[S-GC32] Volatiles in the Earth - from Surface to Deep Mantle

[SGC32-05] Preliminary results of a petrological and geochemical study on mantle xenoliths from the Mediterranean: new constraints on the mantle below Italy

*Antonio Caracausi^1,2, Gabriele Carnevale³, Massimo Coltorti⁴, Barbara Faccini⁴, Laura Italiano³, Giulia Marras⁵, Michele Paternoster^6,2, Silvio Giuseppe Rotolo^3,2, Vincenzo Stagno^5,2, Vittorio Zanon⁷ (1.Departamento de Geología, Universidad de Salamanca, Salamanca, Spain, 2.Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Palermo, Italy, 3.Dipartimento di Scienze della Terra e del Mare, Università di Palermo, Palermo, Italy, 4.Dipartimento di Scienze dell’Ambiente e della Prevenzione, Università di Ferrara, Via Saragat 1, 44121 Ferrara, Italy, 5.Dipartimento di Scienze della Terra, Sapienza Università di Roma, Roma, Italy, 6.Dipartimento di Scienze, University of Basilicata, Potenza, Italy, 7.Instituto de Vulcanologia e Avaliação de Riscos, Universidade dos Açores, Ponta Delgada, Portugal )

Keywords:Mantle, Mediterranean, xenoliths, Fluid inclusions, noble gases, metasomatism

Southern-central Italy is a dynamic tectonic region characterized by active volcanoes (Vesuvio, Ischia and Phlegraean fields) along the western side of the peninsula and a quiescent volcano slightly offset in the central-eastern side (Mt. Vulture). The area in between the two volcanic districts is dominated by active extensional tectonics prone to generate catastrophic earthquakes (Irpinia 1981, M=6.9). The non-volcanic region is also characterized by an extensive outgassing of deep sourced CO₂ (crust vs mantle) that has an active role in the processes related to the genesis of the earthquakes at a regional scale (e.g., Buttitta et al., 2023 and references therein). Therefore, it is crucial to understand the origin of volatiles that actively contribute to the seismogenesis of the area. Recent geochemical investigations highlight that fluids emitted at the surface in these volcanic and seismic areas are strongly affected by physico-chemical processes that modify their pristine composition (e.g., Buono et al., 2023; Buttitta et al., 2023), hence these fluids are not the best candidates to figure out the mantle component in the fluids that reach the surface.
In order to improve the knowledge of the volatiles in the mantle at a regional scale we studied ultramafic xenoliths from Mt. Vulture, that together with those from Hyblean volcanism (e.g., Marras et al., 2023_a) are the rare findings of mantle products in the southern-central Italy volcanic district. Xenolihs are wehrlitic in modal composition and constitute the core of pelletal lapilli that are surrounded by a 3–10 mm thick rim of microphenocrysts (mainly olivine and diopside). The wehrlitic cores are essentially composed of Mg-rich olivine (Fo90-91, NiO varying from 0.35 to 0.38 wt. %),diopside (Wo46-48, En47-48, and Fs4-5) with Cr2O3 ranging from 1.3 to 1.5 wt. % and Cr-spinel. The Mg# values of olivine and clinopyroxene are uniform (0.90–0.92)..
Our study provides the first micro-thermometric data of fluid inclusions (FI) in Mt. Vulture xenoliths (Carnevale et al., 2022). In olivine and clinopyroxene of the ultramafic xenoliths, the early stage FIs are more abundant than late stage FI. These FI contain pure CO₂trapped/re-equilibrated at the local the crustal-mantle transition zone (32 km deep).
The He isotopic signature of fluids trapped in the FI in olivine ranges from 6.0 to 6.2 Ra (Ra is the He isotopic signature in atmosphere) and they are higher than the same ratios in FI within the clinopyroxene (5.6 to 5.7 Ra). This range of the He isotopes (5.6-6.2) overlaps that of the Sub Continental Lithospheric Mantle (6.1 ± 0.9). In addition, the ⁴He/⁴⁰Ar* ratios in the same samples (where the ⁴⁰Ar* is the amount of ⁴⁰Ar corrected for the atmospheric contribution) fall in the typical mantle range, supporting the presence of mantle-derived volatiles within the trapped fluids. We also observed a positive relation between ³He (a mantle component) and ³⁶Ar (an atmospheric component), as previously recorded worldwide in mantle xenoliths, evidencing a possible deep origin of the atmospheric component, recorded in the mantle because of subduction processes that affected the Mediterranean mantle.
Measurements of the Fe3+/Fe_tot ratios, by in-situ synchrotron Mössbauer spectroscopy show ratios of 0.03–0.05 for olivine and 0.40–0.45 for the spinel, being the latter range higher than those reported in literature for mantle spinel harzburgites and lherzolites (Marras et al., 2023_b). Given the evidence of the mantle origin of the spinels and other volatiles trapped within the FIs, we propose that the high fO2 (between 0.8 and 1.0 log units above the fayalite–magnetite–quartz buffer) together with the other geochemical evidences likely could result from the interaction between a pristine spinel lherzolite/harzburgite and a CO₂-rich metasomatic agent at mantle depths.

References
Buono et al., 2023. Geology.
Buttitta et al., 2023. Science of The Total Environment.
Carnevale et al. (2022). Geophys. Res. Lett.
Marras et al., 2023_a. Lithos
Marras et al., 2023_b. European Journal of Mineralogy