1,721,013 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
Residence 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
Non-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 compo
sition 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)
Constraining slab recycling under Vesuvius volcano from combined U-series and non-traditional stable isotope (Mo, 238U/235U) data
No full textThe fate of deeply subducted oceanic crust and overlying
sediments is of great importance for its role in the generation
of magmas in subduction-relatedgeodynamic settings. Italian
volcanoes, and Vesuvius in particular, are good laboratories to
investigate these processes due to their strong enrichment in K
and incompatible trace element that requires a significant
amount of sediment material recycled into the mantle.
Volcanic rocks from Vesuvius display ubiquitous 238U excesses
(up to 27%), a feature that is unusual in such enriched
subduction-related magmas. In addition they have among the
highest (231Pa/235U) and (226Ra/230Th) reported for arc rocks.
These characteristics require a recent addition of a high-U
component to the mantle beneath the Italy. In order to
constrain the origin and nature of this slab-related component
we present new data on non-traditional stable isotopes (Mo,
238U/235U) on both volcanic rocks and possible sedimentary
end-members. Non-traditional isotope systems such as Mo and
238U/235U are sensitive to redox-related isotopic fractionation
on the Earth’s surface, hence they may provide key
information on the type of material recycled from the
subducting slab to the mantle wedge.
The combined use of these different isotope systematics
will provide a wider picture of the mechanism and timescales
of the processes occurring from slab subduction to magmas
generation and ascent above subduction zones
The role of carbon from recycled sediments in the origin of ultrapotassic igneous rocks in the Central Mediterranean
Full text linkThe Central Mediterranean region is one of the most important areas on Earth for studying subduction-related potassic and ultrapotassic magmatism, derived from partial melting of the metasomatised lithospheric mantle wedge. In this region, leucite-free (i.e., lamproite) and leucite-bearing (i.e., kamafugite, leucitite, and plagioleucitite) ultrapotassic rocks closely occur, in a time-related progression, linked to the evolution of both the mantle source and the regional tectonic regime. Time- and space-related magmatism migration followed the roll-back of the subducting slab and the anticlockwise drift of the Italian Peninsula. Leucite-free silica-rich lamproites are restricted to the early stage of magmatism and are associated with ultrapotassic shoshonites and high-K calc-alkaline volcanic rocks. Leucite-bearing (i.e., Roman Province) rocks are erupted consistently later than lamproite-like and associated shoshonitic rocks, with post-leucititic volcanism occurring in the late stage of volcanic activity with eruption of alkali-basaltic to latitic and trachytic rocks, often after major caldera-forming events. Present-day ultrapotassic volcanism is restricted to the Neapolitan area. Central Mediterranean potassic and ultrapotassic rocks are extremely enriched in incompatible trace elements with variable fractionation of Ta, Nb, and Ti in comparison to Th and large ion lithophile elements (LILE). They are also variably enriched in radiogenic Sr and Pb and unradiogenic Nd. The main geochemical and isotopic signatures are consistent with sediment recycling within the mantle wedge via subduction. A twofold metasomatism, induced by the recycle of pelitic sediments and dehydration of lawsonite-bearing schists generates the early metasomatic events that enriched the mantle wedge from which leucite-free ultrapotassic rocks (i.e., lamproite) were generated. Recycling of carbonate-rich pelites played an important role in the shift to silica-undersaturated ultrapotassic rocks (kalsilite- and leucite-bearing) of the classic ‘Roman province’
New <sup>40</sup>Ar-<sup>39</sup>Ar dating and revision of the geochronology of the Monte Amiata Volcano, Central Italy
No full textThe duration of the Mt. Amiata volcanic activity is still a matter of debate, in spite of the presence of several geochronological data in the literature. We performed new 40Ar-39Ar dating on the sanidinegroundmass pairs of the upper stratigraphic units: Dome and massive Lava flows Complex (DLC) and Olivine Latitic final Lavas (OLF). The aim was twofold: to check the reliability of sanidine ages as geochronometer in these products, questioned in the literature, and to better define the chronology of the late activity of this volcano. Ages obtained on coexisting sanidine and groundmass of the Dome and massive Lava flows Complex (DLC) samples overlap within errors, demonstrating that sanidine crystals recorded reliable emplacement ages in these rocks. The Olivine Latite final lavas (OLF) display a different scenario, where the groundmass has an age younger than that of the sanidine, which is xenocrystic and, evidently, retains inherited Ar. Preferred ages for analysed DLC samples are comprised between 301 and 294 ka, an interval of time too short to resolve the ages of the four dome samples taken into account. The Ermeta lava is about 60 ka younger. We propose that the majority of Mt. Amiata volcanic rocks were emplaced in a narrow interval of time, whilst a temporal gap, which needs more detailed constraints, exists with at least one of the Olivine Latite final lavas (OLF)
Constraining slab recycling under Vesusius volcano from combined U-series and non-traditional stable isotope (Mo, 238U/235U) data.
No full textThe fate of deeply subducted oceanic crust and overlying
sediments is of great importance for its role in the generation
of magmas in subduction-related geodynamic settings. Italian volcanoes, and Vesuvius in particular, are good laboratories to
investigate these processes due to their strong enrichment in K
and incompatible trace element that requires a significant
amount of sediment material recycled into the mantle.
Volcanic rocks from Vesuvius display ubiquitous 238U excesses
(up to 27%), a feature that is unusual in such enriched
subduction-related magmas. In addition they have among the
highest (231Pa/235U) and (226Ra/230Th) reported for arc rocks.
These characteristics require a recent addition of a high-U
component to the mantle beneath the Italy. In order to
constrain the origin and nature of this slab-related component
we present new data on non-traditional stable isotopes (Mo,
238U/235U) on both volcanic rocks and possible sedimentary
end-members. Non-traditional isotope systems such as Mo and
238U/235U are sensitive to redox-related isotopic fractionation
on the Earth’s surface, hence they may provide key
information on the type of material recycled from the
subducting slab to the mantle wedge.
The combined use of these different isotope systematics
will provide a wider picture of the mechanism and timescales
of the processes occurring from slab subduction to magmas
generation and ascent above subduction zones
Non-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)
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