CO<sub>2</sub>-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

Article Article scientifique Article CO<sub>2</sub>-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 eng 2019 author Tene Djoukam Joelle Flore 697398 author Nkoumbou Charles author Tchouankoué Jean Pierre author Pintér Zsanett author Moritz Robert 27897 68-85 10.1016/j.jafrearsci.2018.09.016 1464-343X https://linkinghub.elsevier.com/retrieve/pii/S1464343X1830298X 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 (Fo93-88), orthopyroxene (En91-88Fs11-8Wo2.2-0.8), Crdiopside, 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 °C at pressures of 23 to 14 kbar while the results on Nyos xenoliths give temperatures of 1134 to 1044 °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 μm). 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 °C and from −56.8 to −63.0 °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 +31.1 to −48.1 °C in BM xenoliths, and from +31.1 to −38.1 °C in Nyos xenoliths. Densities of CO2 inclusions in minerals range from 1.1 to 0.35 g/cm3 at Nyos, and from 1.15 to 0.2 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. 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. Nyos and Barombi-mbo maars Mantle xenoliths Mantle equilibria CO2-Rich fluid inclusion Microthermometry Raman spectroscopy Dewey 550 Autre Swiss Confederation Scholarship 42 Département des sciences de la Terre 154 false 692 Mineral Resources and Geofluids Journal of African Earth Sciences 150 eb569d15-7cf0-4ead-9f17-148b3b31f430 Article (Published version) Restricted CO2fuidsJoelleTene.pdf 4261219 application/pdf restrictedAccess Faculté des sciences / Section des sciences de la Terre et de l'Environnement / Département des sciences de la Terre

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CO₂-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	Journal of African Earth Sciences			2019	1,073	1

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Tene Djoukam, Joelle Flore