CO_2-rich fluid inclusions in mantle xenoliths from Barombi-Mbo and Nyos maars (Cameroon volcanic line, central Africa): Upper mantle equilibria and history of magma ascension

摘要

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. They are mostly composed of olivine (Fo(93-88)), orthopyroxene (En(91-88)Fs(11.8)Wo(2.2-0.8)), Cr-diopside, aluminous spinel (Cr#: 0.32-0.08) and occasional accessory pargasite and labradorite. Thermobarometric calculations based on the mineral chemistry of the BM xenoliths indicate temperatures of 1205 to 800 degrees C at pressures of 23 to 14 kbar while the results on Nyos xenoliths give temperatures of 1134 to 1044 degrees C at pressures of 12 to 9 kbar consistent with mantle depths of 75-42 km to BM and 36-27 km to Nyos. Olivine and pyroxene crystals contain abundant CO2-rich inclusions (48.1 mu m). In Nyos, Raman spectra allow the detection of CO2 and unknown species containing NH2 while CO2 and traces of nahcolite (NaHCO3) were detected in fluid inclusions in the Barombi-Mbo mantle xenoliths. The melting point (Tm) of CO2 varies from -56.6 to -58.0 degrees C and from -56.8 to -63.0 degrees C respectively in BM and Nyos xenoliths. 95% of inclusions homogenized in the liquid phase (Th-L; L + V - L) and 5% in the vapor phase (Th-V; L + V - V) at temperatures ranging from +31.1 to -48.1 degrees C in BM xenoliths, and from +31.1 to -38.1 degrees C in Nyos xenoliths. Densities of CO2 inclusions in minerals range from 1.1 to 0.35 g/cm(3) at Nyos, and from 1.15 to 0.2 g/cm(3) 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. Taking into account the post-trapping stretch, 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.