1. Deep Earth (Bruce A. Buffett, Dion L. Heinz)
The restless interior of the Earth continually reshapes our environment. Convection in the rocky mantle powers plate tectonics and drives many of the geological processes we observe at the surface. From deeper in the planet we have indirect observations of turbulent motion in the liquid iron core, which generates the Earth's magnetic field. Many aspects of these processes are poorly understood. Part of the difficulty lies in understanding the state of matter at extreme pressures and temperatures. Another challenge arises in using this information to build models and gain insights into the internal dynamics. We are actively pushing the frontier of high-pressure research using state-of-the-art facilities at Argonne National Laboratory. We are also leading the effort to develop a more comprehensive understanding of internal processes.
Cross-section of Earth's interior. A combination of laboratory and computer based research allows scientists to explore the composition and dynamics of the deep Earth, which is otherwise inaccessible to direct sampling. Figure credit: Calvin J. Hamilton.
Campbell A.J., Seagle C.T., Heinz D.L., Guoyin S., Prakapenka V.B. (2007) Partial melting in the iron sulfur system at high pressure: A synchrotron X-ray diffraction study. Physics of the Earth and Planetary Interiors 162, 119-128.
Buffett B.A., Christensen U.R. (2007) Magnetic and viscous coupling at the core-mantle boundary: inferences from observations of the Earth's nutations. Geophysical Journal International 171, 145-152.
Mao W.L.,Campbell A.J., Heinz D.L., Shen G.Y. (2006) Phase relations of Fe-Ni alloys at high pressure and temperature. Phys Earth Planet In 155, 146-151.
Buffett B.A. (2000) Clathrate hydrates. Annu Rev Earth Planet Sci 28, 477-507.
Buffett B.A. (2000) Earth's core and the geodynamo. Science 288, 2007-2012.
2. Early Earth (Bruce A. Buffett, Robert N. Clayton, Nicolas Dauphas, Frank M. Richter)
No terrestrial rocks formed before 4.03 Ga have survived subsequent reworking by impacts, erosion, and tectonic activity. The only direct witnesses of earlier times, which can appropriately be called the dark age of the Earth, are tiny zircon grains accumulated in younger detrital sediments. There is thus a large gap in the geological record which covers the formation of the oceans, the late heavy bombardment of the Earth by comets and asteroids, and the possible emergence of life. In this respect, the large exposures of sedimentary rocks dated at 3.6-3.8 Ga in Greenland and Canada are unique. We study the isotopic and chemical compositions of these rocks to try to understand, among other things, whether the Earth was habitable by 3.7-3.8 Ga, whether life had already emerged, and if yes, what kind of organisms were present.
False color image of a 3.7-3.8 Ga sediment (banded iron formation or BIF) from Nuvvuagittuq (Canada). The horizontal yellow lines are made of magnetite, an iron oxide formed by precipitation of dissolved iron in the ocean. The scale bar is 1 cm. Figure credit: Nicolas Dauphas.
Dauphas N., Cates N.L., Mojzsis S.J., Busigny V. (2007) Identification of chemical sedimentary protoliths using iron isotopes in the >3750 Ma Nuvvuagittuq supracrustal belt, Canada. Earth Planet Sci Lett 254, 358-376.
Dauphas N., van Zuilen M., Wadhwa M., Davis A.M., Marty B., Janney P.E. (2004) Clues from Fe isotope variations on the origin of early Archean BIFs from Greenland. Science 206, 2077-2080.
Richter F.M. (1985) Models for the Archean thermal regime. Earth Planet Sci Lett 73, 350-360.
Becker R.H., Clayton R.N. (1976) Oxygen isotope study of a precambrian banded iron-formation, Hamersley Range, Western Australia. Geochim Cosmochim Acta 40, 1153-1165.
3. Formation of the elements (Robert N. Clayton, Nicolas Dauphas, Andrew M. Davis)
The chemical elements have a variety of origins. For example, of the three most abundant elements in the human body, hydrogen was made in the Big Bang that gave birth to the Universe, carbon was mostly made in red giant stars, and oxygen was mostly made in supernovae. We study the origin of the elements by analyzing samples from stars in our laboratories. This work is made possible by a remarkable discovery made nearly 20 years ago at the University of Chicago, that certain kinds of meteorites contain tiny grains of stardust. These stardust grains, which are made of diamond, silicon carbide and graphite, as well as more common silicates, condensed at high temperature from stellar winds and survived a series of potentially destructive events in the interstellar medium, during the formation of the solar system and on asteroidal meteorite parent bodies. Individual stardust grains have huge isotopic anomalies compared to solar system material and each grain provides a record of nucleosynthesis in its parent star.
