Exceptionally High Emplacement Rate of the Afar Mantle Plume Head

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 head The Waja section was extruded in 10 km3/yr, compared to 0.3–1.2 km3/yr for the e