Mathematics – Logic
Scientific paper
Mar 1976
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1976gecoa..40..283v&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 40, Issue 3, pp.283-295
Mathematics
Logic
59
Scientific paper
The isotopic composition of Pb and Sr and the abundances of Rb, Sr, U, Th, and Pb were determined for whole rock samples from all major volcanic centres of the Cenozoic alkaline volcanism of Central and South Italy, together with some samples from the contemporaneous anatectic Tuscan volcanism. The Sr and Pb isotopic compositions of the alkaline rocks show a negative correlation combined with a regional trend: the 87 Sr / 86 Sr ratios decrease from 0.711 in the north-west to 0.704 in the south-east, while the 206 Pb / 204 Pb ratios increase from 18.7 to 20.0. Variations in both isotopic compositions are generally small throughout erupted rock sequences for any volcanic centre. The Pb and Sr isotopic abundance variations are interpreted on the basis of two alternative models, which correspond to two groups of geological processes: variations can result (i) from a time dependent development in subsystems with different Rb / Sr or U ( Th )/ Pb ratios or, (ii) from mixing of Sr or Pb with different isotopic compositions. Combining both Pb and Sr isotope abundance measurements it is shown that the source of each volcanic centre is formed by various degrees of mixing between two components. One component and the most southern Tuscan anatectic rocks most likely have a common source, whereas the other component of the mixing process is suggested to be a liquid fraction derived from a small degree of partial fusion of a hydrous mantle. Thus at least a two-stage evolution of the Italian alkaline rocks is indicated: first a mixing process leading to the formation of the parental material followed by differentiation processes leading to the formation of the erupted rock sequences. The geodynamic model which explains the data best is that of a lateral inhomogeneous mantle. The lateral inhomogeneities in the mantle would be the result of mixing between originally mantle and crustal derived material. The mixing process itself would not have any primary connection with the Quarternary volcanic activity.
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