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1.
Guilherme A. R. Gualda Alfred T. AndersonJr 《Contributions to Mineralogy and Petrology》2007,153(6):733-742
Little is known about the presence, distribution and size of bubbles in rhyolitic magmas prior to eruption. Using X-ray tomography
to study pumice from early-erupted Bishop rhyolite, we discovered a large vesicle with abundant magnetite crystals attached
to its walls. Attachment of magnetite crystals to bubble walls under pre-eruptive conditions can explain the cluster of magnetite
crystals as a result of bubbles rising and collecting magnetite crystals. Alternatively, bubbles may have nucleated on magnetite
crystals and then coalesced to form one large bubble with multiple magnetite crystals attached to it. We argue that the clusters
of magnetite crystals could not have formed during or after eruptive decompression, and conclude that this vesicle corresponds
to a bubble present prior to eruptive decompression. The inferred presence of pre-eruptive bubbles in the Bishop magma confirms
the interpretation that the magma was volatile-saturated prior to eruption. The pre-eruptive size of this bubble is estimated
based on three independent approaches: (1) the current size of the vesicle, (2) the total cross-sectional area of the magnetite
crystals, and (3) the bubble size required for the aggregate to be neutrally buoyant. These approaches suggest a pre-eruptive
bubble 300–850 μm in diameter, with a preferred value of 600–750 μm.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
2.
Satoshi Okumura Michihiko Nakamura Tsukasa Nakano Kentaro Uesugi Akira Tsuchiyama 《Contributions to Mineralogy and Petrology》2012,164(3):493-504
The gas and fluid transport in magmas via permeable flow through interconnected bubble networks controls the rate of outgassing from magmas ascending in volcanic conduits and the fluid transport in the mushy boundary layer of magma reservoirs. Hence, clarifying its mechanism and rate is crucial to understanding the explosivity of volcanic eruptions and the evolution and dynamics of a magma reservoir. Recent experimental studies have determined the gas permeabilities in crystal-free rhyolite and basalt. However, no experimental study has investigated the effect of the crystal contents on the permeable gas transport in magmas. In this study, we performed decompression experiments for hydrous rhyolitic melts having crystallinities of 30 and 50 vol% to examine the effect of crystals on the bubble microstructure and gas permeability during magma vesiculation. Size-controlled (100-meshed) corundum crystals were used as an analog of the phenocrysts in silicic magmas. Microstructural analyses using X-ray CT showed that bubbles coalesce and their connectivity increases with a decrease in the final pressure after the decompression, that is, an increase in the vesicularity. As long as the vesicularities of melt part in the crystal-free basis (melt vesicularity) were similar, no clear effect of the crystallinity on the degree of bubble coalescence and connectivity was observed at melt vesicularities <68 vol%. The corundum showed a large contact angle with aqueous fluid as well as plagioclase and alkaline feldspar; this failed to induce the efficient heterogeneous nucleation and coalescence of bubbles on its surface. The gas permeabilities of all the run products were lower than the detection limits of the present analysis (the order of 10−16 m2) at melt vesicularities <68 vol%. These results show that silicic magmas containing 30 and 50 vol% phenocrysts with a large contact angle have low gas permeabilities until the vesicularity becomes large (at least >68 vol%). This result indicates that the permeable fluid transport through a deep volcanic conduit, which has been proposed on the basis of the observations of volcanic gases and natural products, is so slow that other processes, like shear deformation or magma convection, may be needed to explain the observations. 相似文献
3.
Nicole C. Lautze Thomas W. Sisson Margaret T. Mangan Timothy L. Grove 《Contributions to Mineralogy and Petrology》2011,161(2):331-347
Diffusive coarsening (Ostwald ripening) of H2O and H2O-CO2 bubbles in rhyolite and basaltic andesite melts was studied with elevated temperature–pressure experiments to investigate
the rates and time spans over which vapor bubbles may enlarge and attain sufficient buoyancy to segregate in magmatic systems.
