Formula omitted] and REE variations along the Easter Microplate boundaries (south Pacific): Application of multivariate statistical analyses to ridge segmentation

Abstract

We report 87 Sr 86 Sr , REE, Sc, V, Cr and Co analyses on 48 basaltic glasses dredged at 47 different localities along the spreading boundaries of the Easter Microplate (EMP). The latitudinal 87 Sr 86 Sr variation along the East and West Rifts closely parallels that of the Pb isotope variation previously published and confirms the strong abnormality of the mantle underlying this part of the East Pacific Rise (EPR). The data also further support the binary mantle mixing model previously proposed. We evaluate the potential of multivariate statistical analyses in studying this and previously published geochemical data obtained on these very same 48 basaltic glasses. The data base includes major and trace elements ranging from compatible (e.g., Cr) to highly incompatible (e.g., La), and the Pb and Sr isotope ratios. Multivariate classification of these geochemical variables in Q-mode (sample space) allows to clearly distinguish major-element grouping from those of moderate to highly incompatible elements, and the isotope ratios reflecting mantle source heterogeneities present in the region. The power of this multivariate geochemical classification is best represented in two-dimensional space by combining principal components analysis (PCA) with cluster analysis (CA) illustrated in the form of a dendrogram. The method is further illustrated by classifying elements of the lanthanide series at different statistical level of affinities. Using the variance-covariance matrix and correlation matrix in R-mode, we further demonstrate a close relationship between segmentation revealed by the multiple variation of the four isotope ratios and the actual tectonic segmentation. Both discriminant analysis and PCA further indicate a close relationship between the geochemical anomaly located on the East Rift of the EMP and Sala y Gomez and Easter Island, to a lesser extent the Tuamotu Chain, whereas there is no close affinity between the EMP and the more distant Juan Fernandez, San Felix or Society Islands in the isotopic space considered. This provides further support that the geochemical anomaly located on the East Rift of the EMP may be related to the influence of a plume located either beneath the East Rift, or in the vicinity of Sala y Gomez, or Easter Island. In the last two models the plume would be laterally deflected at shallow depth towards the EPR-EMP spreading complex. An analysis of statistical generalized distances against geographical distances rules out that such influence is caused by radial dispersion of this plume in the upper mantle. The flow between the plume and the East Rift of the EMP must be more confined or channelled. The possibility that the plume would be located directly beneath the East Rift cannot be ruled out but appears less likely.