products of the explosive and postcaldera activity where only evolved lavas occur (benmoreites and trachytes);
  • the recent basaltic activity of the Horta region;
  • the western fissural activity — recent and historical.
    • The two last units are characterized by exclusively basaltic, frequently picritic, eruptions. The lava groups cannot be distinguished by chemical criteria and have thus been treated as a single suite. Ninety samples have been analysed by X-ray fluorescence, and the mineralogy of 6 representative specimens has been determined by microprobe. The data were used to work out the evolution of the lava. The series is shown to have been produced by crystal fractionation under moderate water pressure from an alkali basalt. Moderate fractionation of amphibole during the last stages allow the liquids to remain weakly undersaturated from initial basalts until final trachytes. Mineralogical and chemical diversity between the most evolved lavas, benmoreites and trachytes, is an evidence of the strong influence ofpH2O and/orfO2 on the composition of such residua.  相似文献   

    2.
    The December 1981 — January 1982 eruption which started in the Christmas night on the SE side of Nyamulagira, gave the longest historical flow (26 km) representing the highest production rate of this volcano in this century (280×106m3 of erupted magmas in 19 days). This eruption built Rugarambiro, a composite spatter cinder-cone. The ejected lava is a K-hawaiite (kivite) whose basicity decreased during the eruption (first emission: D.I. = 40; last products: D.I. = 35). This chemical evolution is reflected by:
  • --the modal composition of lavas. The first emissions are poor in ferromagnesian phenocrysts (olivine + clinopyroxene: 3%) and rich in plagioclase (12%); the contrary is observed in the last ejected lavas (livine + Cpx: 16%; plagioclase: 1%);
  • --the nature of the crystallizing minerals in the groundmass. In fact, only the first ejections include alkaline feldspars, nepheline and Tiphlogopite;
  • --the glassy phase composition which is more differentiated in the first lavas (D.I. 68–84) than in the last ones (D.I. 42–61).
    • A stratification of the Nyamulagira magmatic chamber is proposed where magmatic differentiation has probably occurred for fractional crystallization. Mineralogical thermobarometers lead to locate this magmatic reservoir at the depth of 6–7 km that we had already hypothesized. The presence of phenocrysts of bytownite, basic chrysolite, diopside and salite indicates a basaltic paragenesis which marks a hawaiitic magma chamber feeding.  相似文献   

    3.
    This paper gives a critical analysis of the technical solutions adopted for erecting earthworks to divert lava flows on the basis of the experience obtained recently during the 1983 eruption of Mount Etna. The purpose of the paper is to explain questions of general nature and throw light on problems that must still be solved, which render it difficult to extrapolate the adopted solutions to other volcanic eruptions having radically different characteristics. An account of the earthworks made at the various sites to the east and the west of the lava flow is given, and an analysis is made of the planning choices, the decisions taken during the work, the daily output and thence the relative costs. The works carried out are summarized in Table 1 giving characteristics and costs of the works and times and sizes of the equipments used. The following works were carried out:
  • 4 barriers for a total length of ca. 1700 m with a height variable from 8 to 20 m.
  • 2 channels for a total length of 1000 m.
  • many emergency measures during night and day-time also on fresh lava.
    • During 50 days of 13 working hours per day a total volume of ca. 750,000 m3 of material was moved and ca. 10 km of service roads were built. The total cost of the above mentioned works was 3,678 millions of Italian liras. The final result fulfilled the planning forecasts: all the earthworks erected carried out their task of guiding and diverting the lava flows. Amongst other things, the study of the evolution of eruption has shown that preventive and emergency works can successfully be programmed. Finally, the technological aspects of the works are discussed with particular reference to their planning, site organization, determination of type and number of vehicles to be employed, output to expect and thus, for a correcta priori estimate of the advantage-cost ratio, a forecast of the expenses involved. The paper aims at giving information that may be of use during emergency situations and at stimulating the study of an operations program for mitigating the effects of lava flows in areas of volcanic risk.  相似文献   