Bubble growth and segregation are also considered in terms of classical steady-state and transient (non-steady-state) ripening
theory. Experimental results are consistent with diffusive coarsening as the dominant mechanism of bubble growth. Ripening
is faster in experiments saturated with pure H2O than in those with a CO2-rich mixed vapor probably due to faster diffusion of H2O than CO2 through the melt. None of the experimental series followed the time1/3 increase in mean bubble radius and time−1 decrease in bubble number density predicted by classical steady-state ripening theory. Instead, products are interpreted
as resulting from transient regime ripening. Application of transient regime theory suggests that bubbly magmas may require
from days to 100 years to reach steady-state ripening conditions. Experimental results, as well as theory for steady-state
ripening of bubbles that are immobile or undergoing buoyant ascent, indicate that diffusive coarsening efficiently eliminates
micron-sized bubbles and would produce mm-sized bubbles in 102–104 years in crustal magma bodies. Once bubbles attain mm-sizes, their calculated ascent rates are sufficient that they could
transit multiple kilometers over hundreds to thousands of years through mafic and silicic melt, respectively. These results
show that diffusive coarsening can facilitate transfer of volatiles through, and from, magmatic systems by creating bubbles
sufficiently large for rapid ascent. 相似文献
4.
Generation of Deccan Trap magmas 总被引:1,自引:0,他引:1
Gautam Sen 《Journal of Earth System Science》2001,110(4):409-431
Deccan Trap magmas may have erupted through multiple centers, the most prominent of which may have been a shield volcano-like
structure in the Western Ghats area. The lavas are predominantly tholeiitic; alkalic mafic lavas and carbonatites are rare.
Radioisotope dating, magnetic chronology, and age constraints from paleontology indicate that although the eruption started
some 68 Ma, the bulk of lavas erupted at around 65–66 Ma. Paleomagnetic constraints indicate an uncertainty of ± 500,000 years
for peak volcanic activity at 65 m.y. in the type section of the Western Ghats. Maximum magma residence times were calculated
in this study based on growth rates of “giant plagioclase” crystals in lavas that marked the end phase of volcanic activity
of different magma chambers. These calculations suggest that the > 1.7 km thick Western Ghats section might have erupted within
a much shorter time interval of ∼ 55,000 years, implying phenomenal eruption rates that are orders of magnitude larger than
any present-day eruption rate from any tectonic environment. Other significant observations/conclusions are as follows: (1)
Deccan lavas can be grouped into stratigraphic subdivisions based on their geochemistry; (2) While some formations are relatively
uncontaminated others are strongly contaminated by the continental crust; (3) Deccan magmas were produced by 15–30% melting
of a Fe-rich lherzolitic source at ∼ 3–2 GPa; (4) Parent magmas of the relatively uncontaminated Ambenali formation had a
primitive composition with 16%MgO, 47%SiO2; (5) Deccan magmas were generated much deeper and by significantly more melting than other continental flood basalt provinces;
(6) The erupted Deccan tholeiitic lavas underwent fractionation and magma mixing at ∼ 0.2 GPa. The composition and origin
of the crust and crust/mantle boundary beneath the Deccan are discussed with respect to the influence of Deccan magmatic episode. 相似文献
5.
Matteo Masotta Carmela Freda Mario Gaeta 《Contributions to Mineralogy and Petrology》2012,163(1):49-65
Crystal-poor, differentiated magmas are commonly erupted from shallow, thermally zoned magma chambers. In order to constrain
the origin of these magmas, we have experimentally investigated crystallization, differentiation and crystal-melt separation
in presence of a thermal gradient. Experiments have been designed taking advantage of the innate temperature gradient of the
piston cylinder apparatus and carried out on a phonolitic system at 0.3 GPa and temperature ranging from 1,050 to 800°C. Crystallization
degree and melt composition in experimental products vary as a function of the temperature gradient. In particular, melt composition
differentiates from tephri-phonolite (starting material) to phonolite moving from the hotter, glassy zone (T ≤ 1,050°C) towards the cooler, heterogeneously crystallized zone (T ≤ 900°C) of the charge. The heterogeneously crystallized zone is made up of: (1) a crystal-rich, mushy region (crystallinity
>30 vol%), (2) a rigid crystal framework (crystallinity ≤80 vol%) and (3) glassy belts of phonolitic glass at the top. Thermal
gradient experiments picture crystallization, differentiation and crystal-melt separation processes occurring in a thermally
zoned environment and reveal that relatively large volumes of crystal-poor melt (glassy belts) can originate as a consequence
of the instability and collapse of the rigid crystal framework. Analogously, in thermally zoned magma chambers, the development
and collapse of a solidification front may represent the controlling mechanism originating large volumes of crystal-poor,
differentiated magmas. 相似文献
6.
