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Geochemical and Petrological Aspects of Karthala Volcano

2015; Springer Nature; Linguagem: Inglês

10.1007/978-3-642-31395-0_23

2195-7029

Patrick Bachèlery, Christophe Hémond,

earthquake and tectonic studies

The eruptive activity at Karthala consists of hypersthene-normative and mildly-alkalic basalts and some hawaiites, with MgO contents varying from approximately 4 % to more than 20 % for oceanites and ankaramites. Most of La Grille volcano’s products are basanites and olivine nephelinites, often with a significant proportion of olivine. Although these rocks are silica under-saturated, modally they rarely contain crystals of feldspathoïds. Olivine and augite are the dominant phenocrysts in the Karthala and La Grille lavas (more than 40 % vol. in oceanites and ankaramites). Plagioclase is also present in some phenocryst-rich samples, mainly from the oldest units of the “Karthala ancien” and “M’Badjini” massifs. Plagioclase also occurs in some recent aphyric lavas of the summit caldera. The consistent variations in the Karthala lava compositions observed for both major and trace element contents are in agreement with crystal fractionation or accumulation. Conversely, La Grille lavas do not follow clear fractionation trends. Large variations in incompatible trace elements for a given MgO %, negative correlation between incompatible trace elements and SiO2, and K depletion are symptomatic of magma generation driven by variable degrees of partial melting of a source containing residual amphibole (and/or phlogopite). Mantle-normalized trace elements patterns for Karthala lavas show a pattern typical of Oceanic Island Basalts generated from a mantle source containing garnet and amphibole. The Comoros plume represents a “low 3He/4He” hotspot dominated by recycled 4He-rich material. Grande Comore Island is located on the ~140 Ma-old oceanic crust of the Somali Basin. All Grande Comore melts represent mixtures between plume-derived and peridotitic lithosphere-derived melts. The compositional variability of alkali basalts from Karthala is consistent with the mixing of plume—and lithosphere-derived melts, with a higher contribution of an EM1 mantle source in the “Karthala actuel” and “Karthala recent” lavas. The “Karthala ancien” and “M’Badjini” units require a source containing a predominantly HIMU component, some enriched EM1, and smaller amounts of a depleted DM component. La Grille lavas more likely reflect a source containing a significant involvement of a DM component, in addition to a HIMU component, and a lower degree of melting.