EXPLORATION OF EARTH'S INTERIOR THROUGH TUNGSTEN ISOTOPE GEOCHEMISTRY OF THE HAWAIIAN MANTLE PLUME

Abstract

The advancement of analytical techniques has enabled the discovery of µ182W (the parts-per-million deviation of 182W/184W relative to the presumed composition of the bulk silicate Earth) isotopic differences among Earth materials. Variations in µ182W are particularly useful for probing early Earth events because radiogenic 182W was only produced during the initial ~60 million years of the Solar System, requiring anomalous µ182W in the modern mantle to have formed and been preserved for over 4.5 billion years. Characterization of µ182W in modern ocean island basalts (OIB) has revealed that some mantle plumes contain µ182W deficits in one or more of their source components. Investigation of the well characterized Hawaiian mantle plume, from a single volcano scale to the archipelago scale, provides new insights into the materials and processes that affect the expression of µ182W in erupted OIB. In Chapter 2, a systematic study of the Mauna Kea volcano indicates that the anomalous µ182W is present in small volumes, such that it is not always sampled by melting events, even when present in the mantle source of a volcano. Through a two-step acid leaching procedure on ~5 Ma powders from Kauai, Chapter 3 confirms that the µ182W measured in minimally altered, subaerial OIB is a reliable tracer of the mantle source composition. Throughout fourteen volcanic centers in the archipelago, Chapter 4 concludes there are gaps in the plume, where no anomalous µ182W component is present, which could reflect inconsistent entrainment of the anomalous mantle source. The chemical characteristics of Hawaiian lavas with µ182W deficits, both within and among volcanic centers, suggest that the anomalous component in the Hawaiian plume is relatively refractory, is associated with relatively radiogenic 208Pb/204Pb, and is enriched in Ti and Nb, relative to similarly incompatible elements. The probability of a Hawaiian volcano expressing negative µ182W is related to the thermal and physical structure of the plume, such that higher mantle potential temperature and magmatic flux are more likely to produce lavas with anomalous µ182W.