HISTORY OF THE PAIRED LUNAR METEORITES MAC88104 AND MAC88105 DERIVED FROM NOBLE-GAS ISOTOPES, RADIONUCLIDES, AND SOME CHEMICAL ABUNDANCES

Noble gas isotopes, radionuclides, and chemical abundances were studied in the lunar meteorites MAC88104 and MAC88105 collected in the MacAlpine Hills area of Antarctica. The concentrations of the noble gas isotopes and the radionuclide activities in the two meteorites are essentially identical, proving that the two meteorites are paired. From K-40-Ar-40 dating we obtain a gas retention age of 3550 +/- 400 Ma, typical for lunar surface material. Probably before breccia compaction the MAC88104/5 material resided for 630 +/- 200 Ma at an average shielding depth of 85 g/cm2, that is, about 50 cm below the lunar surface in the lunar regolith, as judged from the concentration of cosmic-ray produced Kr and Xe isotopes. Although this duration of lunar regolith residence is relatively long, MAC88104/5 represent immature regolith material: the concentration of solar wind implanted noble gases are two orders of magnitude lower than those in mature lunar soil. The reason for that might be the relatively high average shielding. The Ar-40/Ar-36 ratio of the trapped component is 5.7 +/- 1.0, indicating an intermediate antiquity of the material; we estimate that the solar wind and lunar atmospheric particles were implanted about 2000 Ma ago. The radionuclide activities allow us to derive the exposure history of the MAC88104/5 material. The duration of the Moon-Earth transfer was less-than-or-equal-to 0.24 Ma. Production of the radionuclides on the lunar surface immediately before ejection is possible, but duration and shielding depth for such an exposure cannot be determined from our data. We obtain a terrestrial age of 0.1 less-than-or-equal-to t(terr) less-than-or-equal-to 0.6 Ma and an ejection time of MAC88104/5 from the Moon of 0.1 less-than-or-equal-to t(ejection) less-than-or-equal-to 0.8 Ma. The exposure histories of the lunar meteorites discussed in this work indicate that at least two impact events are required for their ejection from the Moon. Our first noble gas results for lunar meteorite Yamato-793274 show that it represents mature lunar regolith material with relatively high concentrations of solar wind implanted noble gases and a duration of several hundred million years of exposure to cosmic rays.