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581.
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
Olivier?BachmannEmail author Chad?D.?Deering Janina?S.?Ruprecht Christian?Huber Alexandra?Skopelitis Cedric?Schnyder 《Contributions to Mineralogy and Petrology》2012,163(1):151-166
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
582.
The present paper proposes a new type of two–dimensional turbulence with a deep configuration as an idealization of the dynamics of the global circulations of the Jovian atmospheres under the constraint of the Taylor–Proudman theorem. A major difference of the deep two-dimensional turbulence from a standard shallow version is that the former reverses the sign of the planetary β-effect, leading to an opposite sign for the equatorial jet under the homogenization tendency of the potential vorticity. As a result, a westerly (prograde) equatorial jet is generated, which contrasts with the easterly (retrograde) equatorial jet obtained using the shallow configuration. The results suggest that the zonal flows of Jupiter and Saturn are probably of deep origin, whereas those of Uranus and Neptune are likely to be of shallow origin. 相似文献
583.
This paper describes the application of high‐resolution SPOT data for the structural and thematic mapping of shallow coral reefs. Results are presented as structural and thematic maps of the main reef environment types. Different substrate themes including soft bottom, coral débris, coral rubble, and living coral have been identified. The separation of various living coral themes, as measured by differences in areal coral density, has also been achieved. 相似文献
584.
585.
We address the occurrence of narrow planetary rings and some of their structural properties, in particular when the rings
are shepherded. We consider the problem as Hamiltonian scattering of a large number of non-interacting massless point particles in an effective potential. Using the existence of stable motion
in scattering regions in this set up, we describe a mechanism in phase space for the occurrence of narrow rings and some consequences
in their structure. We illustrate our approach with three examples. We find eccentric narrow rings displaying sharp edges,
variable width and the appearance of distinct ring components (strands) which are spatially organized and entangled (braids).
We discuss the relevance of our approach for narrow planetary rings. 相似文献
586.
The Deep Impact flyby spacecraft includes a 1.05 to 4.8 μm infrared (IR) spectrometer. Although ice was not observed on the surface in the impact region, strong absorptions near 3 μm due to water ice are detected in IR measurements of the ejecta from the impact event. Absorptions from water ice occur throughout the IR dataset beginning three seconds after impact through the end of observations, ∼45 min after impact. Spatially and temporally resolved IR spectra of the ejecta are analyzed in conjunction with laboratory impact experiments. The results imply an internal stratigraphy for Tempel 1 consisting of devolatilized materials transitioning to unaltered components at a depth of approximately one meter. At greater depths, which are thermally isolated from the surface, water ice is present. Up to depths of 10 to 20 m, the maximum depths excavated by the impact, these pristine materials consist of very fine grained (∼1±1 μm) water ice particles, which are free from refractory impurities. 相似文献
587.
Progressive deformation of a zone of magma transfer in a transpressional regime: The Variscan Mérens shear zone (Pyrenees, France) 总被引:1,自引:0,他引:1
The EW-striking Variscan Mérens shear zone (MSZ), located on the southern border of the Aston dome (Pyrenees), corresponds to variously mylonitized gneisses and plutonic rocks that are studied using the Anisotropy of Magnetic Susceptibility (AMS) technique. The plutonic rocks form EW-striking bands with, from south to north, gabbro-diorites, quartz diorites and granodiorites. The MSZ underwent a mylonitic deformation with an intensity progressively increasing from the mafic to the more differentiated rocks. The foliations are EW to NW–SE striking and subvertical. A first set of lineations shows a moderate WNW plunge, with a dextral reverse kinematics. More recent subvertical lineations correspond to an uplift of the northern compartment. To the east, the MSZ was cut by a N120°E-striking late shear band, separating the MSZ from the Quérigut pluton. The different stages of mylonitization relate to Late Variscan dextral transpression. This regime allowed the ascent of magmas along tension gashes in the middle crust. We interpret the MSZ as a zone of magma transfer, which fed a pluton now eroded that was similar to the Quérigut and Millas plutons located to the east. We propose a model of emplacement of these plutons by successive pulses of magmas along en-échelon transfer zones similar to the MSZ. 相似文献
588.
Jacqueline Vander Auwera Michel Bogaerts Olivier Bolle John Longhi 《Contributions to Mineralogy and Petrology》2008,156(6):721-743
Abundant ferroan, metaluminous granitoids (970–950 Ma) emplaced at the end of the Sveconorwegian collisional orogeny (1130–900 Ma)
are dominated by intermediate to silicic compositions with rare mafic facies. Both 73% fractional crystallization of an amphibole-bearing
gabbroic cumulate substracted from the parent mafic composition and 30% non-modal batch melting of an amphibolitic source
equivalent in composition to the mafic facies produce a monzodioritic liquid with appropriate trace element composition. A
better fit is obtained for the partial melting process. Both processes could have occurred simultaneously to produce mafic
cumulates and restites. As there is no evidence for large volumes of dense mafic rocks in the Sveconorwegian upper crust,
these dense mafic rocks were probably produced in the lower crust. Formation of these granitoids, thus, contributed to the
vertical stratification of the Proterozoic continental crust and also to the transfer of water from the lower crust to the
surface. 相似文献
589.
