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Journal of Petrology Advance Access originally published online on December 12, 2007
Journal of Petrology 2008 49(2):195-223; doi:10.1093/petrology/egm076
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© The Author 2007. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Constraints on the Genesis of Potassium-rich Italian Volcanic Rocks from U/Th Disequilibrium

Riccardo Avanzinelli1,*, Tim Elliott2, Simone Tommasini1 and Sandro Conticelli1,3

1Dipartimento Di Scienze Della Terra, Università Degli Studi Di Firenze, Via Giorgio La Pira, 4, I-50121, Firenze, Italy
2Bristol Isotope Group, Department Of Earth Sciences, University Of Bristol, Wills Memorial Building, Queen'S Road, Bristol Bs8 1Rj, Uk
3Istituto Di Geoscienze E Georisorse, Cnr, Via Giorgio La Pira, 4, I-50121, Firenze, Italy

RECEIVED NOVEMBER 8, 2006; ACCEPTED NOVEMBER 12, 2007


  Abstract

We present new U-series isotope, 87Sr/ 86Sr, 143Nd/ 144Nd and trace element data for a set of mafic, K-rich rocks from volcanoes in Central–Southern Italy. These shoshonitic to ultrapotassic lavas display strongly depleted high field strength element (HSFE) abundances with respect to other incompatible trace elements together with high but variable 87Sr/ 86Sr and low but variable 143Nd/ 144Nd values. Such characteristics are thought to be due to addition of subducted crust of variable amount and composition to their mantle sources prior to magma genesis. Rocks from the northernmost region (i.e. Tuscan Magmatic Province and Northern Roman Magmatic Province) display (230Th/ 238U) activity ratios close to radioactive equilibrium, suggesting that metasomatism of their sources occurred before 400 ka and recent melting took place at shallow depths, in the absence of garnet. A 238U excess of up to 27% has been measured in rocks from the Neapolitan District. The occurrence of significant U excesses is a feature of arc magmas, but is typically seen in depleted lavas rather than in highly enriched rocks such as these (~20 ppm Th). This signature requires a recent addition of a U-rich component to the already strongly enriched mantle wedge beneath this region of Italy. We suggest that a supercritical liquid, from deeply subducted carbonate-rich sediments of the still-active Ionian slab, is responsible for generating a high-U, low-Th component, which produces the observed disequilibria. A 30% 230Th excess measured in a single unaltered sample from the Lucanian Magmatic Province, along with a less marked negative HFSE anomaly, suggests the contribution of a deeper, garnet-bearing component in the genesis of these magmas, plausibly related to the upwelling of asthenospheric mantle around the corner of the Ionian slab.

KEY WORDS: U/Th disequilibria; potassic and ultrapotassic rocks; subduction: metasomatism; mantle melting; Central and Southern Italy

*Corresponding author. Present address: Bristol Isotope Group, Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK. Telephone: +44 0117 9545247. Fax: +44 0117 9253385. E-mail: r.avanzinelli{at}bristol.ac.uk


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