A combined study of noble gases, trace elements, and Sr-Nd isotopes for alkaline and tholeiitic lava from the Hyblean Plateau (Italy)

Abstract

Eight samples of Pliocenic/Pleistocenic volcanic rocks from the Hyblean area (south-eastern Sicily, Italy) were investigated for noble gases, trace elements, and Sr-Nd isotopes. The samples consist of tholeiitic basalts, basanites, and nephelinites and display a variable Sr-Nd isotopic composition (87Sr/86Sr = 0.70275–0.70284 and 143Nd/144Nd = 0.51312–0.51316 in the tholeiites; 87Sr/86Sr = 0.70294–0.70331 and 143Nd/144Nd = 0.51293– 0.51308 in the basanites/nephelinites). Most of the investigated lavas show almost constant 3He/4He ratios of about 7.0 Ra, except one basanite and one tholeiite-basalt which exhibit 3He/4He ratios of 3.5 Ra and 8.3 Ra, respectively. The two samples, both characterized by low abundance of gases, are affected by secondary inputs of 4He and 3He that led to a wide deviation from the leading value of 7.0 Ra. We suggest that a heterogeneous lithosphere fed the Quaternary volcanism beneath the Hyblean Plateau. This mantle source is featured by a variable Sr-Nd isotopic composition as a consequence of patchy partial melting or cryptic metasomatism events. Its almost constant isotopic helium is due to the homogenizing effect of deep fluids periodically infiltrating the lithosphere. Although the samples show the same He isotope signatures as mantle harzburgite xenoliths from Hyblean Miocenic diatremes, the distribution of some trace element pairs argues against a close geochemical parentage between the Pliocenic-Pleistocenic volcanic rocks and mantle xenoliths. This can be reconciled if the xenoliths come from mantle portions whose trace element distribution was modified by carbonatitic metasomatic melts responsible for the enrichment of incompatible trace elements but not able to carry significant He amounts.