1,721,180 research outputs found
Magma dynamics in the shallow plumbing system of Mt. Etna as recorded by compositional variations in volcanics of recent summit activity (1995–1999)
No full textWe describe the reactivation and the successive evolution of the shallow plumbing system of Mt. Etna between the end of the largest flank eruption of the last three centuries (1991–1993) and the subterminal eruption from South-East Crater (SEC), which occurred between February and mid-November 1999. Our analysis is based on observations of the volcanic activity and petrological studies of the erupted volcanics. Bulk rock, mineral and glass compositions have been determined for more than 80 samples erupted from the four summit craters between October 1995 and February 1999. These data allow us to recognise significant compositional variations among the products of different craters. In particular, volcanics produced between 1995 and 1999 by Bocca Nuova (BN), Voragine (VOR) and North-East Crater (NEC) show limited compositional variations and are similar to those observed during recent eruptions (e.g., 1991–93). More primitive magmas have been produced during the more vigorous fire fountains episodes. On the contrary, the South-East Crater produced slightly more differentiated volcanics than those of the other summit craters following its reactivation (November 1996) until the end of 1998. Whole rock compositions of products from this crater show low CaO/Al2O3, whereas interstitial glasses have lower MgO and higher alkali contents than those from the other craters. However, since the beginning of 1999, and just before the start of the subterminal eruption from SEC, the volcanics erupted from this crater progressively changed in composition, becoming similar to those of the other craters. This trend indicates that within the conduits of the summit craters, distinct thermal and fluid-dynamical regimes can evolve, controlling the cooling and crystallisation of Etna magmas.Published55-71partially_ope
Petrography, Mineral Composition and Geochemistry of Volcanic and Subvolcanic Rocks of CRP-3, Victoria Land Basin, Antarctica
No full textThe petrography, mineralogy and geochemistry of volcanic and subvolcanic rocks in CRP-3 core have been examined in detail in order to characterise and to compare them with volcanic and subvolcanic rocks cropping out in the Victoria Land area, and to define the clast provenance or to establish possible volcanic activity coeval with deposition.
Clasts with sizes ranging from granule to boulder show geochemical and mineralogical features comparable with those of Ferrar Supergroup rocks. They display a subalkaline affinity and compositions ranging from basalts to dacite.
Three different petrographic groups with distinct textural and grain size features (subophitic, intergranular-intersertal, and glassy-hyalopilitic) are recognised and are related to the emplacement/cooling mechanism.
In the sand to silt fraction, the few glass shards that have been recognised are strongly altered; however chemical analyses show they have subalkalic magmatic affinity. Mineral compositions of the abundant free clinopyroxene grains found in the core, are less affected by alteration processes, and indicate an origin from subalkaline magmas. This excludes the presence, during the deposition of CRP-3 rocks of alkaline volcanic activity comparable with the McMurdo Volcanic Group.
Strong alteration of the magmatic body intruded the Beacon sandstones obliterates the original mineral assemblage. Geochemical investigations confirm that intrusion is part of the Ferrar Large Igneous Provinc
Petrology of some amphibole-bearing volcanics of the pre-ellittico period (102-80 ka) Mt. Etna
Full text linkWe present here petrological and
geochemical data on volcanics cropping out in
southern and northeastern walls of the Valle del
Bove (Mt. Etna), belonging to the Rocche, Serra
Giannicola Grande and Canalone della Montagnola
Units. These units constitute the remnants of several
volcanoes that were active in the time span 102-80
ka, i.e. before the growth of the Ellittico-Mongibello
strato-volcano. Their products, range in composition
from hawaiites to benmoreites. Amphibole
(kaersutite) is present as phenocryst in all the studied
rocks, and commonly shows breakdown coronas
of rhönite ± clinopyroxene and plagioclase formed
during magma ascent. Nevertheless, in mafic rocks,
amphibole occurs as an early liquidus phase enclosed
in a Ca-rich plagioclase (up to An87). We propose that
early cotectic crystallization of amphibole and Caplagioclase
may reflect H2O-rich melts. Variations in
major and trace elements among lavas erupted from
coeval centres, suggest that fractional crystallization
was the principal evolutionary process but at the same
time magmas feeding the various volcanoes belonging
to the Rocche Unit were more heterogeneous with
respect to the younger Units studied here
Mafic and ultra-mafic enclaves from Ustica island lava: inferences on deep magmatic processes
No full textUstica Island, southern Tyrrhenian Sea, is constituted of Quaternary alkaline volcanics. Avariety of enclaves representative of
deep to supra-crustal settings were recently found in a hawaiitic lava flow. Enclaves consist of: (i) Ultramafic meta-cumulates, i.e.
clinopyroxenites and wherlites characterized by variably deformed porphyroclastic to granoblastic textures. (ii) Mafic cumulates,
i.e. gabbros (F amphibole) and troctolites, the first often characterized by frequent amphibole breakdown coronas (olivine+Tiaugite+
plagioclase+magnetite+ilmenite+rhfnite) in response to an H2O decrease during the ascent, while the troctolites
interpreted as meta-cumulates. (iii) Microsyenites, consist of anorthoclase and Fe-clinopyroxene organized in a granular subipidiomorphic
texture. Amphibole is absent in Ustica lavas and is found only in some old, now exposed, sub-intrusive volcanic
bodies. This evidence suggests a late appearance of amphibole on the liquidus, at a high crystal content that inhibits further ascent
of the magma. The importance of the amphibole as a medium pressure liquidus phase in Ustica mafic magmas is in the bearings on
the geochemistry of lavas e.g. in buffering Na and Ti abundances, in trace elements partitioning, etc.