Image of the Murchison meteorite. This sample contains grains that were formed in the envelopes of now extinct stars, before the Sun was born. Figure credit: Nicolas Dauphas
Barzyk J.G., Savina M.R., Davis A.M., Gallino R., Gyngard F., Amari S., Zinner E., Pellin M.J., Lewis R.S., Clayton R.N. (2007) Constraining the 13C neutron source in AGB stars through isotopic analysis of trace elements in presolar SiC. Meteoritics and Planetary Science 42, 1103-1120.
Dauphas N. (2005) The U/Th production ratio and the age of the Milky Way from meteorites and Galactic halo stars. Nature 435, 1203-1205.
Savina M.R., Davis A.M., Tripa C.E. et al. (2004) Extinct technetium in silicon carbide stardust grains: implications for stellar nucleosynthesis. Science 303, 649-652.
Lugaro M., Davis A.M., Gallino R. et al. (2003) Isotopic compositions of strontium, zirconium, molybdenum, and barium in single presolar SiC grains and asymptotic giant branch stars. Astrophys. J. 593, 486-508.
4. Formation of the Sun and planets (Fred Ciesla, Robert N. Clayton, Nicolas Dauphas, Andrew M. Davis, Lawrence Grossman, Frank M. Richter)
The question of the formation of the solar system can be tackled from different points of view. It can be studied by examining meteorites, which are pieces of asteroids that fall on Earth and leave a shooting star in the sky as they heat up and volatilize during atmospheric entry. Some of these meteorites formed at the same time as the Sun and their compositions and properties can therefore be used to infer the conditions that prevailed in the protosolar nebula around the nascent Sun. The science of studying meteorites is called cosmochemistry. Cosmochemistry was born at the University of Chicago in Harold Urey's laboratory and since then scientists at UofC have remained at the forefront of research in this field. Among notable achievments are the development of the theory of condensation in the solar nebula by Larry Grossman, the discovery of isotope anomalies for oxygen by Bob Clayton, and the discovery of presolar grains by Ed Anders.
Dust was collected by NASA's Stardust spacecraft during flight through the coma of Comet 81P/Wild 2 on January 2, 2004. The dust grains carried back to Earth probably represent the most pristine material available for study of the formation of the solar system. Figure credit: NASA/JPL.
Grossman L., Beckett J.R., Fedkin A.V., Simon S.B., Ciesla F.J. (2007) Redox conditions in the solar nebula: Observational, experimental and theoretical constraints. Accepted for publication in Oxygen in Earliest Solar System Materials and Processes.
Clayton R.N. (2007) Isotopes: from Earth to the Solar System. Ann Rev Earth Planet Sci 33, 1-19.
Richter F.M., Mendybaev R.A., Davis A.M. (2006) Conditions in the protoplanetary disk as seen by the type B CAIs. Meteoritics Planet Sci 41, 83-93.
Clayton R.N. (2002) Solar system - Self-shielding in the solar nebula. Nature 415, 860-861.
Grossman L., Ebel D.S., Simon S.B. (2002) Formation of refractory inclusions by evaporation of condensate precursors. Geochim Cosmochim Acta 66, 145-161.
5. The ups and downs of mountains and oceans (Bruce A. Buffett, David B. Rowley, Frank M. Richter)
Plate tectonics provides a very general explanation for the origin of mountain belts. However, more detailed accounts of the timing and style of mountain building are still the subject of considerable controversy and thus remain an area of active contemporary research. Here we have been using sedimentological and geochemical methods to address particular aspects of the temporal evolution of mountain belts. We have used 40Ar/39Ar thermochronometry to determine the unroofing rate of Himalayan plutons. We have used sediment volumes as a function of age in the Bay of Bengal together with changes in the riverine input of strontium and its isotopes into the oceans to determine a spatially averaged history of denudation of Himalaya-Tibet. A more recent major effort involves using oxygen isotopes to determine how elevation has changed with time in particular parts of Tibet. The origin of mountains is a classic geologic theme, and there is good reason to believe that we are in for an exciting and productive time in addressing this theme through a judicious combination of fieldwork and modern geochemical analytical methods.
