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

Determinations of the temperatures and pressures of volcanic products during their formation are helpful for understanding magmatic system behaviour and igneous processes. Whereas estimates of the former give insights into the thermal evolution of magmas, the latter provides estimation of the storage depth of magmatic reservoirs. Together they provide practical constraints for understanding the magma storage conditions beneath active volcanic areas and their implications in other research fields, such as geothermal exploration. 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 rift-in-rift area has witnessed repeated magmatic and tectonic activity over its ~1 Ma evolution. Most recently, in November 1978, a one-week-long basaltic fissure eruption led to the birth of the Ardoukôba volcano. Due to its active and unique location, the area has been subject to geothermal exploration since the 1970s. However, although intensive geological, geochemical, and geophysical studies have been conducted in the area, detailed knowledge is still lacking regarding the evolution of storage conditions in the magmatic system beneath the Asal-Ghoubbet rift. For this study, eleven samples, representing the time span from ~300 ka to today, were collected in the rift, and geochemical and geothermobarometric analyses were conducted. Two samples representing the oldest basalts in Djibouti (BD and BS) were also added for comparison. The rock samples are tholeiitic to transitional basalts with MgO contents between 3.3 and 10.2 wt%. The negative correlations of TiO2 and FeO contents, and positive of CaO, with MgO indicate that fractional crystallization is an important process in the magmatic system. The crystal cargo of the erupted lavas consists of plagioclase, clinopyroxene and olivine, in order of decreasing amount. Anorthite contents of plagioclase vary between 45and 89, with a main population at ~An85. Most crystals are in disequilibrium with their carrier melts. Clinopyroxenes have Mg# ranging from 47 to 87,with crystals of Dalha basalts (BD) being the most differentiated. Olivine cores and mantles are mostly primitive with a main composition of Fo84. The majority of clinopyroxenes and olivines are in equilibrium with carrier melts, except for BD. Based on geothermobarometric calculations, we propose the existence of two magma storage depths beneath the Asal-Ghoubbet rift. A mid-deep crustal reservoir is located at ~13 km with a mean temperature of 1200°C. It represents the main magma body with higher temperatures and where most of the minerals crystallized. The second one is found at shallow crustal level (5 to 7 km) with lower temperatures (~ 1120°C). Few crystals formed at this depth, which could indicate a smaller size reservoir. Textural and chemical variations of minerals suggest a connection between the two magma bodies, forming the magma plumbing system. Magma storage conditions appear to have been maintained over time, which implies the continuous renewal of the geothermal heat source in the Asal-Ghoubbet rift.