GEOCHEMISTRY OF INACCESSIBLE ISLAND VOLCANICS

Inaccessible Island, part of the Tristan da Cunha group, is older than Tristan itself but younger than Nightingale. It comprises an alkali basalt-trachybasalt-phonolitic trachyte series which is similar to but less alkaline than that of Tristan. Chemical and isotopic analyses for a new suite of sixteen samples are presented. Sr-Nd-Pb isotopes and incompatible element ratios exhibit a DUPAL signature (Sr-87/Sr-86 = 0.7050, Nd-143/Nd-144 = 0.51255), underlining the similarity to Tristan. Most of the Inaccessible lavas appear to have been derived by melting of an enriched mantle source. However, one small group of rocks from the island is isotopically distinct and shows progressive uranium depletion with increasing Nd-143/Nd-144 (0.51262-0.51267) and decreasing Sr-87/Sr-86 (0.70454-0.70414). Admixture of a component from a Mid-Ocean Ridge basalt (DMM) source can explain this anomalous group and minor Pb isotope variations appear to support this explanation. These distinct lavas are not confined to any particular stratigraphic level, implying the availability of both sources throughout the growth of the island. The isotopic variability found for Inaccessible is distinct from that on Tristan and from the Walvis Ridge basalts.