    4.
    Origin of rhyolites by anatectic melting of granitic crustal rocks   总被引：1，自引：0，他引：1  
    Rhyolitic pumice deposits on some East Aegean Islands are the remnants of a tuff sheet which covered formerly more than 2500 km2. The rhyolites originated by melting of granitic crustal rocks. The petrological properties leading to this conclusion are as follows:
  • -mineralogical composition of the pumice being 60 wt % of glass and 40 % of relictic granitic minerals (quartz, plagioclase, potash feldspar, biotite) in «phenocrysts» up to 1 cm Ø.
  • -phenomena of strong corrosion, resorption and melting of all light colored «phenocrysts» in the rhyolitic pumice.
  • -mineralogical and chemical identity between pumice and granitic xenolites found in great quantity in the pumice tuff.
  • -structurally transitional types between pumice and the granites in different state of melting.
  • -eutectic composition of the pumice glass for a pressure of 2 kb.
    •   相似文献   

    5.
    In a belt of Quaternary volcanism within the Mojave Desert, California, lavas of alkali-olivine basalt have been extruded from many centers. Also three small volcanoes erupted alkaline lavas similar to basanite, except that the zeolites phillipsite and chabazite crystallized in the groundmass instead of the usual feldspathoid. These zeolite-bearing lavas, more alkaline than the olivine basalts, brought many peridotitic nodules to the surface. Two of the volcanoes built cones 500 to 600 feet high and extruded several short lava flows. The third volcano is smaller and mostly covered by younger alluvium. All three cones are composed of cinders, bombs and blocks, and lava spatter. The lava spatter strengthened the cones, making it possible for one or two flows to pour over the rims. Short flows also issued from fissures cutting the flanks of the cones. The two larger volcanoes each concluded with a flow that began issuing from a fissure that enlarged until one side of the cone was rafted away. Even these final flows are small. The basanitic lavas have an average composition ofSiO 2>=44.5;TiO 2=3.4;Al 2 O 3=14.5;Fe 2 O 3=5.0;FeO=7.3;MnO=0.2;MgO=7.4;CaO=9.0;Na 2 O=4.1;K 2 O=2.3;P 2 O 3=1.1;CO 2=0.1; andH 2 O=1.0. The lavas are very fine-grained and hypocrystalline with microphenocrysts of olivine (Fo 85). The groundmass consists of olivine (Fo 70), plagioclase (An 4:), titanaugite, titanmagnetite, ilmenite, glass (less than 10%), and the zeolites-phillipsite and chabazite. The zeolites occur in small irregular patches only in the nearly holocrystalline centers of the flows. The basanites contain abundant peridotitic nodules, xenoliths of partially fused granite, and xenocrysts (possibly cognate) of titanaugite and kaersutite. The minerals in the inclusions, except for olivine and clinopyroxene, have all reacted with the alkaline melt. In several flows the nodule enstatite has been partially replaced by titaniferous clinopyroxene; this reaction drew so much lime and silica from the surrounding melt that zeolite formed a halo around the reaction rim. Three chemical analyses show about 15% normative nepheline, even though there is no modal nepheline. The feldspathoid is represented instead by the undersaturated zeolites phillipsite and chabazite. A high water content in the magma probably accounts for the occurrence of zeolite. The peridotite nodules do not occur in the nearby, less alkaline lavas, and they appear to be cognate. The separation of olivine and enstatite under high pressures can produce the basanite liquid from the same magma that yields the alkali-olivine basalt liquids by olivine crystallization under low pressures.  相似文献   

