Intellectual merit. Improved knowledge of the pressure, temperature and gas fugacity conditions prevalent during the formation of the Earth and subsequent core formation would substantially enhance to our understanding of the composition of the Earth's core and surrounding silicate shell, particularly with respect to the abundance and distribution of the heat-producing elements U, Th and K. Such insights are essential for understanding the energy sources that drive the geodynamo, mantle convection and plate tectonics, as well as understanding the volatile element inventory of the planet and constraining the nature and degree of core-mantle exchange. This proposal requests funds to analyze the compositions of natural and synthetic samples, with emphasis on precise and accurate determinations of trace abundances of siderophile, chalcophile and key lithophile elements in order to establish solid-liquid, solid-solid and liquid-liquid distribution coefficients for metallic (and to a lesser extent sulfidic) and silicate systems. These data will be used to develop improved estimates of bulk compositional of the core, mantle, crust and bulk silicate Earth, and to provide quantitative insights into the partitioning of elements during core formation, crust-mantle evolution, and core-mantle exchange.
The main goals of the proposed study include determining: (1) the concentration and behavior of siderophile and some chalcophile elements in Earth reservoirs (core-mantle-crust) and magmas, (2) the abundance and behavior of nominally lithophile elements (e.g., Nb, Ta, K, Th, and U) during core formation, and (3) the budget of volatile elements in the mantle and core. Results from these interrelated projects will (a) improve our understanding of the abundance and distribution of these elements in the Earth, (b) yield distribution coefficients for these elements during core-mantle and mantle-crust differentiation, (c) constrain the conditions under which core separation and core-mantle exchange has occurred and/or is occurring, and (d) better define the planetary Urey number (ratio of radiogenic heat production to total heat loss). The most important findings of my group's previous investigations (described in this proposal) include: (1) Modern basalts from ocean ridges, islands and arcs have a weighted average K/U of 20,600+/-3600 (2sd,n = 50). Our K/U value for MORB (19,800 +/- 3400, n=43) differs significantly from that reported by Jochum et al. (1983), reflecting differences in sampling and treatment of the population variance. The modern mantle is estimated to have a K/U value higher than the continental crust and likely contains <50% of the K and U budget of the silicate Earth, indicating a larger radioactive contribution of 40K. (2) Modern basalts have relatively constant W/Ba (0.00136 +/- 86, 2sd), which is comparable to the continental crust and by inference represents the silicate Earth. From this, we can establish the W concentration of the silicate Earth (9.0 +/- 5.8 ppb W). Using this revised value we tested the hypothesis of core-mantle exchange proposed for Hawaiian picrites based on Os isotopes and found that the incorporation of a core component is consistent with W-isotope data, if these lavas are derived from a reservoir with 3x silicate Earth W content (~27 ppb) and a core containing 516 ppb W (i.e., ~96% of the Earth's W budget). (3) New experimental partitioning data (Corgne et al) between molten metal and silicate melt are consistent with core formation under increasing oxygen fugacity (initially near IW-3 and finishing near IW-2). Our results are consistent with a core containing a fraction of the planetary budget of Zn, Ga, K, Cu, Mn, Cr, Nb and Ta. A superchondritic Nb/Ta ratio has been proposed for the core, but our data place the upper limit at < ~15 ppb Ta. Our model predicts a core with at least 10 ppm Ti. Moreover, the core is predicted to contain <10% of the planetary budget of K, but this estimate is likely closer to 1% or less.
Broader impacts. Student participation (at the graduate, undergraduate and high school levels) is a fundamental aspect of my research program at UMD. Students participate in hands-on research, which provides important educational and training opportunities that the students can translate to future work in academia, industry or government. This project will support one PhD student and two undergraduate researchers. My record in education, service and outreach extends well beyond the scientific community and into the public realm (K-12 lectures, Maryland Day open house lab tours and analyses, high school interns on year-long research programs in my mass spectrometry laboratory) and is described, in full, in the body of this proposal.
The research finding of this grant provided new data and novel insights into our understanding of the composition of the Earth and the different layers (core and mantle). These data also provide insights and constraints regarding the amount of power generated inside the Earth from radioactive decay. This energy is used to drive the Earth's engine, including mantle convection, plate tectonics and the geodynamo. The findings were specifically geared to identifying which elements were partitioned into the metallic core during its formation. These fundamental studies were developed to simulate the conditions of core formation during the Earth's ealiest days, some 4.5 billion years ago. The findings show that on average the core formation likely occurred at mid-mantle pressures and relatively high temperatures. Consequently, we showed that under these conditions not all elements were 100% extracted into the core and thus 5% to 50% of some metal-loving elements were left behind in the mantle. Thus, we were able to show how oxidizing the conditions were during core formation. We also demonstrated that gold and other noble metals were added into the Earth's mantle during later stages of final planetary growth, during a period when meteorites were sitll bombaring the Earth, the period is often referred to at the late heavy bombardment period.
