Other
Scientific paper
May 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agusm.p33c..05h&link_type=abstract
American Geophysical Union, Spring Meeting 2004, abstract #P33C-05
Other
6205 Asteroids And Meteoroids, 6215 Extraterrestrial Materials, 6240 Meteorites And Tektites
Scientific paper
Ureilites are carbon-bearing ultramafic achondrites composed primarily of olivine and pyroxene with intergranular fine-grained metal, sulfides, and silicates. Carbon (up to 6.5 wt%) is either amorphous or present as graphite, lonsdaleite, and/or diamond. It has been shown that carbon-silicate redox (i.e. "smelting") reactions are responsible for the positive correlation between modal percent pigeonite and mg# and for the negative FeO-MnO trend seen in the mineral and bulk compositions of ureilites. Carbon redox reactions are strongly exothermic and pressure dependent; so ureilites with the largest mg# are the most reduced, experienced the highest temperatures, and formed at the lowest pressures, i.e. near the surface of the ureilite parent body. Ureilites with the largest mg# have the smallest δ 18O and the largest Δ 17O. To further investigate possible relationships, we performed carbon isotope and electron probe measurements on a suite of 27 ureilites in order to see the type of correlation that exists between mg# and carbon. Mg#s of olivine cores, carbon contents, and δ 13C data were taken from this study and the literature, and averaged. Polymict ureilites were not considered. A well-defined negative correlation is observed between the mg# of olivine cores and Δ 17O. A less well-defined negative correlation may exist between mg# of olivine cores and δ 13C, but there is substantial scatter in the data. However, a well-defined negative correlation exists between mg# of olivine rims and δ 13C. At first glance, this trend is unexpected: if ureilites with the largest mg# experienced the greatest amount of reduction, they should have the largest δ 13C and the correlation between mg# and δ 13C should be positive. A plot of carbon content versus δ 13C seems to show a general trend: the smaller the carbon content, the heavier is the carbon. This general trend is exactly what one would expect if smelting has affected the ureilite parent body: the more C is consumed during smelting, the heavier the residue become. However, mg#s do not support this interpretation: ureilites with the largest mg# have the smallest carbon content and the smallest δ 13C, while ureilites with the largest mg# have the opposite. To explain this apparent contradiction, one needs to consider that ureilites have experienced two reducing events. The first one is recorded in the cores of the olivine crystals while the second is seen in the strongly reduced rims. During the heating of the ureilite parent body, the olivine cores first equilibrated with the carbon and their mg# were fixed according to their depth: the deepest olivine experienced little reduction, had low mg#, relatively light carbon (δ 13C < -10 ‰ ) and high carbon content (about 7-8 wt%); the shallowest olivines experienced the greatest reduction, resulting in high mg#, relatively light carbon (δ 13C < -8 to -10‰ ) and low carbon content (about 4 wt%). The second reducing event was marked by a sudden drop in pressure (possibly due to an impact that disrupted the parent body). During this event, olivines that formed at depth were now strongly reduced along their rims (the cores preserved their initial mg#), their carbon became heavier (δ 13C > -8‰ ) and their carbon content decreased (below about 4 wt%). On the other hand, olivines that formed initially near the surface of the parent body did not experience much change in terms of mg#, δ 13C, and carbon content.
Hudon Pierre
Mittlefehldt Dave W.
Paddock L.
Romanek Christopher
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