Richard J. Walker is the world leader in the field of highly siderophile (or iron-loving) element geochemistry. While this may sound rather esoteric, the impact of Walker’s geochemical and cosmochemical research has changed how we look at Earth’s history, both from the outside in and from the inside out. He developed the rhenium-osmium isotopic method to map—for the first time—the age and evolution of the lithospheric mantle and its domains. Using the same isotopic tool kit, he was able to elucidate the planet’s late accretionary history and was among the first to demonstrate that asteroidal cores, sampled by some iron meteorites, formed very early in solar system history. He further pioneered the use of the platinum-osmium isotope system to test models of Earth’s core-mantle interaction and to refine core crystallization histories of iron meteorite parent bodies. Walker’s group pioneered measurements of tungsten isotope anomalies in early Earth rocks and mantle-derived magmas, demonstrating that primordial materials—formed in the first 50 million years of solar system history—astonishingly survived inside Earth for billions of years and are tapped now by mantle plumes rising from great depths. His group’s most recent discovery that the amount of isotopically anomalous tungsten correlates with that of primordial helium in “hot spot” volcanoes from Hawaii and Iceland has lit the field on fire. This observation implies that primordial materials that formed the first tens of millions of years after initial accretion of the planet remain today sequestered at or near the core-mantle boundary. This research, which requires cutting-edge, high-precision isotope ratio measurements, is a game changer and may reflect diffusive exchange between the deep mantle and the core.
Rich Walker’s impact goes well beyond these scientific discoveries, as he has been an excellent educator and mentor to a myriad of graduate students and postdoctoral scholars, most of whom have gone on to establish themselves in academia, thus representing the next generation of “siderophiles.” Finally, he has devoted significant time, energy, and resources to the University of Maryland (his academic home for the past 30 years), where he is currently the chair of the Department of Geology. Walker has fittingly served as the core of the geochemical community at Maryland, where he has been instrumental in accreting world-class colleagues to the institution, thereby making Maryland a bright center of the geochemical and cosmochemical universe.
For these stellar achievements, we honor Richard J. Walker with the 2019 Harry H. Hess Medal of AGU.
—Alan Jay Kaufman, University of Maryland, College Park; and Roberta L. Rudnick, University of California, Santa Barbara
I’d like to start off by thanking those persons who nominated and selected me for this great award. I am much humbled by this recognition, but I sure do appreciate it! I also want to acknowledge the contributions of numerous postdocs and students, as well as other colleagues at the University of Maryland and beyond. I am fortunate to have worked with a dazzling array of accomplished individuals over the years. All that has been accomplished in my lab has been a result of a team effort. I am not permitted enough words or time to thank everyone individually here, so I offer a blanket, heartfelt thanks!
Perhaps the real stars of this show are the siderophile elements. I am grateful to V. M. Goldschmidt for placing these elements in their own special geochemical category. Due to the difficulties associated with measuring many of these elements, they were somewhat neglected during the first few decades of “modern geochemistry.” During that period, only a few brave souls, such as my early mentor John Morgan, sought to lay the critical groundwork for these elements and map out many of the applications we are pursuing today. The siderophile elements are now a fertile ground for much contemporary geochemical and cosmochemical research.
I began work on two siderophile elements, rhenium and osmium, while a postdoc in the mid-1980s. At the time, there were few data published for the associated isotope system, so exploring it seemed like a good thing to do. Studies involving the system are now quite common, and it is applied to a much wider range of problems than we ever envisioned in the 1980s. Accompanied by improvements to chemical separation techniques and mass spectrometers, my group has grown our list of favorite siderophile elements to include tungsten, ruthenium, and molybdenum. These elements are especially interesting because of their participation in short-lived radioactive decay systems and because their nucleosynthetic variability allows their use in tracing planetary genetics. The future is bright, as there is much yet to be learned from this group of elements.
To end I would like to acknowledge and especially thank my wife, Mary Horan. Much of the research I have been involved with over the decades has been supplemented by data provided by her, all the while many of our dinner conversations have strayed into discussions of arcane geochemical issues. Much of what I’ve accomplished would not have been possible without her contributions.
Thanks again to all!
—Richard J. Walker, University of Maryland, College Park