40Ar-39Ar age of the Shergotty achondrite and implications for its post-shock thermal history |
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| Authors: | DD Bogard Liaquat Husain LE Nyquist |
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| Institution: | 1. NASA Johnson Space Center, Houston, TX 77058, U.S.A.;2. Div. Laboratories and Research, New York State Department of Health, Albany, NY, 12201, U.S.A. |
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| Abstract: | Analyses of 40Ar-39Ar have been made on a whole rock sample and a maskelynite (feldspar) separate of the shocked Shergotty achondrite. The maskelynite gave a plateau age of 254 ± 10 Myr. The whole rock sample gave a complex release with apparent ages between 240 and 640 Myr. The slightly younger Rb-Sr isochron age of 165 Myr for Shergotty (Nyquistet al., 1978) suggests that the maskelynite as well as the whole rock was incompletely degassed. Reasonable Ar diffusion characteristics for Shergotty for shock heating temperatures of <400°C indicate D/a2 of 10?11?10?13 sec?1. The time required to lose 95% of the 40Ar from the plagioclase would be ~103–104 yr. When this gas diffusion time is introduced into a thermal model of a cooling ejecta blanket of variable thickness, a post-shock cooling time of ? 103 yr and a burial depth of ? 300 m are indicated for Shergotty. These conclusions are not seriously affected by uncertainties in the thermal model. Most likely the shock event occurred ~ 165 Myr ago, but no earlier than 250 Myr ago, when the Shergotty parent object experienced a collision in the asteroid belt. As a result of that collision, feldspar was converted to maskelynite, the K-Ar and Rb-Sr ages were completely or nearly completely reset, and the Shergotty meteorite was heated to <400°C and left to cool slowly inside the parent body. |
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