Petrology and fluid inclusions geochemistry of ultramafic xenoliths from Mayotte and the origin of the Comoros Archipelago

Abstract

Ultramafic xenoliths provide valuable insights into the physico-chemical, compositional and thermal characteristics of Earth’s mantle along with its heterogeneities. Integrating petrography and mineral chemistry data with the determination of volatile concentrations and isotopic fingerprints in fluid inclusions (FI) in these xenoliths has become the state-of-the-art approach, as it provides not only important information on the nature and evolution of the lithospheric mantle, in terms of melting and metasomatic processes, but also illustrates the storage and migration pathways of volatiles throughout the lithosphere. This tandem approach makes the Comoros Archipelago (Mozambique Channel, Western Indian Ocean) ideal candidates to explore, because the characteristics of the local lithosphere are intimately tied to the complex regional geodynamic setting. Indeed, the origin of the Comoros magmatism remains enigmatic and controversial despite extensive documentation in the literature, as it has been attributed to either a plume-related hot spot or to a passive response to lithospheric break-up. In this study, we investigate a unique suite of ultramafic xenoliths from Mayotte island by combining petrographic observation, mineral phase major and trace element analysis with the geochemistry of noble gases (He, Ar, Ne) and CO2 hosted in olivine (ol), orthopyroxene (opx) and clinopyroxene (cpx) FI. Mineral major elements results show refractory compositions in terms of MgO (Mg#Ol > 90.5, Mg#Opx > 91 and Mg#Cpx > 91.5) and Al2O3 contents (ranging from about 1.60 to 3.00 wt.% for opx and from 2.50 to 3.60 wt.% for cpx, respectively), with Cr# of spinel falling between about 0.4 and 0.55. Overall, these features indicate that the local lithosphere experienced relatively high degrees of melting, from ~20% to 25%. This is also supported by highly depleted chondrite-normalized rare earth element (REE) patterns for cpx, where HREE are roughly (1.5)N. Volatile concentrations and isotopic fingerprints in FI hosted in Mayotte xenoliths are variable, with CO2 standing out as the most abundant gas species. The air-corrected 3He/4He isotopic ratios (5.6 to 6.8 Ra) are intermediate between the typical signatures of MORB (8±1 Ra) and the SCLM (6.1±2.1 Ra) mantle, as measured in local subaerial (Liuzzo et al., 2021) and submarine (Fani Maoré Seamount, Mastin et al., 2023) gas emissions. The relationships between 3He/4He and the extrapolated air-free mantle 21Ne/22Ne ratios, together with 40Ar/36Ar versus 3He/36Ar systematics, suggest a dominating MORB-like component in the upper mantle below the Comoros archipelago, mixed with recycled crustal and atmospheric components, in agreement with recent data of ultramafic xenoliths from Grande Comore island (Ventura Bordenca et al., 2023).