Geochemistry of basalts from Manda Hararo, Ethiopia: LREE-depleted basalts in Central Afar

Abstract

Major, trace element and isotopic (Sr, Nd and O) data are presented for a series of Quaternary basalts from the Manda Hararo Rift in the Afar depression of Ethiopia. Two types of basalts were erupted. The first and most abundant was a light rare earth element (LREE)-enriched type (La n /Sm n =1.5–3.0) with a restricted range in 87Sr/ 86Sr ratios from 0.70362 to 0.70373. These basalts display homogeneous 143Nd/ 144Nd ratios ( ε Nd=+5.2) and are comparable to those reported for the other Quaternary volcanics from Afar suggesting the involvement of similar mantle sources. More interestingly, the δ 18O values vary significantly between +5.15‰ and +6.1‰ and are strongly correlated with the Ta/Th ratios. It is proposed that this may be due to contamination by hydrothermally altered gabbroic cumulates. The second type of basalt is represented by the rare occurrence of LREE-depleted basalts, which are chemically and isotopically distinct from the N-MORB erupted by the nascent oceanic ridges of the Red Sea and the Aden Gulf. They display small positive Eu anomalies (Eu/Eu*=1.03–1.08) and have high Ba/Rb and Sr/Ce ratios. Such anomalies have been previously reported for the strongly LREE-depleted olivine tholeiites and picrites from Iceland (e.g. [J. Geophys. Res. 98 (1993) 15803]) and in one Oligocene basalt from the Ethiopian Plateau [Geochim. Cosmochim. Acta 63 (1999) 2263]. Trace element, Sr and Nd isotope systematics suggest the involvement of a discrete but minor LREE-depleted component in the genesis of these basalts. The latter is not depleted MORB mantle but is possibly an intrinsic part of the plume. By analogy with the Icelandic case, we tentatively propose that this subordinate component could be a remnant of an old recycled oceanic lithosphere.