Geochemical evidence of mantle reservoir evolution during progressive rifting along the western Afar margin

Abstract

evidence of mantle reservoir evolution during progressive rifting along the western Afar margin. Geochimica et Cosmochimica Acta, Elsevier, 2013Elsevier, , 102, pp.65-88. 10.1016Elsevier, /j.gca.2012 Geochemical evidence of mantle reservoir evolution during progressive rifting along the western Afar margin AbstractThe Afar triple junction, where the Red Sea, Gulf of Aden and African Rift System extension zones converge, is a pivotal domain for the study of continental-to-oceanic rift evolution. The western margin of Afar forms the southernmost sector of the western margin of the Red Sea rift where that margin enters the Ethiopian flood basalt province. Tectonism and volcanism at the triple junction had commenced by $31 Ma with crustal fissuring, diking and voluminous eruption of the EthiopianYemen flood basalt pile. The dikes which fed the Oligocene-Quaternary lava sequence covering the western Afar rift margin provide an opportunity to probe the geochemical reservoirs associated with the evolution of a still active continental margin. 40 Ar/ 39 Ar geochronology reveals that the western Afar margin dikes span the entire history of rift evolution from the initial Oligocene flood basalt event to the development of focused zones of intrusion in rift marginal basins. Major element, trace element and isotopic (Sr-Nd-Pb-Hf) data demonstrate temporal geochemical heterogeneities resulting from variable contributions from the Afar plume, depleted asthenospheric mantle, and African lithosphere. The various dikes erupted between 31 Ma and 22 Ma all share isotopic signatures attesting to a contribution from the Afar plume, indicating this initial period in the evolution of the Afar margin was one of magma-assisted weakening of the lithosphere. From 22 Ma to 12 Ma, however, diffuse diking during continued evolution of the rift margin facilitated ascent of magmas in which depleted mantle and lithospheric sources predominated, though contributions from the Afar plume persisted. After 10 Ma, magmatic intrusion migrated eastwards towards the Afar rift floor, with an increasing fraction of the magmas derived from depleted mantle with less of a lithospheric signature. The dikes of the western Afar margin reveal that magma generation processes during the evolution of this continental rift margin are increasingly dominated by shallow decompressional melting of the ambient asthenosphere, the composition of which may in part be controlled by preferential channeling of plume material along the developing neo-oceanic axes of extension.

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

“…A recent study [Rooney et al, 2012a] suggest that Hf-Pb data from the Asal Rift have affinity to, and overlap with, the East Sheba Ridge (Aden Ridge) data and has also Indian Ocean-like Hf and Pb isotope signatures [Rooney et al, 2012a]. However it has been also shown that these lava flows exhibit values consistent with mixing between Afar plume and regional lithospheric mantle [Schilling et al, 1992;Rooney et al, 2012a;Rooney et al, 2013]]. In the Afar depression, the isotopic signature of a mantle plume is most pronounced toward Djibouti [Schilling et al, 1992;Rooney et al, 2012a], consistent with maximum temperature values recorded in this area [Rooney et al, 2012b].…”

“…[45] However, trace element patterns ( Figure 5) and isotopic compositions [Rooney et al, 2012a;Rooney et al, 2013] show that the Asal Rift lava derived from a fertile source. Because the fertile mantle starts melting deeper and melts more, the calculated final pressures of melting slightly decrease, and the initial pressures and crust thicknesses slightly increase compared to a normal mantle composition.…”

“…Furthermore, the contribution of the upper mantle asthenosphere (mid-ocean ridge basalt (MORB)-like source) is best recognized in the young lavas (<4 Ma), particularly in the Tadjoura area and the Asal Rift (3-0 Ma), where Hf-Pb data have affinity to, and overlap with, the East Sheba Ridge (Aden Ridge) data and has Indian Ocean-like Hf and Pb isotope signatures [Rooney et al, 2012a]. However it has been shown that these lava flows exhibit values consistent with mixing between Afar plume and regional lithospheric mantle [Schilling et al, 1992;Rooney et al, 2012a, Rooney et al, 2013.…”

The Riphean Magmatism Preceding the Opening of Uralian Paleoocean: Geochemistry, Isotopes, Age, and Geodynamic Implications

“…1 -mantle sequence and average composition points of the mantle (N-MORB, E-MORB, OIB) and crustal (UCC and LCC, Late and lower continental types of crust, correspondingly) sources; RSRRed Sea Rift [Rogers, 1993;Volker et al, 1997]; EAR -East African Rift [Deniel et al, 1994;Duffield et al, 1997;Barrat et al, 1998;Lowenstern et al, 2006;Daoud et al, 2010;Teklay et al, 2010;Field et al, 2012;Rooney et al, 2013]; 2 -sill-and-dike swarms; 3 -amphibolites (metabasallts) of the Nazyam mountains. Letters in circles mean the average compositions of basalts of Ai (A) and Mashak (M) formations, trachybasalts of the Sibirka deposit (S).…”

“…1 -мантийная последовательность и точки средних составов мантийных (N-MORB, E-MORB, OIB) и коровых (UCC и LCC, верхняя и нижняя континентальная кора, соответственно) источников; RSR -Красноморский рифт [Rogers, 1993;Volker et al, 1997]; EAR -Восточно-Африканский рифт [Deniel et al, 1994;Duffield et al, 1997;Barrat et al, 1998;Lowenstern et al, 2006;Daoud et al, 2010;Teklay et al, 2010;Field et al, 2012;Rooney et al, 2013]; 2 -силлово-дайковые рои; 3 -амфиболи-ты (метабазальты) назямских гор. Буквы в кружках -сред-ние составы базальтов айской (А) и машакской (M) свит, трахибазальтов месторождения Сибирка (S).…”