Petrogenesis of rift related marginal basalts from the Asal-Ghoubbet rift area, Republic of Djibouti: Implications for magma genesis and mantle source characteristics

Abstract

This research investigates the Marginal basalts in the Asal-Ghoubbet rift, a poorly understood geological feature, with the aim of clarifying magma sources and identifying the mantle reservoir responsible for forming basaltic rocks in the region. The study involved analysing major geochemical elements, rare earth elements and radiogenic isotopes of Sr, Nd and Pb from basalt samples collected in the Marginal basalts of the rift. Additionally, the mineral composition of plagioclase, olivine, pyroxene and FeTi oxides was examined using electron probe microanalysis (EPMA) on polished thin sections. Geochemical results indicate a tholeiitic composition for the Marginal basalts, with evidence of fractional crystallization driven initially by clinopyroxene and olivine, followed by plagioclase crystallization. Thermobarometric and MELTS modelling estimated magma storage depths ranging from the Moho to the upper crust. The basalt is slightly enriched in light rare earth elements (LREEs) and depleted in Heavy Rare Earth Elements (HREEs), similar to the characteristics of enriched Mid-Ocean Ridge Basalt (E-MORB) and Ocean Island Basalt (OIB). Isotopic data of Sr and Nd reveal sources that vary between the high μ mantle source (HIMU) and the dominant prevalent mantle source (PREMA). Furthermore, Pb isotopic signatures suggest an influence from an enriched mantle (EM) component, plotting between the EMI and EMII mantle reservoirs fields. Forward-melting modelling and isotopic evidence suggest that the Afar mantle plume is a significant contributor to the magma source of Marginal basalts in the Asal-Ghoubbet rift. Overall, the study enhances understanding of the magma plumbing system in this area, elucidating the process governing magma chamber evolution and the nature of the mantle reservoirs that generated these basaltic rocks.