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1.
Basaltic pyroclastic volcanism takes place over a range of scales and styles, from weak discrete Strombolian explosions (~102–103 kg s?1) to Plinian eruptions of moderate intensity (107–108 kg s?1). Recent well-documented historical eruptions from Etna, Kīlauea and Stromboli typify this diversity. Etna is Europe's largest and most voluminously productive volcano with an extraordinary level and diversity of Strombolian to subplinian activity since 1990. Kīlauea, the reference volcano for Hawaiian fountaining, has four recent eruptions with high fountaining (>400 m) activity in 1959, 1960, 1969 (–1974) and 1983–1986 (–2008); other summit (1971, 1974, 1982) and flank eruptions have been characterized by low fountaining activity. Stromboli is the type location for mildly explosive Strombolian eruptions, and from 1999 to 2008 these persisted at a rate of ca. 9 per hour, briefly interrupted in 2003 and 2007 by vigorous paroxysmal eruptions. Several properties of basaltic pyroclastic deposits described here, such as bed geometry, grain size, clast morphology and vesicularity, and crystal content are keys to understand the dynamics of the parent eruptions.The lack of clear correlations between eruption rate and style, as well as observed rapid fluctuations in eruptive behavior, point to the likelihood of eruption style being moderated by differences in the fluid dynamics of magma and gas ascent and the mechanism by which the erupting magma fragments. In all cases, the erupting magma consists of a mixture of melt and gaseous bubbles. The depth and rate of degassing, melt rheology, bubble rise and coalescence rates, and extent of syn-eruptive microlite growth define complex feedbacks that permit reversible shifts between fragmentation mechanisms and in eruption style and intensity. However, many basaltic explosive eruptions end after an irreversible shift to open-system outgassing and microlite crystallization in melt within the conduit.Clearer understanding of the factors promoting this diversity of basaltic pyroclastic eruptions is of fundamental importance in order to improve understanding of the range of behaviors of these volcanoes and assess hazards of future explosive events at basaltic volcanoes. The three volcanoes used for this review are the sites of large and growing volcano-tourism operations and there is a public need both for better knowledge of the volcanoes’ behavior and improved forecasting of the likely course of future eruptions. 相似文献
2.
《Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy》1999,24(11-12):957-961
The process of magma ascent along volcanic conduits during explosive eruptions is numerically simulated by solving the transport equations in the one-dimensional, steady, and isothermal assumptions. The physical model considers the non-equilibrium multiphase flow of a mixture of liquid magma, crystals and/or lithic fragments, and exsolving gas. The liquid magma is treated as a mixture of 10 major oxides plus dissolved water and carbon dioxide, and the gas phase as a mixture of water and carbon dioxide. The model is characterized by a strong coupling between fluid dynamic and constitutive equations, whereby the magma properties like viscosity, density, and gas solubility are calculated on the basis of the chemical composition of the liquid magma and its crystal content, and of the physical conditions along the volcanic conduit. The model predictions obtained by considering many different eruptive conditions reveal complex and sometimes non-intuitive relations between the initial conditions and the predicted flow parameter distribution. The composition of the liquid magma and the amount and composition of volatiles are found to be of major importance in determining the dynamics of magma ascent during explosive eruptions. The results of this modeling can be useful to forecast the volcanic hazard at a given volcano, as the conduit exit flow parameters could constitute initial conditions to study the atmospheric dispersion of the gas-pyroclasts and the dynamics of pyroclastic flows. 相似文献
3.
Federica Schiavi Katsura Kobayashi Takuya Moriguti Eizo Nakamura Massimo Pompilio Massimo Tiepolo Riccardo Vannucci 《Contributions to Mineralogy and Petrology》2010,159(4):541-561
During its 1800-year-long persistent activity the Stromboli volcano has erupted a highly porphyritic (HP) volatile-poor scoriaceous
magma and a low porphyritic (LP) volatile-rich pumiceous magma. The HP magma is erupted during normal Strombolian explosions
and lava effusions, while the LP one is related to more energetic paroxysms. During the March–April 2003 explosive activity,
Stromboli ejected two typologies of juvenile glassy ashes, namely highly vesicular LP shards and volatile-poor HP shards.
