Galapagos Hot Spot‐SPreading Center System: 2. ⁸⁷SR/⁸⁶ SR and large ion lithophile element variations (85°W‐101°W)

Thirty eight samples of basalts dredged from the Galapagos spreading center (GSC) between 85°W and 101.5°W longitudes have been analyzed for K, Rb, Cs, Ba, and Sr contents as well as for 87 Sr/ 86 sr ratios. Basalts between 85°W and 89°W segment of the GSC range from 0.7025 to 0.7028 in 87 Sr/ 86 sr and those between 95.5°W and 101.5°W range from 0.7026 to 0.7027. These basalts are characterized by LREE‐depleted patterns. Basalts from 89°W to 95.5°W segment having slightly LREE‐depleted to LREE‐enriched patterns ((La/Sm) E.F. between 0.8 and 2.4) range from 0.7027 to 0.7031 in 87 Sr/ 86 Sr. The longitudinal 87 Sr/ 86 Sr variation shows a maximum at about 92°W, where the GSC is nearest to Darwin Island. The alkali and alkaline earth elements as well as several element ratios also show maxima in their longitudinal profiles, but these occur at about 91°W. The apparent shift of 87 Sr/ 86 Sr maximum with respect to that of the trace elements may be related to differences in the degree of partial melting. The highest 87 Sr/ 86 Sr from the GSC is only slightly lower than the highest 87 Sr/ 86 Sr for tholeiitic basalts from the Galapagos Archipelago. Several possible hypotheses have been examined to account for these geochemical and Sr isotopic variations. In the light of the presently available data, the most promising explanation seems to be a binary mixing of hot spot (or plume‐type) material with the source of ‘normal’ mid‐ocean ridge basalts and resulting rift propagation. Both end‐members seem to be heterogeneous, and therefore it is at present difficult to put severe constraints on this binary mixing. Nevertheless, the flow pattern of mantle material from the hot spot does not appear to be radial but is preferentially channeled along the 91°W and 92.5°W fracture zones toward the rift.