The island of Pantelleria: A case for the development of DMM-HIMU isotopic compositions in a long-lived extensional setting

Abstract

The Sr, Nd and Pb isotopic and trace element compositions of basalts and pantellerites from the island of Pantelleria (Strait of Sicily) have been used to constrain the evolution of these magmas and their sources. All Pantelleria products have isotopic compositions that plot in Sr Pb, Nd Pb and Sr Nd space between DMM and HIMU oceanic mantle sources. The Pantelleria basalts have variable Ce/Pb, Nb/U and Th/U, and some have elevated 207Pb/ 204Pb and 208Pb/ 204Pb, which is indicative of the addition of an older component that resembles old enriched lithosphere sampled by potassic magmas in the circum-Tyrrhenian Sea. It is proposed that the relatively homogeneous pantellerites derive their Ce/Pb, Nb/U, and OIB-like Sr, Nd and Pb isotopic compositions from a well-mixed crustal magma chamber supplied by heterogeneous basalts. We propose further that the radiogenic 206Pb/ 204Pb and 143Nd/ 144Nd, coupled with the unradiogenic Sr isotopic composition of some of the Pantelleria basalts, are inherited in a process involving chemical modification of the thinned lithospheric crust-mantle source by addition of basaltic (MORB) materials, a process that may have occurred intermittently at least since the Permo-Triassic. This process is responsible for lowering the Pb concentration and changing the U/Pb and Th/Pb ratios of portions of the lithospheric crust-mantle boundary. It is envisaged that the increase in μ and ω of the modified lithospheric mantle was caused by the addition of phases such as clinopyroxene and amphibole, crystallized in equilibrium with MORB. After chemical exchange with the asthenospheric mantle ceased during the Hercynian orogeny, this young (or rejuvenated) lithosphere evolved a radiogenic Pb isotopic composition. It is proposed that the isotopic differences observed between the volcanic rocks erupted in the Strait of Sicily and those erupted in the southern margin of the Tyrrhenian abyssal plain are primarily the result of mixing between magmas derived from a variably modified lithospheric source, some of it with OIB-type isotopic characteristics, that were developed by intermittent interaction with asthenospheric melts in a geodynamic regime dominated by extension.