1,721,102 research outputs found
Residence time analysis of active volcanic systems:Rb-Sr isotope study of Ischia and Pantelleria
No full textNumerous active and potentially high-risk volcanoes do occur in the Italian peninsula and therefore understanding their dynamics is crucial for volcanic hazard assessment. Here we present a study on the active volcanic systems of Ischia and Pantelleria, representing two high-silica volcanoes emplaced in subduction related and within-plate geodynamic settings, respectively.
Ischia has erupted in a subduction-related setting and it is characterised by a continuous transition from trachy-basalt to phonolite. The geochemical and radiogenic isotope data of its volcanic products demonstrate a two-step evolutive process: the first step, controlled by fractional crystallization plus crustal assimilation (AFC), drives magma composition from trachy-basalt to moderately differentiated trachyte; the second step, controlled only by fractional crystallisation (FC), drives the magma composition to the more differentiated products (phonolite) determining very low Sr (a few ppm) and high Rb (>500 ppm) contents due to extreme plagioclase and K-feldspar fractionation.
Pantelleria is located in a witin-plate setting and it is characterised by a bimodal magmatism of alkali-balsalt and differentiated products passing from trachyte to peralkaline ryolite (i.e. Pantellerite), through FC processes. Pantelleritic rocks also show extremely low Sr and high Rb contents.
The active volcanic systems of Ischia and Pantelleria, although belonging to different geodynamic settings, are characterized by the occurrence of strongly differentiated products with high Rb/Sr and anomalously high Sr isotope compositions that cannot be justified by the assimilation of crustal material.
This characteristic could be explained by 87Sr in-growth in long-lived magma chambers. To explore this hypothesis we carefully screened a number of evolved samples from Ischia and Pantelleria, on which we separated their rock-forming minerals (sanidine and clinopyroxene) and glass to determine Rb and Sr content by isotope dilution, along with Sr isotope composition. The extremely low diffusion coefficients of Sr in feldspar and clinopyroxene makes them perfect candidates to estimate the timing of crystallisation and, by inference, the magma residence time. The calculated crystallization times are here discussed in terms of the chemical and physical characteristic of the magmas
Potassic and ultrapotassic magmatism in the circum-Tyrrhenian region: the role of sediment recycling at destructive plate margin
No full textResidence Time Analysis of the Active Volcanic System of Ischia, Italy.
Full text linkThe Italian peninsula hosts numerous active and potentially high-risk volcanoes, therefore understanding their dynamics is fundamental for volcanic hazard assessment. Here we present a study on the active volcanic system of Ischia, whose products have a potassic affinity with a subduction-related signature.
Volcanic rocks are characterised by a continuous transition from trachy-basalt to trachyte and minor phonolite. Geochemical and radiogenic isotope (Sr, Nd, Pb, Hf) data are consistent with a closed-system, two-step crystal fractionation process: the first step, drives magma composition from trachy-basalt to moderately differentiated trachyte; the second step drives the magma composition to the more differentiated products (trachyte and minor phonolite) determining very low Sr (a few ppm) and high Rb (>500 ppm) contents due to extreme plagioclase and K-feldspar fractionation.
A number of these highly differentiated trachytes have, along with high Rb/Sr, anomalously high Sr isotope composition that cannot be justified by assimilation of crustal material.
This characteristic could be explained by 87Sr in-growth in a long-lived magma chamber. To explore this hypothesis we carefully screened a number of evolved samples on which we analysed feldspar-glass pairs through Rb-Sr isotope dilution method. The extremely low diffusion coefficients of Sr in feldspars makes them perfect candidates to estimate the timing of crystallisation and, by inference, the magma residence time. The calculated crystallisation times are here discussed in terms of magma chamber dynamics
Mo and stable U isotopes as tracers for subduction components in the Quaternary West-Mediterrean potassic and ultrapotassic magmatism
No full textThe central-western Mediterranean is one of the most important areas on Earth for studying subduction-related potassic and ultrapotassic magmatism. In a very restricted area, leucite-free (lamproite) and leucite-bearing (kamafugite, leucitite, and plagioleucitite) ultrapotassic rocks have been emplaced and are associated with shoshonites and high-K calc-alkaline volcanic rocks.
