Isotopes, DUPAL, LLSVPs, and Anekantavada

Abstract The mantle perspective provided by radiogenic isotope geochemistry has evolved over the last 40 years. Although the idea of an upper mantle depleted in incompatible elements by extraction of partial melt remains, ideas about the lower mantle have evolved from simple primitive material occupying the lower two-thirds of the mantle to a heterogeneous, multicomponent lower mantle. The lower mantle is sampled by plumes rising from it, and although each is compositionally unique, these plumes can be divided into 5 or 6 genera with the compositional similarity within each suggesting they have evolved in similar ways. The dominant signature in these plumes is that of lithospheric material carried into the deep mantle through subduction, subduction erosion, and continental foundering, and these processes have also polluted and refertilized the depleted upper mantle. The stable isotope geochemistry of these plumes confirms the presence of recycled material, including material derived from the continents, oceans and atmosphere. Noble gas isotope ratios provide a dramatically different perspective: that of reservoirs of noble gases isolated from the convecting mantle for the entire history of the Earth. Neither perspective is likely to be wrong and the conclusion to be drawn is that plumes, at least some of them, draw on reservoirs of both recycled and primitive material from the deep mantle. The difference in noble gas content in the primitive and recycled material may well be such that the primitive noble gas isotopic signature dominates in any mixture of these. In the meantime, the suprachondritic 142 Nd/ 144 Nd ratio of the modern mantle has reopened the question of the composition of the Earth and therefore primitive mantle. If this is not entirely a result of nebular isotopic heterogeneity, then “primitive mantle”, defined as the composition that has given rise to the lithosphere and convecting mantle, appears to be depleted in light rare earths, and likely other incompatible elements such as U and Th. This material would have e Nd in the range of 3 to 7, which matches the most common compositions observed in oceanic island basalts, particularly those of the PREMA genera. PREMA cannot be truly primitive mantle, even defined in this way, instead, it appears to consist of balanced mix of recycled enriched and depleted components with only a limited contribution from long-isolated, nearly primitive material. As the geochemical perspective of the mantle has evolved, so has the geophysical one, and it now reveals a highly heterogeneous lowermost mantle dominated by two large low-shear-wave velocity provinces (LLSVPs) beneath Africa and the South Pacific and associated with the DUPAL anomaly. The latter appears to actually be two anomalies, DUPAL and SOPITA, each closely associated with an LLSVP. Mantle plumes have now been shown to preferentially rise from the margins of these structures, while subduction zones overly apparently cold high-seismic velocity regions surrounding the LLSVPs. The LLSVPs appear to be both hot and dense, and may consist of the PRIMA-like mixture of recycled and primitive material present in many mantle plumes. Geoneutrino studies may eventually resolve some of these questions, including both the total amount of U and Th in the Earth and whether these elements are concentrated within the LLSVPs.