Isotopic heterogeneity of oceanic, arc and continental basalts and its implications for mantle dynamics

In order to discuss material recycling within the mantle, isotopic variability of the young basalts, which reflect the composition of the mantle, has been investigated based on a large compiled data set of Sr, Nd and Pb isotopic ratios. In addition to mid-ocean ridge basalts (MORB, 2773 data), oceanic island basalts (OIB, 1515 data) and arc basalts (AB, 1049 data) that have been used in the previous studies, the new data set includes continental basalts (CB, 1517 data) to densely cover the globe and examine the geographical distribution of the mantle heterogeneity. It is found that CB are heavily concentrated in the eastern hemisphere, while MORB, OIB and AB are broadly distributed in the eastern and western hemispheres evenly. Then the independent features hidden in the data have been extracted using the multivariate analysis "Independent Component Analysis (ICA)", which provide information concerning the overall structure of the data and its origin. As a result, three independent components (ICs), which are similar to those deduced in the previous studies on the oceanic and arc basalts, are identified and account for 99% of the sample variance. Of these, the two independent components (IC1 and IC2) span a compositional plane that concentrates most of the data (95%), and the third minor component (IC3) accounts for 4%. Therefore, the oceanic, arc and continental basalts mostly plot on the same plane. The geochemical nature and the statistical properties (mean, standard deviation and skewness) of the ICs suggest that IC1 is related to variability of a long-term 'melt component' inherited in the mantle, whereas IC2 corresponds to a long-term 'aqueous fluid component' in the mantle. Numerical models for a differentiation-recycling system explain most of the statistical features of IC1 and IC2, which have likely been produced by melting and aqueous fluid-rock interactions in ridge, plume and subduction zone. Material recycling and multiple processing in these tectonic settings allow IC1 and IC2 to overlap and create a joint distribution (i.e., the major compositional plane). On the other hand, IC3 exhibits unique features (e.g., a large positive skewness), suggesting that it represents minor and incomplete mixing of continental components from outside the mantle recycling system. In addition, IC2 shows global geographical discrimination, irrespective of the type of basalts, indicating the presence of east-west geochemical hemispheres in the mantle. We have also found a striking geometrical similarity between the IC2 hemispheres and the inner core hemispheric structure: the eastern hemisphere shows positive IC2 in the mantle and high seismic velocities in the inner core. Combining these constraints, we propose top-down hemispherical dynamics involving both the mantle and the core: focused subduction towards the supercontinents has formed a fluid component-rich hemispheric domain that seems to have been anchored to the asthenosphere during the continental dispersal in the past several hundred million years, and may affect the temperature and growth rate of the inner core, resulting in the synchronized hemispherical structures in the mantle and the core. (C) 2014 The Authors. Published by Elsevier B.V. on behalf of International Association for Gondwana Research.