Nd‐Sr‐Pb isotopic variations along the Gulf of Aden: Evidence for Afar Mantle Plume‐Continental Lithosphere Interaction

Abstract

We report on the rare earth and Nd‐Sr‐Pb isotopic composition of basalts dredged along the Sheba Ridge axis in the Gulf of Aden and its extension into the Gulf of Tadjoura and subaerial basalts from the Ardoukoba Rift in east Afar. The sampling profile provides a means to study the evolutionary nature of the mantle sources involved in the melting process associated with the interaction of the head of a starting mantle plume with continental lithosphere and an ocean basin at a nascent stage of formation. An 800‐km‐long Nd‐Sr‐Pb isotopic and La/Sm gradient, sinusoidally modulated, is apparent from the Afar eastward. The first enrichment peak occurs in the Gulf of Tadjoura, where diffuse extension of the Danakil‐Aisha continental lithospheric block and westward rift propagation is currently progressing. The second enrichment peak at 46°E is associated with a mantle buoyancy anomaly and related constructional volcanism. East of 48°E, the MORBs are typically light rare earth element depleted, whereas 206Pb/204Pb and 87Sr/86Sr slightly increase, suggesting recent decoupling. In Nd‐Sr‐Pb isotope ratio space, three distinct vector trends are observed within a plane. The mixing vectors point toward three mantle source end‐members which can be interpreted as Pan‐African continental lithosphere along the Gulf of Tadjoura (a hybrid EM‐l‐EM‐2), a mantle plume (relatively young HIMU‐like) which dominates the 46°E anomaly, and the depleted asthenosphere east of 48°E (DUPAL‐like). Combined data from the Gulf of Aden‐Red Sea‐Afar‐Ethiopian rifted zones suggest a radial pattern of geochemical and isotopic variation about the Afar. A working dynamical‐thermal model is presented for the past 30–40 m.y. history of the Horn of Africa. It invokes both passive rifting/seafloor spreading in the Red Sea/Gulf of Aden and the flattening and interaction of the starting head of a toruslike thermal mantle plume with the Pan‐African continental lithosphere which is slowly moving northeastward with the plume head attached at its base. The plume flattened into a pancakelike form, twice the diameter of the original head which is estimated to be of the order of 700 km in diameter. The thinning of the lithosphere by stretching and thermal erosion by the mantle plume has not yet been completed. A working ternary mixing model constrained by the isotope data indicates that within the 800–1000 km radius of influence of the Afar mantle plume, melting of the lithosphere mantle and the depleted asthenosphere apparently entrained by the ascending mantle plume dominates initially. Only along the three rifting zones intersecting the flattened plume ring, 450±150 km in radius, composed of original HIMU‐like plume material does the original plume component play a more dominant role. Judging from the spatial isotopic composition variation of the basalts, the plume torus may be apparent along (1) the 46°E Gulf of Aden anomaly where seafloor spreading is now well established; (2) the 13°–16°N southern Red Sea segment, which represents a rift zone at a transient st...

Tracing the Indian Ocean Mantle Domain Through Time: Isotopic Results from Old West Indian, East Tethyan, and South Pacific Seafloor

“…Principal MORB data sources include those 1988; Klein et al, 1988;le Roex et al, 1989;Mahoney cited in the text, plus Ito et al (1987), White et al (1987), Hanan & et al, 1989Hanan & et al, , 1992Storey et al, 1989Storey et al, , 1992Pyle et al, Schilling (1989), Dosso et al (1993), Bach et al (1994), Mahoney et al (1994), and other recent studies cited therein. 1992, 1995Weis et al, 1992;Hickey-Vargas et al, 1995;Rehkämper & Hofmann, 1997).…”

“…Schilling et al, 1992;Volker et al, 1993).Indian MORB are characterized, in particular, by lower Ocean region is becoming increasingly well mixed through time, and * …”

Nd, Pb, and Sr isotope composition of Late Mesozoic to Quaternary intra-plate magmatism in NE-Africa (Sudan, Egypt): high-μ signatures from the mantle lithosphere

“…The field of Ethiopian flood basalts (Pik et al 1999) indicates strong crustal contributions to many samples of the low Ti series, whereas many samples of the high Ti series resemble mantle compositions. a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a includes Arabian intra-plate magmatic rocks (Bertrand et al 2003;Shaw et al 2003); Afar flood basalts and younger basalts ascribed to Afar plume activity are in field (b) with high data density and field (c), which includes all data (Deniel et al 1994;Baker et al 1996;Barrat et al 1998;Pik et al 1999;Kieffer et al 2004); proposed Afar plume compositions (1) (Baker et al 1996 mainly based on data of Deniel et al 1994) and (2) (Pik et al 1999); basalts from the Red Sea and Gulf of Aden spreading centres (Altherr et al 1988Schilling et al 1992;Volker et al 1993Volker et al , 1997; field (d) includes data o...…”

“…9). Substantial contributions of material from the Afar-plume indicating young metasomatism according to process (1) are precluded for NE-African and Arabian (discussion in Bertrand et al 2003) intra-plate magmatic rocks, because a high-µ signature of this plume has not been established (e.g., Vidal et al 1991;Schilling et al 1992;Deniel et al 1994;Baker et al 1996Baker et al , 1997Baker et al , 1998Pik et al 1999).…”

“…Altherr et al 1990;Volker et al 1997;Schilling et al 1992). The arrays of Sr-Nd and Pb isotopic compositions of the ridge basalts ( Fig.…”