Their textural and in situ chemical characteristics are used to unravel mutual relationships between HP and LP magmas, as
well as magma dynamics within the shallow plumbing system. The mantle-normalized trace element patterns of both ash types
show the typical arc-lava pattern; however, HP glasses possess incompatible element concentrations higher than LP glasses,
along with Sr and Eu negative anomalies. HP shards are generally characterized by higher Li contents (to ~20 ppm) and lower
δ7Li values (+1.2 to −3.8‰) with respect to LP shards (Li contents of 7–14 ppm and δ7Li ranging between +4.6 and +0.9‰). Fractional crystallization models based on major and trace element compositions, combined
with a degassing model based on open-system Rayleigh distillation and on the assumption that melt/fluidDLi > 1, show that abundant (~30%) plagioclase precipitation and variable degrees of degassing can lead the more primitive LP
magma to evolve toward a differentiated (isotopically lighter) HP magma ponding in the upper conduit and undergoing slow continuous
degassing-induced crystallization. This study also evidences that in March 2003 Stromboli volcano poured out a small early
volume of LP magma that traveled slower within the conduit with respect to later and larger volumes of fast ascending LP magma
erupted during the April 5 paroxysm. The different ascent rates and cooling rates of the two LP magma batches (i.e., pre-
and post-paroxysm) resulted in small, but detectable, differences in their chemical signatures. Finally, this study highlights
the high potential of in situ investigations of juvenile glassy ashes in petrologic and geochemical monitoring the volcanic
activity and of Li isotopes as tracers of degassing processes within the shallow plumbing system. 相似文献
4.
D. Giordano P. Ardia D.B. Dingwell M.W. Schmidt A. Mangiacapra 《Geochimica et cosmochimica acta》2009,73(21):6613-3236
Somma-Vesuvius is considered one of the highest-risk volcanic systems in the world due to its high population density and record of highly destructive explosive activity. Eruptive style at Vesuvius varies greatly, alternating between effusive and explosive activities, and is likely strongly controlled by the evolution of the physical and chemical properties of the magma. Nevertheless, with the exception of the 1631 eruption, the rheological properties of Vesuvius magmas remain largely unconstrained. Here, we investigate the Newtonian shear viscosity (η) of dry and hydrous melts from the Mercato (plinian) and 1906 (violent strombolian) eruptions. These eruptions differ in size, eruptive style and magma chemistry (from phonolite to phono-tephrite). To evaluate the dry liquid viscosity variation covered by the eruptive products of the recent activity at Vesuvius, we measured the melt viscosities of bulk rock compositions and, for highly crystalline samples, of the separated groundmasses of tephras from the Pollena and 1906 eruptions. Hydrated samples with up to 4.24 wt% dissolved water were synthesised in a piston cylinder apparatus at confining pressure up to 10 kbar. The dry high temperature and the dry and hydrous low-temperature viscosities were obtained by combining the concentric cylinder and micropenetration techniques. The measured viscosities were parameterized by a modified Vogel-Fulcher-Tammann equation, accounting for the effect of water content, and were compared with previous measurements and models. At magmatic temperatures, the viscosities of Mercato samples are about four orders of magnitude higher than that of the least viscous investigated products from the 1906 eruption. Complex numerical models to forecast eruptive scenarios and their environmental impact are extremely sensitive to the accuracy of the input parameters and constitutive equations of magma properties. As a consequence, the numerical expressions obtained here are of particular relevance in the context of hazard assessment related to the different possible eruptive scenarios at Vesuvius through numerical simulation tools. The effect of composition on the liquid viscosities is compared to other high-Na (e.g., samples from Teide and Etna) and high-K (e.g., samples from Stromboli and Phlegrean Fields) alkaline magmas. 相似文献
5.
6.
7.
