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A large number of mantle (peridotite and pyroxenite) xenoliths occurs in basaltic diatreme pipes and lava flows from Hyblean Plateau (Sicily, Southern Italy). Peridotites consist mainly of spinel-facies protogranular-textured harzburgites and lherzolites (Fo90, En89, Cr-diopside, Cr-rich spinel). Major elements distribution and abundance are consistent with their depleted character (high MgO, low Al2O3) related to one or more melt extraction events, but evidences of modal and cryptic metasomatism do exist. For instance, phlogopite of metasomatic origin rarely occurs in these peridotites. Geochemical evidences of metasomatism consist of LREE and some LILE enrichment (e.g. LaN/YbN = 9-19), Sr-Nd and He isotopic signatures. We performed in situ Os isotope analyses on 11 sulfide grains from 4 mantle peridotites. Sulfides in these peridotites are Ni-rich, often rimmed by, or enclosing, a Cu-rich phase. Sulfides occur mainly related to the widespread serpentine network. A few sulfides are olivine-enclosed but their size is too small (< 50 m) to be analysed. The studied sulfides has sub-chondritic 187Os/188Os, ranging from 0.110408 to 0.124398. 187Re/188Os ranges from 0.0296463 to 1.41346, being sub-chondritic in all samples but one (sample GE12). The occurrence of supra-chondritic 187Re/188Os indicates the secondary, probably recent, addition of Re to the system, but the Os isotope composition attests its provenance from a low-187Os/188Os (< PUM) reservoir. The negative correlation between 187Os/188Os and 187Re/188Os in the other samples also supports the Os metasomatic origin, rather suggesting the mixing between two metasomatic reservoirs with different Re-Os signatures. Calculated Re-depletion age (TRD) gives Neo and Mesoproterozoic ages that is the minimum melt-depletion age. Hyblean peridotites all experienced melt extraction event(s) – determining removal of Re from the system - and underwent metasomatic Re-enriching event(s). The occurrence of sulfides with different Os isotope signatures in sample GE12 confirms the importance of using in situ technique in such petrological study

Morphological and trace element studies of a hundred and twenty-one zircon grains separated from a chlorite–alkali feldspar xenolith of metasomatic origin collected from Valle Guffari (Hyblean Plateau, south-eastern Sicily, Italy) indicated the coexistence of igneous and hydrothermal zircons within the same sample. The subdivision into igneous and hydrothermal zircons may be consistent with in situ LAM-ICPMS U-Pb and Lu-Hf data. Hf isotope analyses were carried out on fifty-nine out of sixty-six zircon grains already analysed for U-Pb isotopes. U-Pb ages for fifty-eight (hydrothermal) zircons scatter from 184 to 283 Ma, with ~ 72% data clustering around 245 ± 9 Ma and lying close to Concordia. Their Hf isotope signature ranges from Hf +8.5 to –1.2. This trend may indicate the mixing of low- Hf (old crustal) components and a high- Hf (juvenile) source. The Hf isotope composition is considered to be that of hydrothermal fluid, whose F-rich composition is highly probable. The relatively narrow range of U-Pb ages indicates that the age is geologically meaningful, and corresponds to the timing of the hydrothermal event generating these zircons. U-Pb ages for eight igneous-like zircons are scattered (622 to 2687 Ma, Proterozoic to Archean age), but Lu-Hf isotope signatures allow us to distinguish the grains that can be genetically linked. Five out of eight igneous-like zircons show scattered U-Pb ages (from 1939 to 2687 Ma) but they have very similar Lu-Hf signature (176Hf/177Hfi ~ 0.2811), and hence they should be genetically related. Their position on Concordia diagram confirms this observation: the Archean age for three zircons is geologically meaningful since they are concordant, while the other two ages are unreliable (discordant values on Concordia), and reflect non-zero-age Pb loss. Hence, these five zircons are all interpreted as igneous, but some of them were affected by incomplete U-Pb resetting, and the oldest (Archean) age represents the crystallization age. The Lu-Hf system seems to have retained the original signature, given its better resistance to thermal disturbance. For remaining igneous zircon grains a scrupulous comparison between the Concordia plot and a plot of 176Hf/177Hfi vs U-Pb age distinguishes between geologically meaningful (two samples: Proterozoic) and unreliable (one sample) ages. This study reveals the presence of distinct zircon populations, characterised by clearly distinguishable U-Pb and Lu-Hf isotope signatures, coexisting in the same sample. The isotopic results testify to (1) the presence of a few Archean zircon grains of igneous origin in rare metasomatic xenoliths from Hyblean area and (2) the occurrence of a hydrothermal event during Early Triassic time.

Rare chlorite-alkali feldspar xenoliths of metasomatic origin occur among the crustal xenolith suite of the Hyblean Plateau (SE Sicily). These rocks consist of fractured alkali feldspar porphyroclasts sit in a pseudo-fluidal matrix of alkali-feldspar neoblasts, chlorites and other clay minerals. The alkali feldspar, exhibiting chessboard extinction and a low-temperature structural state, consists of albite crosscut by a complex set of K-feldspar veinlets. Abundant zircon grains occur in the phyllosilicate matrix. These rocks have SiO2 = 55-58 wt%, low MgO and CaO contents (1.5-2.7 and ~1.3 wt% respectively), P2O5 below detection limits, high alkali contents (Na2O ~ 4.6 wt%; K2O = 3.4-4.7 wt%). Loss on Ignition (LOI) is relatively high (7-10 wt%) and reflect the high modal abundance of hydrated phases. Zr (511-639 ppm) contents reflect the modal abundance of zircon. Ni content is 12-61 ppm, Cr is below detection limits. The spoon-shaped REE patterns and the oxygen isotope ratios ( 18OSMOW = 14-15 ‰) are compatible with the presence of secondary hydrated phases. Backscattered electron-cathodoluminescence (BSE/CL) images of a hundred twenty-one zircon grains from sample FBZ have been hand-picked under binocular microscope to look for its morphology and internal structure. U and Th data are available for sixty-six grains, while the LAM-ICPMS trace element analyses were performed on eight grains. The external morphology, internal texture and geochemical features allow the recognition of three zircon types. Type 1 zircons are reddish and show near euhedral shapes and weak zoning with REE distribution (LaN < CeN, HREE > LREE; Ce/Ce* > 1 with Ce* = CeN/(LaN+PrN)/2) compatible with the igneous nature. Type 2 zircons are opaque and whitish, elongated and ovoid-shaped. These zircons are internally structureless. REE patterns (spoon-shaped from Ce to Gd, HREE > LREE) suggest that some of them may have suffered at least partial hydrothermal alteration. Th/U ratio in type 1 and 2 zircons ranges from 0.2 to 1.66. Type 3 zircons are turbid opaque sugar-like anhedral fragments with different colors (brownish, pinkish and purplish). These grains are fractured and pitted, with variable degrees of porosity due to abundant tiny milky inclusions (spongy texture). The REE distribution (similar to type 2 zircons), the spongy texture and anomalously high Th/U ratio (up to ~ 34) of type 3 zircons may indicate a hydrothermal origin. Type 1 zircons are considered to represent igneous relics, while type 2 zircons probably are partially altered igneous zircons. Geochemical features of type 3 zircons (namely REE patterns, Th/U ratio) suggest their derivation through hydrothermal processes with the active participation of a F-rich aqueous fluid, which can carry Th and U (and maybe Zr). The hydrothermal nature of type 3 zircons is in agreement with the geochemical and petrographic features of the whole rock.