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.
In the first hundred million years of the solar system the Earth accreted to about its current size through gravitationally-driven accretion of smaller bodies with rocky mantles and iron metal cores. That simple process predicts that elements that prefer to join a metal phase should reside in the Earth's core, and those that prefer silicate minerals should be in the mantle and crust. The most sensitive modern analytic methods show that some elements are either more or less abundant in the mantle and crust than expected. Tungsten is an example; it is metal-loving and should be in the core, but traces of a certain tungsten isotope is found in unexpected quantities in volcanic rocks around the world. Over the course of this one-year grant, we have written a new computer code that allows us to calculate the compositions of the Earth's mantle that would result from solidifying the Earth from its likely original molten state. By adding to our code the elements samarium, neodymium, hafnium, and tungsten, we can compare our code's predictions to measurements made in Prof. Rich Walker's and others' labs. This initial study lays the groundwork to test several hypotheses for the Earth's geochemistry: (1) that is was set in an initial magma ocean, as described here; (2) that later meteoroid bombardment added trace elements that would otherwise have joined the core; and (3) that there exists a dense layer of material at the core-mantle boundary, whose composition is added only in small incremenets to the convecting mantle of the Earth. Our initial magma ocean code indicates that the samarium, neodymium, tungsten, and hafnium measurements in Earth rocks can all be explained by processing in a magma ocean, perhaps the one produced by the Moon-forming impact. Further, this magma ocean would produce a deep dense layer consistent with geophysical observations of the Earth. The code was written by Stephanie Brown, who started as a post-baccelaureate research scholar and is now a graduate student at MIT. She has presented her results at the Fall American Geophysical Union Meeting, in a prestigious and well-attended Union session, and so this grant has provided a great experience and excellent training for her. She is now in the process of publishing a first-authored paper.