Intra-Grain Sr Isotope Evidence for Crystal Recycling and Multiple Magma Reservoirs in the Recent Activity of Stromboli Volcano, Southern Italy 总被引:3,自引:1,他引:3
FRANCALANCI LORELLA; DAVIES GARETH R.; LUSTENHOUWER WIM; TOMMASINI SIMONE; MASON PAUL R. D.; CONTICELLI SANDRO 《Journal of Petrology》2005,46(10):1997-2021
Over the last several hundred years, Stromboli has been characterizedby steady-state Strombolian activity. The volcanic productsare dominated by degassed and highly porphyritic (HP-magma)black scoria bombs, lapilli and lava flows of basaltic shoshoniticcomposition. Periodically (about one to three events per year),more energetic explosive eruptions also eject light colouredvolatile-rich pumices with low phenocryst content (LP-magma)that have more mafic compositions than the HP-magma. An in situmajor and trace element and Sr isotope microanalysis study ispresented on four samples chosen to characterize the differentmodes of activity at Stromboli: a lava flow (19851986effusive event), a scoria bomb from the normalpresent-day activity of Stromboli (April 1984), and a scoriaand coeval pumice sample from a recent more explosive eruption(September 1996). Plagioclase (An6290) and clinopyroxene(Mg-number between 0·69 and 0·91) phenocrystsin all samples record marked major element variations. Largeand comparable Sr isotope variations have been detected in plagioclaseand clinopyroxene. HP-magma crystals have resorbed cores, witheither high 87Sr/86Sr (0·706350·70630)or low 87Sr/86Sr (0·706140·70608); thelatter values are similar to the values of the outer cores.Mineral rims and glassy groundmasses generally have intermediate87Sr/86Sr (0·706280·70613). Similarly,mineral growth zones with three groups of 87Sr/86Sr values characterizeminerals from the LP-pumice, with the lowest values presentin mineral rims and groundmass glass. These results define amixing process between HP- and LP-magmas, plus crystallizationof clinopyroxene, plagioclase and olivine, occurring in a shallowmagma reservoir that feeds the present-day magmatic activityof Stromboli. An important observation is the presence of athird component (high 87Sr/86Sr in mineral cores) consideredto represent a pre-AD 1900 cumulus crystal mush reservoir situatedjust below the shallow magma chamber. These cumulus phases areincorporated by the LP-magma arriving from depth and transportedinto the shallow reservoir. A rapid decrease of 87Sr/86Sr inthe replenishing LP-magma immediately prior to eruption of theAD 1985 lava flow is associated with an increased volume ofLP-magma in the shallow magma chamber. The HP-magma in the shallowreservoir is not fully degassed when it interacts with the LP-magma,making efficient mixing possible that ultimately produces awell overturned homogeneous magma. Further degassing and crystallizationoccur at shallower levels as the HP-magma moves through a conduitto the surface. KEY WORDS: isotopic microsampling; mineral recycling; mixing; Sr isotope disequilibria; Stromboli 相似文献
8.
R. Gertisser S.J. Charbonnier V.R. Troll J. Keller K. Preece J.P. Chadwick J. Barclay R.A. Herd 《Geology Today》2011,27(2):57-62
Merapi is Indonesia's most dangerous volcano with a history of deadly eruptions. Over the past two centuries, the volcanic activity has been dominated by prolonged periods of lava dome growth and intermittent gravitational or explosive dome failures to produce pyroclastic flows every few years. Explosive eruptions, such as in 2010, have occurred occasionally during this period, but were more common in pre‐historical time, during which a collapse of the western sector of the volcano occurred at least once. Variations in magma supply from depth, magma ascent rates and the degassing behaviour during ascent are thought to be important factors that control whether Merapi erupts effusively or explosively. A combination of sub‐surface processes operating at relatively shallow depth inside the volcano, including complex conduit processes and the release of carbon dioxide into the magmatic system through assimilation of carbonate crustal rocks, may result in unpredictable explosive behaviour during periods of dome growth. Pyroclastic flows generated by gravitational or explosive lava dome collapses and subsequent lahars remain the most likely immediate hazards near the volcano, although the possibility of more violent eruptions that affect areas farther away from the volcano cannot be fully discounted. In order to improve hazard assessment during future volcanic crises at Merapi, we consider it crucial to improve our understanding of the processes operating in the volcano's plumbing system and their surface manifestations, to generate accurate hazard zonation maps that make use of numerical mass flow models on a realistic digital terrain model, and to utilize probabilistic information on eruption recurrence and inundation areas. 相似文献
9.
Lorella Francalanci Riccardo Avanzinelli Isabella Nardini Massimo Tiepolo Jon P. Davidson Riccardo Vannucci 《Contributions to Mineralogy and Petrology》2012,163(1):109-131
In situ Sr-isotope data by microdrilling, coupled with major and trace element analyses, have been performed on plagioclase
and clinopyroxene from seven samples collected during the 2002–2003 eruptive crisis at Stromboli volcano (Aeolian Islands,
Italy). On 28 December 2002, the persistent moderate explosive activity was broken by an effusive event lasting about 7 months.