Glass Mountain, California, consists of >50 km3 of high-silica rhyolite lavas and associated pyroclastic deposits that erupted over a period of >1 my preceding explosive
eruption of the Bishop Tuff and formation of the Long Valley caldera at 0.73 Ma. These “minimum-melt” rhyolites yield Fe-Ti-oxide
temperatures of 695–718°C and contain sparse phenocrysts of plagioclase+quartz+magnetite+apatite±sanidine, biotite, ilmenite,
allanite, and zircon. Incompatible trace elements show similar or larger ranges within the Glass Mountain suite than within
the Bishop Tuff, despite a much smaller range of major-element concentrations, largely due to variability among the older
lavas (erupted between 2.1 and 1.2 Ma). Ratios of the most incompatible elements have larger ranges in the older lavas than
in the younger lavas (1.2–0.79 Ma), and concentrations of incompatible elements span wide ranges at nearly constant Ce/Yb,
suggesting that the highest concentrations of these elements are not the result of extensive fractional crystallization alone;
rather, they are inherited from parental magmas with a larger proportion of crustal partial melt. Evidence for the nature
of this crustal component comes from the presence of scarce, tiny xenocrysts derived from granitic and greenschist-grade metamorphic
rocks. The wider range of chemical and isotopic compositions in the older lavas, the larger range in phenocryst modes, the
eruption of magmas with different compositions at nearly the same time in different parts of the field, and the smaller volume
of individual lavas suggest either that more than one magma body was tapped during eruption of the older lavas or that a single
chamber tapped by all lavas was small enough that the composition of its upper reaches easily affected by new additions of
crustal melts. We interpret the relative chemical, mineralogical, and isotopic homogeneity of the younger Glass Mountain lavas
as reflecting eruptions from a large, integrated magma chamber. The small number of cruptions between 1.4 and 1.2 ma may have
allowed time for a large magma body to coalesce, and, as the chamber grew, its upper reaches became less affected by new inputs
of crustal melts, so that trace-element trends in magmas erupted after 1.2 Ma are largely controlled by fractional crystallization.
The extremely low Sr concentrations of Glass Mountain lavas imply extensive crystallization in chambers at least hundreds
of cubic kilometers in volume. The close similarity in Sr, Nd, and Pb isotopic ratios between the younger Glass Mountain lavas
and unaltered Bishop Tuff indicates that they tapped the same body of magma, which had become isotopically homogenous by 1.2
Ma but continued to differentiate after that time. From 1.2 to 0.79 Ma, volumetric eruptive rates may have exceeded rates
of differentiation, as younger Glass Mountain lavas become slightly less evolved with time. Early-erupted Bishop Tuff is more
evolved than the youngest of the Glass Mountain lavas and is characterized by slightly different trace element ratios. This
suggests that although magma had been present for 0.5 my, the composiional gradient exhibited by the Bishop Tuff had not been
a long-term, steady-state condition in the Long Valley magma chamber, but developed at least in part during the 0.06-my hiatus
between extrusion of the last Glass Mountain lava and the climactic eruption. 相似文献
7.
Yang Liu Alfred T. Anderson Colin J. N. Wilson Andrew M. Davis Ian M. Steele 《Contributions to Mineralogy and Petrology》2006,151(1):71-87
Large pyroclastic rhyolites are snapshots of evolving magma bodies, and preserved in their eruptive pyroclasts is a record
of evolution up to the time of eruption. Here we focus on the conditions and processes in the Oruanui magma that erupted at
26.5 ka from Taupo Volcano, New Zealand. The 530 km3 (void-free) of material erupted in the Oruanui event is comparable in size to the Bishop Tuff in California, but differs
in that rhyolitic pumice and glass compositions, although variable, did not change systematically with eruption order. We
measured the concentrations of H2O, CO2 and major and trace elements in zoned phenocrysts and melt inclusions from individual pumice clasts covering the range from
early to late erupted units. We also used cathodoluminescence imaging to infer growth histories of quartz phenocrysts. For
quartz-hosted inclusions, we studied both fully enclosed melt inclusions and reentrants (connecting to host melt through a
small opening). The textures and compositions of inclusions and phenocrysts reflect complex pre-eruptive processes of incomplete
assimilation/partial melting, crystallization differentiation, magma mixing and gas saturation. ‘Restitic’ quartz occurs in
seven of eight pumice clasts studied. Variations in dissolved H2O and CO2 in quartz-hosted melt inclusions reflect gas saturation in the Oruanui magma and crystallization depths of ∼3.5–7 km. Based
on variations of dissolved H2O and CO2 in reentrants, the amount of exsolved gas at the beginning of eruption increased with depth, corresponding to decreasing
density with depth. Pre-eruptive mixing of magma with varying gas content implies variations in magma bulk density that would
have driven convective mixing.
Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users. 相似文献
8.
Summary Mineral compositions in leucite-bearing and leucite-free rocks from Vico volcano are reported. FeO/MgO partitioning (Kdol/liq) between olivine and latite (0.14–0.22), and between olivine and trachyte (0.06–0.10) indicates a lack of equilibrium between
mineral and host rock. This suggests that mingling and/or mixing between magmas was a leading process during magmatic differentiation.
In addition, a phono-tephrite olivine population with high (0.84) and equilibrium (0.23–0.29) Kdol/liq values has been produced by the interaction of differently evolved magmas. Zoning in clinopyroxene and plagioclase from these
rocks recorded the same processes. In addition, resorbed quartz xenocrysts with coronas of clinopyroxene microlites indicate
that digestion of crustal rocks occurred during the residence of magma in a shallow level reservoir. Increasing Fe coupled
with decreasing Ca in diopside crystals from some phonolites, together with the petrographic and trace element data, indicate
that polybaric fractional crystallisation also may be involved in the genesis of magmas of the second period of Vico activity.
Leucite-free trachybasalts erupted in a late stage contain highly forsteritic olivine phenocrysts (forsterite 84–88 mol.%)
in-equilibrium (Kdol/liq = 0.24–0.35) with the host rock, which indicate that they did not suffer chemical modification at low pressure.
Received November 28, 2000; revised version accepted September 27, 2001 相似文献
9.
Nicole A. Stroncik Andreas Klügel Thor H. Hansteen 《Contributions to Mineralogy and Petrology》2009,157(5):593-607
A thermobarometric and petrologic study of basanites erupted from young volcanic cones along the submarine portions of the
three El Hierro rift zones (NE-Rift, NW-Rift and S-Ridge) has been performed to reconstruct magma plumbing and storage beneath
the island. Mineral-melt thermobarometry applied to naturally quenched glass and clinopyroxene rims yields pressures ranging
from 350 to 1070 MPa with about 80% of the calculated pressures being in the range of 600–800 MPa. This corresponds to a depth
range of 19–26 km, implying that the main level of final crystal fractionation is within the uppermost mantle. No systematic
dependence between sample locality and fractionation pressures could be observed. Olivine and clinopyroxene crystals in the
rocks are complexly zoned and have, on an inter-sample as well as on an intra-sample scale, highly variable core and rim compositions.
This can best be explained by mixing of multiply saturated (olivine, magnetite, clinopyroxene, ilmenite), moderately evolved
magmas with more mafic magmas being either only saturated with olivine + spinel or with olivine + spinel + clinopyroxene.
The inter-sample differences indicate derivation from small, isolated magma chambers which have undergone distinct fractionation
and mixing histories. This is in contrast to oceanic intraplate volcanoes situated on plumes with high melt supply rates,
e.g. Kilauea Volcano (Hawaii), where magma is mainly transported through a central conduit system and stored in a shallow
magma chamber prior to injection into the rift zones. The plumbing system beneath El Hierro rather resembles the magma storage
systems beneath, e.g. Madeira or La Palma, indicating that small, intermittent magma chambers might be a common feature of
oceanic islands fed by plumes with relatively low fluxes, which results in only limited and periodic magma supply. 相似文献
10.
