Paul Asimow

2005 James B. Macelwane Medal Winner

California Institute of Technology

Paul Asimow received the James B. Macelwane Medal at the AGU Fall Meeting Honors Ceremony, which was held on 7 December 2005 in San Francisco, Calif. The medal is given for significant contributions to the geophysical sciences by a young scientist of outstanding ability.


It is my great pleasure to present my friend and colleague, Paul Asimow, recipient of one of this year’s three James B. Macelwane Medals. Paul is a petrologist interested in the origins and evolution of basaltic magmas, and he is being recognized for a series of profoundly insightful papers on the energetics of decompression melting and how it controls the compositions of the oceanic crust and upper mantle.

The significance of what Paul has done comes from the simplicity of the question that first inspired him: How should we describe the way the mantle melts as it upwells during convection? The importance of the problem is obvious; this is how the Earth makes most of its crust, and so it is the starting point for most of geology. But does it sound like something we already understand? Wasn’t I taught this as an undergraduate? Paul’s first and perhaps most important contribution was to recognize, as a second-year graduate student working with Ed Stolper [California Institute of Technology (Caltech), Pasadena], that the explanation of mantle melting we were telling each other was a Rube Goldberg device masquerading as physical theory.

Paul started solving this problem by stripping the issue to its essentials—an analysis of the energetics of decompression melting of the simplest imaginable chemical system. He began his analysis by identifying the thermodynamic quantities that are conserved during decompression, defining the thermodynamic coordinates in which decompressing systems should be considered, and sorting out the behaviors of solid-state and melting reactions in those coordinate systems. Actually, it is even harder than it sounds. I’m glad there is no question period after this presentation. Paul then added layers of complexity until these principles could be shown to apply to natural systems. I was a postdoc at Caltech during these years and so could look over Paul’s shoulders while he laid these foundations. Unfortunately, all I remember clearly is the uncomfortable feeling of coming to work knowing the person next to me would spend the day recreating the theoretical underpinnings of my field.

During the transition from the end of his graduate studies to a postdoctoral fellowship at Lamont-Doherty [Columbia University, Palisades, N.Y.], Paul went on to show how one could combine his thermodynamic analysis with a quantitative algorithm that incorporates experimental and natural chemical data so that the principles he had revealed could be put to work on practical problems. Collaboration between Paul, Mark Ghiorso [OFM Research Inc., Redmond, Wash.]—creator of the ‘MELTs’ algorithm—and Mark Hirschmann [University of Minnesota, Minneapolis] was key to this work.

Finally, in the years after Paul’s return to Caltech as a professor, he illustrated the power of his analysis by using it to interpret the compositions of peridotites and mid-ocean ridge basalts. Highlights include a reconciliation of apparent inconsistencies in the chemical and mineralogical properties of mantle peridotites, and an explanation of the relationship between the abundance of hydrogen in the upper mantle, the abundances of minor elements in mantle melts, and the differentiation of those melts to create erupted lavas. Paul has subsequently grown several new appendages in the form of field programs and experimental labs, turning him into a sort of petrologic Vishnu.

In summary, the Macelwane Medal is a fitting recognition of Paul’s intellectual leadership and creativity, and I’m sure you will find him a deserving and articulate recipient. Ladies and gentlemen, I give you Paul Asimow.

—JOHN EILER, California Institute of Technology, Pasadena


Thank you, John, and thank you also to the AGU and to the Macelwane Medal committee for this recognition.

At some level I find it quite surprising to be recognized by a geophysical society because I don’t consider myself a geophysicist. However, this apparent curiosity in fact reflects quite well, I think, on the breadth of scientific endeavor that AGU has grown to encompass and on the interactions among disciplines that have resulted.

I began my work on mantle melting as a petrologist, seeking to explain something about rocks, namely, the variations in basalt composition among different mid-ocean ridges. I soon found that I needed the tools of geochemistry in order to bring rigor to the analysis and to make quantitative statements derived from conservation of mass and energy. Then, because melting at mid-ocean ridges affects phenomena ranging from oceanic crustal structure to mantle rheology and beyond, I found my work could be used to constrain a number of geophysical questions. Finally, when I had an opportunity to go to sea and collect samples from the field, I would say I began to qualify as a geologist as well. If this award helps to bring my work on mantle melting to the attention of a broader range of disciplines within geophysics, then it will contribute to an aspect of AGU’s mission much more important than the recognition of individuals and the wearing of tuxedos.

I have previously recognized the intellectual debts I owe to J. W. Gibbs, John Verhoogen, Dan McKenzie, Ian Carmichael, Mark Ghiorso, and Marc Hirschmann. I also want to acknowledge important ideas I have picked up from Charlie Langmuir, who with Emily Klein gave us a sound theory for connecting the chemical and physical variables of mid-ocean ridges; Henry Dick for his tireless and nearly thankless work on abyssal peridotites and their significance in understanding mantle melting; Peter Kelemen and Marc Spiegelman for blazing numerous petrological and fluid dynamical paths that I have felt compelled to retread; and Richard Sack for his incomprehensible yet essential solid solution models.

I also want to thank a number of mentors and collaborators for their faith in me and the time and resources they have committed to my success: James B. Thompson Jr., John Wood, Dave Stevenson, and especially my incomparable thesis advisor, Ed Stolper; Charlie Langmuir, Dave Walker, and John Longhi for hosting my postdoctoral respite at Lamont-Doherty Earth Observatory with yet another set of free reins; Ed Stolper again and the rest of the Caltech Geological and Planetary Science faculty for breaking their own rule against hiring their own students to take me back; my colleagues junior and senior for numerous collaborations, especially John Eiler, George Rossman, and Tom Ahrens; my right-hand man Jed Mosenfelder and my graduate students and postdocs for getting things done despite my lax management; and my wife, Colette, and my sons, Lev and Dante, for helping me keep my balance and find the time for science, life, love, work, and play.

—PAUL ASIMOW, California Institute of Technology, Pasadena