A more violent explosion (paroxysm) occurred on 5 April 2003. Two magma types were erupted, namely a volatile-poor and highly
porphyritic magma (HP-magma) poured out as scoria or lava and a volatile-rich, phenocryst-poor magma (LP-magma) found as pumice.
LP-magma differs from the HP-magma also for its slightly less-evolved chemistry, the groundmass composition and the lower
Sr-isotope ratios. Micro-Sr-isotope data show the presence of zoned minerals in strong isotope disequilibrium, as previously
found in products erupted in 1984, 1985 and 1996 AD, with 87Sr/86Sr values generally decreasing from cores to rims of minerals. Only some outer rims testify for equilibrium with the host
groundmass. The internal mineral zones with high Sr-isotope ratios (0.70665–0.70618) are interpreted as ‘antecrysts’, crystallised
during the previous activity and recycled in the present-day system since the opening shoshonitic activity of the Recent Period,
which occurred at about 2.5 ka ago. This result has implications for the dynamics of the present-day plumbing system of Stromboli
at intermediate pressure (about 2–3 km depth) and allows us to propose a model whereby an HP-magma reservoir is directly interconnected
at the bottom with a cumulate crystal much reservoir. Efficient mixing between residing HP- and input LP-magmas can occur
in this reservoir, due to more similar rheological characteristics of the two magmas than in the conduit, where crystallisation
is enhanced by degassing. Antecrysts (and possibly melts) re-enter in the HP-magma reservoir both from the bottom, recycled
by ascending LP-magmas crossing the crystal mush, and from the top, recycled by descending degassed and dense HP-magma, residual
of the periodic Strombolian explosions at the surface. The isotope variation measured in the groundmasses allows calculating
the proportion of the LP-magma entering the shallow HP-magma reservoir at ~20%. From this proportion, we estimate that the
total volume of LP-magma input during 2002–2003 closely matches the magma volume erupted in the effusive event, suggesting
a steady-state system at broadly constant volume. The comparison with estimates of the LP-magma volume ejected by the paroxysm
indicates that the LP-magma amount directly reaching the surface during the 5 April paroxysm is minimal with respect to that
entering the system. 相似文献
10.
Major changes in volcano behaviour after a sector collapse: insights from Stromboli, Italy 总被引:1,自引:0,他引:1
Alessandro Tibaldi 《地学学报》2004,16(1):2-8
In recent years, increasing numbers of volcanoes are being recognized as having undergone large sector collapses. How far such events can change the succeeding behaviour of the volcano is poorly known, despite the importance for natural hazards and economic applications. By analysing the last 100‐kyr geological history of Stromboli (Italy), which has involved multiple collapses, this paper illuminates the feedback effect between sector collapse and magma upwelling and emplacement. The collapsed zone of Stromboli is episodically unbuttressed, draining magma preferentially with formation of eruptive centres in the collapse amphitheatre and dyking along the collapse shoulders. After a collapse, there were no further linear eruptions across the cone on opposing middle–lower slopes, but these occurred again after rebuilding of the cone. The time‐averaged eruption rate increased immediately before the first collapse and remained higher during the succeeding period of frequent collapses. 相似文献
11.
