Isotopic variability of oceanic, arc and continental basalts and its implications on mantle dynamics
Wednesday, 17 December 2014
Based on a large compiled data set of Sr, Nd and Pb isotopic ratios, including mid-ocean ridge basalts (MORB, 2773 data), oceanic island basalts (OIB, 1515 data), arc basalts (AB, 1049 data) and continental basalts (CB, 1517 data), we examine the geographical distribution of the mantle heterogeneity. It has been 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. Using the multivariate analysis “Independent Component Analysis (ICA)”, three independent components (ICs) are identified, as have been deduced in the previous studies on the oceanic and arc basalts, which 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%). Therefore, the young basalts from almost all tectonic settings on Earth mostly plot on a single compositional 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, likely produced by melting and aqueous fluid-rock interactions at ridge, plume and subduction zone. Material recycling and multiple processing in these tectonic settings allows IC1 and IC2 to overlap and create the major compositional plane. On the other hand, the third component (IC3) accounts for 4% of the sample variance and 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 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.