Heterogeneous mantle domains: signatures, genesis and mixing chronologies

Abstract

A Sr Nd Pb isotope data base is available now which includes over 300 samples from some 43 oceanic islands or island groups. This data base supports the identification by Zindler and Hart [20] of four principal end-member isotopic components in oceanic basalts: depleted MORB mantle (DMM), high U/Pb mantle (HIMU) and two enriched mantle components (EMI, EMII). Linear mixing arrays (in five isotopic dimensions) between EMI and HIMU and between DMM and HIMU are documented and argue for similar proportions of Sr Nd Pb in these three components. EMI is argued to be a slightly modified bulk-earth component; HIMU is a component with a greatly enhanced U/Rb ratio, probably generated by intra-mantle metasomatism. It is unlikely that either EMI or HIMU are recycled oceanic crust or sediment. EMII is easily reconciled with a recycled (subducted) sediment protolith; this leads to strong Pb enrichment and consequently to markedly curved mixing arrays between EMII and the other components. The DUPAL anomaly is demonstrated to be unequivocally real, and is marked by basalts with enhanced contents of EMI, EMII, or HIMU; polar regions are marked by only weak or absent signatures from these components. Not all basalts from the DUPAL belt show a strong DUPAL signature, but 95% of all basalts showing a strong DUPAL signature lie between 0° and 50° south latitude. The mesosphere boundary layer model of Allègre and Turcotte [21] is shown to be consistent with available data, provided a somewhat ad-hoc plea is made for preferential recycling of sediments and delaminated continental lithosphere within the DUPAL latitudes. An alternative model is proposed which links the DUPAL with other deep mantle geophysical anomalies, and utilizes a core/mantle boundary layer as a source of the DUPAL components. The low-degree mantle circulation is viewed as whole mantle quadrupolar convection, with equatorial upwelling; this serves to concentrate DUPAL components (and hotspots themselves) into the low-latitude regions.