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An enriched mantle source for Italy's melilitite-carbonatite association as inferred by its Nd-Sr isotope signature

Published online by Cambridge University Press:  05 July 2018

F. Castorina ,
F. Stoppa ,
A. Cundari  and
M. Barbieri
F. Castorina*
Affiliation:
Dipartimento di Scienze della Terra, Università “La Sapienza”, P. le A. Moro, 5, 00185 Roma, Italy
F. Stoppa
Affiliation:
Dipartimento di Scienze della Terra, Università G. d'Annunzio, 66013 Chieti Scalo, Italy
A. Cundari
Affiliation:
Dipartimento di Geofisica e Vulcanologia, Università “Federico II”, Largo S. Marcellino, 10, 80100, Napoli, Italy
M. Barbieri
Affiliation:
Dipartimento di Scienze della Terra, Università “La Sapienza”, P. le A. Moro, 5, 00185 Roma, Italy
*
*E-mail: castorina@uniroma1.it
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Abstract

New Sr-Nd isotope data were obtained from Late Pleistocene carbonatite-kamafugite associations from the Umbria-Latium Ultra-Alkaline District of Italy (ULUD) with the aim of constraining their origin and possible mantle source(s). This is relevant to the origin and evolution of ultrapotassic (K/Na ≫2) and associated rocks generally, notably the occurrences from Ugandan kamafugites,Western Australian lamproites and South African orangeites. The selected ULUD samples yielded 87Sr/86Sr and 143Nd/144Nd ranging from 0.7100 to 0.7112 and from 0.5119 to 0.5121 respectively, similar to cratonic potassic volcanic rocks with higher Rb/Sr and lower Sm/Nd ratios than Bulk Earth. Silicate and carbonate fractions separated from melilitite are in isotopic equilibrium, supporting the view that they are cogenetic. The ULUD carbonatites yielded the highest radiogenic Sr so far reported for carbonatites. In contrast, sedimentary limestones from ULUD basement formations are lower in radiogenic Sr, i.e. 87Sr/86Sr = 0.70745–0.70735. The variation trend of ULUD isotopic compositions is similar to that reported for Ugandan kamafugites and Western Australian lamproites and overlaps the values for South African orangeites in the εSr-εNd diagram. A poor correlation between Sr/Nd and 87Sr/86Sr ratios in ULUD rocks is inconsistent with a mantle source generated by subduction-driven processes, while the negligible Sr and LREE in sedimentary limestones from the ULUD region fail to account for a hypothetical limestone assimilation process. The Nd model ages of 1.5–1.9 Ga have been inferred for a possible metasomatic event, allowing further radiogenic evolution of the source, a process which may have occurred in isolation until eruption time. While the origin of this component remains speculative, the Sr-Nd isotope trend is consistent with a simple mixing process involving an OIB-type mantle and a component with low εNd and high εSr.

Keywords

ItalyUmbriaLatiumcarbonatitesmelilititesSr-Nd isotopesenriched mantle
Type
Research Article
Information
Mineralogical Magazine , Volume 64 , Issue 4 , August 2000 , pp. 625 - 639
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2000

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