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Magmatism plays a key role in accommodating and localizing extension during continental breakup. However, how the crustal magmatic systems evolve at the continental-ocean transition is poorly understood. We address these questions by studying the evolution of the magmatic system in the rift of Central Afar (Ethiopia), currently marking the transition from continental rifting to oceanic spreading. We focus on the voluminous and widespread Upper Stratoid Series (2.6–1.1 Ma) and the following Central Afar Gulf Series (1.1–0.6 Ma), the latter corresponding to localization of volcanism in narrow magmatic segments. We carried out the first systematic study of major and trace element mineral chemistry for these two Series and integrated it with geothermobarometry estimates and geochemical modeling, to reconstruct the evolution of the magmatic system architecture during rift localization. The Upper Stratoid magmas evolved by fractional crystallization in a melt-rich, moderately zoned, middle-lower crustal (10–18 km) magmatic system, from where they rose directly to the surface. Polybaric plagioclase convection and dissolution of a plagioclase-rich crystal mush is recorded in the phenocryst texture and chemistry. The Central Afar Gulf magmas evolved at similar depth in a more complex and dynamic storage system, with magma rising and mixing through multiple, relatively small, crystal-rich and interconnected reservoirs. Our study documents the transition during the continental breakup, from an overall stable and melt-rich magmatic system feeding the voluminous and homogeneous Upper Stratoid eruptions to a more dynamic, interconnected and crystal-rich situation feeding small-volume eruption while the rift localizes. [ABSTRACT FROM AUTHOR]

The relationship between rifting and continental flood basalt eruptions is debated, and a control by mantle plume is commonly invoked for flood basalts production. In this work, we investigate the relationship between magmatism and rifting by studying the flood basalts erupted in Afar (4.5–0.6 Ma), known as the Stratoid and Gulf Series. We present new field observations and petrography, major and trace elements analyses and mineral chemistry of lavas collected during a regional campaign in Afar. The Series are characterized by E‐MORB magmatism and residual amphibole in the mantle source, consistent with the contribution of metasomatized sub‐continental lithospheric mantle during partial melting. Differences in garnet‐compatible elements indicate a shallower melting column for the oldest and youngest products (4.5–2.6 Ma Lower Stratoid Series; 1.1–0.6 Ma Gulf Series), and deeper for the products erupted at 2.6–1.1 Ma (Upper Stratoid Series). Incompatible element ratios (Th/Nb, Th/Zr) indicate a higher degree of partial melting for the Gulf with respect to the Upper Stratoid Series. Accordingly with independent geophysical and stratigraphic evidence, we explain our results with rift re‐localization: the Pliocene rift caused thinning of the lithosphere and the Lower Stratoid eruptions in Southern Afar, then the Pleistocene rift jumped to Central Afar under a less‐extended lithosphere, producing the Upper Stratoid and, subsequently, as stretching of the lithosphere localized, the Gulf Series formed. Our findings suggest that rift migration and localization can exert a fundamental control on the spatial variability and character of flood basalts without requiring variations on the activity of the mantle plume. Plain Language Summary: Afar is a volcanically active low‐lying area in Ethiopia, where the separation of the tectonic plates is breaking up the African continent and leading to the formation of new oceans (i.e., Red Sea, Gulf of Aden). It is therefore an excellent place to study the role of volcanism and related subsurface movements of magma during this transition from continent to ocean. In this work, we studied the Stratoid Series, a huge succession of basaltic lava flows covering much of the Afar depression, and the younger Afar Gulf Series lava flows, which were jointly erupted from 4.5 to 0.6 Ma. This study allows us to identify different mantle sources producing the magmas erupted during the breaking‐up process and to recognize two distinct episodes of "break‐up" in Afar. We interpret the process that guides the passage from one to the other and identify the "evolutionary stages" of the break‐up process toward what is going to be a new ocean. Key Points: Metasomatized, amphibole‐bearing lithospheric component in the mantle sources of the 4.5–0.6 Ma basaltic magmatism in AfarDeeper partial melting column for the Upper Stratoid Series mantle source with respect to the Lower Stratoid and the Gulf SeriesStratoid Series flood basalt volcanism related to rift jump of the Red Sea rift branch and not to a single magmatic event [ABSTRACT FROM AUTHOR]

We investigated a 1‐km‐thick sequence of lava flows that erupted over the Afar plume axis in order to better understand the emplacement history of the ∼30 Ma Ethiopia‐Yemen Traps. Geochemical analyses reveal high‐titanium concentrations (TiO2 3.9 ± 0.5 wt%) in basalts close to picritic compositions. Indistinguishable 40Ar/39Ar ages throughout the section define a weighted‐mean of 31.18 ± 0.28 Ma (95% confidence). This date, together with solely normal polarity magnetization directions in 68 geomagnetically independent horizons, constrain the eruption to within chron C12n, with a maximum duration of a few hundreds of kyr for the entire 1‐km‐thick section. The rate of geomagnetic secular variation used as a chronometer refines the duration to only a few tens of kyr, leading to a local extrusion rate of 4–13 km3/yr for the Afar plume head, which greatly exceeds the average rate of 0.3–1.2 km3/yr for the entire Ethiopia‐Yemen Traps. Plain Language Summary: The Ethiopia‐Yemen Traps are the expression of an important geodynamic process where a mantle plume pierced the lithosphere, resulting in the massive outpouring of a few million km3 of basaltic lava over a geologically brief (1–3 Myr) interval. Here, we provide constraints on the extrusion rate of lava flows from the 1‐km‐thick Waja section, which originated from the Afar mantle plume head. Radioisotopic and magnetostratigraphic results indicate that the outpouring of the lava spanned only a few hundreds of kyr. Moreover, incessant eruption provides a continuous record of changes in the ancient magnetic field direction, called secular variation, which can also be used as a chronometer. A limited amount of secular variation at Waja refines the maximum duration to only a few tens of kyr. The corresponding extrusion rate of 4–13 km3/yr greatly exceeds the average rate of 0.3–1.2 km3/yr for the entire Ethiopia‐Yemen Traps. Consistent with geodynamic models predicting that plume head eruptions are the most intense, our results suggest that 20% of the trap basalts were emplaced within ∼0.02–0.2 Myr, whereas the remaining 80% were erupted over ca. 1.5 Myr. Key Points: 1‐km‐thick picritic basalts at the Waja section were emplaced at the onset of chron C12n (∼31 Ma) above the Afar mantle plume headThe Waja section was extruded in <260 kyr according to Ar‐Ar dating and at least 20 kyr using a secular variation geochronometerExtrusion rates above the Afar plume head were >10 km3/yr, compared to 0.3–1.2 km3/yr for the entire Ethiopia‐Yemen Traps [ABSTRACT FROM AUTHOR]