Petrology and Geochemistry of Basalts from the Red Sea Axial Rift at 18° North

Detailed studies by submersible were carried out in the axial zone of the Red Sea Rift near 18"N during the Soviet Red Sea expedition of the Oceanological Institute of the Academy of Sciences (December 1979-March 1980). The initial bathymetric, magnetic and seismic surveys established the general organization of the symmetric tectonic steps (1-3) descending towards the axial rift. The 45 km wide inner floor of the rift was explored during 21 dives. It is occupied by 100-300 m high, young pillowed volcanoes, isolated or grouped to form elongated hills, frequently cut by open fissures except in the zone of most recent extrusion. The 42 samples collected are typical plagioclase k olivine 5 clinopyroxene & spinel, more or less porphyritic mid-ocean ridge basalts whose compositions were mainly controlled by polybaric fractionation of plagioclase, olivine and minor clinopyroxene. They have been separated into porphyritic and sub-alfhyric groups using modes andmineralogical criteria. Mineral-liquid equilibria, crystal zonation, andmodal proportions indicate some magma mixing but probably only of closely related magma batches within each described group, as can occur inside a single magma chamber. Crystal accumulation is believed to have played a significant role in only a few porphyritic samples. Three sub-groups (from less to more evolved; (a) FeO*/MgO 1.49) were distinguished on the basis of glass and whole-rock major element chemistry. Glass compositions follow the multisaturated cotectic-like curve for MORB-type basalts and show a general evolution very comparable to what is observed on the Mid-Atlantic Ridge near 36"N, but are less diverse than in the FAMOUS area. 87Sr/86Sr, 143Nd/144Nd, and zo6Pb/204Pb data for 4 samples show strong similarities to those from the Mid-Atlantic and East Pacific Ridges, and indicate no continental contamination despite the fact that they have been produced during recent continental break-up and ocean opening. 206Pb/204Pb values, Th/Ta vs. Th/Tb correlations, and rare earth element patterns allow recognition of three different groups of samples, indicating that the Red Sea Rift near 18"N is fed by a heterogeneous mantle source. The chondrite-normalized LREE