1,721,063 research outputs found
Photoelectron spectroscopy of reactive intermediates with synchrotron radiation
No full textEThOS - Electronic Theses Online ServiceGBUnited Kingdo
La classificazione delle rocce vulcaniche e subvulcaniche: schema operativo per il progetto CARG
No full text
Magmatism from Mesozoic to Present: petrogenesis, time-space distribution and geodynamic implications
No full textGeochemical features of eastern Sicily lithosphere as probed by Hyblean xenoliths and lavas
No full textUpper mantle and lower crust xenoliths of Quaternary basanitoid lavas and Miocene tuff-breccias of the Hyblean Volcanic Plateau (HVP), southeastern Sicily, have been studied for their REE contents and Sr-Nd isotope composition. Mantle rocks mainly consist of spinel-harzburgites, spinel-websterites and monomineralic assemblages of Al-augite, Plagioclase-bearing xenoliths are two-pyroxene mafic granulites and subordinate anorthosites. Pressure estimates point to a depth ranging from the lower crust down to the spinel-peridotite field (at least 45 km); temperature varies between 800 and 1050 degrees C. Mantle xenoliths have LREE(n) > HREE(n), high Nd (0.51289-0.51299) and low Sr (0.70298-0.70334) isotopic ratios, suggesting the occurrence of LREE enrichment in relatively recent times. The possibility that contamination is induced in the xenoliths by host lavas is discussed against the hypothesis of a Permo-Triassic age for the metasomatic event. Latter model is preferred due to the occurence of igneous activity in SE Sicily since Trias, to whole-rock T-DM(Nd) (200-270 Ma) and to the linear correlation of mantle samples in the Sm/Nd vs. Nd-143/Nd-144 diagram pointing to an ''age'' of 170 +/- 110 Ma. In this case, initial epsilon(Nd) = 6.8 +/- 0.9 denotes an asthenospheric origin for the metasomatic agent. Plagioclase-bearing xenoliths have Nd and Sr isotope ratios ranging between 0.51297-0.51288 and 0.70278-0.70292, respectively. In these lower crust mafic granulites, Sm/Nd whole-rock and mineral pairs yield ''ages'' ranging between 370 +/- 110 and 190 +/- 40 Ma. This suggests that, since Hercynian time, products of subalkaline and alkaline affinity have been adding to the crust and merges the evolution of the lower crust of SE Sicily to the cycle of igneous activity linked to the extensional tectonics affecting the northern margin of the African Plate. Sr-Nd isotopic and trace element data on Plio-Quaternary primitive lavas from the HVP provide a further sampling of the upper mantle of southeastern Sicily. Nd-Sr isotopic data of tholeiitic (Nd-143/Nd-144 = 0.51315-0.51306; Sr-87/Sr-86 = 0.70273-0.70285) and alkaline rocks (Nd-143/Nd-144 = 0.51299-0.51293; Sr-87/Sr-86 = 0.70302-0.70307) suggest that the genesis of HVP lavas is controlled by both a component akin to the source of MORB and a relatively enriched source similar to mantle xenoliths. A model is proposed in which both sources are present in the Hyblean lithosphere due to veining of the Thermal Boundary Layer
Genesis and evolution of Mt Etna alkaline lavas: petrological and Sr-Nd-B isotope constraints
No full textMt. Etna lies at the northern margin of the African plate, on the accretionary prism of the Africa-Europe subduction system. Differential roll-back of the Ionian oceanic lithosphere has created a vertical slab window through which the passive rise of asthenosphere causes partial melting and magma genesis. In this rapidly evolving geodynamic context, the increase of alkalinity in time, accompanied by variations of Sr and Nd isotopes, is a first order feature. Fluid mobile elements and B systematics reveal the fundamental role played by fluids released by the dehydrating oceanic lithosphere, even if they are added to the mantle source in amounts no larger than 1 wt.%. This induces relatively high H2O contents in Etna magmas that exert a strong control on phase relationships. Compositions of alkaline primary melts have been reconstructed and used to constrain the depth of origin and fractionation sequences at various pressures. It was found that phase relations at the crust-mantle boundary can only produce basaltic compositions akin to the less evolved sub-aphyric lavas, whereas trachybasaltic compositions are generated in the depth range 12-3 km together with cumulate bodies detected through geophysical investigations and here related to massive pyroxene and plagioclase fractionation. Detailed monitoring of Sr-isotope equilibrium between pyroxene and host rock revealed both the increase of 87Sr/86Sr over time and the occurrence of mixing processes between distinct batches of magma in the plumbing system. Sr, Nd and B systematics also reveal contamination effects due to interactions with lower crust and sporadic interaction with the sedimentary basement
): The Montecristo Monzogranite (Northern Tyrrhenian Sea Italy): a collisional pluton in an extensional setting
No full textThe Montecristo monzogranite (MM) is a near-circular peraluminous monzogranite pluton occupying the entire 10 km(2) of Montecristo Island. Outcrops of country rock are scarce, and are mainly roof pendants of metagabbros and calcsilicate hornfels of the Apenninic ophiolite sequence. Emplacement of the pluton (Rb-Sr age = 7.1 +/- 0.2 Ma), following the early Miocene onset of continental collision, occurred during an extensional phase which migrated eastward via a combined process of subduction-delamination. The MM rocks are strongly porphyritic, the assemblage being composed of alkali-feldspar, quartz, plagioclase (all occurring as mega- or phenocrysts), biotite and minor cordierite. Accessory minerals include tourmaline, apatite, zircon, ilmenite, allanite, monazite, rutile and hellandite. Reconstructed crystallization histories for the mineral phases reveal a polybaric crystallization starting at about 5 kb. Textural variations of MM occur in sharp contact with each other; darker types often form globular masses containing fewer megacrysts and more abundant mafic microgranular enclaves. Geochemical, isotopic and petrographic data indicate that the MM magma was produced by anatectic melting of an intermediate to deep pelitic crustal source. On the basis of the geochemical and mineralogical characteristics of the enclaves, modification of their parent magma occurred by crystal fractionation coupled with mixing and mingling of components from the MM magma. The limited geochemical variation in MM is interpreted as due to crystal fractionation processes during the magma's ascent. Younger porphyritic dykes with more potassic and alkaline affinities cut the pluton; these dykes are concentrated in a major fracture zone and are associated with contemporaneous pseudotachylites
- …
