Scribano, V.

We analyzed major and trace elements, Sr and Nd isotopes in ultramafic xenoliths in Miocenic age Hyblean diatremes, along with noble gases of CO2-rich fluid inclusions hosted in the same products. The xenoliths consist of peridotites and pyroxenites, which are considered to be derived from the upper mantle. Although the mineral assemblage of peridotites and their whole-rock abundance of major elements (e.g., Al2O3 = 0.8–1.5 wt.%, TiO2 = 0.03–0.08 wt.%) suggest a residual character of the mantle, a moderate enrichment in some incompatible elements (e.g., LaN/YbN = 9–14) highlights the presence of cryptic metasomatic events. In this context a deep silicate liquid is considered the metasomatizing agent, which is consistent with the occurrence of pyroxenites as veins in peridotites. Both the Zr/Nb and 143Nd/144Nd ratios of the investigated samples reveal two distinct compositional groups: (1) peridotites with Zr/Nb ≈ 4 and 143Nd/144Nd ≈ 0.5129, and (2) pyroxenites with Zr/Nb ≈ 20 and 143Nd/144Nd ≈ 0.5130. The results of noble-gas analyses also highlight the difference between the peridotite and pyroxenite domains. Indeed, the 3He/4He and 4He/40Ar* ratios measured in the fluid inclusions of peridotites (respectively 7.0–7.4 ± 0.1 Ra and 0.5–8.2, where Ra is the atmospheric 3He/4He ratio of 1.38 × 10− 6) were on average lower than those for the pyroxenites (respectively 7.2–7.6 Ra and 0.62–15). This mantle heterogeneity is interpreted as resulting from a mixing between two end-members: (1) a peridotitic layer with 3He/4He ≈ 7 Ra and 4He/40Ar* ≈ 0.4, which is lower than the typical mantle ratio (~ 1–4) probably due to melt extraction events, and (2) metasomatizing mafic silicate melts that gave rise to pyroxenites characterized by 3He/4He ≈ 7.6 Ra, with a variable 4He/40Ar* due to degassing processes connected with the ascent of magma at different levels in the peridotite wall rock. The complete geochemical data set also suggests two distinct mantle sources for the xenolithic groups highlighted above: (1) a HIMU (high-μ)-type source for the peridotites and (2) a DM (depleted mantle)-type source for the pyroxenites.

The extensive study of a great number of deep-seated xenoliths from Tortonian tuff-breccia pipes in the Hyblean area (Sicily) revealed the following fundamental evidence: (1) typical continental crust rocks are completely absent in the entire xenolith suite; (2) mantle ultramafics are more abundant than gabbroids; (3) sheared oxide–gabbros, closely resembling those from oceanic fracture zones, are relatively common; (4) secondary mineral assemblages, compatible with alteration processes in serpentinite-hosted hydrothermal systems, occur both in peridotites and gabbros. Among the products of this hydrothermal activity, organic compounds, having abiotic origin via Fischer–Tropsch synthesis, occur in some hydrothermally altered gabbro and ultramafic xenoliths, as well as in hydrothermal clays. Moreover, the U–Pb dating of hydrothermal zircon grains, hosted in a xenolith of metasomatized tectonic breccia, indicated an Early–Middle Triassic age of the fossil hydrothermal system. Another line of evidence for the oceanic nature of the Hyblean–Pelagian basement is the complete absence of continental crust lithologies (granites, felsic metaigneous, and metasedimentary rocks) in outcrops and in boreholes, and the oceanic affinity of the Tertiary volcanic rocks from the Hyblean Plateau and the Sicily Channel (Pantelleria and Linosa Islands), which lack of any geochemical signature for continental crust contamination. A reappraisal of existing geophysical data pointed out that serpentinites form the dominant lithologies in the lithospheric basement of the Hyblean–Pelagian area down to a mean depth of 19 km, which represents the regional Moho considered as the serpentinization front, marking the transition from serpentinites to unaltered peridotites. On these grounds, we confirm that Hyblean xenoliths contain mineralogical, compositional, and textural evidence for tectonic, magmatic, and hydrothermal processes indicating the existence of fossil oceanic core complexes, in the geotectonic framework of the Paleo–Mesozoic, ultra-slow spreading, Ionian–Tethys Ocean forming the present Ionian–Hyblean–Pelagian domain.

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.

In our paper (Manuella et al. 2015), which arose from our 30-year-long research on the Hyblean xenoliths, we discussed an exhaustive dataset retrieved from the literature. We concluded that the unexposed basement of southeastern Sicily and neighboring areas consists of a remnant of the Paleo–Mesozoic Ionian–Tethyan oceanic lithosphere. Our viewpoint is opposite to the most popular theory that the Hyblean–Pelagian foreland domain is part of the Africa continental plate. We acknowledge some comments by Beccaluva et al. (2015) since they prompted us to explicit some background information given as implicit in our paper and hence to reaffirm with more emphasis fundamental aspects of our research, strongly confirming our previous conclusions.

In this study, we derived the first 3-D P-wave seismic attenuation images (QP) as well as new 3-D VP and VP/VS models for the crust in southeastern Sicily.We used a large data set of local seismic events occurring in the time span 1994–2013. The results of this tomographic study have important implications on the seismic behaviour of the region. Based on velocity and attenuation images, we identified distinct volumes characterized by different fluid content, which correlate well with seismicity distribution. Moreover, the obtained velocity and attenuation tomographies help us to provide a more complete picture of the crustal structure of the area. High VP, high QP and high VP/VS values have been obtained in the crustal basement, below a depth of 8 km, and may be interpreted as due to the presence of serpentinized peridotites. Accordingly, the new model for the degree of serpentinization, retrieved from VP values, shows that the basement has an average serpentinization value of 96 ± 3 vol.% at 8 km, decreasing to 44 ± 5 vol.% at about 18–20 km. Our joint interpretation of geophysical and petrophysical evidence suggests that the nature and composition of the Hyblean upper lithosphere may differ from accepted and longestablished geological models, which consider this lithospheric block a continuation of the Africa continental plate.