EVOLUTION. Toward a prospective molecular evolution.


SCIENCE sciencemag.org far-side effect generates heterogeneity on a hemispherical scale, because an impact hits only one side of the planet. Second, portions of the mantle may have been isolated from mixing with the rest of the mantle. Perhaps such a hidden reservoir has existed at the base of the mantle (see the figure). Alternatively, self-gravitating cores may have plunged through the mantle without appreciable emulsification, thus preventing enough metal to mix with the silicate mantle down to small enough length scales for chemical equilibration to occur (9). What is surprising is that such an initial W isotope heterogeneity could be preserved in the mantle today. The mantle source reservoir for the flood basalts is a matter of contention. One candidate is the large low– shear velocity provinces (LLSVPs) at the base of the mantle (see the figure). These regions consist of dense, hot material (10) and are thought to be stable at their present antipodal position on the core-mantle boundary close to the equator (11). Rizo et al. suggest that these regions have existed for essentially all of Earth’s history. Previous analyses of oceanic basalts thought to be derived from deep mantle plumes, including samples from the Ontong Java Plateau, found no W excesses (±5 ppm) (2). If we accept both studies at face value, then the chemical heterogeneity apparently exists within a single source domain, excluding W heterogeneity created by the far-side effect. Future studies must first rule out the possibility of analytical artifacts, because several processes may obscure high-precision isotope analyses of W poor samples (for example, lavas), including isotope fractionation in the laboratory, contamination, and interferences. In the end, identifying primitive signatures in Earth’s mantle today elucidates the style of Earth’s core formation and requires certain parts of the mantle to stay sufficiently buoyant to escape convective mixing throughout essentially all of Earth’s geodynamical history. j

DOI: 10.1126/science.aaf7543

Cite this paper

@article{He2016EVOLUTIONTA, title={EVOLUTION. Toward a prospective molecular evolution.}, author={Xionglei He and Li Liu}, journal={Science}, year={2016}, volume={352 6287}, pages={769-70} }