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 4–5 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 ± olivine ± 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-aphyric groups using modes and mineralogical criteria. Mineral-liquid equilibria, crystal zonation, and modal 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<1·22; (b) 1·16<FeO^*/MgO < 1·48; and (c)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 arc less diverse than in the FAMOUS area. ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, and ²⁰⁶Pb/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 patterns strongly suggest that variations in the degree of melting also played a significant role. A Tb/Th vs. Th diagram shows that the crystal fractionation inside each'group of samples cannot exceed 20%. Transition element versus hygromagmatophile element correlations show that: (a) these magmas have already fractionated olivine; (b) no mixing between a very primitive magma and a more evolved one occurred; and (c) clinopyroxene did not play a major role in the fractionation. The geochemical and petrological data clearly identified two magma groups related to two distinct volcanic ridges and a third magma group related to the rift floor. The new data set, compared with the FAMOUS-Narrowgate set for the Mid-Atlantic Ridge provides some constraints on magma chamber evolution beneath IOW spreading rate ridges. It suggests the possibility of ephemeral ( 102-103 y) magma chambers producing magma batches which build the volcanic ridges, alternating with more steady-state magma chambers (on the scale of less than a few lo5 y), less than 2 km wide, which could produce larger volumes of magma erupting over much larger surfaces, smoothing the topography of the axial rift. Each of these magmas would have its own geochemical and petrological signature while remaining within a restricted range of MORB-type compositions.

Présentation de l'étude pétrographique et géochimique des échantillons prélevés dans la zone axiale du rift de la Mer Rouge (18°N). Il s'agit de basaltes de dorsale océanique (MORB) qui peuvent être séparés en deux groupes (porphyrique et subaphyrique) par des critères pétrographiques et minéralogiques. Les données géochimiques et pétrographiques permettent d'identifier deux groupes de magmas en relation avec deux dorsales volcaniques et un troisième groupe en relation avec la base du rift.