Pétrologie des nodules de péridotites des maars de nyos et barombi-mbo (ligne volcanique du Cameroun) et étude de leurs inclusions fluides

Abstract

A detailed study of fluid inclusions hosted within minerals of mantle xenoliths from the Barombi-Mbo (BM) and Nyos maars provides information on the trapping conditions of fluids and subsequent upper mantle equilibria and the history of the host en route to the surface. These porphyroclastic to protogranular spinel lherzolites and their fluid inclusions were investigated using petrography, microprobe, microthermometry and Raman Microspectroscopy. The mineral chemistry of the BM xenoliths indicates temperatures of 800 to 1205°C and pressures of 14 to 23 kbar while the results on Nyos xenoliths give temperatures of 1044 to 1134 °C and pressures of 9 to 12 kbar pointing to mantle depths of 42-75 km to BM and 27-36 km to Nyos. Olivine and pyroxene crystals contain abundant CO2-rich inclusions (1-48 μm). In Nyos, Raman spectra show the presence of CO2 and NH2 while CO2 and nahcolite (NaHCO3) were detected in fluid inclusions in the Barombi-Mbo mantle xenoliths. The melting point (Tm) of CO2 varies from -58.0 to -56.6 °C and from -63.0 to -56.8 °C respectively in BM and Nyos xenoliths. 95% of inclusions homogenized in the liquid phase (ThL; L+V →L) and 5% in the vapor phase (ThV; L+V →V) at temperatures ranging from -48.1 to +31.1 °C in BM xenoliths, and from -38.1 to +31.1 °C in Nyos xenoliths. Densities of CO2 inclusions in minerals range from 0.35 to 1.1 g/cm3 at Nyos, and from 0.2 to 1.15 g/cm3 at BM. The high fluid pressures deduced and the corresponding depths are 11 to 8 kbar (33-24 km) at Nyos and 12 to 10 kbar (36-30 km) at BM. The corrected inclusions densities and corresponding pressures and depths set the minimum xenoliths entrainment depths to 50 km beneath BM and 36 km under Nyos maars. Fluid inclusion pressures also reveal that the Moho and Conrad discontinuities lie at depths of 24 and 15 km and of 30 and 13.5 km, respectively at Nyos and BM. The estimated Moho depths are consistent with the geophysical observations along the CVL. Low densities and relative depths probably indicate short magma stops in intermediate magmatic chambers. Before eruption, the last short magma storage and fluid mixing occurred at a depth of less than 1.8 km at BM and 3 km at Nyos.