Olivier Jacquat André Villard Ruben Kretzschmar 《Geochimica et cosmochimica acta》2008,72(20):5037-5054
Recent studies demonstrated that Zn-phyllosilicate- and Zn-layered double hydroxide-type (Zn-LDH) precipitates may form in contaminated soils. However, the influence of soil properties and Zn content on the quantity and type of precipitate forming has not been studied in detail so far. In this work, we determined the speciation of Zn in six carbonate-rich surface soils (pH 6.2-7.5) contaminated by aqueous Zn in the runoff from galvanized power line towers (1322-30,090 mg/kg Zn). Based on 12 bulk and 23 micro-focused extended X-ray absorption fine structure (EXAFS) spectra, the number, type and proportion of Zn species were derived using principal component analysis, target testing, and linear combination fitting. Nearly pure Zn-rich phyllosilicate and Zn-LDH were identified at different locations within a single soil horizon, suggesting that the local availabilities of Al and Si controlled the type of precipitate forming. Hydrozincite was identified on the surfaces of limestone particles that were not in direct contact with the soil clay matrix. With increasing Zn loading of the soils, the percentage of precipitated Zn increased from ∼20% to ∼80%, while the precipitate type shifted from Zn-phyllosilicate and/or Zn-LDH at the lowest studied soil Zn contents over predominantly Zn-LDH at intermediate loadings to hydrozincite in extremely contaminated soils. These trends were in agreement with the solubility of Zn in equilibrium with these phases. Sequential extractions showed that large fractions of soil Zn (∼30-80%) as well as of synthetic Zn-kerolite, Zn-LDH, and hydrozincite spiked into uncontaminated soil were readily extracted by 1 M NH4NO3 followed by 1 M NH4-acetate at pH 6.0. Even though the formation of Zn-precipitates allows for the retention of Zn in excess to the adsorption capacity of calcareous soils, the long-term immobilization potential of these precipitates is limited. 相似文献
590.
Origin of Pyroxenite-Peridotite Veined Mantle by Refertilization Reactions: Evidence from the Ronda Peridotite (Southern Spain) 总被引:4,自引:0,他引:4
Bodinier Jean-Louis; Garrido Carlos J.; Chanefo Ingrid; Bruguier Olivier; Gervilla Fernando 《Journal of Petrology》2008,49(5):999-1025
The Ronda orogenic peridotite (southern Spain) contains a varietyof pyroxene-rich rocks ranging from high-pressure garnet granulitesand pyroxenites to low-pressure plagioclase–spinel websterites.The asthenospherized part of the Ronda peridotitecontains abundant layered websterites (group Cpyroxenites), without significant deformation, that occur asswarms of layers showing gradual modal transitions towards theirhost peridotites. Previous studies have suggested that theselayered pyroxenites formed by the replacement of refractoryspinel peridotites. Here, we present a major- and trace-element,and numerical modelling study of a layered outcrop of groupC pyroxenite near the locality of Tolox aimed at constrainingthe origin of these pyroxenites after host peridotites by pervasivepyroxene-producing, refertilization melt–rock reactions.Mg-number [= Mg/(Mg + Fe) cationic ratio] numerical modellingshows that decreasing Mg-number with increasing pyroxene proportion,characteristic of Ronda group C pyroxenites, can be accountedfor by a melt-consuming reaction resulting in the formationof mildly evolved, relatively low Mg-number melts (0·65)provided that the melt fraction during reaction and the time-integratedmelt/rock ratio are high enough (>0·1 and > 1,respectively) to balance Mg–Fe buffering by peridotiteminerals. This implies strong melt focusing caused by melt channellingin high-porosity domains resulting from compaction processesin a partial melted lithospheric domain below a solidus isothermrepresented by the Ronda peridotite recrystallization front.The chondrite-normalized rare earth element (REE) patterns ofgroup C whole-rocks and clinopyroxenes are convex-upward. Numericalmodeling of REE variations in clinopyroxene produced by a pyroxene-forming,melt-consuming reaction results in curved trajectories in the(Ce/Nd)N vs (Sm/Yb)N diagram (where N indicates chondrite normalized).Based on (Ce/Nd)N values, two transient, enriched domains betweenthe light REE (LREE)-depleted composition of the initial peridotiteand that of the infiltrated melt may be distinguished in thereaction column: (1) a lower domain characterized by convex-upwardREE patterns similar to those observed in Ronda group C pyroxenite–peridotite;(2) an upper domain characterized by melts with strongly LREE-enrichedcompositions. The latter are probably volatile-rich, small-volumemelt fractions residual after the refertilization reactionsthat generated group C pyroxenites, which migrated throughoutthe massif—including the unmelted lithospheric spinel-tectonitedomain. The Ronda mantle domains affected by pyroxenite- anddunite- or harzburgite-forming reactions (the layeredgranular subdomain and plagioclase-tectonitedomain) are on average more fertile than the residual, coarsegranular subdomain at the recrystallization front. Thisindicates that refertilization traces the moving boundariesof receding cooling of a thinned and partially melted subcontinentallithosphere. This refertilization process may be widespreadduring transient thinning and melting of depleted subcontinentallithospheric mantle above upwelling asthenospheric mantle. KEY WORDS: subcontinental mantle; refertilization; pyroxenite; peridotite; websterite; melt–rock reaction; plagioclase; trace elements 相似文献