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1.
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
2.
Three main types of recent volcanism may be distinguished in the Afar Depression: 1) oceanic volcanism of the axial ranges; 2) volcanism along the margins where an attenuated sialic crust probably occurs; 3) mainly fissural volcanism of Central-Southern Afar, with associated central volcanoes, similar as a whole to the volcanism of the Ethiopian Rift Valley. Peralkaline silicic volcanic rocks are found in all the three groups but showing some different characteristics which seem related to their geological location and which probably reflect different sources. Moreover emplacement of peralkaline granitic bodies, associated with volcanics of the same composition, marks the first stage of formation of the Afar Depression, in the Early Miocene. Axial Ranges: Erta’Ale and Boina volcanic ranges indicate that peralkaline rocks are the final liquids produced by fractionation of basalt in shallow magma chambers of central volcanoes. The parental magma is a transitional type of basalt with a mildly alkalic affinity, which fractionated under lowpH2O-pO2 conditions. Transition to peralkaline liquids is realized without passing a «true» trachytic (low silica) stage. The first peralkaline liquid is a low silica comendite and evidence exists that «plagioclase effect» was active in determining the first peralkalinity. Within the peralkaline field a fractionation mainly controlled by alkali feldspar progressively increases the peralkalinity and silica oversaturation of residual liquids (transition from comendites to pantellerites). The most peralkaline pantellerites of Boina are produced by fractionation of an alkali feldspar of constant composition (Ab65–68 Or35–32) suggesting that these liquids lie on a «low temperature zone» of the peralkaline oversaturated system. Marginal Units: On the borders of the depression peralkaline silicics are found in volcanic massifs mainly made of metaluminous silicic products. Petrology and geochemistry suggest a complex origin. Crystal fractionation, contamination with sialic crust and chemical changes related to a volatile rich phase, all these processes probably played a role in the genesis of these peralkaline silicic rocks. Central-Southern Afar Fissural Volcanism: Mildly alkaline basalts are associated with peralkaline and metaluminous silicics; intermediate rocks are very scanty. Fractionation from deep seated magmatic bodies with selective eruptivity and partial melting at depth of associated basalts or of a common source material are possible genetic mechanisms. 相似文献
3.
《Journal of Volcanology and Geothermal Research》1988,35(3):179-194
What controls the intrusion and extrusion frequencies associated with volcanoes is still poorly understood. I propose that for volcanoes at divergent plate boundaries, these frequencies may be largely determined by the tensile stress concentration around the magma chambers that feed them. This stress concentration is mainly a function of the applied tensile stress, associated with spreading, and the aspect (height/width) ratios of the chambers. High spreading rates and/or aspect ratios lead to high rates of tensile stress concentration around the chambers and to an increase in their intrusion frequencies. It is found that for chambers at the same depth in a volcanic zone, the one of the highest aspect ratio will normally intrude magma most frequently. Also, if the chambers are of equal aspect ratios, the one at the greatest depth will intrude magma most frequently. Because the extrusion frequency of a volcano is a fraction of its intrusion frequency, the extrusion frequency may also be largely determined by the rate of tensile stress concentration around the magma chamber that feeds the volcano. These results are applied to the divergent plate boundary in Iceland, where many of the volcanoes appear to be fed by “double chambers”, that is, shallow chambers fed by deep-seated chambers. It is found that, except when the aspect ratio of the deep-seated chamber is much less than that of the shallow chamber, the intrusion frequency of the shallow chamber is normally largely controlled by that of the deep-seated chamber. It is concluded that whereas the short-term (i.e., ≤103 yrs) extrusion frequencies of volcanoes at the plate boundary in Iceland may be similar to the dike intrusion frequencies of the source chambers, the long-term (i.e., ≥104 yrs) extrusion frequencies may be about ten times lower than the intrusion frequencies. 相似文献
4.
The influence of volcanic processes on magmatic differentiation can be evidenced by the study of some of the most typical volcanoes of post-orogenic magmatism of Central Italy. It has been recognized that a close relationship exists between degree and type of differentiation on one hand, and structure and evolution of volcanic edifices as well as shape of their magmatic chambers on the other. The effect of the structural features of volcanic apparata on the magmatic differentiation is often so strong as to obliterate the original genetic characters of the magma. It was seen that, in Central Italy, magmas of «atlantic» affinities differentiating from basalt to trachyte, can turn to magmas of strong « mediterranean » affinities in the more superficial volcanic environments. 相似文献
5.
