COMPOSITION OF SOLAR-WIND NOBLE-GASES RELEASED BY SURFACE OXIDATION OF A METAL SEPARATE FROM THE WESTON METEORITE

Surface-sited noble gases were released from metal separates from the Weston meteorite by step-wise oxidation at temperatures below 400-degrees-C. Gas release from residual silicates in the separates was successfully suppressed by prior pyrolysis at somewhat higher temperatures than the combustions. Helium, neon and argon extracted from the metal show clear evidence of a solar wind signature. Xenon and nitrogen, on the other hand, were dominated by air, so that no solar component could be identified for these gases. Uncertainties on krypton isotopes were too large to yield useful conclusions. Helium, neon and argon elemental and isotopic ratios appear to depend on depth within the metal grains, in most cases becoming heavier with increasing oxidation of the metal. The cause of this dependence on amount of oxidation is not yet known. Ratios derived for the surface-sited component are therefore somewhat model dependent. Our best estimates are that, at the time of Weston's exposure, the solar wind He-4/Ar-36 ratio was around 3.6 x 10(4), Ne-20/Ar-36 was close to 45, He-4/He-3 was about 2500, Ne-20/Ne-22 about 13.3-13.4, Ne-21/Ne-22 about 0.033-0.035, and Ar-36/Ar-38 about 5.5-5.7. These ratios indicate that Weston did not acquire its solar wind gases from a recent exposure to solar wind, but more probably at a time in the past similar to or even earlier than the exposure time of Apollo 17 breccias such as 79035. The Ar-36/Ar-38 ratio, in conjunction with other recent determinations of this value, suggests that it is no longer appropriate to assume that the solar and terrestrial values are equivalent.