Despite their alkaline characteristics, the least evolved Italian ultrapotassic rocks associated with destructive plate margins invariably show a strong depletion of Nb and Ta along with the highest levels of incompatible trace elements ever seen in any volcanic arc. These characteristics are thought to be derived through the recycling of sediments via subduction within the mantle wedge, and their extreme trace element enrichments make them unique for understanding the roles of different subduction-related metasomatic agents (e.g. silico-clastic vs carbonate). In fact, the variable compositions of the sedimentary materials, subducted along the Adriatic slab and transported into the overlying mantle forming a vein network, could explain the distinct geochemical signature of each Italian magmatic region (Avanzinelli et al., 2009).
We propose to investigate this issue considering two stable isotopic systematics that are perceptive to redox condition-related isotopic fractionation. We measured Mo and stable U isotopes with the high-resolution MC-ICP-MS (Neptune), using a double-spike technique, on selected volcanic rocks from three Italian magmatic provinces and representative samples of subducting sediments.
Molybdenum has seven stable isotopes, which have been shown to fractionate during the incorporation into oceanic sediments. Under oxic conditions, Mo adsorbs to particles into the sediment, particularly when Fe-Mn oxides are present, producing lighter isotopic composition (δ98Mo/95Mo), whilst is quantitatively removed in anoxic conditions, leaving sediments with a heavier isotopic signature. The recently observed variability in natural 238U/235U values (different from the widely used “consensus” value of 137.88) due to isotopic fractionation during the redox transition between the U(IV) and U(VI) oxidation states, produces as well as Mo isotopes, a lighter isotopic composition (δ238U/235U) in oxic sediments compared to a heavier composition in anoxic sediments. We interpret those results in order to recognize the U and Mo isotopic signature of sediments, with different lithology and chemical composition, recorded into the selected volcanic rocks, and to set new constraints on the metasomatic agents responsible for the transition from silica oversaturated lamproite-like to strongly silica undersaturated HKS magmas
Shift from lamproite-like to leucititic rocks: Sr-Nd-Pb isotope data from the Monte Cimino volcanic complex vs. the Vico stratovolcano, Central Italy
No full textIn the Italian peninsula leucite-free and –bearing ultrapotassic rocks occur intimately
associated but well separated in time. Silica-saturated ultrapotassic and associated shoshonitic
magmas erupted during the Pliocene to Lower Pleistocene. Silica under-saturated leucite-bearing
ultrapotassic rocks, mainly leucitites, were emplaced sometime later, during the Middle-Upper
Pleistocene. The transition from leucite-free to – bearing rocks is diachronous and in some cases is
complicated by the occurrence of crustal-derived magmas coeval with early leucite-free magma.
The Monte Cimino-Vico area is a key locality for studying the transition from leucite-free to –
bearing ultrapotassic rocks. There a volcanic complex is overlain by some 500 ka younger leucitebearing
rocks of Vico stratovolcano. Most of the potassic and ultrapotassic rocks of the Monte
Cimino volcanic complex are characterised by high Mg-# (66.4-77.8), and compatible element
abundances in spite of their high silica contents (52.9-58.0 wt.%). Ultrapotassic leucite-free rocks
transitional to lamproites are olivine-bearing orthopyroxene- and plagioclase-free latites and Ktrachyandesite
and they are confined in the final mafic activity of the Monte Cimino volcanic
complex. Two-pyroxene parageneses are observed in the early Monte Cimino volcanic complex
giving equilibration temperature as high as 1050°C suggesting a sub-crustal derivation of their
parental magmas. A two-pyroxene high-K calc-alkaline component is then inferred for the genesisof the early Monte Cimino activity (e.g. dome complex and silicic lava flows and ignimbrites),
which was modelled through crystal-fractionation from a high-K basaltic andesitic magma plus
mixing with crustal anatectic rhyolitic magma.