This paper presents new data on the upper mantle characteristics, and on seismicity and volcanism in Kamchatka. It is shown that the seismic activity in the Pacific focal layer decreases sharply below that narrow line on which the foci of the active volcanoes are situated. A map of longitudinal wave velocitiesV p in the mantle upper layers under Kamchatka is given. The lowest values ofV p (7.3–7.6 km/sec) are found near the volcanic belt. The graphs Θ=lg (Es/Ep) (h) for the Kamchatka earthquakes indicate that Θmin at the depths of 120–250 km may be caused by a concentration of magmatic melts. A map of bodies (magma chambers?) screening S- and P-waves at the depths of 30–100 km under Kamchatka has been compiled. These bodies are mainly located under the belt of active volcanoes. 相似文献
6.
D. Schimozuru 《Bulletin of Volcanology》1981,44(3):499-504
Inflation patterns based on water-tube tiltmeter and levelling observation show different features for Krafla Volcano in Iceland and Kilauea Volcano in Hawaii. Monotonous sawtooth shape inflation is observed at Krafla, while inflation curves at Kileauea are more or less complicated. The difference was attributed to differences in the system of magma reservoir for the two volcanoes. By using the electrical equivalent of a magma reservoir and volcanic conduit as a capacitor and a resistor, an electrical oseillator was considered to be a possible model for a magma reservoir system. In the case of Krafla, the magma reservoir system is replaced with one electric oscillator called «Single system» or «Icelandic type» system. The complicated inflation pattern of Kilauea was interpreted as the assembly of a main magma reservoir and the group of surrounding small reservoirs. The equivalent electric analogue is the composite parallel and serial connection of a single oscillator which generates irregular output voltage during a charging process. The proposed magma reservoir system of Kilauea is called «Multi-coupled system» or «Hawaiian type system» which also help in interpreting the wondering of the uplift center and tidal phenomena of the Halemaumau lava lake. 相似文献
7.
The eruption centres of late volcanism in Chile are situated in two separate areas in the northern and southern High Cordillera. In the north, the ignimbrites of the Rhyolite Formation and the rocks of the « Andesite » Formation occur in about equal proportions, and recent activity is meagre. In the south, the rocks of the « Andesite » Formation predominate, and many volcanoes are in a highly explosive phase of activity. Field relationships, petrological and geochemical data show that the rocks of both Formations are closely related to each other. There is evidence that the magmas of the Rhyolite Formation were formed by fusion of sialic material in the upper parts of the crust. The data for the volcanics of the « Andesite » Formation are inconsistent with their derivation by fractional crystallization of a basaltic parent or by direct mantle derivation involving a single stage process. The authors suggest that the « andesitic » magmas are products of a primary andesitic magma originated by partial fusion of material of the lower crust. Assuming that the « andesitic » magmas of the central parts of the Andes are derived from the upper mantle, this would mean — in the light of the Sr87/Sr86 data — that the upper mantle in the central region of the Andes is essentially more radiogenic than in other orogenic areas; moroever, it should be very similar in its chemical and Sr87/Sr86 composition to that of the lower crust. 相似文献
8.
This paper discusses the methods and techniques of observation that can at present be applied to a seismic refraction study of active volcanoes with a view of determining the magma chamber location. A system of magma chambers has been outlined in the earth’s crust and transition layer between the crust and the mantle under the Tolbachik Volcano group. Magma chambers are dynamically related to each other. The feeding magma chambers of the newly-formed Tolbachik volcanoes and Plosky Tolbachik volcano are assumed to be interrelated through the transition zone between the crust and the mantle. 相似文献
9.
Excess degassing of magmatic H2O and SO2 was observed at Izu-Oshima volcano during its latest degassing activity from January 1988 to March 1990. The minimum production rate for degassed magma was calculated to be about 1×104 kg/s using emission rates of magmatic H2O and SO2, and H2O and S contents of the magma. The minimum total volume of magma degassed during the 27-month period is estimated to be 2.6×108 m3. This volume is 20 times larger than that of the magma ejected during the 1986 summit eruption. Convective transport of magma through a conduit is proposed as the mechanism that causes degassing from a magma reservoir at several kilometers depth. The magma transport rate is quantitatively evaluated based on two fluid-dynamic models: Poiseuille flow in a concentric double-walled pipe, and ascent of non-degassed magma spheres through a conduit filled with degassed magma. This process is further tested for an andesitic volcano and is concluded to be a common process for volcanoes that discharge excess volatiles. 相似文献
10.
