Submarine basalts from the Red Sea: New Pb, Sr, and Nd isotopic data

Abstract

High‐precision Pb isotope analyses are reported for Holocene basalts from the axial trough of the Red Sea in order to resolve the discrepancy between Pb isotope data published in two recent papers [Dupré et al., 1988; Altherr et al., 1990]. Carefully handpicked glassy rock chips were leached in strong acid prior to dissolution to minimize contamination and alteration effects. In addition, Sr and Nd isotopes were analysed from the same sample solution to obtain a complete isotopic profile along this nascent oceanic basin. Red Sea basalts show a wide range in Pb isotopes with 206 Pb/ 204 Pb ratios extending from 18.1 to 19.6. In a plot of 207 Pb/ 204 Pb versus 206 Pb/ 204 Pb, the basalts lie next to, or slightly above, the Northern Hemisphere Reference Line with Δ7/4 values between −1.5 and +3.1, in accord with the data from Dupré et al. [1988]. High Δ7/4 values (up to +12) reported by Altherr et al. [1990] are most probably due to severe sample contamination. Basalts with chemical and isotopic signatures typical for N‐type MORBs ( 206 Pb/ 204 Pb = 18.1–18.3; 87 Sr/ 86 Sr ≈ 0.70269; 143 Nd/ 144 Nd ≈0.51315) occur in the central region of the Red Sea at about 21.4°N. To the North and South of this central region, Pb and Sr isotope ratios show a systematic increase accompanied by a decrease in Nd isotope ratios. N‐type MORBs from the central part of the Red Sea display Pb, Sr, and Nd isotopic signatures indistinguishable from basalts from the Carlsberg Ridge of the Indian Ocean. Thus there is no evidence that these two oceanic spreading centres tap chemically different convective systems in the asthenosphere as has been suggested previously.