Douglas Rumble III

Oxygen isotope geochemistry is the single most powerful research tool available for the study of interactions between water and rocks. Water alters rocks by chemical reactions such as solution, replacement, and precipitation, and at the same time indelibly changes the rocks' oxygen isotope composition. Oxygen has three stable, nonradioactive isotopes -- ¹⁶O, ¹⁷O, and ¹⁸O. Seawater, rainwater, glacier ice, groundwater, and extraterrestrial water each have distinctive and identifiable ratios of the isotopes. By measuring the oxygen isotope ratios in rocks and minerals, the sources and amounts of ancient waters long drained away can be estimated.

Projects under way in Doug Rumble's laboratory accurately illustrate the wide applicability and power of oxygen isotope geochemistry. Rumble is analyzing oxygen isotopes in high-pressure minerals from a continental collision zone in northern Kazakhstan. The samples are unique to the planet because they contain diamonds in a matrix of metamorphosed sediments rather than the more typical occurrence in volcanic rocks. Together with geologists from the Tokyo Institute of Technology (TIT), Waseda University, Yokohama University, the Geological Survey of Japan, and Stanford University, Rumble participated in mapping and sampling diamond-bearing rocks of the Kokchetav massif in August 1999. Graduate students from Waseda and TIT are now analyzing the samples in Rumble's laboratory. The goal of the study is to identify the source of the diamond-containing rocks. The isotopic information will be used to understand how surface rocks can be subducted to depths of 125 kilometers, or more, and return to Earth's surface without losing their definitive surface characteristics. It should be possible to reconstruct ancient environmental conditions recorded by sediments before they were driven into the Earth's mantle by continental collision.

NSF postdoctoral fellow Henry Fricke is using the ultraviolet (UV) laser oxygen isotope microprobe and other analytical techniques to investigate the record of ancient climate stored in dinosaur teeth. These methods work because the oxygen isotope ratio of rainwater is influenced by local climate. Animals drink the rainwater, which in turn influences the oxygen isotopic composition of their growing teeth. A UV laser study of several small dinosaur teeth indicates that they grew rapidly, and may be used to study seasonal changes in temperature during the Cretaceous period. Analyses of dinosaur teeth from different localities in North America using conventional techniques indicate that temperatures were much warmer during this same time period, particularly in the polar regions.

Richard Ash, NASA postdoctoral fellow, is studying meteorites with the UV laser oxygen isotope microprobe. He has found isotopic evidence of the effects of 4.5-billion-year-old ice that accumulated with the metallic and rocky components of meteorites early in the history of the solar system. Continuing accretion from the solar nebula grew planetesimals big enough that the internal radioactive decay melted ice chunks. Liquid water percolated through the metal-rock mixture, altering the isotopic composition as it moved, and leaving behind a record that survived even the fiery flight through Earth's atmosphere.

None of the projects would be possible without the use of state-of-the-art equipment, much of which must be crafted in-house. The infrared laser oxygen isotope probe was invented in this laboratory by postdoctoral fellow Zach Sharp 10 years ago and an updated instrument remains in demand. The new UV laser oxygen isotope microprobe was developed through the efforts of many researchers, including former German Science Foundation postdoctoral fellow Uwe Wiechert, former Carnegie fellows Ed Young and James Farquhar, and Rumble. The UV microprobe is opening up new research opportunities because of its capability for in situ, spatially