Crozet Rise, Indian Ocean: Petrology, Geochemistry, and Geodynamic Setting

Abstract One of the widely discussed issues of genesis of oceanic rises is the possibility of their occurrence due to the interaction of hot spots and mid-ocean ridges. The Crozet Rise (less than 9 million years old) is located in the western Indian Ocean and consists of a group of volcanic islands with alkaline lavas. The paper studies basanites of the largest island of the archipelago, Possession Island. Obtained results of petrological and geochemical studies in combination with available geophysical and numerical models’ data were used to determine the conditions of the formation of the Possession Island lavas, as well as the relationship between the features of the origin of the Crozet Rise and other rises of the western Indian Ocean located near South Africa. The olivine–clinopyroxene–spinel phenocryst assemblage observed in the lavas of Possession Island corresponds to deeper crystallization conditions compared to the typical oceanic magmas, which are usually characterized by the olivine–plagioclase–clinopyroxene assemblage. The fractionation of melts could have occurred in the intermediate chamber as a result of the magma replenishment at pressures of 8–10 kbar and temperatures of 1200–1300°C during the volcanic edifice formation 9 million years ago. The geochemical characteristics of the Possession Island lavas, including the Sr, Nd, and Pb isotopic composition, indicate an enriched source, possibly with an admixture of a HIMU-type component (with the high initial U/Pb and U/Th ratios), and are close to the composition of enriched magmas of the Indian Ocean rises (Crozet, Marion, and Bouvet), but differ from the composition of basalts of the Conrad and Afanasiy Nikitin rises and Mozambique Ridge in the western Indian Ocean. The enriched HIMU-type source is associated mainly with Gondwanan ancient continental mantle. The fragments of continental crust or oceanic mantle enriched at the early evolution stages could be involved in melting, since the Crozet Rise is an intraplate structure recently formed under the influence of a hotspot on a relatively ancient oceanic lithosphere. In this case, the fractionation of alkaline magmas occurred at a depth of ~ 25–30 km. The formation of the eastern part of the Crozet Rise occurred under the influence of the Crozet–Marion hot spot, which can be considered a satellite of the large African plume, which determined the entire evolution of the Southern Ocean.