Petrology and Geochemistry of the Bandas del Sur Formation, Las Canadas Edifice, Tenerife (Canary Islands) 总被引:1,自引:2,他引:1
The Bandas del Sur Formation preserves a Quaternary extra-calderarecord of central phonolitic explosive volcanism of the LasCañadas volcano at Tenerife. Volcanic rocks are bimodalin composition, being predominantly phonolitic pyroclastic deposits,several eruptions of which resulted in summit caldera collapse,alkali basaltic lavas erupted from many fissures around theflanks. For the pyroclastic deposits, there is a broad rangeof pumice glass compositions from phonotephrite to phonolite.The phonolite pyroclastic deposits are also characterized bya diverse, 78-phase phenocryst assemblage (alkali feldspar+ biotite + sodian diopside + titanomagnetite + ilmenite + noseanhaüyne+ titanite + apatite) with alkali feldspar dominant, in contrastto interbedded phonolite lavas that typically have lower phenocrystcontents and lack hydrous phases. Petrological and geochemicaldata are consistent with fractional crystallization (involvingthe observed phenocryst assemblages) as the dominant processin the development of phonolite magmas. New stratigraphicallyconstrained data indicate that petrological and geochemicaldifferences exist between pyroclastic deposits of the last twoexplosive cycles of phonolitic volcanism. Cycle 2 (0·850·57Ma) pyroclastic fall deposits commonly show a cryptic compositionalzonation indicating that several eruptions tapped chemically,and probably thermally stratified magma systems. Evidence formagma mixing is most widespread in the pyroclastic depositsof Cycle 3 (0·370·17 Ma), which includesthe presence of reversely and normally zoned phenocrysts, quenchedmafic glass blebs in pumice, banded pumice, and bimodal to polymodalphenocryst compositional populations. Syn-eruptive mixing eventsinvolved mostly phonolite and tephriphonolite magmas, whereasa pre-eruptive mixing event involving basaltic magma is recordedin several banded pumice-bearing ignimbrites of Cycle 3. Theperiodic addition and mixing of basaltic magma ultimately mayhave triggered several eruptions. Recharge and underplatingby basaltic magma is interpreted to have elevated sulphur contents(occurring as an exsolved gas phase) in the capping phonoliticmagma reservoir. This promoted noseanhaüyne crystallizationover nepheline, elevated SO3 contents in apatite, and possiblyresulted in large, climatologically important SO2 emissions. KEY WORDS: Tenerife; phonolite; crystal fractionation; magma mixing; sulphur-rich explosive eruptions 相似文献
12.
The behavior of chlorine and sulfur during differentiation of the Mt. Somma-Vesuvius magmatic system 总被引:4,自引:0,他引:4
Summary Reheated silicate melt inclusions in volcanic rock samples from Mt. Somma-Vesuvius, Italy, have been analyzed for 29 constituents
including H2O, S, Cl, F, B, and P2O5. This composite volcano consists of the older Mt. Somma caldera, formed between 14 and 3.55 ka before present, and the younger
Vesuvius cone. The melt inclusion compositions provide important constraints on pre-eruptive magma geochemistry, identify
relationships that relate to eruption behavior and magma evolution, and provide extensive evidence for magmatic fluid exsolution
well before eruption.
The melt inclusion data have been categorized by groups that reflect magma compositions, age, and style of eruptions. The
data show distinct differences in composition for eruptive products older than 14.0 ka (pre-caldera rocks) versus eruptive
products younger than 3.55 ka. Moreover, pre-caldera eruptions were associated with magmas relatively enriched in SiO2, whereas eruptions younger than 3.55 ka (i.e., the syn- and post-caldera magmas which generated the Somma caldera and the
Vesuvius cone) were derived from magmas comparatively enriched in S, Cl, CaO, MgO, P2O5, F, and many lithophile trace elements.
Melt inclusion data indicate that eruptive behavior at Vesuvius correlates with pre-eruptive volatile enrichments. Most magmas
associated with explosive plinian and subplinian events younger than 3.55 ka contained more H2O, contained significantly more S, and exhibited higher (S/Cl) ratios than syn- and post-caldera magmas which erupted during
relatively passive interplinian volcanic phenomena.
Received January 10, 2000 Revised version accepted July 17, 2000 相似文献
13.