    6.
    One of the problems associated with massif-type anorthosite in high grade metamorphic terranes is the absence of anorthositic extrusives. However, if anorthosite formed by crystal segregation, volcanic equivalents of the final fluid differentiate might occur. Massive, stratiform hornblende-garnet granitic gneisses occur between two metasedimentary sequences in asynclinorium south of the main Adirondack anorthosite massif and north of the Thirteenth Lake anorthosite dome. The granitic gneisses are divided into three stratigraphic units on the basis of mineralogy. The entire stratigraphic configuration has been traced 70 kilometers laterally, and the interpretation of the literature indicates that it may extend for over 100 kilometers. Earlier workers considered the granitic gneisses to be sill-like intrusives. The following evidence suggests a volcanic origin for these gneisses:
    1. A thin (3 to 5 meters) but continuous zone of cale-silicates and quartzite occurs within the sequence of granitic gneisses at the same horizon for some 25 kilometers laterally and does not appear to be truncated nor intruded by the gneisses. This observation virtually precludes an intrusive origin.
    2. 2V, of plagioclase (An24,25) in granulated, augen-shaped zones in the granitic gneisses variesfrom 109° to 120°, whereas 2V, for groundmass plagioclase (also An20, 25) is 84° to 88°. According to the data of Slemmons, the 2V of the granulated augen is characteristic of volcanic (high temperature) plagioclase and that of the groundmass is characteristic of plutonic (low temperature) plagioclase.
    3. The great lateral extent of the granitic gneisses suggests that the materials were deposited as volcanic ash or pyroclastics.
    4. The stratigraphic coherency of metasedimentary sequences and granitic gneisses indicates that the gneisses are an integral part of the stratigraphy.
    A jotunite-mangerite-charnockite suite of intrusives in this area is related to the main anorthosite massif. Since plagioclase augen or their remnants exist in both the granitic gneisses and in the intrusive suite, it is possible that the granitic gneisses and the intrusives were derived from the same magma. It has been suggested that differentiation of the anorthosite suite occurs at considerable depth whereas differentiation of the same magma at shallow depth produces the rapakivi-laboradorite porphyry association of the Fennoscandian region. If the stratiform granitic gneisses and the anorthosite suite in the Adirondacks are comagmatic, differentiation of the magma must have occurred at moderate depth and the parental magma may have been granodioritic and thus more silicic than presumed by most workers.  相似文献   

    7.
    For most previous geomagnetic surveys, the time variations of the observatory closest to the site of observation have been used for reduction. The accuracy of repeat station measurements depends upon the distance between the station and the observatory. Nowadays, the requirements for the accuracy of repeat station measurements are much higher, especially with respect to local secular variation anomalies. On the other hand, transportable variograph stations are available, by means of which time variations can be recorded closer to the station site. The reduction steps for this method are:
    1. Reduction of measured values to the base-line of the nearby variograph station;
    2. Computation of the nearest quiet night value using the baseline of the variograph;
    3. Computation of the difference between this value and the corresponding one at the observatory.
    The problems are:
  • - the stability of the base-line values of the variographs;
  • - the question as to what degree a quiet night value represents the normal value at that time (e.g. influence of aD st field and/or other fields).
    • A repeat station survey of the Federal Republic of Germany was carried out in 1982. During the same time, eleven stations recorded the time variations in that area. We discuss the aforementioned aspects by way of an example taken from this survey.  相似文献   

    8.
    Study of these submarine pyroclastic flows is important for two reasons:
    1. From a regional point of view, submarine pyroclastic flows partially explain the origin of widespread argillized glasses in the Eocene sedimentary series of Central Iran, which have diversely been described by earlier workers as Green Tuffs, Tuffaceous Muds, Green Series.
    2. More generally, in contrast to basaltic volcanism, submarine acid volcanism is still very poorly-known. The mechanism here proposed is as follows: starting with a submarine, highly vesiculated lava containing idiomorphic phenocrysts, gas expansion gives rise to a vitroclastic shardy facies, whereas phenocrysts are fragmented. The whole makes up a gravitational flow that is rich in pumice, glassy shards, broken crystals, and gases; it becomes increasingly turbulent and able to tear-up sea-bottom sediments in its down-stream course. This flow evolves into turbidity currents at its distal reaches.
      相似文献   

    9.
    The Nevado de Toluca, in the middle of the Mexican volcanic belt, has been built by two very dissimilar phases. The first one that lasted more than one million years is mainly andesitic. Numerous massive and autobrecciated lava flows of this phase pass outwards into thick conglomeratic formations. The volume of this primitive volcano represents the essential part of the Nevado. After an intense periode of erosion, the second phase is of very short duration (about 100.000 years) and is dacitic in nature. Three main episode can be distinguished:
    1. Eruption of important ash and pumice pyroclastic flows related to caldera collapse above a shallow magmatic reservoir.
    2. Extrusions of several dacitic domes within and outside the caldera with numerous associated «nuées ardentes» surrounding the volcano.
    3. Plinian eruption leading to widespread pumiceous air-fall and to the opening of the present crater inside the caldera. Extrusion of a new small dacitic dome and late phreatic explosions.
    This second sequence of events can be interpreted as the progressive emptying of the crustal magmatic chamber without refilling by a new magma supply. The most recent activity in the area is represented by monogenic cones and flows of basic andesites outside the central vent system of the Nevado.  相似文献   

