Ar-39-Ar-40 "ages" and origin of excess Ar-40 in Martian shergottites

作者：Bogard Donald^*; Park Jisun; Garrison Daniel

来源：Meteoritics & Planetary Science, 2009, 44(6): 905-923.

We report new Ar-39-Ar-40 Measurements oil 15 plagioclase, pyroxene, and/or whole rock samples of 8 Martian shergottites. All age spectra suggest ages older than the meteorite formation ages, as defined by Sm-Nd and Rb-Sr isochrons. Employing isochron plots, only Los Angeles plagioclase and possibly Northwest Africa (NWA) 3 17 1 plagioclase give ages in agreement with their formation ages. Isochrons for all shergottite samples reveal the presence of trapped Martian Ar-40 (Ar-40(xs)), which exists in variable amounts ill different lattice locations. Some Ar-40(xs) is uniformly distributed throughout the lattice, resulting ill a positive isochron intercept, and Other Ar-40(xs) Occurs ill association with K-bearing minerals and increases the isochron slope. These samples demonstrate Situations where linear Ar isochrons give false ages that are too old. After Subtracting Ar-40-* that would accumulate by K-40 decay since meteorite formation and small amounts of terrestrial Ar-40, all Young age samples give similar Ar-40(xs) concentrations of similar to 1-2 x 10(-6) cm(3)/g, but a variation in K content by a factor of similar to 80. Previously reported NASA Johnson Space Center data for Zagami, Shergotty, Yamato (Y-) 000097, Y-793605, and Queen Alexandra Range (QUE) 94201 shergottites show similar concentrations of Ar-40(xs) to the new meteorite data reported here. Similar Ar-40(xs) in different minerals and meteorites cannot be explained as arising from Martian atmosphere carried ill strongly shocked phases Such as melt veins. We invoke the explanation given by Bogard and Park (2008) for Zagami, that this Ar-40(xs) in shergottites, was acquired from the magma. Similarity in Ar-40(xs) among shergottites may reveal common magma Sources and/or similar magma generation and emplacement processes.

• 出版日期2009-6