Mike Walter is an experimental petrologist who, like Bowen, asks major questions about Earth. His career began with the resolution of a large number of questions surrounding the compositions of melts and residue produced during partial melting of upper mantle peridotite. Starting with a simplified system, Mike elucidated the key melting reactions; the stability fields of garnet, plagioclase, and spinel; and the changing compositions of melts with increasing pressure and temperature. His paper is an experimental classic, painstaking in execution, remarkably rich in detail and insight. It provided a powerful framework for the interpretation and extrapolation of data on the more complex natural systems that he addressed next. Mike showed that partial melts of fertile peridotite shift from basaltic at low pressure through picritic to komatiitic at approximately 6 gigapascals. In this paper (1998) he also showed the compositional relationships between melts and peridotitic residues for carefully estimated fractions of melting. This is still the classic and definitive work on the pressure dependence of anhydrous peridotite melting (>1,100 citations).

Mike was the first to demonstrate the profound effect of pressure on core-mantle partitioning of nickel (Ni) and cobalt (Co). This led to the model of core segregation at the base of a deep magma ocean, the now generally accepted explanation for the apparent overabundance of siderophile Ni and Co in Earth’s mantle.

Moving into the lower mantle, Mike’s paper on trace element partitioning between perovskite and silicate melt (2004) showed that despite numerous assertions to the contrary, there was no isolation of a large perovskite-rich reservoir in the lower mantle at the end of the magma ocean phase of Earth’s accretion. The need for natural samples to compare with experiments led him into the study of inclusions in superdeep diamonds. In 2011, Mike showed that compositions of some inclusions matched precisely those expected for minerals formed from oceanic crust subducted into the lower mantle. When coupled with isotope measurements indicating formation of the diamonds from surficial carbon, this provided the first direct evidence that surface material is subducted to these depths and later exhumed. This and his succeeding studies on diamond inclusions have had a remarkable impact on our perceptions of both Earth’s convective cycle and the deep carbon cycle, providing some of the only direct constraints on both processes. Cast from the same mold as Bowen himself, Mike Walter asks the important questions and gets the right answers.

—Bernard J. Wood, University of Oxford, Oxford, U.K.

I thank Bernie Wood for his gracious citation, the generous colleagues who gave their precious time to support my nomination, and the Volcanology, Geochemistry, and Petrology committee for acknowledging me when there are so many worthy folks in our community. I am ever thankful for the scientific and personal freedoms I have enjoyed throughout my journey, especially when these gifts are not available to everyone. No one achieves anything without the help and encouragement of others, and there are so many people who have taught, guided, and supported me along the circuitous path I have trodden across three continents. It would be impossible to name each in the space available, and I sincerely thank all of you. Yet there are those whose indelible imprint must be highlighted: Harmon Maher for believing in a late-blooming kid; Dean Presnall for instilling a deep respect and understanding of the power of phase equilibria as a tool for problem-solving in the tradition of Bowen; Ikuo Kushiro for inviting and welcoming me to the high-pressure paradise that is Japan and encouraging me to explore new horizons while tolerating my many egregious shitsugen; Eiji Ito for his kindness and for revealing the dark arts of the multianvil; Dave Walker for letting me in on his “rules”; my most excellent colleagues and friends in Bristol and throughout the United Kingdom from whom I learned and gained immensely, both scientifically and personally, not to mention the many poker nights at Blundy Towers when petrology and straight flushes comingled like crustal magmas; and not least of all the many outstanding students and postdocs who have done the lion’s share, kept me on my toes, and made synchrotron trips so enjoyable. It has been a great privilege to work at the institution that crystallized Bowen himself, first as a postdoc at the Geophysical Lab when Yingwei Fei gave me carte blanche in his high-pressure lab allowing me to become truly independent. I am grateful to my colleagues at Carnegie for welcoming me back into the fold after many years in exile and for their boundless scientific curiosity and enthusiasm. Science has evolved tremendously over the decades, becoming necessarily multidisciplinary to answer the complex questions of planetary formation and evolution. I can only imagine what Bowen would do with the tools and diversity of talented people we enjoy today to explore the Earth, planets, and new worlds beyond our solar system. —Michael J. Walter, Carnegie Institution for Science, Washington, D.C.

For advances in understanding the formation of Earth and its core, the petrology of the mantle, and the phase relationships of the deep Earth.