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U-Th magma residence times of the Plinian Mercato eruption at Mt. Somma-Vesuvius (Southern Italy)
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Isotopic evidence for open system processes within the Campanian Ignimbrite (Campi Flegrei-Italy) magma chamber
No full textNew Sr and Nd isotope data for whole rocks, glasses and minerals are combined to reconstruct the nature and origin of mixing end-members of the 200 km(3) trachytic to phonolitic Campanian Ignimbrite (Campi Flegrei, Italy) magmatic system. The least-evolved magmatic end-member shows equilibrium between host glass and the majority of the phenocrysts and is less radiogenic in Sr and Nd than the most-evolved magma. On the contrary, only the Fe-rich pyroxene from the most-evolved erupted magma is in equilibrium with the matrix glass, while all other minerals are in isotopic disequilibrium. These magmas mixed prior to and during the Campanian Ignimbrite eruption and minerals were freely exchanged between the magma batches. Combining the results of the geochemical investigations on magma end-members with geophysical and geological data, we develop the following scenario. In stage 1, a parental, less differentiated magma rose into the middle crust, and evolved through combined crustal assimilation and crystal fractionation. In stage 2, the differentiated magma rose to shallower depth, fed the pre-Campanian Ignimbrite activity and evolved by further open-system processes into the most-evolved and most-radiogenic Campanian Ignimbrite end-member magma. In stage 3, new trachytic magma, isotopically distinct from the pre-Campanian Ignimbrite magmas, rose from ca. 6 km to shallower depth, recharged the most-evolved pre-Campanian Ignimbrite magma chamber, and formed the large and stratified Campanian Ignimbrite magmatic system. During the course of the Campanian Ignimbrite eruption, the two layers were tapped separately and/or simultaneously, and gave rise to the range of chemical and isotopic values displayed by the Campanian Ignimbrite pumices, glasses and minerals.INGV-DPC Project-Campi Flegrei; [EVG1-2001-00046-ERUPT
U-Th magma residence times of the Plinian Mercato eruption at Mt. Somma-Vesuvius (Southern Italy)
No full textU-Th disquilibrium and Rb-Sr age constraints on the magmatic evolution of peralkaline rhyolites from Kenya
Full text linkMildly peralkaline rhyolites of the Olkaria Volcanic Complex, located in the Kenyan sector of the East African rift valley, have low Sr concentrations and elevated Rb/Sr ratios (Sr 1.3-2 ppm; Rb-87/Sr-86 = 748-1769) that potentially allow the resolution of time differences on the order of 1 ka by conventional Sr isotope determination. Because of their young eruption ages (less than or equal to20 ka), a chemically independent assessment of the Sr isotope results has been obtained by U-series dating. Rb-Sr isochron ages of pristine glasses and phenocrysts from the most chemically evolved rhyolites pre-date the eruption ages and are best defined by a mineral isochron of 24 +/- 1 ka. The glasses are in secular U-Th equilibrium so that no age information can be obtained. In contrast, glasses and minerals yield U-Th isochrons of 25 +/- 10 ka and are probably controlled by the Th-enriched accessory phase chevkinite. We therefore ascribe the pre-eruptive age information to crystallization of the observed phenocryst phases. Inferred high magma fractionation rates of up to 2.5 x 10(-3) km(3)/yr are comparable with those for much larger metaluminous silicic magma systems. Magma storage times (similar to22 ky), however, are much shorter and may best be accounted for by the specific size, longevity and thermal gradient of the silicic magma system
The Fate of High-Angle Dipping Slabs in the Subduction Factory: an Integrated Trace Element and Radiogenic Isotope (U, Th, Sr, Nd, Pb) Study of Stromboli Volcano, Aeolian Arc, Italy
No full textThe subaerial part of the Stromboli stratovolcano was built up in the last 100 kyr through six periods of activity; the erupted magmas record the largest compositional variation of all the Aeolian arc volcanoes (calc-alkaline, shoshonitic, and potassic alkaline magma series). The trace element characteristics of the less evolved magmas of each period of activity are coherently correlated with their radiogenic isotope (Sr, Nd, Pb) composition, and are typical of volcanic arc rocks. In terms of U-series isotopes, samples from the different magma series have both U-238 and Th-230 excesses, and this distinctive feature provides additional constraints on source enrichment processes within the mantle wedge and on the mechanism of partial melting. Overall the complete set of data demonstrates that the genesis of the different magma series at Stromboli can be accommodated in a mantle source that experienced two distinct enrichment processes by different parts of the subducting oceanic crust of the Ionian slab. The first was caused by supercritical liquids originating from the basaltic and sedimentary parts of the subducting slab at >5 GPa and similar to 900 degrees C. The second was induced by aqueous fluids, again originating from the basaltic and sedimentary parts of the slab, released from a shallower part of the subducted Ionian slab (435 ka, whereas the second event (Stage II: aqueous fluids) occurred at similar to 100 ka. The high-angle dip of the Ionian slab (similar to 70 degrees) caused the superimposition of the metasomatizing agents of the two enrichment processes in the same volume of the mantle wedge, explaining the occurrence of such different magma series in a single volcanic edifice. The U-Th disequilibria provide evidence for dynamic melting of the metasomatized mantle wedge combined with an ageing effect resulting from the restoration of secular equilibrium after the perturbation caused by the U-rich aqueous fluids of Stage II. The trace element and radiogenic isotope (U, Th, Sr, Nd, Pb) signature of the mantle source of the magmas at Stromboli is thus dependent upon the amount of supercritical liquids and aqueous fluids released by the two components of the subducted slab, whereas the distinctive U-238 and Th-230 excesses of the magmas result from a combination of mantle ageing and time-dependent dynamic melting. The geochemical and radiogenic isotope signature of the mantle source beneath Stromboli places important constraints on the isotopic polarity from Southern Latium to the Aeolian arc attributed to the effect of a HIMU mantle component following either lateral inflow of foreland mantle material or upwelling of a mantle plume in the centre of the Tyrrhenian basin. Our geochemical model demonstrates that the high Pb-206/Pb-204 of the putative 'HIMU' mantle component could be equally formed during metasomatism of the pre-existing mantle wedge by either the supercritical liquid (Stage I) or aqueous fluid (Stage II) released by the subducted altered basalt of the Ionian plate
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