J. Roberge P. J. Wallace A. J. R. Kent 《Contributions to Mineralogy and Petrology》2013,165(2):237-257
To investigate the origin of compositional zonation in the Bishop Tuff magma body, we have analyzed trace elements in the matrix glass of pumice clasts and in quartz-hosted melt inclusions. Our results show contrasting patterns for quartz in different parts of the Bishop Tuff. In all samples from the early part of the eruption, trace element compositions of matrix glasses are similar to but slightly more evolved than quartz-hosted melt inclusions. This indicates a cogenetic relationship between quartz crystals and their surrounding matrix glass, consistent with in situ crystallization. The range of incompatible element concentrations in melt inclusions and matrix glass from single pumice clasts requires 16–20 wt% in situ crystallization. This is greater than the actual crystal content of the pumices (<15 % crystals). In contrast to the pattern for the early pumices, pyroclastic flow samples from the middle part of the eruption show contrasting trends: In some clasts, the matrix is more evolved than the inclusions, whereas in other clasts, the matrix is less evolved. In the late Bishop Tuff, all crystal-rich samples have matrix glasses that are less evolved than the melt inclusions. Trace element abundances indicate that the cores of quartz in the late Bishop Tuff crystallized from more differentiated rhyolitic magma that was similar in many ways, yet distinct from the early-erupted Bishop Tuff. Our results are compatible with a model of secular incremental zoning (Hildreth and Wilson in Compositional zoning of the Bishop Tuff. J Petrol 48(5):951–999, 2007), in which melt batches from underlying crystal mush rise to various levels in a growing magma body according to their buoyancy. Early- and middle-erupted quartz crystallized from highly evolved rhyolitic melt, but then some parts of the middle-erupted magma were invaded by less differentiated rhyolite such that the matrix melt at the time of eruption was less evolved than the melt inclusions. A similar process occurred but to a greater extent in magma that erupted to form the late Bishop Tuff. In addition, there was a final, major magma mixing event in the late magma that formed Ti-rich rims on quartz and Ba-rich rims on sanidine, trapped less evolved rhyolitic melt inclusions, and resulted in dark and swirly crystal-poor pumice that is a rare type throughout much of the Bishop Tuff. 相似文献
11.
Fidel Costa Laurence A. Coogan Sumit Chakraborty 《Contributions to Mineralogy and Petrology》2010,159(3):371-387
We have studied the chemical zoning of plagioclase phenocrysts from the slow-spreading Mid-Atlantic Ridge and the intermediate-spreading
rate Costa Rica Rift to obtain the time scales of magmatic processes beneath these ridges. The anorthite content, Mg, and
Sr in plagioclase phenocrysts from the Mid-Atlantic Ridge can be interpreted as recording initial crystallisation from a primitive
magma (~11 wt% MgO) in an open system. This was followed by crystal accumulation in a mush zone and later entrainment of crystals
into the erupted magma. The initial magma crystallised plagioclase more anorthitic than those in equilibrium with any erupted
basalt. Evidence that the crystals accumulated in a mush zone comes from both: (1) plagioclase rims that were in equilibrium
with a Sr-poor melt requiring extreme differentiation; and (2) different crystals found in the same thin section having different
histories. Diffusion modelling shows that crystal residence times in the mush were <140 years, whereas the interval between
mush disaggregation and eruption was ≤1.5 years. Zoning of anorthite content and Mg in plagioclase phenocrysts from the Costa
Rica Rift show that they partially or completely equilibrated with a MgO-rich melt (>11 wt%). Partial equilibration in some
crystals can be modelled as starting <1 year prior to eruption but for others longer times are required for complete equilibration.
This variety of times is most readily explained if the mixing occurred in a mush zone. None of the plagioclase phenocrysts
from the Costa Rica Rift that we studied have Mg contents in equilibrium with their host basalt even at their rims, requiring
mixing into a much more evolved magma within days of eruption. In combination these observations suggest that at both intermediate-
and slow-spreading ridges: (1) the chemical environment to which crystals are exposed changes on annual to decadal time scales;
(2) plagioclase crystals record the existence of melts unlike those erupted; and (3) disaggregation of crystal mush zones
appears to precede eruption, providing an efficient mechanism by which evolved interstitial melt can be mixed into erupted
basalts. 相似文献
12.
