Petrological constraints on melt generation beneath the Asal Rift (Djibouti) using quaternary basalts

Abstract

[1] The temporal evolution of the mantle melting processes in the Asal Rift is evaluated from the chemical composition of 56 new lava flows sampled along 10 km of the rift axis and 9 km off-axis (i.e., erupted within the last 620 kyr). Petrological and primary geochemical results show that most of the samples of the inner floor of the Asal Rift are affected by plagioclase accumulation. Trace element ratios and major element compositions corrected for mineral accumulation and crystallization show a symmetric pattern relative to the rift axis and preserved a clear signal of mantle melting depth variations. While FeO, Fe 8.0 , Zr/Y, and (Dy/Yb) N decrease from the rift shoulders to the rift axis, SiO 2 , Na/Ti, Lu/Hf increase and Na 2 O and Na 8.0 are constant across the rift. These variations are qualitatively consistent with shallow melting beneath the rift axis and deeper melting for off-axis lava flows. Na 8.0 and Fe 8.0 contents show that beneath the rift axis, melting paths are shallow, from 81 6 4 to 43 6 5 km. These melting paths are consistent with adiabatic melting in normal-temperature fertile asthenosphere, beneath an extensively thinned mantle lithosphere. On the contrary, melting on the rift shoulders (from 107 6 7 to 67 6 8 km) occurred beneath thicker lithosphere, requiring a mantle solidus temperature 100 6 40 C hotter. In this geodynamic environment, the calculated rate of lithospheric thinning appears to be 4.0 6 2.0 cm yr À1 , a value close to the mean spreading rate (2.9 6 0.2 cm yr À1 ) over the last 620 kyr.

Using Ambient Noise to Image the Northern East African Rift

“…Mantle velocities of 3.60-4.10 km/s, a 9-16% reduction from the global average, are likely accommodated at least in part by an increase in mantle temperature and/or grain size changes. Petrological estimates and models suggest a mantle potential temperature for this region of between 1350°C (Armitage et al, 2015;Pinzuti et al, 2013;Rychert et al, 2012) and 1490°C (Armitage et al, 2015;Ferguson et al, 2013;Rooney et al, 2012) and therefore ranging from normal ambient mantle tõ 140°C hotter than average (Rooney et al, 2012). We use a Burgers model relating shear velocity and temperature (Jackson & Faul, 2010), for a peridotite mantle at grain sizes between 1-20 mm, (based on xenoliths in the MER; Rooney et al, 2005) with an estimate for the geotherm for the MER, to determine the mantle temperature required to match our observations.…”

Upper mantle temperature and the onset of extension and break-up in Afar, Africa

“…(B) Mean melt fraction given by F mean = Fφ melt dxdz/ F dxdz, where F is the local melt fraction (Plank et al, 1995). Dashed lines are for hydrous-only melt production.…”

Flip-flop detachment tectonics at nascent passive margins in SE Afar

“…1). The presence of oceanic-type lithosphere in SE Afar is documented beneath only the Asal axis, based on petrogenetic studies of its <1 Ma old volcanic rocks, which suggest relatively shallow melting in the depth range of 40-80 km (Pinzutti et al 2013). Outside the currently active segments, seismic refraction and receiver function data recorded throughout eastern and central Afar suggest the existence of a 20-26 km thick crust of transitional character, comprising a continental lower crust intensively intruded by magma (e.g.…”

“…1). Asthenosphere depth after Pinzutti et al (2013). Vertical scale applies to only the upper crust.…”