Earth Sciences

Earth Sciences

Volcanic Outgassing of the Earth's Atmosphere

Some scenarios for the formation of life on Earth involve prebiotic chemical reactions occurring in the presence of a chemically reducing atmosphere (e.g., H2, CO, CH4, NH3). The goal of this project is to develop a geochemical oxybarometer that can be applied to mantle-derived basalts to infer the oxidation state of the Earth's upper mantle through geological time. Since the oxidation state of the upper mantle determines the molecular species of gases released during basaltic volcanism, the composition of the early atmosphere would have in part been coupled to the nature of gases emanating from the mantle. Although the present upper mantle is too oxidized to release highly reduced gas species, was the upper mantle ever sufficiently reducing to provide these components to the atmosphere? If so, when? What has been the evolutionary history of the mantle's oxidation state?

The transition elements, chromium (Cr) and vanadium (V), undergo valence changes within the range of oxidation states that is of interest for planetary mantles. The consequence of these redox changes is that the geochemical partitioning of Cr and V between basaltic magmas and the solid residuum in the Earth's mantle changes significantly depending on the oxidation state. The purpose of this research is to (a) experimentally calibrate the melt/solid partition coefficients of Cr and V as a function of oxidation state, and (b) apply those data to natural basaltic rocks of known age through geological time to constrain the oxidation state of the Earth's mantle through time. Preliminary experimental data suggest that the oxidation state of the Earth's upper mantle has been constant (within experimental uncertainties) during the last 3.5 billion years (Gyr). If correct, these data suggest that highly reduced gas species have not been emitted into the atmosphere by mantle-derived volcanism for at least 3.5 Gyr.

This work will be performed in association with Dr. John Jones (NASA Johnson Space Center, Houston, TX).

Extinction by Impacts

The bombardment of the Earth by large asteroids and comets during accretion would have presented a formidable barrier to the origin and sustainability of life. Although current models for the time-dependent flux of meteorites during the first 1 billion years (Gyr) of Earth history are based largely on interpretations of crater-populations on the Moon, isotopic analyses of lunar rocks and minerals returned by the Apollo astronauts do not seem readily consistent with the commonly held view that bombardment of the Earth-Moon system can be described by a simple decline in flux during the first 700 million years. The potential implications for this current uncertainty are significant. For example, was life not possible until about 3.9 Gyr? If so, prebiotic chemical reactions during the first 700 million years would have been 'for nothing' since episodic impacts would have destroyed all progress toward formation of life. But is that flux model really true?

To better constrain the nature of the early bombardment of the Earth-Moon system, research is proposed whereby hundreds of lunar glasses produced by impact events will be individually dated using Ar isotopes. High-potassium lunar glasses from the Apollo 14 and 17 landing sites on the Moon will be individually analyzed to determine the age of each impact event that caused them. By performing this work for hundreds of glasses, the flux of cosmic debris onto the Moon's surface through time will be better understood (e.g., the larger the number of impact glasses of a given age, the higher the inferred flux).

This work will be performed in association with Dr. Graham Ryder (Lunar and Planetary Institute, Houston, TX) and Dr. Paul Renne (Berkeley Geochronology Center, Berkeley, CA).

Associated Scientists

Dr. Graham Ryder is a Staff Scientist at the Lunar and Planetary Institute in Houston, TX. He has been active in lunar sample analysis since 1975, and is currently interested in determining the bombardment history of the Moon from isotopic analyses of lunar impact melt-rocks collected by the Apollo astronauts.

Dr. John Jones is a Staff Scientist at the NASA Johnson Space Center in Houston, TX. His research interests include experimental trace element partitioning studies for the purpose of understanding the geochemical evolution of Mars and the parent bodies of differentiated meteorites. In addition, Dr. Jones has been an active investigator into the early history of the Earth-Moon system.

Dr. Paul Renne is the President and Director of the Berkeley Geochronology Center in Berkeley, CA. His research interests include the development and application of 40-Ar/39-Ar geochronology to problems in planetary and biotic evolution (e.g., mass extinctions; paleoclimate; bolide impacts).

Other Research Areas:

Astrophysics

Biology

Chemistry