The collision of India with Asia is responsible for the uplift of the Himalayas and Tibetan Plateau. The rise of the Himalayas was initiated ~50 My ago and had global climatological consequences. Figure credit: D.B. Rowley.
Rowley D.B., Garzione C.N. (2007) Stable isotope-based paleoaltimetry. Annual Review of Earth and Planetary Sciences 35, 463-508.
Buffett B.A., Rowley D.B. (2006) Plate bending at subduction zones: consequences for the direction of plate motions. Earth Planet Sci Lett 245, 359-364.
Rowley D.B., Currie B.S. (2006) Paleo-altimetry of the late Eocene to Miocene Lunpola basin, central Tibet. Nature 439, 677-681.
Rowley D.B., Pierrehumbert R.T., Currie B.S. (2001) A new approach to stable isotope-based paleoaltimetry: implications for paleoaltimetry and paleohypsometry of the High Himalaya since the Late Miocene. Earth Planet Sci Lett 188, 253-268.
Richter F.M., Rowley D.B., DePaolo D.J. (1992) Sr isotope evolution of seawater - The role of tectonics. Earth Planet Sci Lett 109, 11-23.
6. Volcanoes and plutons (Alfred T. Anderson, Andrew M. Davis)
Ours is truly an active planet. Every month there are some 10 to 20 volcanic eruptions on earth, and a handful of these, like Kilauea, Hawaii have been continuously erupting for years. Most of these go unnoticed as they are in poorly populated areas remote from where we live. Volcanoes and volcanic rifts mark the locations of plate boundaries and reveal both the places where mantle rises up as well as where crust and lithosphere decend into Earth’s interior. Volcanic rocks contain large crystals that formed before eruption, and these record the evolution of the magma including the formation of bubbles that cause buoyancy and drive eruptions. Students and I study the crystals and their inclusions of glass (formerly melt) for clues to understand why some silicic magmas erupt and others crystallize at depth. I think granitic magmas that crystallize at depth are necessary for the formation of continental crust. Granitic magmas are almost invariably buoyant and should erupt if eruptibility were mainly dependent on their relatively low density. Granitic magmas that do erupt form mainly superficial fragmental rhyolites that are easily eroded. Our studies thus seek factors that either deter eruption or that enhance crystallization at depth. Field and structural relations show that erupted rhyolites and granites may form different parts of the same magma body: the most buoyant parts may erupt leaving behind a largely crystallized and deeply buried part.
The largest quartz crystal in this group is about 1mm long. It is bounded by several crystal faces and two fractures. About a dozen melt (glass) inclusions occur in the crystal and one of these has a neck that extends all the way to a crystal face. It contains a bubble about 25 microns in diameter that is mostly dark, but with a pinprick of light in its center. The pinprick of light shows that the bubble is transparent, and the dark part results from refraction of light around the low-refractive-index gas in the bubble. Plausibly the bubble is a primary bubble that was accidentally captured as the quartz crystal grew. Study of such hourglass inclusions constrains the decompression history of the magma and its pre-eruptive gas content. Figure credit: A.T. Anderson.
Anderson A.T., Liu Y., Wilson C.J.N., (2005) Magma ascent rates using melt pockets in phenocrysts. Eos Trans. AGU, 86(52), V21E-0671.Gualda G.A.R., Cook D.L., Chopra R., Qin L, Anderson A.T., Rivers M. (2004) Fragmentation, nucleation, and migration of crystals and bubbles in the Bishop Tuff magma: evidence from crystal size distributions. Trans. R. Soc. Edinburgh, Earth Sciences, 95, 375-390.
Anderson A.T., Davis A.M., and Lu F. (2000) Evolution of the Bishop Tuff rhyolitic magma based on melt and magnetite inclusions and zoned phenocrysts. J. Petrol., 41, 449-473.
Anderson, A.T. & Brown, G.G., (1993) CO2 contents and formation pressures of some Kilauean melt inclusions. Am. Mineral., 78, 794-803.
Anderson, A.T.,Jr., (1991) Hourglass inclusions: Theory and application to the Bishop rhyolitic tuff. Am. Mineral., 76, 530 – 547.