Abstract
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. SrNdPb isotopes and incompatible element ratios exhibit a DUPAL signature ( 87 Sr 86 Sr = 0.7050 , 143 Nd 144 Nd = 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 143 Nd 144 Nd (0.51262–0.51267) and decreasing 87 Sr 86 Sr (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.
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