DANYUSHEVSKY LEONID V.; SOKOLOV SERGUEI; FALLOON TREVOR J. 《Journal of Petrology》2002,43(9):1651-1671
A technique is described for determining the cooling historyof olivine phenocrysts. The technique is based on the analysisof the diffusive re-equilibration of melt inclusions trappedby olivine phenocrysts during crystallization. The mechanismof re-equilibration involves diffusion of Fe from and Mg intothe initial volume of the inclusion. The technique applies toa single crystal, and thus the cooling history of differentphenocrysts in a single erupted magma can be established. Weshow that melt inclusions in high-Fo olivine phenocrysts frommantle-derived magmas are typically partially re-equilibratedwith their hosts at temperatures below trapping. Our analysisdemonstrates that at a reasonable combination of factors suchas (1) cooling interval before eruption (<350°C), (2)eruption temperatures (>1000°C), and (3) inclusion size(<70 µm in radius), partial re-equilibration of upto 85% occurs within 35 months, corresponding to coolingrates faster than 12°/day. Short residence timesof high-Fo phenocrysts suggest that if eruption does not happenwithin a few months after a primitive magma begins cooling andcrystallization, olivines that crystallize from it are unlikelyto be erupted as phenocrysts. This can be explained by efficientseparation of olivine crystals from the melt, and their rapidincorporation into the cumulate layer of the chamber. Theseresults also suggest that in most cases erupted high-Fo olivinephenocrysts retain their original composition, and thus compositionsof melt inclusions in erupted high-Fo olivine phenocrysts donot suffer changes that cannot be reversed. Short residencetimes also imply that large unzoned cores of high-Fo phenocrystscannot reflect diffusive re-equilibration of originally zonedphenocrysts. The unzoned cores are a result of fast efficientaccumulation of olivines from the crystallizing magma, i.e.olivines are separated from the magma faster than melt changesits composition. Thus, the main source of high-Fo crystals inthe erupted magmas is the cumulate layers of the magmatic system.In other words, olivine-phyric rocks represent mixtures of anevolved transporting magma (which forms the groundmass of therock) with crystals that were formed during crystallizationof more primitive melt(s). Unlike high-Fo olivine phenocrysts,the evolved magma may reside in the magmatic system for a longtime. This reconciles long magma residence times estimated fromthe compositions of rocks with short residence times of high-Foolivine phenocrysts. KEY WORDS: melt inclusions; olivine; picrites; residence time; diffusion 相似文献
13.
Evolution of Bishop Tuff Rhyolitic Magma Based on Melt and Magnetite Inclusions and Zoned Phenocrysts 总被引:15,自引:7,他引:8
The evolution of large bodies of silicic magma is an importantaspect of planetary differentiation. Melt and mineral inclusionsin phenocrysts and zoned phenocrysts can help reveal the processesof differentiation such as magma mixing and crystal settling,because they record a history of changing environmental conditions.Similar major element compositions and unusually low concentrationsof compatible elements (e.g. 0·45–4·6 ppmBa) in early-erupted melt inclusions, matrix glasses and bulkpumice from the Bishop Tuff, California, USA, suggest eutectoidfractional crystallization. On the other hand, late-eruptedsanidine phenocrysts have rims rich in Ba, and late-eruptedquartz phenocrysts have CO2-rich melt inclusions closest tocrystal rims. Both features are the reverse of in situ crystallizationdifferentiation, and they might be explained by magma mixingor crystal sinking. Log(Ba/Rb) correlates linearly with log(Sr/Rb)in melt inclusions, and this is inconsistent with magma mixing.Melt inclusion gas-saturation pressure increases with CO2 fromphenocryst core to rim and suggests crystal sinking. Some inclusionsof magnetite in late-erupted quartz are similar to early-eruptedmagnetite phenocrysts, and this too is consistent with crystalsinking. We argue that some large phenocrysts of late-eruptedquartz and sanidine continued to crystallize as they sank severalkilometers through progressively less differentiated melts.Probable diffusive modification of Sr in sanidine phenocrystsand the duration of crystal sinking are consistent with an evolutionaryinterval of some 100 ky or more. Crystal sinking enhanced thedegree of differentiation of the early-erupted magma and pointsto the importance of H2O (to diminish viscosity and enhancethe rate of crystal sinking) in the evolution of silicic magmas. KEY WORDS: crystal settling; differentiation; melt inclusions; rhyolite; trace elements 相似文献
14.