    10.
    A modified formula of the cumulative frequency-magnitude relation has been formulated and tested in a previous paper by the authors of this study. Based on the modified relationship, the following reoccurrence formulas have been obtained.
    1. For the ‘T-years period’ larger earthquake magnitude,M T $$M_T = \frac{1}{{A_3 }}ln\frac{{A_2 }}{{(1/T) + A_1 }}.$$
    2. For the value of the maximum earthquake magnitude, which is exceeded with probabilityP inT-years period,M PT $$M_{PT} = \frac{{ln(A_2 .T)}}{{A_3 }} - \frac{{ln[A_1 .T - ln(1 - P)]}}{{A_3 }}.$$
    3. For the probability of occurrence of an earthquake of magnitudeM in aT-years period,P MT $$P_{MT} = 1 - \exp [ - T[ - A_1 + A_2 \exp ( - A_3 M)]].$$
    The above formulas provide estimates of the probability of reoccurrence of the largest earthquake events which are significantly more realistic than those based on the Gutenberg-Richter relationships; at least for numerous tested earthquake samples from the major area of Greece.  相似文献   

    11.
    This paper gives a complete commentary on some popular gradings which based on the 2×2 contingency table, for one—dimensional seismologic prediction. It contains:
    1. In view of the statistical correlation, the Wallen gradingV=√QS is the most equitable one amomg all gradings.
    2. Each one of the three gradings: Zhenchao Gu gradingS, Obuhov gradingQ and Shaoxie Xu gradingR, has its own inadequacy although they are so easy for use.
    3. A set of gradings can be derived fromV. The harmonic mean ofS andQ, H(S,Q) and the Hedike gradingS H are the members of this set. The order of some popular gradings are given in Theorem 5. 3.
    4. In view of the measure of the overlapping part a gradingW is introduced. Another set of gradings may be derived from it. The index of efficiencyh is the severest one among this set.
      相似文献   

    12.
    Nisyros is a totally volcanic island located at the eastern limit of the quaternary calc-alkaline island arc system of the South Aegean Sea. Its age is rather young since K/Ar dating has given an age of 0.2 m.y. B.P. for one of the oldest outcropping products of this volcano. The volcanological evolution of Nisyros has tentatively been reconstructed as follows:
    1. after a period (from 3.0? to 0.2 m.y. B.P.) of submarine activity, evidenced by the presence of pillow-lavas and hyaloclastites, the volcano grew above sea level;
    2. effusive and explosive subaerial activity from different vents built up a complex stratovolcano, probably around 0.2 m.y. B. P.;
    3. at the end of an intense explosive activity (between 0.2 m.y. B.P. and Present) the top of the volcano collapsed forming a caldera which is still perfectly preserved. A post-caldera activity with eruptions of huge and viscous domes and lava flows of uniform composition, both inside and outside the caldera, concluded this stage of the volcano evolution;
    4. in historical times, spectacular phreatic explosion craters formed on the caldera floor;
    5. presently, a large area of the caldera floor is affected by a considerable hydrothermal activity. The hypothesis is formed that Nisyros volcano is not yet extinct.
    Four small volcanic islets — Yali, Stronjili, Pakia and Perigusa — located a few miles on the North and West of Nisyros, although volcanologically independent of one another, are composed of products which are attributable, from the petrologic point of view, to the Nisyros magma. The volcanic rocks of Nisyros and of its neighbouring minor islands as well as the volcanics of the coeval volcanoes of the South Aegean Sea arc (Aegina, Milos, Santorini, etc.) belong to a typical orogenic calc-alkaline series (from basic andesites through andesites-dacites-rhyodacites to strongly silicic rhyolites) with normal K2O contents. The potassium contents of these rocks are compatible with the depth of 150 km (as inferred from geophysical data) for the inclined seismic zone underneath the active volcanic arc. The existence of a top-caldera as well as the occurrence of a huge amount of xenoliths (hornblende-rich cumulates and contact-metamorphic calcareous rock derivatives) suggest the presence of a magma chamber at a relatively shallow depth beneath the volcano. The rhyolitic obsidians of Yali can be considered as residual liquids from the Nisyros rhyodacites, thus representing the end-members of a fractionation process. Volcanological and petrological arguments are in favour of fractional crystallization as the most probable genetic process for the calc-alkaline differentiation series of Nisyros and of its neighbouring minor islands. However, the lack of any rock with a high alumina basalt composition makes it difficult to define exactly the nature of the parent magma. According to recent geophysical data, continental collision is already in progress at the Hellenic trench. Therefore, Nisyros and the other active volcanoes of the South Aegean Sea arc are approaching the senile stage. What would follow could be a transition to shoshonitic magmatism as a consequence of the deepening of the lithospheric slab under the Aegean microplate. The limited extension and the relatively short-lived calc-alkaline activity of the South Aegean Sea arc could be related to the particular geodynamic pattern of the Mediterranean area which is characterized by a microplates mosaic between the two converging African and Eurasian major plates.  相似文献   

