Inferences on melt and fluid migration trough the Patagonin mantle wedge are provided by the ultramafic xenoliths occurrence of Tres Lagos (TL; lat. 49.13°S, long. 71.18°W). It is placed at the eastern border of the Meseta de la Muerte backarc basaltic plateau, where a post-plateau volcanic diatreme contains mantle xenoliths in both pyroclastites and lavas. Its latitude corresponds with the Northern limit of the Austral Volcanic Arc (AVZ), which is separated from the Southern Volcanic Zone (SVZ) by a gap in the arc magmatism between 49° and 46°30′ latitude S. The selected xenoliths have been distinguished into two groups (Group 1 & 2). Group 1 consists of lherzolites and harzburgites, whereas Group 2 is constituted only by harzburgites. The Sr, Nd and Pb isotopic compositions of the Group I clinopyroxenes (Cpx) form arrays from DM to the TL basaltic lavas, according to the presence of EM1 component. Group 2 Cpx have much more radiogenic Sr and less radiogenic Nd values, approaching those of EM1 and EM2 end-members: these features are associated to very unradiogenic lead isotopic compositions (206Pb/204Pb = 17.4-18.1; 207Pb/204Pb = 15.55-15.60; 208Pb/204Pb = 37.3-38.5). The combination of petrographic, trace element and isotopic fetures indicate that TL harzburgites are residua after melt-assisted partial melting triggered by melt/fluid migration in the hottest, and perhaps deeper, parts of the pristine DM lithosphere. The interpretation of the Pb isotope composition of Group 2 Cpx is not trivial. In analogy with the interpretation proposed for SWIR, it could unravel the occurrence of mantle sources which incorporated ancient crust and failed to homogenise with the DM mantle. Alternatively, it could be the evidence for ancient continental crust of the South America plate dragged down into the mantle by slab motion.
|Data di pubblicazione:||2009|
|Titolo:||Evidence for ultra-depleted Pb isotopic component in the backarc Patagonian mantle wedge.|
|Autori:||RIVALENTI G.; MAZZUCCHELLI M.; ZANETTI A.; HÉMOND C.; CIPRIANI A.|
|Nome del convegno:||Goldschmidt Conference 2009|
|Luogo del convegno:||Davos (CH)|
|Data del convegno:||21-26/06/09|
|Appare nelle tipologie:||Abstract in Rivista|
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