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Rapid Temporal Changes in Ocean Island Basalt Composition: Evidence from an 800 m Deep Drill Hole in Eiao Shield (Marquesas) |
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| Authors: | CAROFF MARTIAL; MAURY RENE C; VIDAL PHILIPPE; GUILLE GERARD; DUPUY CLAUDE; COTTEN JOSEPH; GUILLOU HERVE; GILLOT PIERRE-YVES |
| Institution: | 1LABORATOIRE DE PTROLOGIE, URA NO. 1278 ET GDR GEDO, UNIVERSIT DE BRETAGNE OCCIDENTALE B.P. 809, 29285 BREST, FRANCE 2UA NO. 10 CNRS ET UNIVERSIT BLAISE PASCAL 5 RUE KESSLER, 63038 CLERMONT-FERRAND, FRANCE 3CEA/DAM/DE/MES/LDG, B.P. 12, 91680 BRUYRES LE CHATEL, FRANCE 4BTIMENT DE GOLOGIE, USTL 34060 MONTPELLIER, FRANCE 5CENTRE DES FAIBLES RADIOACTIVITS, LABORATOIRE MIXTE CEA-CNRS, DOMAINE DU CNRS 91198 GIF-SUR-YVETTE, FRANCE |
| Abstract: | The Dominique drill hole has penetrated the volcanic shieldof Eiao island (Marquesas) down to a depth of 800 m below thesurface and 691•5 m below sea-level with a percentage ofrecovery close to 100%. All the lavas encountered were emplacedunder subaerial conditions. From the bottom to the top are distinguished:quartz and olivine tholeiites (800–686 m), hawaiites,mugearites and trachyte (686–415 m), picritic basalts,olivine tholeiites and alkali basalts (415–0 m). The coredvolcanic pile was emplaced between 5•560•07 Ma and5•220•06 Ma. Important chemical changes occurred during this rather shorttime span (0•34 0•13 Ma). In particular, the lowerbasalts differ from the upper ones in their lower concentrationsof incompatible trace elements and their Sr, Nd and Pb isotopicsignature being closer to the HIMU end-member, whereas the upperbasalts are EM II enriched. The chemical differences betweenthe two basalt groups are consistent with a time-related decreasein the degree of partial melting of isotopically heterogeneoussources. It seems unlikely that these isotopic differences reflectchanges in plume dynamics occurring in such a short time span,and we tentatively suggest that they result from a decreasingdegree of partial melting of a heterogeneous EM II–HIMUmantle plume. Some of the intermediate magmas (the uppermost hawaiites andmugearites) are likely to be derived from parent magmas similarto the associated upper basalts through simple fractionationprocesses. Hawaiites, mugearites and a trachyte from the middlepart of the volcanic sequence have Sr–Nd isotopic signaturessimilar to those of the lower basalts but they differ from themin their lower 206Pb/204Pb ratios, resulting in an increasedDMM signature. Some of the hawaiites-mugearites also displayspecific enrichments in P2O5, Sr and REE which are unlikelyto result from simple fractionation processes. The isotopicand incompatible element compositions of the intermediate rocksare consistent with the assimilation of MORB-derived wall rocksduring fractional crystallization. The likely contaminant correspondsto Pacific oceanic crust, locally containing apatite-rich veinsand hydrothermal sulphides. We conclude that a possible explanationfor the DMM signature in ocean island basalts is a chemicalcontribution from the underlying oceanic crust and that studiesof intermediate rocks may be important to document the originof the isotopic features of plume-derived magmas. KEY WORDS: alkali basalt; assimilation; mantle heterogeneity; Marquesas; tholeiile *Corresponding author |
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