Geochemical evidence for a lithospheric origin of the Comoros Archipelago (Indian Ocean) as revealed by ultramafic mantle xenoliths from La Grille volcano

Author(s): Claudio Ventura Bordenca, Barbara Faccini, Antonio Caracausi, Massimo Coltorti, Andrea Di Muro, Theodoros Ntaflos, Raphaël Pik, Andrea Luca Rizzo, Marco Liuzzo, Alessandro Aiuppa
Abstract: Petrology and fluid inclusions (FI) geochemistry are increasingly used in tandem to constrain the compositional features and evolution of the lithospheric mantle. In this study, we combine petrography and mineral chemistry with analyses of noble gases (He, Ne and Ar) and CO₂ in olivine, orthopyroxene- and clinopyroxene-hosted FI, as well as radiogenic isotope (Sr-Nd-Pb) systematics of ultramafic xenoliths collected at La Grille volcano in Grande Comore Island, aiming at better characterizing one of the most enigmatic and controversial portions of the western Indian Ocean lithospheric mantle. Xenoliths have been divided in three groups on the basis of their textural features: Group 1 (Opx-bearing), Group 2 (Opx-free) and Group 3 (Cumulate). Overall, petrographic observations and mineral phase compositions indicate that the sampled lithospheric portion experienced variable degrees of melting (from 5% to 35%), recorded by Group 1 most refractory harzburgites and lherzolites, as well as modal metasomatic processes as evidenced by the crystallization of cpx at the expense of opx in Group 1 fertile lherzolites and wehrlite and by Group 2 xenoliths. Crystallization of slightly oversaturated basic silicate melts seems also to have occurred, as shown by Group 3 xenoliths. A positive trend between temperature and ƒO₂ is evident, with Group 2 and 3 xenoliths testifying for hotter and more oxidised conditions than Group 1. The variability of the ⁴He/⁴⁰Ar* ratio (0.02–0.39) in Group 1, significantly below typical values of a fertile mantle (⁴He/⁴⁰Ar* = 1–5), can be explained by the variable degrees of partial melting coupled to metasomatic enrichment that may account for modifying ⁴He/⁴⁰Ar*, as also indicated by the mineral composition. He-Ar-CO₂ relationships support the presence of a metasomatic CO₂-rich process post-dating the melt extraction and the cumulate formation. The air-corrected ³He/⁴He isotopic ratios (6.30 to 7.36 Ra) are intermediate between the MORB mantle signature (8 ± 1Ra) and the SCLM (6.1 ± 0.9 Ra). The Ne and Ar isotopic signatures (²⁰Ne/²²Ne, ²¹/Ne/²²Ne and ⁴⁰Ar/³⁶Ar) are consistent with mixing between an air-derived component and a MORB-like mantle, supporting the hypothesis for a lithospheric origin of the Comoros magmas, and arguing against any deep mantle plume-related contribution. This is also corroborated by combining Ne with He isotopes, showing that La Grille ultramafic xenoliths are far from the typical plume-type compositions. Sr-Nd-Pb isotope systematics in opx and cpx from La Grille additionally support a MORB-type signature for the lithospheric mantle beneath the area.
Organisation(s): Department of Lithospheric Research
External organisation(s): Università degli Studi di Palermo, Universitá degli studi di Ferrara, Istituto Nazionale di Geofisica e Vulcanologia (INGV) Sezione di Palermo, Universidad de Salamanca, Université de Paris, Université de Lorraine, Università degli Studi di Milano-Bicocca, Istituto Nazionale di Geofisica e Vulcanologia
Journal: Lithos
Volume: 462-463
ISSN: 0024-4937
DOI: https://doi.org/10.1016/j.lithos.2023.107406
Publication date: 12-2023
Peer reviewed: Yes
Austrian Fields of Science 2012: 105120 Petrology, 105105 Geochemistry
Keywords
ASJC Scopus subject areas: Geology, Geochemistry and Petrology
Portal url: https://ucrisportal.univie.ac.at/en/publications/7e851858-5c6d-46bb-a1b5-7fee2bdcb622

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