Regolith Exposure of Lunar Meteorites Based on Neutron Capture Induced Shifts in Samarium Isotopic Composition

Introduction. Most lunar meteorites have unique cosmic-ray exposure (CRE) histories with long exposure times on the Moon, followed by very short transfer times from the Moon to Earth, typically 10-10 yr. Due to these short transfer times of lunar meteorites in space, the inventories of most cosmogenic radionuclides with half-lives of 10-10 yr contain contributions from exposure in the lunar regolith. The radionuclide results indicate that about half of the lunar meteorites were irradiated at depths of <1000 g/cm (<5 m) during the last 10 Myr on the Moon [1]. While the cosmogenic radionuclides only constrain the irradiation conditions during the last few Myr, the concentrations of stable cosmogenic noble gases in lunar meteorites suggest CRE ages of ~100 Myr to 2 Gyr [2]. However, the details of this exposure on the Moon are not well constrained, because the cosmogenic noble gases do not provide reliable information on the average irradiation depth. Consequently, it is not clear how long the lunar meteorites were exposed near the surface of the Moon and if they were exposed at a constant depth, or if their exposure conditions in the lunar regolith changed with time as a function of lunar impact events. To fill this gap in knowledge we measured the stable isotope composition of Samarium (Sm). Since Sm has an exceptionally large neutroncapture cross section (σth=41,000b), the Sm/Sm ratio increases from their average solar system abundance if Sm is subjected to bombardment by lowenergy neutrons. The degree of this shift is a function of the integrated low-energy neutron dose, which is much higher for 2π exposure on the lunar surface than for 4π exposure as a small object in space. The depth dependence of the lunar neutron flux is well understood based on (previous) measurements of the lunar neutron flux [3] and of neutron-capture produced Ca in the Apollo 15 drill core [4]. In this study, we measured the isotopic composition of Sm in 5 lunar meteorites and discuss their exposure history on the lunar surface. Meteorite samples. We selected ALH 81005, MAC 88105, LAP 02205, QUE 93069 and QUE 94281. The cosmogenic radionuclide concentrations [5-8] show that they were ejected from different depths on the lunar surface, ranging from 75 ± 15 g/cm for QUE93, 165 ± 15 g/cm for ALH81, 295 ± 15 g/cm for QUE94, 380 ± 20 g/cm for MAC88 and 700 ± 30 g/cm for LAP02. Cosmogenic noble gas concentrations in four of these meteorites suggest that they had CRE ages in the lunar regolith ranging from 400 ± 60 Myr for QUE94 to 1000 ± 400 Myr for QUE93 [2]. Chemical separations. We dissolved aliquots of 60-100 mg in HF/HNO3. After complete dissolution of the samples, we separated the REE’s from the major elements using a 5 ml Dowex AG50W-X8 cation column and then separated Sm and Gd from the other REE’s using a 1.4 ml Eichrom LN column [9]. We typically loaded 100-200 ng of the Sm fractions on a Re double filament and 100 ng of a terrestrial Sm standard from Ames high purity metal. TIMS measurements. We measured the isotopic composition of Sm on a Thermo Finnigan Triton multi-collector instrument. We simultaneously measured the seven isotopes of Sm as well as interfering species of Nd on the low-mass side and Gd on the highmass side. We corrected the Sm, Sm and Sm signals for Nd interferences, based on the measured Nd/Sm ratios, resulting in reliable Sm/Sm Sm/Sm ratios consistent with terrestrial Sm. Typical blank levels of our chemical separation procedures are ~20 pg Sm. Results for Sm/Sm and Sm/Sm ratios are shown in Table 1. Errors represent 2σ uncertainties.