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Feldspar phenocrysts of silicic volcanic rocks are commonly in Sr-isotopic disequilibrium with groundmass. In some cases the feldspar is more radiogenic, and in others it is less radiogenic. Several explanations have been published previously, but none of these is able to accommodate both senses of disequilibrium. We present a model by which either more- or less-radiogenic feldspar (or even both within a single eruptive unit) can originate. The model requires a magma body open to interaction with biotite- and feldspar-bearing wall rock. Magma is incrementally contaminated as wall rock melts incongruently. Biotite preferentially melts first, followed by feldspar. Such melting behavior, which is supported by both field and experimental studies, first contaminates magma with a relatively radiogenic addition, followed by a less-radiogenic addition. Feldspar phenocrysts lag behind melt (groundmass of volcanic rock) in incorporating the influx of contaminant, thus resulting in Sr-isotopic disequilibrium between the crystals and melt. The sense of disequilibrium recorded in a volcanic rock depends on when eruption quenches the contamination process. This model is testable by isotopic fingerprinting of individual feldspar crystals. For a given set of geologic boundary conditions, specific core-to-rim Sr-isotopic profiles are expectable. Moreover, phenocrysts that nucleate at different times during the contamination process should record different and predictable parts of the history. Initial results of Sr-isotopic fingerprinting of sanidine phenocrysts from the Taylor Creek Rhyolite are consistent with the model. More tests of the model are desirable. 相似文献
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Victoria Bannister Peter Roeder Alexei Poustovetov 《Journal of Volcanology and Geothermal Research》1998,87(1-4)
Small euhedral chromite crystals are found in olivine macrophenocrysts (Fo80–84) from the basaltic andesites (150 ppm Cr) erupted in 1943–1947, and in orthopyroxene macrophenocrysts of the andesites (75 ppm Cr) erupted in 1947–1952. The majority of the chromite octahedra are 5–20 μm in diameter, and some are found in clusters and linear chains of three or more oriented chromite crystals. The composition of the majority of the chromite grains within olivine and orthopyroxene macrophenocrysts is Fe2+/(Fe2++Mg)=0.5–0.6, Cr/(Cr+Al)=0.5–0.6 and Fe3+/(Fe3++Al+Cr)=0.2–0.3. The chromite crystals in contact with the groundmass are larger, subhedral, and grade in composition from chromite cores to magnetite rims. Comparison of the composition of chromite with those of other volcanic rocks shows that the most primitive Paricutin chromite is richer in total iron and higher in Fe3+/(Fe3++Al+Cr) than primary chromite in most lavas. The linear chains of oriented chromite octahedra are found in olivine and orthopyroxene macrophenocrysts, and in the groundmass. These chromite chains are thought to result from diffusion-controlled crystallization because of the very high partition coefficient (1000) of Cr between chromite and melt. We conclude that chromite was a primary phase in the lavas at the time of extrusion and that magnetite only crystallized after extrusion during cooling of the lava flows. The presence of chromite microphenocrysts in andesitic lavas containing as little as 70 ppm Cr can be explained by dissolved H2O in the melt depressing the liquidus temperature for orthopyroxene such that chromite becomes a liquidus phase. The influence of dissolved H2O can also explain the lack of plagioclase macrophenocrysts in most of the lavas and the relatively high partition coefficient (20) of Ni between olivine and melt and the high partition coefficient (40) of Cr between orthopyroxene and melt. The liquidus temperature of the basaltic andesite is estimated to have been less than 1140°C, assuming H2O>1 wt.%, and the log fO2 to have been above that of the QFM buffer. The chromite and orthopyroxene liquidus temperature of the andesites, assuming H2O>1 wt.%, is estimated to have been 1100°C or less. The derivation of the later andesites from the earlier basaltic andesites has been explained by a combination of fractional crystallization of olivine, orthopyroxene and plagioclase, and assimilation of xenoliths. The significantly lower Cr, Ni and Mg of the andesites may have been in part due to the separation of olivine macrophenocrysts plus enclosed chromite crystals from the earlier basaltic andesites. 相似文献