Olivier Bachmann Paul J. Wallace Julie Bourquin 《Contributions to Mineralogy and Petrology》2010,159(2):187-202
The >60 km3 rhyolitic Kos Plateau Tuff provides an exceptional probe into the behavior of volatile components in highly evolved arc magmas:
it is crystal-rich (30–40 vol% crystals), was rapidly quenched by the explosive eruptive process, and contains abundant homogeneous
melt inclusions in large quartz crystals. Several methods for measuring major, trace and volatile element concentrations (SIMS,
FTIR, Raman spectroscopy, electron microprobe, LA–ICPMS) were applied to these melt inclusions. We found a ~2 wt% range of
H2O contents (4.5–6.5 wt% H2O, measured independently by SIMS, FTIR, and Raman spectroscopy) and relatively low CO2 concentrations (15–140 ppm measured by FTIR, with most analyses <100 ppm). No obvious correlations between H2O, CO2, major and trace elements are observed. These observations require a complex, protracted magma evolution in the upper crust
that included: (1) vapor-saturated crystallization in a chamber located between 1.5 and 2.5 kb pressure, (2) closed-system
degassing (with up to 10 vol% exsolved gas) as melts percolated upwards through a vertically extensive mush zone (2–4 km thick),
and (3) periodic gas fluxing from subjacent, more mafic and more CO2-rich magma, which is preserved as andesite bands in pumices. These processes can account for the range of observed H2O and CO2 values and the lack of correlation between volatiles and trace elements in the melt inclusions. 相似文献
15.
Buoyancy controls the ability of magma to rise, its ascent rate and the style of the eruptions. Geophysical, geological and petrological data have been integrated to evaluate the buoyancy of magmas at Mt Etna. The density difference between host rocks and magmas is mainly related to the amount of H2O dissolved in the magma and to the bubble‐liquid separation processes. In the depth interval 22–2 km b.s.l. highly hydrated (H2O ~ 3%) basaltic magmas or mixtures of bubbles + liquid have positive buoyancy and rise rapidly. Conversely, bubble‐depleted liquids, with an intermediate H2O content (~ 1.5%), having neutral buoyancy, will spread out and form magmatic reservoirs at different depths until cooling/crystallization further modify composition and density. These different processes account for the magma compositions, location of magmatic reservoirs as determined by geophysical methods, and the complex eruptive cycles (slow effusions, fire fountains and Plinian eruptions) that have been observed in the history of the volcano. 相似文献
16.
Georg F. Zellmer Kenneth H. Rubin Peter Dulski Yoshiyuki Iizuka Steven L. Goldstein Michael R. Perfit 《Contributions to Mineralogy and Petrology》2011,161(1):175-176
Profiles of a total of 23 plagioclase crystals erupted within the 1982–1991 and 1993 flows of the Coaxial segment of the Juan
de Fuca ridge, the 1996 flow of the North Gorda ridge, and from the Western Volcanic Zone of the ultra-slow spreading Gakkel
Ridge, have been studied for variations in major and trace element concentrations. We derive equilibration times for the relatively
rapidly diffusing Sr in mid-ocean ridge basalt (MORB) plagioclase crystals of the order of months to a few years in each case.
All crystals preserve diffusive disequilibria of strontium and barium. Crystal residence times at MORB magmatic temperatures
are thus significantly shorter, of the order of days to a few months at most, precluding prolonged crystal storage in axial
magma chambers and instead pointing to rapid crystal growth (up to ~10−8 cm s−1) and cooling (up to ~1°C h−1) shortly prior to eruption of these samples. Growth of these crystals is therefore inferred to occur almost entirely within
oceanic layer 2 during dike injection. Crystals that grew at lower crustal levels or earlier in the differentiation sequence
appear to have been excluded from the erupted magmas, as might occur if most of the gabbroic rocks in oceanic layer 3 formed
an interlocking crystal framework, with viscosities that are too high to carry earlier formed crystals with the melt. The
vertical extent of eruptible, crystal-poor melt lenses within the gabbroic zone is constrained to ~1 m or less by considering
the width of local equilibrium growth zones, equilibration times, and crystal settling velocities. This lengthscale is consistent
with field evidence from ophiolites. Finally, crystal aggregates within the Gakkel ridge sample studied here are the result
of synneusis within the propagating dike during melt ascent. 相似文献
17.
18.
Bernard Déruelle Oscar Figueroa A. Eduardo Medina T. Jose Viramonte G. Mario Maragaño C. 《地学学报》1996,8(2):191-199
Lascar Volcano (Atacama, Chile) erupted on 18–20 April 1993. Several sub-Plinian explosions occurred, and some were mushroom-shaped. The highest column rose up to 23 km. Ash clouds crossed South America eastwards. Dacite pumice falls made of blocks and ashes were deposited on the flanks of the volcano as a result of collapsed columns. The pumice contains phenocrysts of plagioclase, enstatite, augite, biotite, magnetite and ilmenite and small crystals of apatite. The 1992 previous andesite dome inside the crater was destroyed. Banded blocks resulting from mingling of the dacitic pumice and andesite from the dome are found in the pumice flow. Both the lava dome and the pumice are representative of the Lascar high-K magma unit. Dacitic pumice is a product of crystal fractionation of the andesitic magma. 相似文献
19.