Toshikazu Ebisuzaki Hiroko Miyahara Ryuho Kataoka Tatsuhiko Sato Yasuhiro Ishimine 《Gondwana Research》2011,19(4):1054-1061
Volcanoes with silica-rich and highly viscous magma tend to produce violent explosive eruptions that result in disasters in local communities and that strongly affect the global environment. We examined the timing of 11 eruptive events that produced silica-rich magma from four volcanoes in Japan (Mt. Fuji, Mt. Usu, Myojin-sho, and Satsuma-Iwo-jima) over the past 306 years (from AD 1700 to AD 2005). Nine of the 11 events occurred during inactive phases of solar magnetic activity (solar minimum), which is well indexed by the group sunspot number. This strong association between eruption timing and the solar minimum is statistically significant to a confidence level of 96.7%. This relationship is not observed for eruptions from volcanoes with relatively silica-poor magma, such as Izu-Ohshima. It is well known that the cosmic-ray flux is negatively correlated with solar magnetic activity, as the strong magnetic field in the solar wind repels charged particles such as galactic cosmic rays that originate from outside of the solar system. The strong negative correlation observed between the timing of silica-rich eruptions and solar activity can be explained by variations in cosmic-ray flux arising from solar modulation. Because silica-rich magma has relatively high surface tension (~ 0.1 Nm?1), the homogeneous nucleation rate is so low that such magma exists in a highly supersaturated state without considerable exsolution, even when located relatively close to the surface, within the penetration range of cosmic-ray muons (1–10 GeV). These muons can contribute to nucleation in supersaturated magma, as documented by many authors studying a bubble chamber, via ionization loss. This radiation-induced nucleation can lead to the pre-eruptive exsolution of H2O in the silica-rich magma. We note the possibility that the 1991 Mt. Pinatubo eruption was triggered by the same mechanism: an increase in cosmic-ray flux triggered by Typhoon Yunya, as a decrease in atmospheric pressure results in an increase in cosmic-ray flux. We also speculate that the snowball Earth event was triggered by successive large-scale volcanic eruptions triggered by increased cosmic-ray flux due to nearby supernova explosions. 相似文献
14.
A. Yu. Ozerov 《Petrology》2009,17(7):653-668
Processes in the feeders of basaltic volcanoes during Strombolian-type eruptions were examined with the use of a complex apparatus
for modeling basaltic eruptions (CAMBE), which was designed and manufactured by the authors for this purpose. The experimental
setup consists of modeling and registering units and has a height of 18 m. It was designed with regard for the geometric dimensions
of a natural feeding volcanic system: the ratio of the inner diameter of the feeder to its height is approximately 1: 1000.
CAMBE was the first modeling equipment making possible passing a flow of gas-saturated liquid through the conduit, which allowed
us to study the nucleation of gas bubbles, their growth, coalescence, transformations of the gas structures, and the kinetics
of the gas phase. The experiments were carried out in a manner that made it possible to eliminate effects of structural barriers
and fluctuations in the liquid flow velocity. As a result of the experiments, a new (previously unknown) regime in the flow
of two-phase systems through a vertical conduit was discovered: the cluster regime, which is characterized by systematically
alternating dense accumulations of gas bubbles (bubble clusters) and liquid devoid of a free gas phase. It is demonstrated
that the liquid, bubbly, cluster, and slug regimes systematically grade into one another and are polymorphic modifications
of gas-saturated liquids moving through vertical conduits. Our data led us to propose a new model for the gas-hydrodynamic
movement of magmatic melt through the conduit of a basaltic volcano: depending on the gas-hydrodynamic regime in the volcanic
vent, various types of eruptive activity (up to explosions) may take place. The analyses of basaltic magma explosions allowed
us to describe them from a new standpoint and recognize the following four major modes of their manifestations at the surface:
(1) weak ash explosions early during the cluster regime, (2) strong ash explosions during the mature cluster regime, (3) bomb
explosions during the slug regime, (4) bomb grading to ash explosions during the slug regime associated with trains of small
bubbles. 相似文献
15.
San Martin Tuxtla Volcano, the largest and highest edifice in the Tuxtla Volcanic Field, had explosive eruptions in 1664 and
1793. This volcano poses the highest hazard for the growing population centers surrounding it. The impacts and deposits of
the 1793 eruption have been studied recently, and some of its characteristics, such as eruptive mass, grain-size distribution
of the products and plume height, have been estimated. These data, together with daily data on wind velocities taken in the
nearby city of Veracruz, have been used to conduct numerical modeling of ash distribution from an eruptive column comparable
to that estimated during the 1793 event. Using wind velocities of randomly chosen days, we constructed probability maps of
areas likely to receive, at least 1, 5, or 10 cm of ash. Our results indicate that an area about 1.3 × 104 km2 has a 12% probability of being covered by an ash fall more than 1 cm thick, if a mass of 1014 g was erupted and deposited from a 10-km-high eruptive plume. The results presented in this paper serve as a scientific basis
to estimate volcano risk in the area from a possible eruption, consisting of one or several large explosive events, at San
Martin Tuxtla volcano. 相似文献
16.
The major element chemical compositions of lava from four eruptionson the east rift zone of Kilauea between August 1968 and October1971 reflect three petrologic processes:
- Production of chemically distinct batches of magma in the mantle.
