Deep recycling of carbonate rich sediments and mantle melting: U-Th-Pa constraints from Vesuvius historical volcanic rocks
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Ischia, a volcanic island located 18 miles SW of Naples (Southern Italy), is a densely populated active caldera that last erupted in ad 1302. Melt inclusions in phenocrysts of the Vateliero and Cava Nocelle shoshonite–latite eruptive products (6th to 4th centuries bc) constrain the structure and nature of the Ischia deep magmatic feeding system. Their geochemical characteristics make Ischia a natural borehole for probing the physico-chemical conditions of magma generation in mantle contaminated by slab-derived fluids or melts, largely dominated by CO2. Volatile concentrations in olivine-hosted melt inclusions require gas–melt equilibria at between 3 and 18 km depth. In agreement with what has already been demonstrated at the other neighboring Neapolitan volcanoes (Procida, Campi Flegrei caldera and Somma–Vesuvius volcanic complex), a major crystallization depth at 8–10 km has been identified. The analyzed melt inclusions provide clear evidence for CO2-dominated gas fluxing and consequent dehydration of magma batches stagnating at crustal discontinuities. Gas fluxing is further supported by selective enrichment in K owing to fluid-transfer during magma differentiation. This takes place under oxidized conditions (Fe3+/ΣFe ≥ 0·3) that can be fixed by an equimolar proportion of divalent and trivalent iron in the melt if post-entrapment crystallization of the host olivine is discarded. The melt inclusion data, together with data from the literature for other Neapolitan volcanoes, show that magmatism and volcanism in the Neapolitan area, despite differences in composition and eruption dynamics, are closely linked to supercritical CO2-rich fluids. These fluids are produced by devolatilization of subducting terrigenous–pelagic metasediments and infiltrate the overlying mantle wedge, generate magmas and control their ascent up to eruption. Geochemical characteristics of Ischia and the other Neapolitan volcanoes reveal that the extent of fluid or melt contamination of the pre-subduction asthenospheric mantle wedge was similar among these volcanoes. However, differences in the isotopic compositions of the erupted magmas (more enriched in radiogenic Sr at Ischia, Campi Flegrei and Somma–Vesuvius with respect to Procida) and the amount of H2O in the plumbing system of these volcanoes (almost double at Ischia, Campi Flegrei and Somma–Vesuvius than at Procida) reflect the different flow-rates of deep slab-derived fluids or melts through the mantle wedge, which, in turn, control the amount of generated magma. The high bulk permeability of the lithosphere below Ischia, Campi Flegrei and Somma–Vesuvius, determined by the occurrence of intersecting NW–SE and NE–SW regional fault systems, favours fluid ascent and accumulation at crustal levels, with consequent larger magma production and storage than at Procida, located along the NE–SW system.
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Abstract Quaternary calc-alkaline andesitic to dacitic lavas effusively erupted on top of about 30 km thick accreted continental crust at Methana peninsula in the western Aegean arc. We present new data of major and trace element concentrations as well as of Sr–Nd–Pb isotope ratios along with mineral compositions of Methana lavas and their mafic enclaves. The enclaves imply a parental basaltic magma and fractional crystallization processes with relatively little crustal assimilation in the deep part of the Methana magma system. The composition of amphibole in some mafic enclaves and lavas indicates deeper crystallization at ∼25 km depth close to the Moho compared with the evolved lavas that formed at <15 km depth. The presence of amphibole and low Ca contents in olivine suggest high water contents of ∼4 wt% in the primitive magmas at Methana. The compositions of andesitic and dacitic lavas reflect fractional crystallization, assimilation of sedimentary material, and magma mixing in the upper 15 km of the crust. The Methana magmas have fO2 of FMQ + 1 to FMQ + 2 (where FMQ is the fayalite–magnetite–quartz buffer) at temperatures of 1200 to 750 °C and the fO2 does not vary systematically from mafic to felsic compositions, suggesting that the mantle wedge was oxidized by sediment subduction. Amphibole is an important fractionating phase in the more evolved Methana magmas and causes significant changes in incompatible element ratios. Although xenocrysts and mineral compositions indicate magma mixing, the major and trace element variation implies only limited mixing between dacitic and basaltic melts.
Fractional crystallization (geology)
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Igneous differentiation
Adakite
Incompatible element
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Fractional crystallization (geology)
Igneous differentiation
Trace element
Magma chamber
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The active volcano of Ischia, the well-known island off-shore the city of Naples, has had a discontinuous volcanic activity characterised by caldera-forming paroxysmal eruptions, lava flows, and lava domes for >150 kyr. The overall geochemical composition of erupted magmas includes shoshonite, latite, and trachyte/trachyphonolite. In a complementary study, we demonstrated that the evolution of Ischia trachytes with Sr
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