Geochemistry, Mineralogy, Volcanology Tribute

Gerald J. Wasserburg (1927–2016)

A leading geochemist who became well known for creating the first high-precision mass spectrometer, called Lunatic I, that measured isotope ratios in lunar samples from the Apollo missions.

By , Dimitri A. Papanastassiou, and Donald J. DePaolo

Gerald J. “Jerry” Wasserburg, a pioneer in the fields of geochemistry and cosmochemistry, died on 13 June 2016. He was 89. Wasserburg made fundamental and enduring contributions to Earth and planetary sciences using the methods of isotope geochemistry and cosmochemistry.

Jerry was born in New Brunswick, N.J. After enlisting in the U.S. Army and seeing combat in World War II, he graduated from high school and spent 2 years at Rutgers University. Following the advice of Henri Bader, he transferred to the University of Chicago, where he obtained a B.Sc. in physics in 1951, a M.Sc. in geology in 1952, and a Ph.D. under H. C. Urey and M. G. Inghram III in 1954. His Ph.D. thesis was on the branching ratio of potassium-40 decay, an essential step in the development of potassium-argon (K-Ar) age dating.

Jerry joined the faculty of the California Institute of Technology (Caltech) in 1955 as a professor of geology and geophysics and retired from that position in 2001. He initially focused his research on the search for now extinct iodine-129 (129I) in meteorites and problems in rubidium-strontium (Rb-Sr) and uranium-lead (U-Pb) geochronology. He published a classic paper on the duration of nucleosynthesis based on Reynold’s discovery of 129I. He was the first to recognize the significance of the difference of Earth’s potassium/uranium (K/U) ratio from that measured in primitive meteorites and the implications for the thermal history of the Earth.

Lunatic Asylum

Jerry’s signature approach was to improve chemical separation and mass spectrometric methods for isotopic measurements. He led the way for these developments, and the Caltech laboratory he established in the late 1960s in anticipation of lunar sample return set the standard for the field. There, Jerry built a mass spectrometer of extraordinarily high precision, which he called Lunatic I. He dubbed the lab the “Lunatic Asylum.” His analytical contributions, combined with keen insight, led to a long line of discoveries and developments.

One of the first of those results was the measurement of the initial ratio of strontium-87 to strontium-86 (87Sr/86Sr) for the solar system on the basis of meteorites that we now believe are samples of basaltic lavas from the asteroid Vesta. This value is still in use and is essential for any considerations of planetary evolution based on the Rb-Sr system.

Solar System Clues in a Meteorite

In 1969, when the meteorite Allende fell in Mexico, Jerry rushed to the field to collect pieces of it. The shards contained white, high-temperature inclusions, with lower 87Sr/86Sr than those seen previously in the basaltic meteorites. His studies of these inclusions led to the discovery that short-lived radioactive aluminum-26 was present in the early solar system. This finding required that fresh nucleosynthetic material had been injected into the solar system’s parent molecular cloud, perhaps by a supernova triggering the formation of the solar system.

Jerry subsequently found that some of the Allende white inclusions had measurable isotopic heterogeneities in heavy elements (calcium, titanium, strontium, barium, neodymium, and samarium). These anomalies could be explained only by incomplete mixing of presolar nucleosynthetic components. That realization made it clear for the first time that the materials that now make up the terrestrial planets and the asteroid belt did not all form by condensation from a hot solar gas. Instead, some grains must have survived the event that formed the solar system, retaining direct evidence for the individual stellar nucleosynthetic sources that contributed to the newborn solar system. These extraordinary results were followed by the discovery that palladium-107 had been present in the early solar system. Research following up on these discoveries from the late 1970s and early 1980s is continuing today at full pace.

Concurrently, Jerry continued to pursue research on the implications of long- and short-lived radionuclides for the evolution of the Milky Way and showed that long-lived chronometers date the mean age of the universe. Work by him and his colleagues on the U-Pb isotopic system in lunar samples, together with Rb-Sr and K-Ar data, led Jerry to propose that about 3.9 billion years ago the frequency of impacts on the Moon rose sharply, an intensification he described as a “terminal lunar cataclysm.” There is now evidence that this event, referred to today as the Late Heavy Bombardment, affected the entire inner solar system.

The Apollo program provided a golden age for planetary science and isotope cosmochemistry. Jerry identified and mentored many talented scientists in the Lunatic Asylum during those years, and his group generated an impressive cascade of publications that are testament to Jerry’s imagination, inspiration, drive, uncompromising dedication to high-quality data, and ability to choose exciting and important scientific problems. Jerry also helped to ensure that the last two Apollo missions proceeded as planned and included science.

From Earth’s Interior to Its Ancient Climate

In the mid-1970s a major focus of Jerry’s laboratory was the development of the samarium-neodymium isotopic system. This system forms the cornerstone of the part of modern geochemistry devoted to understanding the chemical evolution of Earth’s interior. Jerry’s work initiated an ongoing debate on how best to interpret and integrate the geophysical and geochemical evidence for the structure and evolution of Earth’s interior. A later important contribution was to point out that helium isotope variations are consistent with a deep primitive reservoir in Earth.

Jerry was also the first to develop rhenium-osmium (Re-Os) and thorium-230 (230Th) measurements by thermal ionization mass spectrometry. The Re-Os technique resulted in a method for mapping the age structure of the subcontinental lithospheric mantle and age dating of black shales. The 230Th technique has led to enormous advances in the field of paleoclimatology.

Award-Winning Pioneer

Jerry received many prestigious honors. In 1978, he was awarded the V. M. Goldschmidt Medal, now called the V. M. Goldschmidt Award, which is the highest honor of the Geochemical Society. In 1984, the American Geophysical Union (AGU) selected him as the first recipient of its Hess Medal. In 1986 he was jointly awarded, with Claude Allègre, the Crafoord Prize for his pioneering work on isotope geology, the top honor given by the Royal Swedish Academy of Sciences to geoscientists. AGU recognized Jerry again in 2008 with the Bowie Medal, the highest honor given by the organization.

Jerry’s extensive efforts over 60 years defined the fields of isotope geochemistry and cosmochemistry and contributed substantially to the field of space and planetary sciences. The pioneering techniques he developed remain essential tools today in many subfields of Earth and planetary science and astrophysics.

—Stein B. Jacobsen (email: [email protected]), Department of Earth and Planetary Sciences, Harvard University, Cambridge, Mass.; Dimitri A. Papanastassiou, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena; and Donald J. DePaolo, Department of Earth and Planetary Science, University of California, Berkeley

Citation: Jacobsen, S. B., D. A. Papanastassiou, and D. J. DePaolo (2017), Gerald J. Wasserburg (1927–2016), Eos, 98, Published on 03 May 2017.
© 2017. The authors. CC BY-NC-ND 3.0