    13.
    Some recent calc-alkaline andesites and dacites from southern and central Martinique contain basic xenoliths belonging to two main petrographic types:
  • The most frequent one has a hyalodoleritic texture (« H type ») with hornblende + plagioclase + Fe-Ti oxides, set in an abundant glassy and vacuolar groundmass.
  • The other one exhibits a typical porphyritic basaltic texture (« B type ») and mineralogy (olivine + plagioclase + orthopyroxene + clinopyroxene + Fe-Ti oxides and scarce, or absent hornblende).
    • Gradual textural and mineralogical transitions occur between these two types (« I type ») with the progressive development of hornblende at the expense of olivine and pyroxenes. Mineralogical and chemical studies show no primary compositional correlations between the basaltic xenoliths and their host lavas, thus demonstrating that the former are not cognate inclusions; they are remnants of basaltic liquids intruded into andesitic to dacitic magma chambers. This interpretation is strengthened by the typical calc-alkaline basaltic composition of the xenoliths, whatever their petrographic type (« H », « I » or « B »). The intrusion of partly liquid, hot basaltic magma into colder water-saturated andesitic to dacitic bodies leads to drastic changes in physical conditions. The two components; the basaltic xenoliths are quenched and homogeneized with their host lavas with respect to To;fO2 andpH2O conditions. « H type » xenoliths represent original mostly liquid basalts in which such physical changes lead to the formation of hornblende and the development of a vacuolar and hyalodoleritic texture. The temperature increase of the acid magma depends on the amount of the intruding basalt and on the thermal contrast between the two components. The textural diversity which characterizes the xenoliths reflects the cooling rate of the basaltic fragments and/or their position relative to the basaltic bodies (chilled margins or inner, more crystallized, portions). In addition to physical equilibration (T, fO2) between the magmas, mixing involves:
  • mechanical transfer of phenocrysts from one component to another, in both directions;
  • volatile transfer to the basaltic xenoliths, with chemical exchanges.
    • It is here demonstrated that a short period of time (some ten hours to a few days) separates the mixing event from the eruption, outlining the importance of magma mixing in the triggering of eruption. The common occurrence of basaltic xenoliths (generally of « H » type) in calc-alkaline lavas is emphasized, showing that this mechanism is of first importance in calc-alkaline magma petrogenesis.  相似文献   

    14.
    Using an acoustic system in a 9 m deep enclosure, the daily vertical distribution patterns of whitefish larvae were studied from hatching to the age of two months. Four patterns of daily vertical distribution and migration were observed in accordance to different developmental stages:
    1. Newly hatched larvae dispersed in the enclosure during the day and congregated at the water surface at night.
    2. After 2 weeks of age larvae stayed close to the water surface during day and night.
    3. At 15 to 18 mm length, larvae showed distinct vertical migration at dawn and dusk, they stayed close to the surface during the day and dispersed at night.
    4. At 28 mm length the depth pattern was reversed. These fish frequented greater depths during the day than at night.
    No relation was found between larval depth distribution or vertical migration and food availability or water temperature. Low levels of solar radiation triggered the dawn and dusk vertical migration behaviour of the more developed larvae. Possible reasons for the different behaviour patterns occurring over time are discussed.  相似文献   

