The genesis of the first group of rhyolites can be easily explained by
closed-systemfractional crystallization fromparentmaficmagmas similar
to the Gedemsa basaltswith the lowest 87Sr/86Sr and highest 143Nd/
144Nd. The second group, instead, cannot be explained by closed-system
processes because it is characterized by a strong enrichment in
radiogenic Sr. Given the low Sr concentration in the range 2–
23 ppm (Tables S1, S2), the possibility that the measured Sr isotopic
Afar mantle plume average composition, and several other samples
from both Gedemsa and Fanta 'Ale volcanoes fall in its variation field
(Table 1; Fig. 6).
Not any among the investigated mafic volcanic rocks has the geochemical
features of a primary magma. Generally, the analyzed mafic
rocks from both volcanoes have Mg# ≤56, exept for the two Gedemsa
basalts of trend 1 in Fig. 8, with Mg# ~63 (Table S1). Using these primitive
samples it can be worthwhile to make an attempt to model the
source composition. In order to reproduce the incompatible element
distribution of the two primitive basalts, a mixture between a depleted
(88%) and an enriched (12%) mantle components must be chosen as
source composition; a pure enriched mantle source would result in
melts too much enriched in incompatible elements. The subcontinental
lithospheric mantle of McDonough (1990) has been taken
as representative of the enrichedmantle component,whereas the composition
of DMM has been taken from Salters and Stracke (2004). Nonmodal
batch melting of 7% of such a source originates a melt of which
the incompatible element distribution matches with good agreement that of the two Gedemsa primitive basalts