V. S. Trofimov 《Bulletin of Volcanology》1970,34(3):767-776
Diamantiferous diatremes usually occur in the old platforms and shields where deep fractures are «blind»,i.e., these fractures do not come out to the earth surface. Alkaline-ultrabasic magma ascending along these fractures and encountering an impervious cap of sedimentary and/or volcanic rocks had formed, between the cap and the basemnet rocks, intermediate chambers in which the crystallization of diamonds took place. Under the influence of the increasing pressures in these chambers, the roofs were destroyed and diamantiferous diatremes, dykes and veins of kimberlite have been formed. These diatremes are filled with a typical eruptive breccia in which the fragmental material, formed by the destructive explosion of the magma chamber roof, is cemented by a porphyritic, alkaline-ultrabasic rock known under the name of kimberlite. 相似文献
11.
F. Barberi H. Bizouard R. Clocchiatti N. Metrich R. Santacroce A. Sbrana 《Bulletin of Volcanology》1981,44(3):295-315
The volcanic history of Somma-Vesuvius indicates that salic products compatible with an origin by fractionation within a shallow magma chamber have been repeatedly erupted («Plinian» pumice deposits). The last two of these eruptions, (79 A.D. and 3500 B.P.) were carefully studied. Interaction with calcareous country rocks had limited importance, and all data indicate that differentiated magmas were produced by crystal-liquid fractionation within the undersaturated part of petrogeny’s residua system at about 1 kb water pressure. The solid-liquid trend indicates that the derivative magmas originated by fractionation of slightly but significantly different parental liquids. Some lavas of appropriate composition were selected as parental liquids to compute the entity of the fractionation. Results suggest that in both bases a fractionation of about 70 weight % was needed to produce liquids with the composition of the pumice. The combination of all data indicates that the two Plinian eruptions were fed by a magma chamber (3–4 km deep) having a volume of approx. 2.0–2.5 km3. The temperature of the magma that initially entered the chamber was about 1100°C, whereas the temperature of the residual liquids erupted was Plinian pumice was 800° and 850°C respectively. There is no evidence that such a magma chamber existed at Vesuvius after the 79 A.D. eruption. These results have relevant practical implications for volcanic hazard and monitoring and for geothermal energy. 相似文献
12.
Characteristics of widespread pyroclastic deposits formed by the interaction of silicic magma and water 总被引：1，自引：0，他引：1
We have recognized a type of pyroclastic deposit formed by the interaction of water and silicic magma during explosive eruptions. These deposits have a widespread dispersal, similar to plinian tephra, but the overall grain size is much tiner. Several deposits studied can be associated with caldera lakes or sea water and water/magma interaction is proposed to account for the fine grain size. Several examples have been studied, including the Oruanui Formation, N.Z., and the Askja 1875 deposit. Both show little downwind decrease in median diameter, a downwind decrease in sorting (σφ) (more evident in the Askja deposit) and coarse tail grading. The Askja example has base surge deposits near source and some Oruanui members show multiple thin beds near source; both are common features of phreatomagmatic deposits. Isopachs of the Askja deposit indicate a source under Lake Oskjuvatn in Askja Caldera and those of the Oruanui indicate a source under the NW part of Lake Taupo. In terms of dispersal area, volume and calculated eruption column heights, these deposits are similar to plinian. However, their extreme fragmentation due to magma/water interaction, superimposed on fragmentation imparted by carlier vesiculation, gives a much finer and more complex grain size distribution than plinian counterparts. The field of phreatomagmatic equivalents to plinian pumice deposits was unoccupied onWalker’s (1973) classification of explosive volcanic eruptions. Such deposits are the phreatomagmatic analogue of plinian deposits and the name « phreatoplinian » is proposed. 相似文献
13.
In volcanic areas occurring in zones of extension, basaltic magma rises up to the surface, through fissures, to form dykes which in depth are connected with one or several magmatic chambers located in the crust or the upper mantle. Starting form this geological situation, we propose models of increasing complexity to study cooling by heat conduction of a system composed of parallel dykes and underlying magmatic chamber. This work has been carried out 1) by numerical methods which take into account the variation with temperature of the thermic parameters and 2) by using an analytical solution of the Fourier equation (an initial fictive temperature is then calculated). The thermic individuality of the dykes disappears quickly and the dyke system may be replaced by a single « intrusion » which cools slowly from a temperature = =2/β θ0 much lower than θ0 (θ0=initial temperature; 1/β=injection density). The temperature gradient due to the dyke system has been estimated for different time intervals after the intrusion. For the calculation of the thermal effect of a magmatic chamber, we have taken into account its size, depth and age. Numerical application for appropriate geological cases have been carried out and allow one to estimate the respective effects of dykes and magma chamber. 相似文献
14.