    15.
    This paper summarizes seismic and rockburst research activities related to South African deep-level gold mines over the period 1983 to 1987. It covers continued research in directions that were considered in the Seismicity in Mines Symposium in 1982 as well as in several new areas of research. Five broad areas are identified:
    1. Seismic data acquisition and processing. Improved seismic systems are being developed. Velocity models related to known stratigraphy are being used to provide more accurate estimates of seismic locations.
    2. Source mechanisms and near-source effects on seismic wave transmission. This work provides fundamental insights into seismicity and rock behaviour and is being applied in rockburst prediction research.
    3. Mine layouts. Excess shear stress is being investigated as a design parameter by analyzing mining configurations and resultant seismicity. In addition, better understanding of the behaviour of highly stressed remnants and pillars is also being obtained from seismic studies.
    4. Strong ground motion studies. Evaluation of the performance of support elements, including recently developed backfill materials, requires better knowledge of ground motion around underground excavations during seismic events and rockbursts.
    5. Rockburst prediction and control. Rockburst prediction research continues with some reported success. In addition, the feasibility of actively triggering fault slip or conditioning the rock ahead of the stope face to ameliorate the rockburst hazard is currently being investigated.
      相似文献   

    16.
    Gabbro xenoliths in a tholeiitic lava of Kahoolawe Island, Hawaii, a 1.3–1.4 Ma shield volcano, are 1–3 cm in size and comprised of plagioclase, clinopyroxene, and orthopyroxene. Gabbro textures — while intergranular and in part subophitic-are open due to 28–48 vol.% of vesicular basalt occupying xenolith space. Vesicles in and around the xenoliths are lined or filled with rhyolitic glass (segregation vesicles). The host is evolved tholeiite (MgO 6.1 wt%) with phenocrysts, microphenocrysts, and glomerocrysts of olivine, clinopyroxene, orthopyroxene, and plagioclase, and megacrysts (1 cm) of plagioclase. The Sr-isotope ratio of one xenolith is 0.70489; the host basalt ratio is 0.70460. Xenolith isotope composition, grain resorption, and clinopyroxene (Fs12.5–15Wo38–35.5), orthopyroxene (Fs19.5–24Wo4.1), and plagioclase (An68–65Or0.8–1.2) compositions suggest that these gabbros crystallized from Kahoolawe tholeiitic magma of essentially the same composition as the host basalt, but pre-dating the magma represented by the host. Based on the absence of intergranular Fe–Ti oxide phases from the pl+cpx+opx assemblages, and the open, vuggy textures, we envision crystallization on a reservoir roof at temperatures >1100°C. Entrainment of gabbro assemblages and plagioclase megacrysts from a roof mush/suspension zone occurred during convection associated with replenishment of the magma reservoir. These open-textured gabbro xenoliths are therefore not fragments of preexisting coarse-grained bodies such as sills or segregation veins. Rhyolitic glass in vesicles represents a gas-effervescence filtration process that forced fractionated residual liquids from the groundmass into voids associated with the xenoliths.Sirrine Environmental Consultants, Fremont, CA 94538  相似文献   

    17.
    A particular nuée ardente type (Merapi-type avalanche nuée) has been defined at the Merapi volcano because of its prominent role in the recent activity of the volcano: gravity plays a significant role during the eruption. However, some other eruption styles occur too producing surges and ashfalls. Three types of tephra, deposited in a very short time-span (15 years) are compared: chemistry and mineralogy are similar, but grain-size analyses are different. There is no vesicular glass, and it is concluded that there is an absence of new magma. This example shows clearly the variety of volcanic styles, with similar chemistry in a very short period. Avalanche nuées from collapsed domes or flows are separated into two types:
    1. Merapi-typesensu stricto, without any fresh glass, derived from a wholly solidified dome.
    2. Arenal-type, containing pumiceous glass, derived from a dome, the interior of which is still liquid.
      相似文献   

