Plumbing system architecture, magma differentiation, and volatile element evolution in an active rift segment: Constraints from melt inclusions in Asal rift (Djibouti, Afar).

The Asal Rift, situated within the Afar region, presents a unique opportunity to study continental rifting and ongoing break-up mechanisms. Here we present a comprehensive study based on volatile element contents of magmas within the Asal rift's segment. Samples were gathered from various volcanic sub-segments within the active part of the Asal rift, and a subset was collected to document the successive steps of the recent 1978 Ardoukoba eruption. Altogether our new sampling is offering a chronological framework crucial to understanding this magmatic system and the eruptive sequences. Quenched pyroclastic deposits (scoria) were used as they are more likely to preserve the magma's volatile content without significant degassing upon cooling at surface than lava flows. By analyzing over 400 melt inclusions within plagioclase and olivine crystals of 15 new samples, we provide insight into the pre-eruptive volatile content of Asal magmas. The melt inclusions volatile contents (H₂O, CO₂, Cl, and S) were quantified through SIMS analyses at CRPG. Sample preparation for SIMS measurements was carefully achieved to avoid any contamination or volatile loss. After estimation of the volatile migration through the shrinkage bubbles of the melt inclusions by Raman spectroscopy, and correction from post-entrapment crystallization processes, we reconstructed the initial magmatic volatile content at the reservoir depth. This content, combined with detailed petrographic study, field observations, and new dating, allows us to propose a comprehensive picture of the Asal Rift plumbing system architecture, and to discuss its spatial variability and temporal evolution. The wealth of data allowed us to highlight a relative homogeneity of the reservoir volatile contents over the recent Asal rift segment erupted magmas, and to discern the depth range of the Asal igneous reservoir that spans from approximately 5 km to 25 km (based on solubility models of VESIcal (1)). Furthermore, volatile data, combined with in-situ major and trace element analysis, provides insights into magma differentiation, degassing, and into the volatile content of the mantle source. We investigate 3 potential steps of degassing and their effect on volatile, trace and major element: within the plumbing system (during the differentiation), during magma ascent, and at the surface during the eruption (unlikely given the sampling method). Finally, our study focused on the 1978 Ardoukoba eruption within the Asal rift. With a sampling of the entire eruptive sequence, we were able to highlight the evolution of the volatile content during this eruption, showing that the initial differentiated magma reservoir underwent a recharge event before eruption. In conclusion, this new in-situ high-resolution volatile, trace and major element extended dataset 1/ delineated the extensive depth range of the magmatic reservoir, 2/ allows a better understanding of the dynamics of magma reservoirs that feed continental rift systems, and 3/ provides new constraints on the magma evolution, differentiation, and degassing at depth within such a system.

(1) Iacovino, K., Matthews, S., Wieser, P. E., Moore, G. M., & Bégué, F. (2021). VESIcal Part I: An Open‐Source Thermodynamic Model Engine for Mixed Volatile (H₂O‐CO₂) Solubility in Silicate Melts. Earth and Space Science, 8(11).