HELIUM ISOTOPE GEOCHEMISTRY OF MIDOCEAN RIDGE BASALTS FROM THE SOUTH-ATLANTIC

We report new helium isotope results for 49 basalt glass samples from the Mid-Atlantic Ridge between 1-degree-N and 47-degrees-S. He-3/He-4 in South Atlantic mid-ocean ridge basalts (MORB) varies between 6.5 and 9.0 R(A) (R(A) is the atmospheric ratio of 1.39 X 10(-6)), encompassing the range of previously reported values for MORB erupted away from high He-3/He-4 hotspots such as Iceland. He, Sr and Pb isotopes show systematic relationships along the ridge axis. The ridge axis is segmented with respect to geochemical variations, and local spike-like anomalies in He-3/He-4, Pb and Sr isotopes, and trace element ratios such as (La/SM)N are prevalent at the latitudes of the islands of St. Helena, Tristan da Cunha and Gough to the east of the ridge. The isotope systematics are consistent with injection beneath the ridge of mantle "blobs" enriched in radiogenic He, Pb and Sr, derived from off-axis hotspot sources. The variability in He-3/He-4 along the ridge can be used to refine the hotspot source-migrating-ridge sink model. MORB from the 2-7-degrees-S segment are systematically the least radiogenic samples found along the mid-ocean ridge system to date. Here the depleted mantle source is characterized by Sr-87/Sr-86 of approximately 0.7022, Pb isotopes close to the geochron and with Pb-206/Pb-204 of approximately 17.7, and He-3/He-4 of 8.6-8.9 R(A). The "background contamination" of the subridge mantle, by radiogenic helium derived from off-ridge hotspots, displays a maximum between approximately 20 and 24-degrees-S. The He-Pb and He-Sr isotope relations along the ridge indicate that the He-3/He-4 ratios are lower for the hotspot sources of St. Helena, Tristan da Cunha and Gough than for the MORB source, consistent with direct measurements of He-3/He-4 ratios in the island lavas. Details of the He-Sr-Pb isotope systematics between 12 and 22-degrees-S are consistent with early, widespread dispersion of the St. Helena plume into the asthenosphere, probably during flattening of the plume head beneath the thick lithosphere prior to continental breakup. The geographical variation in the He/Pb ratio deduced from the isotope systematics suggests only minor degassing of the plume during this stage. Subsequently, it appears that the plume component reaching the mid-Atlantic ridge was partially outgassed of He during off-ridge hotspot volcanism and related melting activity. Overall, the similar behavior of He and Pb isotopes along the ridge indicates that the respective mantle sources have evolved under conditions which produced related He and Pb isotope variations.