Computer Science
Scientific paper
Aug 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002psrd.repte..62t&link_type=abstract
Planetary Science Research Discoveries
Computer Science
Mars, Chemistry, Martian Meteorite, Oxygen Fugacity
Scientific paper
Studies have inferred that the oxidation state of Martian basaltic meteorites (the shergottites) is correlated with diagnostic geochemical parameters. For example, Meenakshi Wadhwa (Field Museum, Chicago) showed that as the ratio of triply-charged europium to doubly-charged europium increases in a group of shergotties, the ratio of strontium-87 to strontium-86 also increases, and neodynmium-143 to neodynmium-144 decreases. [See PSRD article: Gullies and Canyons, Rocks and Experiments: The Mystery of Water on Mars]. Eu3+/Eu2+ is a measure of the oxidation state and can be used to infer the availability of oxygen to react chemically, a property called the oxygen fugacity. Christopher Herd, Lars Borg, and Jim Papike (University of New Mexico) and John Jones (Johnson Space Center) decided to measure the oxygen fugacity more directly by making very careful and painstaking analyses of oxide minerals in Martian meteorites.
Herd and his co-workers find that as oxygen fugacity increases in a group of shergottites, the ratios of strontium-87 to strontium-86 and of lanthanum (La) to ytterbium (Yb) also increase, while neodynmium-143 to neodynmium-144 decreases. They suggest these trends indicate that, compared to the Martian mantle, the crust is more oxidizing, has higher strontium-87 to strontium-86 and La/Yb, and lower neodynmium-143 to neodynmium-144. Magmas formed in the mantle would have low oxygen fugacity. As magmas rose through the crust, they reacted with the surrounding rocks to varying extents, producing the observed chemical trends. How did the crust become more oxidizing than the mantle? They suggest that circulating hot water oxidized the crust. Alternatively, water-rich magmas might have crystallized in the crust, forming deposits of hydrous minerals. Subsequent magmas could react with the hydrated minerals to become more oxidizing. Whatever the details, the work by Herd and colleagues indicates that the mantle and crust differ significantly, that the crust has significant deposits of water, and many pristine magmas are modified by interaction with the crust.
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