U-series, new $ˆ{87}$Sr/$ˆ{86}$Sr and $ˆ{143}$Nd/$ˆ{144}$Nd isotopic ratios beside new trace elements data have been obtained on a well-constrained selection of mafic rocks from volcanoes of the peri-Tyrrhenian region, in Southern Italy, and from Sicily Channel. Samples have a wide petrological diversification, from rifting-related Na-alkaline to peralkaline rocks, from Pantelleria and Linosa volcanoes (Sicily Channel) and Ustica volcano (Southern Tyrrhenian Sea), to subduction-related ultrapotassic rocks from the Roman Magmatic Region (Central Italy), through shoshonitic rocks of Aeolian Arc and Tuscan and Roman Magmatic regions (Central-Southern Italy) and typical calc-alkaline rocks from Aeolian Arc (Southern Tyrrhenian Sea). Mafic rocks from the southernmost volcanoes (e.g., Pantelleria, Linosa, Ustica) show a clear intraplate signature with no HFS element depletion. The lowest $ˆ{87}$Sr/$ˆ{86}$Sr (0.70305-0.70352) and the highest $ˆ{143}$Nd/$ˆ{144}$Nd (0.51293-0.51297) isotopic ratios are also observed. 15-20{\%} $ˆ{230}$Th excess and high ($ˆ{230}$Th/$ˆ{232}$Th) activity ratios (1.13-1.06) have been found in Pantelleria mafic rocks. These magmas have been interpreted as generated by partial melting in a within-plate setting on an upper mantle showing no crustal metasomatism. On the opposite hand, calc-alkaline, shoshonitic and ultrapotassic mafic rocks from Aeolian Arc and peninsular Italy (Roman and Tuscan Magmatic Regions) strong negative Nb and Ta negative anomalies, with variable $ˆ{87}$Sr/$ˆ{86}$Sr and $ˆ{143}$Nd/$ˆ{144}$Nd isotopic values, respectively, increasing and decreasing northward. For these magmas the clear crustal signature shown is explained through a genesis in a lithospheric upper mantle metasomatised by sediment-derived, either fluids or melts, metasomatised agents. A southward increasing of high-$\mu $ component, however, is also found in mafic rocks from Southern Tyrrhenian sea (Aeolian Arc) and adjoning areas (Campanian district). This feature has been used to suggest the presence of an up-rising plume in this area, in contrast to geophysical evidence. Trace element data obtained on mafic rocks from this area provide further evidence on the presence of crustal component, which cannot be acquired by magmas \textit{en-route} to surface. High Th/Ce, and low Ba/La, and Ba/Th ratios strongly suggest that subducted sedimentary materials play a major role in the signature of the mantle source of peri-Tyrrhenian Italian magmas. U-series isotope data fit the above hypothesis. ($ˆ{230}$Th/$ˆ{232}$Th) activity ratios and U/Th elemental ratios, indeed, decrease northwestward, pointing to the mafic potassic and ultrapotassic rocks of the Tuscan Magmatic Region, which are, at comparable degree of primitivity, the most enriched magmas. This confirms the geographic isotopic and geochemical variation trends, which are correlated to primary diversities in the composition of the mantle sources underlying Italian area. U-excess has been observed only in Campanian district (5-27{\%}), with maximum values found in the recent to present-day Vesuvius lavas (11-27{\%}). Aeolian mafic rocks plot close to the equiline as well as most of the other shoshonitic to ultrapotassic rocks of the Tuscan and Roman Magmatic Regions. The occurrence of U-excesses coupled to high U and Th contents (U= 6.2-7.5 pmm, Th=17-20ppm in Vesuvius lavas) is uncommon in any magma worldwide. Extremely large amount of U are needed to be add to the upper mantle source of the Campanian magmas to explain the observed disequilibria.

U-series disequilibria in recent day mafic rocks from Southern Italy: inferences on their genesis and to the role of fluid versus melt sedimentary derived metasomatic agent / R. AVANZINELLI; T. ELLIOTT; S. TOMMASINI; S. CONTICELLI. - In: GEOPHYSICAL RESEARCH ABSTRACTS. - ISSN 1607-7962. - STAMPA. - 6(2004), pp. 5096-5096. ((Intervento presentato al convegno EGU General Assembly 2004 tenutosi a Nice nel 25–30 Aprile 2004.