Crystal fractionation of granitic magma during its non-transport processes: A physics-based perspective 总被引：2，自引：0，他引：2
Chen Chen Xing Ding Rui Li WeiQi Zhang DongJian Ouyang Lei Yang WeiDong Sun 《中国科学:地球科学(英文版)》2018,61(2):190-204
Granitic continental crust distinguishes the Earth from other planets in the Solar System. Consequently, for understanding terrestrial continent development, it is of great significance to investigate the formation and evolution of granite.Crystal fractionation is one of principal magma evolution mechanisms. Nevertheless, it is controversial whether crystal fractionation can effectively proceed in felsic magma systems because of the high viscosity and non-Newtonian behavior associated with granitic magmas. In this paper, we focus on the physical processes and evaluate the role of crystal fractionation in the evolution of granitic magmas during non-transport processes, i.e., in magma chambers and after emplacement. Based on physical calculations and analyses, we suggest that general mineral particles can settle only at tiny speed(~10~(-9)–10~(-7) m s~(-1))in a granitic magma body due to high viscosity of the magma; however, the cumulating can be interrupted with convection in magma chambers, and the components of magma chambers will tend to be homogeneous. Magma convection ceases once the magma chamber develops into a mush(crystallinity, F~40–50%). The interstitial melts can be extracted by hindered settling and compaction, accumulating gradually and forming a highly silicic melt layer. The high silica melts can further evolve into high-silica granite or high-silica rhyolite. At various crystallinities, multiple rejuvenation of the mush and the following magma intrusion may generate a granite complex with various components. While one special type of granites, represented by the South China lithium-and fluoride-rich granite, has lower viscosity and solidus relative to general granitic magmas, and may form vertical zonation in mineral-assemblage and composition through crystal fractionation. Similar fabrics in general intrusions that show various components on small lengthscales are not the result of gravitational settling. Rather, the flowage differentiation may play a key role. In general, granitic magma can undergo effective crystal fractionation; high-silica granite and volcanics with highly fractionated characteristics may be the products of crystal fractionation of felsic magmas, and many granitoids may be cumulates. 相似文献
15.
S. A. Khubunaya L. I. Gontovaya A. V. Sobolev I. V. Nizkous 《Journal of Volcanology and Seismology》2007,1(2):98-118
A 3D velocity model of the Earth’s crust beneath the Klyuchevskoy volcanic group has been constructed using the seismic tomography method. Anomalies of the velocity parameters related to the zones of magma supply to active volcanoes have been distinguished. Petrological data on the composition, temperature, and pressure of generation and crystallization of parental melts of Klyuchevskoy volcano magnesian basalts have been obtained. The parental melt corresponds to picrite (MgO = 13–14 wt %) with an ultimate saturation of SiO2 (49–50 wt %), a high H2O content (2.2–2.9%), and incompatible elements (Sr, Rb, Ba). This melt is formed at pressures of 15–20 kbar and temperatures of 1280–1320°C. Its further crystallization proceeds in intermediate magma chambers at two discrete pressure levels (i.e., greater than 6, and 1–2 kbar). The results of the petrological studies are in good agreement with the seismotomographic model. 相似文献
16.
A model for the origin of large silicic magma chambers: precursors of caldera-forming eruptions 总被引：10，自引：2，他引：8
The relatively low rates of magma production in island arcs and continental extensional settings require that the volume of silicic magma involved in large catastrophic caldera-forming (CCF) eruptions must accumulate over periods of 10 5 to 10 6 years. We address the question of why buoyant and otherwise eruptible high-silica magma should accumulate for long times in shallow chambers rather than erupt more continuously as magma is supplied from greater depths. Our hypothesis is that the viscoelastic behavior of magma chamber wall rocks may prevent an accumulation of overpressure sufficient to generate rhyolite dikes that can propagate to the surface and cause an eruption. The critical overpressure required for eruption is based on the model of Rubin (1995a). An approximate analytical model is used to evaluate the controls on magma overpressure for a continuously or episodically replenished spherical magma chamber contained in wall rocks with a Maxwell viscoelastic rheology. The governing parameters are the long-term magma supply, the magma chamber volume, and the effective viscosity of the wall rocks. The long-term magma supply, a parameter that is not typically incorporated into dike formation models, can be constrained from observations and melt generation models. For effective wall-rock viscosities in the range 10 18 to 10 20 Pa s –1, dynamical regimes are identified that lead to the suppression of dikes capable of propagating to the surface. Frequent small eruptions that relieve magma chamber overpressure are favored when the chamber volume is small relative to the magma supply and when the wall rocks are cool. Magma storage, leading to conditions suitable for a CCF eruption, is favored for larger magma chambers (>10 2 km 3) with warm wall rocks that have a low effective viscosity. Magma storage is further enhanced by regional tectonic extension, high magma crystal contents, and if the effective wall-rock viscosity is lowered by microfracturing, fluid infiltration, or metamorphic reactions. The long-term magma supply rate and chamber volume are important controls on eruption frequency for all magma chamber sizes. The model can explain certain aspects of the frequency, volume, and spatial distribution of small-volume silicic eruptions in caldera systems, and helps account for the large size of granitic plutons, their association with extensional settings and high thermal gradients, and the fact that they usually post-date associated volcanic deposits. 相似文献
17.