- Separation of olivine, augite, and plagioclase from liquidduringflow in the rift-zone conduits.
- Mixing of differentmagmas during ascent to the surface.
17.
A. Peccerillo 《Mineralogy and Petrology》2001,73(1-3):93-105
Summary Vesuvius and Stromboli are two active and extensively studied volcanoes that traditionally have been considered as having
different styles of eruption, rock composition and tectonic setting. Data reveal close compositional affinities between these
two volcanoes. The abundant 13–15 Ka old Stromboli leucite-tephritic rocks have radiogenic isotope signatures, and abundances
and ratios of incompatible elements with the exception of Rb and K, which are identical to those of Vesuvius. The Phlegraean
Fields also show close affinities to these volcanoes.
The similarity between Stromboli leucite-tephrites and Vesuvius rocks cannot be the result of low pressure processes, given
the differences between the two volcanoes in terms of structural features, eruptive behaviour and type of basement rocks.
Instead, the observed geochemical signatures are likely to represent a primary magma composition and reveal a common homogeneous
source for the two suites. The higher K and Rb contents in the Vesuvius rocks suggest either selective enrichment during magma
ascent or a role for phlogopite melting during mantle anatexis.
The most primitive rocks from Vesuvius, Phlegraean Fields and Stromboli reveal intermediate compositions between arc and intraplate
volcanics. It is suggested that the mantle sources beneath these volcanoes consist of a mixture of intraplate- and slab-derived
components. Intraplate material was probably provided by inflow of asthenosheric mantle into the wedge above the subducting
Ionian Sea plate, either from the Apulian plate and/or from the Tyrrhenian Sea region. Fluids or melts released from the sinking
slab and associated sediments generated metasomatic modification of the intraplate material, whose melting gave rise to the
Stromboli, Vesuvius and Phlegraean Fields magmas.
The present study demonstrates how comparative investigations of various volcanic centres from southern Italy allow clarification
of a number of problems involving magma genesis and evolution, composition of mantle sources and geodynamic significance,
which have been long debated and are difficult to solve if individual volcanoes are considered in isolation.
Received July 20, 2000; revised version accepted March 19, 2001 相似文献
18.
Hans Keppler 《Geochimica et cosmochimica acta》2010,74(2):645-7631
The distribution of sulfur between haplogranitic melt and aqueous fluid has been measured as a function of oxygen fugacity (Co-CoO-buffer to hematite-magnetite buffer), pressure (0.5-3 kbar), and temperature (750-850 °C). Sulfur always strongly partitions into the fluid. At a given oxygen fugacity, pressure and temperature, the distribution of sulfur between melt and fluid can be described by one constant partition coefficient over a wide range of sulfur concentrations. Oxygen fugacity is the most important parameter controlling sulfur partitioning. While the fluid/melt partition coefficient of sulfur is 468 ± 32 under Co-CoO buffer conditions at 2 kbar and 850 °C, it decreases to 47 ± 4 at an oxygen fugacity 0.5-1 log unit above Ni-NiO at the same pressure and temperature. A further increase in oxygen fugacity to the hematite-magnetite buffer has virtually no effect on the partition coefficient (Dfluid/melt = 49 ± 2). The dependence of Dfluid/melt on temperature and pressure was systematically explored at an oxygen fugacity 0.5-1 log units above Ni-NiO. At 850 °C, the effect of pressure on the partition coefficient is small (Dfluid/melt = 58 ± 3 at 0.5 kbar; 94 ± 9 at 1 kbar; 47 ± 4 at 2 kbar and 68 ± 5 at 3 kbar) and temperature also has only a minor effect on partitioning.The data show the “sulfur excess” observed in many explosive volcanic eruptions can easily be explained by the presence of a small fraction of hydrous fluid in the magma chamber before the eruption. The sulfur excess can be calculated as the product of the fluid/melt partition coefficient of sulfur and the mass ratio of fluid over melt in the erupted material. For a plausible fluid/melt partition coefficient of 47 under oxidizing conditions, a 10-fold sulfur excess corresponds to a 17.6 wt.% of fluid in the erupted material. Large sulfur excesses (10-fold or higher) are only to be expected if only a small fraction of the magma residing in the magma chamber is erupted.The behavior of sulfur, which seems to be largely independent of pressure and temperature under oxidizing conditions is very different from chlorine, where the fluid/melt partition coefficient strongly increases with pressure. Variations in the SO2/HCl ratio of volcanic gases, if they reflect primary processes in the magma chamber, therefore provide an indicator of pressure variations in a magma. In particular, major increases in the S/Cl ratio of an aqueous fluid coexisting with a felsic magma suggest a pressure reduction in the magma chamber and/or magma rising to the surface. 相似文献
19.