U-series disequilibria in recent day mafic rocks from Southern Italy: inferences on their genesis and to the role of fluid versus melt sedimentary derived metasomatic agent

AVANZINELLI, RICCARDO;TOMMASINI, SIMONE;CONTICELLI, SANDRO
2004

Abstract

U-series, new $ˆ{87}$Sr/$ˆ{86}$Sr and $ˆ{143}$Nd/$ˆ{144}$Nd isotopic ratios beside new trace elements data have been obtained on a well-constrained selection of mafic rocks from volcanoes of the peri-Tyrrhenian region, in Southern Italy, and from Sicily Channel. Samples have a wide petrological diversification, from rifting-related Na-alkaline to peralkaline rocks, from Pantelleria and Linosa volcanoes (Sicily Channel) and Ustica volcano (Southern Tyrrhenian Sea), to subduction-related ultrapotassic rocks from the Roman Magmatic Region (Central Italy), through shoshonitic rocks of Aeolian Arc and Tuscan and Roman Magmatic regions (Central-Southern Italy) and typical calc-alkaline rocks from Aeolian Arc (Southern Tyrrhenian Sea). Mafic rocks from the southernmost volcanoes (e.g., Pantelleria, Linosa, Ustica) show a clear intraplate signature with no HFS element depletion. The lowest $ˆ{87}$Sr/$ˆ{86}$Sr (0.70305-0.70352) and the highest $ˆ{143}$Nd/$ˆ{144}$Nd (0.51293-0.51297) isotopic ratios are also observed. 15-20{\%} $ˆ{230}$Th excess and high ($ˆ{230}$Th/$ˆ{232}$Th) activity ratios (1.13-1.06) have been found in Pantelleria mafic rocks. These magmas have been interpreted as generated by partial melting in a within-plate setting on an upper mantle showing no crustal metasomatism. On the opposite hand, calc-alkaline, shoshonitic and ultrapotassic mafic rocks from Aeolian Arc and peninsular Italy (Roman and Tuscan Magmatic Regions) strong negative Nb and Ta negative anomalies, with variable $ˆ{87}$Sr/$ˆ{86}$Sr and $ˆ{143}$Nd/$ˆ{144}$Nd isotopic values, respectively, increasing and decreasing northward. For these magmas the clear crustal signature shown is explained through a genesis in a lithospheric upper mantle metasomatised by sediment-derived, either fluids or melts, metasomatised agents. A southward increasing of high-$\mu $ component, however, is also found in mafic rocks from Southern Tyrrhenian sea (Aeolian Arc) and adjoning areas (Campanian district). This feature has been used to suggest the presence of an up-rising plume in this area, in contrast to geophysical evidence. Trace element data obtained on mafic rocks from this area provide further evidence on the presence of crustal component, which cannot be acquired by magmas \textit{en-route} to surface. High Th/Ce, and low Ba/La, and Ba/Th ratios strongly suggest that subducted sedimentary materials play a major role in the signature of the mantle source of peri-Tyrrhenian Italian magmas. U-series isotope data fit the above hypothesis. ($ˆ{230}$Th/$ˆ{232}$Th) activity ratios and U/Th elemental ratios, indeed, decrease northwestward, pointing to the mafic potassic and ultrapotassic rocks of the Tuscan Magmatic Region, which are, at comparable degree of primitivity, the most enriched magmas. This confirms the geographic isotopic and geochemical variation trends, which are correlated to primary diversities in the composition of the mantle sources underlying Italian area. U-excess has been observed only in Campanian district (5-27{\%}), with maximum values found in the recent to present-day Vesuvius lavas (11-27{\%}). Aeolian mafic rocks plot close to the equiline as well as most of the other shoshonitic to ultrapotassic rocks of the Tuscan and Roman Magmatic Regions. The occurrence of U-excesses coupled to high U and Th contents (U= 6.2-7.5 pmm, Th=17-20ppm in Vesuvius lavas) is uncommon in any magma worldwide. Extremely large amount of U are needed to be add to the upper mantle source of the Campanian magmas to explain the observed disequilibria.
Geophysical Research Abstracts, Vol. 6, 2004
EGU General Assembly 2004
Nice
25–30 Aprile 2004
R. AVANZINELLI; T. ELLIOTT; S. TOMMASINI; S. CONTICELLI
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/2158/712726
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