Geochemical portray of the Pacific Ridge: New isotopic data and statistical techniques

Cédric Hamelin (1) , Laure Dosso (1) , Barry B. Hanan (2) , Manuel Moreira (3) , Andrew Kositsky (4) , Marion Y. Thomas (4)

1 LDO - Domaines Océaniques
2 Geology SDSU - Department of Geological Sciences [San Diego State Univ]
3 IPGP - Institut de Physique du Globe de Paris
4 Tectonics Observatory

Samples collected during the PACANTARCTIC 2 cruise fill a sampling gap from 53° to 41° S along the Pacific Antarctic Ridge (PAR). Analysis of Sr, Nd, Pb, Hf, and He isotope compositions of these new samples is shown together with published data from 66°S to 53°S and from the EPR. The recent advance in analytical mass spectrometry techniques generates a spectacular increase in the number of multidimensional isotopic data for oceanic basalts. Working with such multidimensional datasets generates a new approach for the data interpretation, preferably based on statistical analysis techniques. Principal Component Analysis (PCA) is a powerful mathematical tool to study this type of datasets. The purpose of PCA is to reduce the number of dimensions by keeping only those characteristics that contribute most to its variance. Using this technique, it becomes possible to have a statistical picture of the geochemical variations along the entire Pacific Ridge from 70°S to 10°S. The incomplete sampling of the ridge led previously to the identification of a large-scale division of the south Pacific mantle at the latitude of Easter Island. The PCA method applied here to the completed dataset reveals a different geochemical profile. Along the Pacific Ridge, a large-scale bell-shaped variation with an extremum at about 38°S of latitude is interpreted as a progressive change in the geochemical characteristics of the depleted matrix of the mantle. This Pacific Isotopic Bump (PIB) is also noticeable in the He isotopic ratio along-axis variation. The linear correlation observed between He and heavy radiogenic isotopes, together with the result of the PCA calculation, suggests that the large-scale variation is unrelated to the plume-ridge interactions in the area and should rather be attributed to the partial melting of a marble-cake assemblage. Research Highlights ► New Sr, Nd, Pb, Hf, and He isotopes data fill a sampling gap along the Pacific Ridge. ► We examine geochemical variation in MORB using a principal component analysis. ► A progressive change in the depleted matrix is recognized along the Pacific ridge. ► In samples devoid of plume influence, He isotopes correlates with Pb isotopes.

Mots clés: oceanic basalts; Pacific-Antarctic Ridge; mantle heterogeneity; Principal Component Analysis; Sr Nd Pb Hf isotopes

Domaines: Géochimie; Milieux et Changements globaux

Cédric Hamelin, Laure Dosso, Barry B. Hanan, Manuel Moreira, Andrew Kositsky, et al.. Geochemical portray of the Pacific Ridge: New isotopic data and statistical techniques. Earth and Planetary Science Letters, 2011, 302 (1-2), pp.154-162. ⟨10.1016/j.epsl.2010.12.007⟩. ⟨insu-00588804⟩