The emerging portrait of an ancient, heterogeneous and continuously evolving mantle plume source

Abstract

Heterogeneity in the lithophile isotopic compositions of ocean island basalts (OIBs) has long been ascribed to the incorporation of recycled materials into the plume source. OIB heterogeneity indicates that plumes do not sample a pristine primordial reservoir, but rather sample an inhomogeneous mixture of primordial and recycled material generated by convective processes over Earth history. Here we present a synthesis of new insights into the characteristics and nature of the plume mantle source.Recent high precision noble gas data demonstrate that the origin of the reservoir supplying noble gases to plumes is fundamentally distinct from that of the mid-ocean ridge basalt (MORB) mantle reservoir: the two reservoirs cannot be related simply by differential degassing or incorporation of recycled atmospheric volatiles. Based on differences observed in the extinct 129I-129Xe system (t1/2 of 15.7 Ma), the mantle source supplying noble gases to plumes differentiated from the MORB source within ~100 Ma of the start of the Solar System, and the two sources have not been homogenized by 4.45 Ga of mantle convection. Thus, the 129I-129Xe data require a plume source that has experienced limited direct mixing with the MORB source mantle.Analysis of mantle source Xe isotopic compositions of plume-influenced samples with primordial He and Ne indicates that the plume source Xe budget is dominated by regassed atmospheric Xe. He and Ne isotopes are not sensitive to regassing due to low overall concentrations of He and Ne in recycled material relative to primordial material. Therefore, plume-influenced samples with primitive He and Ne isotopic compositions do not necessarily reflect sampling of pristine primordial mantle and the lithophile compositions of these samples should not be taken to represent undifferentiated mantle. In addition to recycled atmospheric Xe, the plume mantle source exhibits high ratios of Pu-fission Xe to U-fission Xe. The high proportion of Pu-fission Xe independently confirms a low extent of degassing of the plume source relative to the MORB source.Heavy noble gases illustrate that the mantle reservoir sampled by plumes is fundamentally distinct from the MORB mantle and reflects ongoing degassing of, and incorporation of recycled material into, an ancient (>4.45 Ga) primordial source. If plumes are derived from large low shear-wave velocity provinces (LLSVPs), then these seismically-imaged structures are ancient and long-lived.