Global Structure of Isotopic Compositional Space of Oceanic Basalts by Independent Component Analysis and its Implications for Mantle Dynamics

Based on a large database such as GEOROC or PetDB, statistical analyses may decode essential features inherited in the compositional space of igneous rocks, and provide useful insights concerning differentiation and dynamics of the Earth"fs mantle-crust system. Among statistical analyses, independent component analysis (ICA) is particularly useful to extract independent features from such databases. Iwamori and Albarède (2008) have demonstrated that the Sr-Nd-Pb isotopic compositional space of oceanic basalts from Atlantic and Indian Oceans forms a joint non-Gaussian distribution which is spanned by two independent components (ICs), rather than by mixing of various mantle end-members with specific compositions. Here we present results of ICA on a global dataset mainly from GEOROC and PetDB with Sr-Nd-Pb isotopic ratios. Two ICs, similar to those from Atlantic and Indian Oceans, are also present in the global dataset and cover more than 90% of the population variance: IC1 roughly discriminates OIB from MORB and IC2 maps geographically regional isotopic properties. The geochemical characteristics indicate independent but overlapping differentiation processes, which are likely to reflect the variations due to melting versus those caused by interaction with aqueous fluid with the mantle at ridges and subduction zones. The 3rd component, IC3, corresponds to a narrow trend leaving off significantly the IC1-IC2 plane towards EM II that is often interpreted as recycled sedimentary/crustal materials (e.g., Jackson et al., 2007). However, it covers less than 10% of the population variance, and is geographically limited mainly to southern Pacific, being in contrast with the global variations of IC1 and IC2. IC2 is partially similar to the DUPAL signature, but is different in that in addition to its Southern Ocean type locality, it also distributes itself broadly in the Northern Hemisphere, as well as in the middle to SW Pacific Ocean, including Hawaii. The distribution of IC2 has similarity to seismic velocity structures near CMB (e.g., Takeuchi, 2007), suggesting a large-scale geochemical segmentation contaminated/uncontaminated by a fluid-rich component, extending from a shallow (MORB-source region) to deep mantle.