Earth Mantle's Isotopic Record of Progressive Chemical Depletion

Mid ocean ridge basalts are partial melts of mantle rocks that have become incompatible element depleted prior to recent sub-ridge melting. This prior extent of melt extraction and concomitant incompatible element depletion is often inferred from the radiogenic isotope ratios of ridge basalts, such as hafnium and neodymium. However, hafnium isotope ratios by far exceeding those in ridge basalts are observed sporadically in exposed mantle rocks, abyssal peridotites. At least locally, therefore, Earth's mantle has evolved with much larger extents of incompatible element depletion over geologic time periods (10(8)-10(9) years) than inferred from ridge basalts. Here we show that such "ultra-depleted " hafnium isotope signatures also characterize peridotites exposed at the Doldrums Fracture Zone in the equatorial Mid Atlantic. These peridotites are closely associated with peridotites characterized by hafnium-neodymium isotope ratios similar to ridge basalts, but extensive incompatible element depletion. Our results demonstrate that Earth mantle's isotopic record of prior melt extraction is dampened by ubiquitous past and recent reaction with migrating melts. Hence, a better understanding of chemically reactive flow is required for deducing the mantle's rate of incompatible element depletion by recurring processing through melting regions in the shallow mantle, which is the driving force for silicate Earth evolution. Ultimately, better constraining the rate of incompatible element depletion from Earth's mantle is therefore of first-order importance for understanding our planet's basic mode of operation.