We propose to conduct collaborative, synergistic research combining observations of siderophile element compositions with geodynamic modeling to study the evolution of elemental and isotopic heterogeneities in the mantle. Siderophile elements are those with a preference to enter metal rather than silicates, and are thus, elements that are largely concentrated in planetary cores. The objectives of this study will be to develop a better understanding of the global processes that led to the establishment of siderophile element abundances in the mantle, assess the extent of and possible mechanisms for the long-term preservation of the siderophile elemental and isotopic heterogeneities observed in the terrestrial rock record, and combine this information with geodynamic modeling to provide new insights to develop a better understanding of the formation and early chemical evolution of the Earth. In part, this project will be accomplished via study of the petrologic and chronologic extents of 182W isotopic anomalies in terrestrial systems using a newly-developed, high precision mass spectrometry technique. Ancient rocks, such as komatiites for which we have already identified isotopic anomalies, as well as modern rocks, such as MORB, oceanic peridotites, and ocean island basalts, will be examined as part of this task. Where necessary, complementary concentration and isotopic data for other elements, such as Os and Nd, will be generated. To better understand the causes and implications of the isotopic systematics revealed, we will test, via different geodynamic models, mechanisms for the formation and dispersal of the geochemical reservoirs with the appropriate characteristics. By tracking hafnium and tungsten in models of a range of possible early Earth states, then comparing model results with observations, we will be able to rule out some early Earth formation models and also better understand mantle mixing over the evolution of the Earth.
For this project we studied the isotopic composition of the element tunsten, as well as the abundances of tungsten and a group of elements known as highly siderophile elements ("iron-loving"), in the Nuvvuagittuq rocks from Quebec, Canada, that may be as old as 4.3 billion years. The main goal of the project was to provide new insights into how the Earth formed and changed over its first billion years. The isotopic composition of tungsten is of particular interest because the abundance of the isotope 182-tungsten varied somewhat among early solar system materials (e.g., planets and asteroids) during the first ~50 million years of solar system history as a consequence of decay of 182-Hf. Thus, any variations require processes that occurred very early in Earth history. All of the Nuvvuagittuq samples we examined had a higher proportion of 182-tungsten than modern rocks. This means that the very old samples were made from a portion of the Earth that is either hidden from us today (does not contribute to any modern rocks), or was mixed away by convective stirring of the mantle over hunders of millions to billions of years. The results of this study suggest that during the final stage of Earth's assembly, although planetessimals were continually accreted onto Earth they were not very well stirred into the mantle until relatively late in Earth history. It remains a mystery why the mantle during the first billion years of Earth histroy (when it was presumably much hotter than today), was so inefficient at convectively mixing itself.
