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2016 Nier Prize for Gregory A. Brennecka

I first came to know of Greg Brennecka in 2007 through my good friend and colleague Lars Borg. Lars mentioned to me that he knew this student intern at the Nuclear Forensics Group at the Lawrence Livermore National Lab who had applied to the Ph.D. program at Arizona State University. Lars obviously thought very highly of Greg, and suggested strongly that we admit him—and I am glad we heeded that bit of advice! Growing up, Greg was fond of spending time outdoors, and he credits his parents for instilling in him the importance of understanding the natural world around him. So it is no surprise that Greg obtained not one, but two Bachelor of Science degrees—the first majoring in Natural Science/Chemistry from Columbia College (1998–2002) and the second in Geological Sciences from the University of Missouri-Columbia (2000–2003). He then went on to get a Master of Science degree from Oregon State University (2003–2006). In 2005, while he was still at OSU, he decided to spend the summer working in the Nuclear Forensics Group at the Lawrence Livermore National Lab. He enjoyed his time at LLNL so much that he continued his collaborative projects there during summer internships for several years, even while he was a graduate student at Arizona State University. Greg was admitted in 2007 as a Ph.D. candidate in the newly established School of Earth and Space Exploration at ASU. The centerpiece of Greg’s doctoral work was the careful documentation, and discussion of the implications, of variation in the U/U ratio in meteoritic and terrestrial samples. During his first year, Greg took a course from me on “Meteorites and Cosmochemistry.” For the final project for this course, he independently developed the idea of measuring the uranium isotope compositions of refractory inclusions in chondritic meteorites to search for evidence of the shortlived radionuclide Cm (half-life ~15.6 Ma). Now this is exactly the sort of project that Greg loves to dig into: a big question that can be answered with a novel newly developed analytical method. At the time, Dr. Stefan Weyer was visiting Professor Ariel Anbar’s laboratory at ASU. Dr. Weyer helped to develop the methodologies for high-precision U isotope analyses using multicollector inductively coupled plasma mass spectrometry and Greg had the opportunity to interact with him. It is to Greg’s credit, his innovative thinking, and motivation that he initiated the work on the U isotope systematics in a suite of refractory inclusions from the Allende CV3 chondrite in the Isotope Cosmochemistry and Geochronology Laboratory at ASU. He found that these inclusions recorded a significant variation in the U/U ratio (~3.5&) that correlated with the Nd/U and Th/U ratios. These results were the first definitive evidence for the presence of live Cm in the early solar system (with an initial Cm/U ratio of 1.1–2.4910 ) and were published in a paper in Science (Brennecka et al. 2010a). The implications of U isotope variation in meteoritic materials were profound, the basis of high-precision PbPb chronology had up to this point relied on the assumption of an invariant U/U ratio (=137.88) in all natural samples. Greg showed that the variation he measured in the U isotope ratios of Allende refractory inclusions required an age adjustment of up to 4–5 Ma for these objects. This in turn has implications for ages obtained using high-resolution extinct chronometers that are anchored to high-precision Pb-Pb ages. Therefore, Meteoritics & Planetary Science 51, Nr 9, 1734–1736 (2016) doi: 10.1111/maps.12723