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In order to define the risk from explosive eruptions, one must constrain both the probability of explosive events and the effects, or consequences, of those events. This paper focuses on the effects of pyroclastic flows and surges (here termed ‘pyroclastic density currents', or PDCs) on buildings, infrastructure elements, and to some extent on vehicles. PDCs impart a lateral force to such structures in the form of dynamic pressure, which depends on the bulk density of the PDC (which in turn depends mainly on particle concentration) and its velocity. For reasonable ranges of particle concentration (10−3 to 0.5) and velocities (10 to 300 m/s), dynamic pressure on the upstream face of a structure ranges from 0.1 kPa to 104 kPa. Lateral loads ranging up to about 100 kPa were produced during nuclear weapons tests in the 1940s and 1950s that were designed to study the effects of such loading on a variety of structures for civil defense and emergency response purposes in the event of nuclear war. Although considerable simplifications are involved, the data from these weapon tests provide useful analog information for understanding the effects of PDCs. I reviewed data from the nuclear tests, describing the expected damage from different loadings. Tables are provided that define the response of different structural elements (e.g., windows, framing, walls) and whole structures to loading in probabilistic terms, which in principle account for variations in construction quality, orientation, and other factors. Finally, damage documented from historical eruptions at Mt. Lamington (1951), Herculaneum (AD 79 Vesuvius eruption), and St. Pierre (1902 Mt. Pelee eruption) is reviewed. Damage patterns, combined with estimates of velocity, provide an independent estimate of particle concentration in the PDCs. Details of structural damage should be recorded and mapped around future eruptions in order to help refine this aspect of consequence analysis. Another fruitful approach would be to combine numerical simulations of eruption scenarios, which can produce simulated maps of dynamic pressure, with GIS-based data on structures for a given region; the result would be predictions of consequences that could be used for planning and emergency response training. 相似文献
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A low aspect ratio, decimeter-thick ash deposit, axisymmetrically distributed around the Latera Caldera (Western Vulsini Volcanoes, central Italy) has been studied by means of field and laboratory investigations. Field studies comprise facies analysis at centimeter scale and maximum clast size and deposit thickness measurements. Grain size and component distribution, chemical composition and particle morphoscopic features have been determined on selected samples. We discuss the co-ignimbrite ash fall vs. pyroclastic surge origin of the deposit and the hydrovolcanic vs. magmatic eruption nature. Complex facies association, textural features and grain size data rule out an ash fall origin for the whole deposit. The hydrovolcanic nature of the eruption has been discarded on the grounds of componentry and morphoscopic features of vitric fragments. We propose that the main body of the ash deposit formed from a radially expanding, dilute, turbulent pyroclastic density current, originated by a continuous collapse of a low-altitude (a few kilometers) eruptive column with a possible radial jet component. 相似文献
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The Hasan Dagi stratovolcano (Central Anatolia, Turkey): evolution from calc-alkaline to alkaline magmatism in a collision zone 总被引：1，自引：0，他引：1
Catherine Deniel Erkan Aydar Alain Gourgaud 《Journal of Volcanology and Geothermal Research》1998,87(1-4)
