RAPID TEMPORAL CHANGES IN OCEAN ISLAND BASALT COMPOSITION - EVIDENCE FROM AN 800 M DEEP DRILL HOLE IN EIAO SHIELD (MARQUESAS)

The Dominique drill hole has penetrated the volcanic shield of Eiao is land (Marquesas) down to a depth of 800 m below the surface and 691.5 m below sea-level with a percentage of recovery close to 100%. All the lavas encountered were emplaced tinder subaerial conditions. From the bottom to the top are distinguished: quartz and olivine tholeiites (8 00-686 m), hawaiites, mugearites and trachyte (686-415 m), picritic ba salts, olivine tholeiites and alkali basalts (415-0 m). The cored volc anic pile was emplaced between 5.56 +/- 0.07 Ma and 5.22 +/- 0.06 Ma. Important chemical changes occurred during this rather short time span (0.34+/-0.13 Ma). In particular, the lower basalts differ from the up per ones in their lower concentrations of incompatible trace elements and their Sr, Nd and Pb isotopic signature being closer to the HIMU en d-member, whereas the upper basalts are EM II enriched. The chemical d ifferences between the two basalt groups are consistent with a time-re lated decrease in the degree of partial melting of isotopically hetero geneous sources. It seems unlikely that these isotopic differences ref lect changes in plume dynamics occurring in such a short time span, an d we tentatively suggest that they result from a decreasing degree of partial melting of a heterogeneous EM II-HIMU mantle plume. Some of th e intermediate magmas (the uppermost hawaiites and mugearites) are lik ely to be derived from parent magmas similar to the associated upper b asalts through simple fractionation processes. Hawaiites, mugearites a nd a trachyte from the middle part of the volcanic sequence have Sr-Nd isotopic signatures similar to those of the lower basalts but they di ffer from them in their lower Pb-206/Pb-204 ratios, resulting in an in creased DMM signature. Some of the hawaiites-mugearites also display s pecific enrichments in P2O5, Sr and REE which are unlikely to result f rom simple fractionation processes. The isotopic and incompatible elem ent compositions of the intermediate rocks are consistent with the ass imilation of MORB-derived wall rocks during fractional crystallization . The likely contaminant corresponds to Pacific oceanic crust, locally containing apatite-rich veins and hydrothermal sulphides. We conclude that a possible explanation for the DMM signature in ocean island bas alts is a chemical contribution from the underlying oceanic crust and that studies of intermediate rocks may be important to document the or igin of the isotopic features of plume-derived magmas.