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Solar and cosmogenic argon in dated lunar impact spherules |
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| Authors: | Jonathan Levine Paul R Renne Richard A Muller |
| Institution: | a Chicago Center for Cosmochemistry and Department of Geophysical Sciences, University of Chicago, Chicago, IL 60637, USA b Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA c Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA d Department of Physics, University of California, Berkeley, CA 94720, USA e Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA |
| Abstract: | We have studied lunar impact spherules from the Apollo 12 and Apollo 14 landing sites, examining the isotopic composition of argon released by stepwise heating. Elsewhere, we reported the formation ages of these spherules, determined by the 40Ar/39Ar isochron method. Here, we discuss solar and cosmogenic argon from the same spherules, separating these two components by correlating their partial releases with the releases of calcium-derived 37Ar on a “cosmochron” diagram. We use the abundances of cosmogenic argon to derive a cosmic ray exposure age for each spherule, and demonstrate that single scoops of lunar soil contain spherules which have experienced very different histories of exposure and burial. The solar argon is seen to be separated into isotopically lighter and heavier fractions, which presumably were implanted to different depths in the spherules. The abundance of the isotopically heavy solar argon is too great to explain as a minor constituent of the solar particle flux, such as the suprathermal tail of the solar wind. The fact that the spherules have been individually dated allows us to look for possible variations in the solar wind as a function of time, over the history of the Solar System. However, the isotopic composition and fluence of solar argon preserved in the lunar spherules appear to be independent of formation age. We believe that most of the spherules are saturated with solar argon, having reached a condition in which implantation by the solar wind is offset by losses from solar-wind sputtering and diffusion. |
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