The Hasan Dagi volcano is one of the two large Plio-Quaternary volcanoes in Cappadocia (Central Anatolia, Turkey). Three stages of edifice construction have been identified for this volcano: Paleovolcano, Mesovolcano and Neovolcano. Most samples from Hasan Dagi volcano are calc-alkaline and define an almost complete trend from basaltic andesite to rhyolite. However, the more recent (Neovolcano) mafic samples are alkaline basalts. The mineralogical and geochemical characteristics of the oldest lavas (Keçikalesi (13 Ma) and Paleo-Hasan Dagi (7 Ma)) are significantly different from those of the younger lavas (Meso- and Neo-Hasan Dagi (<1 Ma)). Calcic plagioclase and pigeonite are typically observed in these older lavas. The Paleovolcano basalts are depleted in alkalis and display a tholeiitic tendency whereas the differentiated lavas are depleted in Na2O but enriched in K2O compared to younger lavas. There is an evolution through time towards higher TiO2, Fe2O3*, MgO, Na2O and K2O and lower Al2O3 and SiO2 which is reflected in the basalt compositions. All the basalts display multi-element patterns typical of continental margin magmas with a significant enrichment in LILE (K, Rb, Ba and Th) and LREE and strong (Paleovolcano) to moderate (Meso- and Neovolcano) negative Nb, Zr and Ti anomalies. However, the younger basalts are the most enriched in incompatible elements, in agreement with their alkaline affinities and do not systematically display negative HFSE anomalies. REE data suggest an hydrous amphibole-bearing crystallization history for both Meso- and Neovolcano lavas. The distinction between the older and younger lavas is also apparent in trace element ratios such as Nb/Y, Ti/Y and Th/Y. These ratios indicate the role of a subducted component±crustal contamination in the genesis of the Hasan Dagi lavas, particularly for the oldest lavas (Keçikalesi and Paleo-Hasan Dagi). The decreasing influence of this component through time, over the last 6–7 m.y., has been accompanied by an increasing contribution of melt-enriched lithosphere. Although the range of variation of Sr, Nd and Pb isotopic ratios is small (0.70457–0.70515; 0.51262–0.51273; 18.80–18.94; 15.64–15.69; 38.87–39.10), it also reflects the evolution of the magma sources through time. Indeed, the youngest (Neovolcano) and most primitive basalts display significantly lower 87Sr/86Sr than the Paleo- and Mesovolcano basalts, whereas the Mesovolcano basalts display more radiogenic Pb than Paleovolcano samples. Magma mixing processes between initially heterogeneous and/or variably contaminated magmas may account for the genesis of the less differentiated and intermediate lavas (48–57% SiO2). Meso- and Neovolcano differentiated lavas (60–68% SiO2) are either derived from the analyzed basalts or from more primitive and more depleted magmas by fractional crystallization±some crustal contamination (AFC). Furthermore, the highly differentiated samples (72–75% SiO2) are not strongly contaminated. The strong calc-alkaline character of Hasan Dagi lavas, in the absence of contemporaneous subduction, must reflect the heritage of the early subduction of the Afro–Arabian plate under the Eurasian plate. The evolution towards alkaline compositions through time is clearly related to the development of extensional tectonics in Central Anatolia in the Late Miocene. 相似文献
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Sonia Calvari Lawrence H. Tanner Gianluca Groppelli 《Journal of Volcanology and Geothermal Research》1998,87(1-4)
Previously undescribed debris-avalanche deposits occur in two locations downslope from the open end of the Valle del Bove. These outcrops comprise unstratified, ungraded deposits of metre-scale lava blocks in a matrix of weathered and fractured lava clasts. The avalanche deposits are unconformably overlain by matrix- to clast-supported conglomerates, representing debris-flow and interbedded fluvial deposits, that constitute most of the Milo Lahar sequence. We present evidence that the Milo Lahar sequence, which crops out just at the exit of the Valle del Bove, formed during the opening and enlargement of this depression. The presence of the avalanche deposits at the base of the Milo Lahar sequence indicates that catastrophic landslides were involved in the formation of the Valle del Bove. The composition of lavas in the debris avalanche deposits is similar to that of most of the Ellittico volcanic sequence exposed along the northern wall of the Valle del Bove. Radiocarbon dates of 8400 and 5300 years BP from the base and top, respectively, of the debris-flow sequence indicate that the Milo Lahars are correlative with the exposed part of the Chiancone deposit. The basal lahars of the Chiancone, which contain lava blocks whose compositions partially overlap that of blocks in the avalanche deposits, may have formed by water concentration in the distal end of the avalanche causing transformation to debris, or alternatively by reworking of the avalanche deposit. 相似文献
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