We use the crystallographic orientations of quartz crystals, as determined with EBSD, to provide new evidence for the formation of clustered quartz crystals during magma crystallization. Vinalhaven is dominated by granite, with minor porphyry that formed when granite remelted during input of coeval basalt. CL zoning suggests that most quartz clusters in granite and porphyry formed by synneusis, the “swimming together” of preformed crystals. In granite, most quartz pairs in clusters have random orientations—only about 10% have parallel or Esterel twin orientations. Porphyry has fewer quartz clusters, and all pairs have approximately parallel or Esterel twin orientations. CL zoning of quartz pairs in porphyry indicates that they attached prior to a major remelting event. Interpretation of the Vinalhaven quartz clusters leads us to propose that oriented synneusis occurs during crystal accumulation on a magma chamber floor. During hindered settling, some quartz crystals should have come into contact along their dipyramidal faces. Once in contact, continued settling and loss of interstitial melt may have rotated some quartz crystals such that lattices on their dipyramidal faces matched—producing parallel and Esterel twin orientations and creating strong bonds between pairs. Only a small proportion of pairs with matched dipyramidal faces formed in the granite and, during rejuvenation to produce porphyry, only these oriented pairs survived. Hence, the presence of oriented synneusis in a plutonic rock may demonstrate a history of crystal accumulation. 相似文献
20.
Evolution of the magma chamber at Mount Mazama involved repeated recharge by two types of andesite (high-Sr and low-Sr), crystal
fractionation, crystal accumulation, assimilation, and magma mixing (Bacon and Druitt 1988). This paper addresses the modal
compositions, textures, mineral chemistry and magmatic temperatures of (i) products of the 6845±50 BP climactic eruption,
(ii) blocks of partially fused granitoid wallrock found in the ejecta, and (iii) preclimactic rhyodacitic lavas leaked from
the chamber in late Pleistocene and early Holocene time. Immediately prior to the climactic eruption the chamber contained
≳ 40 km3 of rhyodacite (10 vol% plag + opx + aug + hb + mt + ilm, ∼880° C) overlying high-Sr andesite and cumulus-crystal mush (28–51
vol% plag + hb ± opx ± aug + mt ± ilm, 880° to ≥950° C), which in turn overlay low-Sr crystal mush (50–66 vol% plag + opx
+ aug ± hb ± ol + mt + ilm, 890° to ≥950† C). Despite the well known compositional gap in the ejecta, no thermal discontinuity
existed in the chamber. Pre-eruptive water contents of pore liquids in most high-Sr and low-Sr mushes were 4–6 wt%, but on
average the high-Sr mushes were slightly richer in water. Although parental magmas of the crystal mushes were andesitic, xenocrysts
of bytownite and Ni-rich magnesian olivine in some scoriae record the one-time injection of basalt into the chamber. Textures
in ol-bearing scoriae preserve evidence for the reactions ol + liq = opx and ol + aug + liq(+ plag?) = hb, which occurred
in andesitic liquids at Mount Mazama. Strontium abundances in plagioclase phenocrysts constrain the petrogenesis of preclimactic
and climactic rhyodacites. Phenocryst cores derived from high-Sr and low-Sr magmas have different Sr contents which can be
resolved by microprobe. Partition coefficients for plagioclase in andesitic to rhyolitic glasses range from 2 to 7, and increase
as glass %SiO2 increases. Evolved Pleistocene rhyodacites (∼30–25,000 BP) and rhyodacites of the Holocene Llao Rock center (7015±45 BP)
contain Sr-poor plagioclase and are derivatives from low-Sr magma. Rhyodacites of the Pleistocene Sharp Peak domes, Holocene
Cleetwood flow (∼6850 BP), and climactic ejecta contain discrete Sr-rich and Sr-poor plagioclase phenocryst populations and
are hybrids produced by mixing low-Sr rhyodacite (containing Sr-poor plag + opx + aug) with a more mafic high-Sr magma (with
Sr-rich plag [ + hb?]). The data reinforce the conclusions of crystal-liquid mixing calculations (Bacon and Druitt 1988),
and suggest some important refinements to the magma chamber model. 相似文献