Density measurements pointed out higher values for clinopyroxenites (3160 to 3300 kg/m3) than for gabbros (ca. 2900 kg/
m3). Given the density contrast between enclaves and host lavas (2790 kg/m3) and assuming appropriate rheological models, we
calculated a minimum ascent rate of 0.01 m/s, corresponding to an ascent time in the range of 5–29 days for a depth of
entrapment of 25 km
Petrology of enclaves from Ustica Island (Italy): insights on time scale of magma ascent
No full textIdentifying recycled ash in basaltic eruptions
No full textDeposits of mid-intensity basaltic explosive eruptions are characterized by the coexistence of different types of juvenile clasts, which show a large variability of external properties and texture, reflecting alternatively the effects of primary processes related to magma storage or ascent, or of syn-eruptive modifications occurred during or immediately after their ejection. If fragments fall back within the crater area before being re-ejected during the ensuing activity, they are subject to thermally- and chemically-induced alterations. These ‘recycled’ clasts can be considered as cognate lithic for the eruption/explosion they derive. Their exact identification has consequences for a correct interpretation of eruption dynamics, with important implications for hazard assessment. On ash erupted during selected basaltic eruptions (at Stromboli, Etna, Vesuvius, Gaua-Vanuatu), we have identified a set of characteristics that can be associated with the occurrence of intra-crater recycling processes, based also on the comparison with results of reheating experiments performed on primary juvenile material, at variable temperature and under different redox conditions.Published58513V. Dinamiche e scenari eruttiviJCR Journalrestricte
Petrology and Sr-Nd isotope geochemistry of recent lavas from Mt. Etna: bearing on the volcano feeding system
No full text
Geochemistry of minerals in CRP sediment cores from the Ross Sea, Antarctica
No full textSixty-four volcanic chists, sandstones and tephras between 5.95 and 618.19 meters below sea floor (mbsf) in the Cape Roberts Project cores 2 and 2A cores (CRP-2/2A) were examined for Cenozoic and Mesozoic volcanic components, using optical and Scanning Electron Microscopy. Minerals and glass shards in a selection of samples were analysed by electron microprobe fined with an EDAX detector. Laser-Ablation ICP-Mass-Spectrometry (ICP-MS) was used to determine rare earth elements and 14 additional trace elements in glass shards, pyroxenes and feldspars in order to pin-point the onset of McMurdo Volcanic Group (MVG) activity in the stratigraphic column. Pumices in tephra layers of peralkaline phonolite composition in Unit 7.2 -between 108 and 114 mbsf - were also analysed for trace elements by ICP-MS. This tephra unit is not reworked and its isotopic age (21.44 ± 0.05 Ma) is the age of deposition. The height of the eruptive column responsible for the deposition of the tephra was probably less than 8 km; the source was local, probably within 30 km from the drill site. Phonolite of unit 7.2 of CRP-2/2A has no direct petrogenetic relation with the peralkaline trachyte in the tephra-enriched layer of CRP-1 at 116.55 mbsf.
Volcanic clasts and sand grains (glass shards, aegirine-augite, anorthoclase) related to Cenozoic activity of MVG were observed only starting from Unit 9.8, where they are dated at 24.22 ± 0.06 Ma at c. 280 mbsf. In this unit the lowest- occurring basaltic glass shard is found at 297.54 mbsf. Sampled McMurdo volcanics are generally vesicular and vary in composition from alkali basalt to trachyte and peralkaline phonolite. By contrast, below 320 mbsf, aphyric or slightly-porphyritic volcanic clasts become more abundant but they are all non-vesiculated, pigeconite and ilmenite-bearing basalts and dolerite of tholeiitic affinity. These rocks are considered to be related to lava flows and associated intrusions of Jurassic age (Kirkpatrick basalts and Ferrar dolerite).
As in CRP-1, McMurdo volcanics appear to derive from a variety of lithologics. Besides glaciers, a dominant role of wind transportation from exposed volcanic rocks may be inferred from the contemporary occurrence of glass shards of different compositions at depths above 297.54 mbsf. These data confirm that the onset of magmatic activity in southern Victoria Land is considerably delayed (by about 24 Ma) with respect to northern Victoria Land
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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