Characterization and evolution of the basaltic magma reservoir beneath the Asal-Ghoubbet rift, Republic of Djibouti

Abstract

The Asal-Ghoubbet rift is one of the emergent segments of the Aden Gulf oceanic ridge, which spreads westward on land into the triple junction zone of the Afar depression. The studied rift has been characterized by repeated magmatic and tectonic activity over its ∼1 Ma evolution, and during the latest volcano-tectonic episode in November 1978, a one-week-long basaltic fissure eruption led to the formation of the Ardoukôba volcano. This study investigates the evolution of the basaltic magma reservoir beneath the rift over an eruptive time span from ∼300 ka to the present. In total, eleven basaltic samples were collected and analyzed. We carried out petrography and major and minor element analyses for whole rock, microcrystalline groundmass, glass, and mineral phases. The lava compositions range from transitional to tholeiitic basalts and their chemical variability can largely be explained by fractional crystallization, along with concurrent mixing. The most common macrocryst phase is bytownite plagioclase, whereas olivine and clinopyroxene macrocrysts are found in lesser proportions. Mineral-melt equilibria suggest that olivine and clinopyroxene are dominantly phenocrystic in all formations, crystallizing from their respective carrier liquid. Most plagioclase macrocrysts are antecrysts and too primitive to have crystallized from their carrier liquid. Clinopyroxene-melt and melt-only barometric results indicate that magmas of the last ∼300 ka were extracted from a constant mid- to deep crustal zone at a depth between 7 and 18 km. In contrast, crystallization temperatures indicate progressive cooling and thus evolution of the degree of differentiation, which are interrupted by periodic replenishment of fresh primitive melts. Overall, similar magma storage conditions appear to have been maintained over time, which implies continuous renewal of the geothermal heat source in the Asal-Ghoubbet rift. These results are consistent with previous geophysical studies and thus can be used to update and strengthen our knowledge of the studied magma plumbing system.