Evolution of silicic magmas in the Kos-Nisyros volcanic center,Greece: a petrological cycle associated with caldera collapse 总被引:2,自引:1,他引:1
Multiple eruptions of silicic magma (dacite and rhyolites) occurred over the last ~3 My in the Kos-Nisyros volcanic center
(eastern Aegean sea). During this period, magmas have changed from hornblende-biotite-rich units with low eruption temperatures
(≤750–800°C; Kefalos and Kos dacites and rhyolites) to hotter, pyroxene-bearing units (>800–850°C; Nisyros rhyodacites) and
are transitioning back to cooler magmas (Yali rhyolites). New whole-rock compositions, mineral chemistry, and zircon Hf isotopes
show that these three types of silicic magmas followed the same differentiation trend: they all evolved by crystal fractionation
and minor crustal assimilation (AFC) from parents with intermediate compositions characterized by high Sr/Y and low Nb content,
following a wet, high oxygen fugacity liquid line of descent typical of subduction zones. As the transition between the Kos-Kefalos
and Nisyros-type magmas occurred immediately and abruptly after the major caldera collapse in the area (the 161 ka Kos Plateau
Tuff; KPT), we suggest that the efficient emptying of the magma chamber during the KPT drew out most of the eruptible, volatile-charged
magma and partly solidified the unerupted mush zone in the upper crust due to rapid unloading, decompression, and coincident
crystallization. Subsequently, the system reestablished a shallow silicic production zone from more mafic parents, recharged
from the mid to lower crust. The first silicic eruptions evolving from these parents after the caldera collapse (Nisyros units)
were hotter (up to >100°C) than the caldera-forming event and erupted from reservoirs characterized by different mineral proportions
(more plagioclase and less amphibole). We interpret such a change as a reflection of slightly drier conditions in the magmatic
column after the caldera collapse due to the decompression event. With time, the upper crustal intermediate mush progressively
transitioned into the cold-wet state that prevailed during the Kefalos-Kos stage. The recent eruptions of the high-SiO2 rhyolite on Yali Island, which are low temperature and hydrous phases (sanidine, quartz, biotite), suggest that another large,
potentially explosive magma chamber is presently building under the Kos-Nisyros volcanic center. 相似文献
20.
The youngest volcanic eruptions in East‐Central Europe—new findings from the Ciomadul lava dome complex,East Carpathians,Romania
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Violent explosive eruptions occurred between c. 51 and 29 thousand years ago—during the Last Glacial Maximum in East‐Central Europe—at the picturesque volcano of Ciomadul, located at the southernmost tip of the Inner Carpathian Volcanic Range in Romania. Field volcanology, glass geochemistry of tephra, radiocarbon and optically stimulated luminescene dating, along with coring the lacustrine infill of the two explosive craters of Ciomadul (St Ana and Mohos), constrain the last volcanic activity to three subsequent eruptive stages. The explosivity was due to the silicic composition of the magma producing Plinian‐style eruptions, and the interaction of magma with the underlying, water‐rich rocks resulting in violent phreatomagmatic outbursts. Tephra (volcanic ash) from these eruptions are interbedded with contemporaneous loess deposits, which form thick sequences in the vicinity of the volcano. Moreover, tephra layers are also preserved in the older Mohos crater infill, providing an important archive for palaeoclimate studies. Identifying the final phreatomagmatic eruption of Ciomadul at c. 29.6 ka, which shaped the present‐day landform of the 1600‐m‐wide St Ana explosion crater, we were able to correlate related tephra deposits as far as 350 km from the source within a thick loess‐palaeosol sequence at the Dniester Delta in Roxolany, Ukraine. A refined tephrostratigraphy, based on a number of newly found exposures in the Ciomadul surrounding region as well as correlation with the distal terrestrial and marine (e.g. Black Sea) volcano‐sedimentary record, is expected from ongoing studies. 相似文献