Final mafic lavas fill the isotopic and chemical gap between shoshonite-like and lamproite-like
rocks, and each lava flow could be considered as a mantle-derived term from a heterogeneous subcontinental
lithospheric mantle. Final mafic lavas are high-silica and -MgO ultrapotassic rocks due
to crystallisation of primitive magmas generated by partial melting of a vein metasomatised network
within a lithospheric upper mantle source, at low pressure and high PH2O.
The shift to the younger Vico volcano, in which leucite-bearing magmas prevail, is thought to
be due to the arrival in the mantle source of newly formed metasomatic agent from the undergoing
slab characterised by the increasing of a sedimentary carbonate recycled component. This new
component appears in the Middle Pleistocene and it is responsible for the composition of the
magmatism of the Roman region. The recycling of carbonate-bearing pelitic sediments within the
mantle wedge produces a carbonate-rich metasomatic agent. The latter is responsible for
stabilisation of phlogopite-bearing wehrlitic veins within the mantle wedge. Partial melting of this
newly formed vein network under high XCO2 generated silica under-saturated, and then leucitebearing,
ultrapotassic primary melts. A further shift is observed among the Vico post-caldera
magmas, in which the appearance of a deep asthenospheric component is argued to be channelled
late through slab-tear
Metasomatism and melting in subduction-related volcanics: U-Th-Pa constraints from Vesuvius
No full textNon-Traditional Isotope Tracers (238U/235U and 98Mo/95Mo) of Subduction Processes in the Central-Mediterranean Magmatism.
No full textNon-traditional isotope systems such as Mo and 238U/235U are notably fractionated by redox-related processes on the Earth’s surface. Such distinctive signatures may be carried into the mantle wedge via subduction and provide valuable tracers of components involved in magma genesis. Thus we measured Mo isotopes and 238U/235U on a suite of central-western Mediterranean calc-alkaline to ultra-potassic rocks (both silica-oversaturated and under-saturated). The studied rocks are associated with destructive plate margins, showing strong depletions in Nb and Ta, highly radiogenic Sr isotopes and most notably extreme enrichment in incompatible trace elements with respect to other volcanic arcs. These features have been long related to recycling of sedimentary material of different compositions into their mantle sources, making these rocks particularly suitable to investigate the role and nature of recycled sediments in subduction related magmatism. The data show an extremely wide spread of 98Mo/95Mo values, especially for the silica under-saturated products, that is significantly larger than any volcanic rocks suites reported so far. Smaller variations have been measured for 238U/235U.
We discuss the isotope composition of the studied volcanic rocks and possible sedimentary end-members with the aim of constraining the lithology of the recycled sediments as well the mechanism of element transport from the slab to the mantle (i.e. fluids vs. melts)
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
Quartz-bearing rhyolitic melts in the Earth’s mantle
No full textThe occurrence of rhyolite melts in the mantle has been predicted by high
pressure-high temperature experiments but never observed in nature. Here we
report natural quartz-bearing rhyolitic melt inclusions and interstitial glass
within peridotite xenoliths. The oxygen isotope composition of quartz crystals
shows the unequivocal continental crustal derivation of these melts, which
approximate the minimum composition in the quartz-albite-orthoclase sys tem. Thermodynamic modelling suggests rhyolite was originated from partial
melting of near-anhydrous garnet-bearing metapelites at temperatures
~1000 °C and interacted with peridotite at pressure ~1 GPa. Reaction of rhyolite
with olivine converted lherzolite rocks into orthopyroxene-domains and
orthopyroxene + plagioclase veins. The recognition of rhyolitic melts in the
mantle provides direct evidence for element cycling through earth’s reser voirs, accommodated by dehydration and melting of crustal material, brought
into the mantle by subduction, chemically modifying the mantle source, and
ultimately returning to surface by arc magmatism
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