    18.
    Data obtained from Lakes Baldegg and Sempach, two artificially mixed and oxygenated lakes, show that it is technically possible to significantly improve redox conditions, even in large eutrophic lakes. It is discussed why
  • - the well-known release of phosphorus from anoxic sediments does not necessarily result from an abiotic reduction of phosphorus containing iron compounds,
  • - net phosphorus retention by sediments is not simply linearly related to the phosphorus content of a lake,
  • - artificial oxygenation of a previously anoxic hypolimnion does not permanently increase phosphorus retention capacity of lake sediments.
    • From this it is concluded that improvement of hypolimnic redox conditions by lake internal measures such as aeration or oxygenation may accelerate the rate of recovery induced by a reduction of the external phosphorus loading, but that oxygenation per se will hardly be able to cause a reduction of trophic state.  相似文献   

    19.
    It is shown that when the travel-time curve of a refracted wave from a surface source is known and at least one of the following conditions is satisfied, i.e. when
    1. the travel-time curve of a wave reflected from a horizontal interface lying below the deepest low velocity layer is known, or
    2. the travel-time curve of a wave from a deep source situated below the deepest low velocity layer is known, or
    3. the measureH(u)=mes {z∶z≥0,v ?1 (z)≥u} is analytical in some segment [c, d], where \(0< c< d< \infty , c< a_n , H(a_n ) = \bar z_n ,\bar z_n\) is the depth of the lower end of the deepest low velocity layer and in the interval [c, ∞) an analytical functionH(u)) exists which providesH(u)≡H(u)) ifu∈[c, d], then (1) velocityv(z) outside the low velocity layers and (2) the measureH k (u)=mes {z∶z∈L k,v ?1 (z)≥u} for each low velocity layerL k,k=1, 2, ..., n, are defined unambiguously.
      相似文献   

    20.
    The Miocene Tejeda caldera on Gran Canaria erupted ~ 20 rhyolite–trachyte ignimbrites (Mogán Group 14–13.3 Ma), followed by ~ 20 phonolitic lava flows and ignimbrites (Fataga Group 13–8.5 Ma). Upper-Mogán tuffs have been severely altered immediately within the caldera margin, whereas extra-caldera Mogán ignimbrites, and overlying Fataga units, are apparently unaltered. The altered intra-caldera samples contain minerals characteristic of secondary fluid–rock interaction (clays, zeolites, adularia), and relics of the primary mineral assemblage identified in unaltered ignimbrites (K-feldspar, plagioclase, pyroxene, amphibole, and groundmass quartz). Major and trace-element data indicate that Si, Na, K, Pb, Sr, and Rb, were strongly mobilized during fluid–rock interaction, whereas Ti, Zr, and Nb behaved in a more refractory manner, experiencing only minor mobilization. The δ18O values of the altered intra-caldera tuffs are significantly higher than in unaltered extra-caldera ignimbrites, consistent with an overall low-temperature alteration environment. Unaltered extra-caldera ignimbrites have δD values between − 110‰ and − 173‰, which may reflect Rayleigh-type magma degassing and/or post-depositional vapour release. The δD values of the altered intra-caldera tuffs range from − 52‰ to − 131‰, with ambient meteoric water at the alteration site estimated at ca. − 15‰. Interaction and equilibration of the intra-caldera tuffs with ambient meteoric water at low temperature can only account for whole-rock δD values of around − 45‰, given that ?Dclay–water is ca. − 30‰ at 100 °C, and decreases in magnitude at higher temperatures. All altered tuff samples have δD values that are substantially lower than − 45‰, indicating interaction with a meteoric water source with a δD value more negative than − 15‰, which may have been produced in low-temperature steam fumaroles. Supported by numerical modeling, our Gran Canaria data reflect the near-surface, epithermal part of a larger, fault-controlled hydrothermal system associated with the emplacement of the high-level Fataga magma chamber system. In this near-surface environment, fluid temperatures probably did not exceed 200–250 °C.  相似文献   

    设为首页 | 免责声明 | 关于勤云 | 加入收藏

    Copyright©北京勤云科技发展有限公司  京ICP备09084417号