Stable Isotope Thermometry: There is More to it Than Temperature

2004 Geochemical Society of Japan Award Lecture and Dedicated to the late Matsuo-sensei, whose passion for geochemistry has proven to be contagious

Juske Horita Chemical Sciences Division Oak Ridge National Laboratory Oak Ridge, Tennessee 37831-6110, U.S.A.

Prologue: Since its inception more than half a century ago, the concept of stable isotope thermometry has been the heart and soul of stable isotope geochemistry and cosmochemistry:

α equilibrium( A−B) = f (T ) (1)

Armed with the statistical-mechanical theoretical foundation for equilibrium isotope fractionation and then state-of-the-art dual-inlet, double-collector mass spectrometry for precision isotope ratio measurements of light elements, the Chicago group let by Nobel laureate Harold Urey launched a historical study of the carbonate paleothermometry in the late 1940s and the early 1950s. This study has become perhaps the most prolific and successful field of isotope geochemistry. Soon the same notion of isotope thermometry was applied to virtually all geoscience disciplines, ranging from paleoclimatology to sedimentology to petrology to cosmochemistry, which has then become known as isotope geothermometry. It is the simplicity and elegance of this concept why isotope thermometry has become widely utilized as one of geochemical tools. It is also true that from the beginning the potential effects of other variables (pressure, fluid composition, etc.) on equilibrium isotope partitioning have constantly been debated. The study of these “other” effects has proven to be very elusive, and took us half a century to finally grasp an outline of the issues. Here, I provide a brief, historical and somewhat personal perspective, including significant developments in the past decade, which has led to this year’s Geochemical Society of Japan Award.

Isotope Salt Effect: The effect of dissolved salts on D/H and 18O/16O ratios of aqueous solutions was first discovered at room temperature in the early 1950s by Henry Taube, another Nobel laureate in chemistry, and his student. Realizing the importance of the isotope salt effect in complex aqueous fluids encountered in numerous geologic settings, geochemists started in earnest in the 1970s to investigate the effect of various salts of geochemical interest (NaCl, CaCl2, etc.) over a wide range of temperature. Large magnitudes and complex behaviors of the isotope salt effects to elevated temperatures, reported by Truedsell (1974), have been in the center of hot debates and the community of isotope geochemistry has been divided over two decades. This debate and struggle finally came to an end, when a series of systematic experimental studies conducted by investigators at Oak Ridge National Laboratory provided high-quality data and resolved the two-decade old problem of the isotope salt effects (Horita et al., 1993a, b; 1995). Even potentially a more important issue is the effect of dissolved minerals in aqueous fluids at elevated temperatures and pressures. A recent study by Bob Clayton and his student (Hu and Clayton, 2003) demonstrated that dissolved minerals (calcite, quartz, etc.) at 750°C and 1.5 GPa increased mineral-water 18O/16O fractionations up to 2 ‰, which are extremely large considering the magnitude of typical mineral – water fractionations at elevated temperatures. They also observed complex interplays between the effects of dissolved salts and minerals. Given the magnitude of the observed effects by dissolved minerals and the lack of our knowledge of the solubility of many minerals at elevated temperatures and pressures, we are now faced with even a greater challenge than that of the isotope salt effect two decades ago.

Isotope Pressure Effect: The effect of pressure on isotope fractionation has been even more elusive. The intrinsic similarity of isotopes of an element has led us to assume negligible pressure effects on isotopic fractionation. Both theoretical and experimental studies in the past decades, most notably those by the Chicago group, this time led by Goldschmidt and Urey medalists Bob Clayton (e.g., Clayton et al., 1975), have shown that the pressure effect is very likely within analytical errors of isotopic analysis under typical geologic conditions. Their works set a tone in this issue. Then, the 1990s witnessed resurgence in the study of the isotope pressure effect, both theoretical and experimental. Veniamin Polyakov and his colleague at Vernadsky Institute of Geochemistry and Analytical Chemistry, Russia, published a series of theoretical calculations on the isotope pressure effect (Polyakov and Karlashina, 1994; Polyakov, 1998). Their studies, which are theoretically most advanced to date, provided a strong impetus to investigate and verify the isotope pressure effect experimentally. It was again our team at ORNL who conducted a series of carefully designed experimental studies for the isotope pressure effect. Horita et al. (1999) was the first to demonstrate experimentally that a D/H fractionation between brucite and water increased with increasing pressure (>12 ‰ over a pressure range from 15 to 800 MPa at 380°C). We started a collaboration with Veniamin Polyakov to understand fundamental causes of the observed isotope pressure effects. In our systematic experimental and theoretical study, Horita et al. (2002) not only reported experimental data of large D/H pressure effects over a wide temperature and pressure range, but also provided a framework for quantitative, theoretical understanding of the pressure effect on D/H partitioning. The isotope pressure effect has finally come within our reach.

More than temperature: The last decade has witnessed perhaps the most significant and exciting developments in our understanding of principles of isotope partitioning since the foundation of isotope geochemistry more than half a century ago, including non-mass dependent fractionation. The concept of isotope thermometry has gone through a revolutionary change from Eq- 1 to:

α equilibrium( A−B) = f (T, P, X fluid ) (2)

In fact, in some geologic settings the effect of pressure and dissolved salts/minerals seem greater than that of temperature. Perhaps we need a new terminology beyond isotope thermometry.

Epilogue: With his classic theoretical work in 1947 and a subsequent application to the carbonate thermometry, Harold Urey literally founded and demonstrated the concept of isotope thermometry. Bob Clayton, who assumed the same position at University of Chicago after Urey’s retirement, has been the major force in the development of isotope geothermometry. Perhaps, the rest is history. It is a strange twist of life that I happened to largely focus my efforts on this fundamental, yet poorly understood issue of isotope geochemistry. And if there is any reason why I deserve to receive a prestigious Geochemical Society of Japan Award this year, it is perhaps my and my colleagues’ contributions at ORNL and elsewhere that played a pivotal role in the revolutionary change in the concept of isotope thermometry last decade. Although it is somewhat unclear where my career would be shifting in the future (hopefully plenty of time left!), I take my pride and appreciation in the recognition of my works for this award.