Volcanic gases vs. mantle fluids: clues from mineral-hosted fluid inclusions in ultramafic xenoliths from Mayotte island (Comoros archipelago, Indian Ocean)

Combining the geochemistry of gas emissions in active volcanic regions with the signature of mineral-hosted fluid inclusions in mantle-derived xenoliths is the next frontier in geodynamics and volcano monitoring and can provide important clues on: i) the nature and evolution of the lithospheric mantle; ii) the storage and mobility of fluids through the lithosphere; and iii) the origin of fluids migrating within the mantle and in the plumbing system underneath active volcanoes.

In this study, we present new mineral and fluid inclusion chemistry (noble gases and CO2) data on a unique suite of mantle-derived xenoliths hosted in phonolite pyroclastic deposits in Mayotte island (Comoros archipelago, Indian Ocean), which was the scene of one of the largest submarine eruptions ever documented from 2018 to 2021 (Jacques et al. 2024).

The studied samples are spinel-bearing harzburgites and lherzolites, and are composed of Cr-spinel (Cr# = 0.4-0.55), Mg-rich olivine (Fo90-92, NiO = 0.3-0.5 wt%), orthopyroxene (Mg# = 91-92; Al2O3 = 1.5-3.0 wt%), and clinopyroxene (Mg# = 91-94; Al2O3 = 2.0-3.5 wt%). The mineral major and trace element distribution indicates that the xenoliths represent fragments of a residual lithospheric mantle which experienced 20 to 25% partial melting.

Olivine-, orthopyroxene-, and clinopyroxene-hosted fluid inclusions are CO2-dominated and have air-corrected 3He/4He isotopic ratios of 5.6-6.8 Ra that are intermediate between the typical signature of Mid-Ocean Ridge Basalt (MORB = 8±1 Ra) and Sub-Continental Lithospheric Mantle (SCLM = 6±2 Ra). Such He isotopic signature is similar to that of subaerial and submarine gaseous emissions in the Mayotte area (Liuzzo et al. 2021; Mastin et al. 2023).

With respect to the mantle xenoliths from the neighbouring Grande Comore Island (Coltorti et al. 1999; Bordenca et al. 2023), the peridotites from Mayotte lie within a narrower compositional range, being moderately depleted and not showing significant metasomatic enrichment. Despite comparable 3He/4He ratios, fluid inclusions in the Mayotte samples have higher 4He/40Ar* values than those of the refractory mantle (Rizzo et al. 2021), likely indicating a shallow overprint by magmatic fluids.

Mantle xenoliths and hosted fluid inclusion data are used here to model the melt-fluid/rock reactions in the lithospheric mantle, the genesis and ponding of magmas linked to the recent volcanic activity at Mayotte and the geodynamic setting of the Comores archipelago.

 

 

References

Coltorti, M., Bonadiman, C., Hinton, R. W., Siena, F., & Upton, B. G. J. (1999). Journal of Petrology, 40(1), 133-165.

Jacques, E., Hoste-Colomer, R., Feuillet, N., Lemoine, A., van der Woerd, J., Crawford, W. C., ... & Bachèlery, P. (2024). Earth and Planetary Science Letters, 647, 119026.

Liuzzo, M., Di Muro, A., Rizzo, A. L., Caracausi, A., Grassa, F., Fournier, N., ... & Italiano, F. (2021). Geochemistry, Geophysics, Geosystems, 22(8), e2021GC009870.

Mastin, M., Cathalot, C., Fandino, O., Giunta, T., Donval, J. P., Guyader, V., ... & Rinnert, E. (2023). Chemical Geology, 640, 121739.

Rizzo, A. L., Faccini, B., Casetta, F., Faccincani, L., Ntaflos, T., Italiano, F., & Coltorti, M. (2021). Chemical Geology, 581, 120400.