Evaluating links between primordial mantle domains and deep mantle structures using 182W and 3He/4He

University Of Maryland, College Park, College Park MD

Investigators

Abstract

Many mantle plume-derived ocean island basalts entrain multiple mantle components, representing recycled, ambient, and primordial isotopic domains. The mixtures of these components, sampled by intraplate volcanoes, define isotopic mixing arrays unique to each plume. One of these components, FOZO (“Focal Zone”), is unusual in that it is common to many mantle plumes. The FOZO component is characterized by a specific and relatively narrow compositional range in lead, neodymium, strontium, and helium isotopes, where isotopic compositions of most plumes overlap. Notably, FOZO-type reservoirs retain primordial helium isotopic compositions that have not been as significantly modified by subsequent processes compared to other parts of the mantle. While the strontium-lead-neodymium compositional definition of FOZO has undergone several revisions since it was first identified, primordial helium isotopes have remained an important defining characteristic of this reservoir type. Primordial helium ratios have recently been linked to seismically-detectable structures in the deep mantle as well as parameters in plume systems such as material buoyancy and temperature, suggesting that reservoirs with elevated primordial helium are seismically, thermally, and chemically distinct from “typical” lower mantle material. The findings from this study will improve our understanding of the geochemical evolution of the early Earth. In collaboration with geophysicists and geodynamicists, this study will provide constraints on the size, shape, and development of primordial mantle domains that frequently contribute to deeply-sourced volcanism. In addition, this project will provide two years of support for the PI and broaden her expertise in isotope geochemistry to include short-lived isotope systematics. Recently, variations in tungsten isotopes, driven by the decay of an extinct isotope of hafnium, were discovered in plume-derived volcanism. In plumes, anomalous tungsten is found in samples with primordial helium, implying that a second primordial isotopic constraint may exist for FOZO-type melts. Tentatively, FOZO-type material carrying anomalous tungsten and helium have been linked to ultra-low velocity zones detected near the core-mantle boundary. Other compositions thought to sample ambient mantle domains found in plumes lack distinct helium and tungsten anomalies, but additional data are needed to fully test the links between seismic structures and isotopic composition. To test this scenario, plumes that lack evidence of any recycled crustal material, a possible contaminant, will be analyzed for helium and tungsten isotopes. The Louisville, Juan Fernandez, Bowie, Cobb, and Caroline plumes have been selected for their lack of recycled componentry and variable helium isotopic compositions. The new data will then be linked to spatial correlations (or lack thereof) with seismically-resolved lower mantle structures to determine the relationship of FOZO-type melts, ambient lower mantle compositions, and the structure of the deep mantle. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.