Eisuke Fujita Tomofumi Kozono Hideki Ueda Yuhki Kohno Shoichi Yoshioka Norio Toda Aiko Kikuchi Yoshiaki Ida 《Bulletin of Volcanology》2013,75(1):1-14
Crustal deformation by the M w 9.0 megathrust Tohoku earthquake causes the extension over a wide region of the Japanese mainland. In addition, a triggered M w 5.9 East Shizuoka earthquake on March 15 occurred beneath the south flank, just above the magma system of Mount Fuji. To access whether these earthquakes might trigger the eruption, we calculated the stress and pressure changes below Mount Fuji. Among the three plausible mechanisms of earthquake–volcano interactions, we calculate the static stress change around volcano using finite element method, based on the seismic fault models of Tohoku and East Shizuoka earthquakes. Both Japanese mainland and Mount Fuji region are modeled by seismic tomography result, and the topographic effect is also included. The differential stress given to Mount Fuji magma reservoir, which is assumed to be located to be in the hypocentral area of deep long period earthquakes at the depth of 15 km, is estimated to be the order of about 0.001–0.01 and 0.1–1 MPa at the boundary region between magma reservoir and surrounding medium. This pressure change is about 0.2 % of the lithostatic pressure (367.5 MPa at 15 km depth), but is enough to trigger an eruptions in case the magma is ready to erupt. For Mount Fuji, there is no evidence so far that these earthquakes and crustal deformations did reactivate the volcano, considering the seismicity of deep long period earthquakes. 相似文献
18.
Frederico Machado 《Bulletin of Volcanology》1965,28(1):375-380
Comparison between observed and theoretical seismic intensities of the Messina earthquake of 1908 shows an area of negative anomalies, which seems to correspond to the «shadow» of the Etna magma chamber. This magma chamber has probably an elongated form, being 40 km wide and about 100 km long; the mean depth appears to be close to 5 km, but the thickness cannot be estimated from the present data. 相似文献
19.
F. Le Guern 《Bulletin of Volcanology》1982,45(3):197-202
Volcanic contribution to the atmosphere can be re-evaluated as 5×107 ton/year of CO2, which is less than recent industrial contribution. Degassing is more abundant from volcanoes where magma convections are significant. Effective degassing occurs before the magma reaches the surface, that is, when the magma is fluid. Volcanic gas in the atmosphere can affect the population or the vegetation around volcanoes. 相似文献
20.
《Journal of Volcanology and Geothermal Research》2001,105(3):207-224
A new model is proposed for passive degassing from sub-volcanic magma chambers. The water content in stably stratified shallow magma chamber will be equated to its solubility at the upper boundary by convection. Water from a lower layer high in water content can enrich the contact zone of the upper layer and lead to further convective overturn of this boundary layer. A complete set of equations describing convection with bubble formation and dissolution is reduced to a simplified form by assuming a small bubble content. The development and pattern of flow driven by vesiculation is modeled numerically in a 2D magma chamber for relatively low Raleigh numbers (5×105). Bubbles rising from the magma will collect near the roof in a layer of 8–10 vol% and then escape upward to fumaroles. The Stokes flux of bubbles escaping from an andesitic magma with viscosity 104 P and a top surface of about 500×500 m corresponds with observed total magmatic water fluxes of 35 kg/s. Pressure within the chamber is buffered by elastic (and local visco-elastic) deformations in the solid rocks bounding the chamber to the range between ambient and close to lithostatic values. In a chamber closed to fresh magma inputs, the decrease in volume due to such gentle volatile escape lowers the reference pressure. Bubbles flux from the lower layer induced by variation of the saturation level around stratification boundary may be efficient mechanism for the water transport between layers. 相似文献