Osmium isotope and highly siderophile element systematics of the lunar crust |
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| Authors: | James MD Day Richard J Walker Odette B James Igor S Puchtel |
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| Institution: | 1. Department of Geology, University of Maryland, College Park, MD 20742, USA;2. Emeritus, U.S. Geological Survey, 926A National Center, Reston, VA 20192, USA;1. Steinmann-Institut, Universität Bonn, 53115 Bonn, Germany;2. School of Earth Sciences, The University of Queensland, Brisbane QLD 4072, Australia;3. Institut für Planetologie, Westfälische Wilhelms-Universität Münster, 48149 Münster, Germany;1. Dep. Lagrange, UNSA, CNRS, OCA, Nice, France;2. Center for Lunar Origin & Evolution, SwRI, Boulder, CO, USA;3. Center for Lunar Science & Exploration, USRA, Lunar and Planetary Institute, Houston, TX, USA;1. The Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston, TX 77058, USA;2. NASA Lunar Science Institute;3. Curtin University of Technology, Perth, WA 6845, Australia;4. Swedish Museum of Natural History, S-104 05 Stockholm, Sweden;5. University of Houston, Department of Earth and Atmospheric Sciences, TX, USA;6. Jacobs Technology, JETS, Mail Code JE23, 2224 Bay Area Blvd, Houston, TX 77058, USA;7. Astromaterials Research and Exploration Science, NASA Johnson Space Center, Houston, TX 77058, USA;1. Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany;2. Institute for Geochemistry and Petrology, Department of Earth Science, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland;3. Institut für Mineralogie, Westfälische Wilhelms-Universität Münster, Corrensstrasse 24, 48149 Münster, Germany;1. Department of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden;2. Department of Applied Geology, Curtin University, Perth, WA 6845, Australia |
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| Abstract: | Coupled 187Os/188Os and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundance data are reported for pristine lunar crustal rocks 60025, 62255, 65315 (ferroan anorthosites, FAN) and 76535, 78235, 77215 and a norite clast in 15455 (magnesian-suite rocks, MGS). Osmium isotopes permit more refined discrimination than previously possible of samples that have been contaminated by meteoritic additions and the new results show that some rocks, previously identified as pristine, contain meteorite-derived HSE. Low HSE abundances in FAN and MGS rocks are consistent with derivation from a strongly HSE-depleted lunar mantle. At the time of formation, the lunar floatation crust, represented by FAN, had 1.4 ± 0.3 pg g? 1 Os, 1.5 ± 0.6 pg g? 1 Ir, 6.8 ± 2.7 pg g? 1 Ru, 16 ± 15 pg g? 1 Pt, 33 ± 30 pg g? 1 Pd and 0.29 ± 0.10 pg g? 1 Re (～ 0.00002 × CI) and Re/Os ratios that were modestly elevated (187Re/188Os = 0.6 to 1.7) relative to CI chondrites. MGS samples are, on average, characterised by more elevated HSE abundances (～ 0.00007 × CI) compared with FAN. This either reflects contrasting mantle-source HSE characteristics of FAN and MGS rocks, or different mantle–crust HSE fractionation behaviour during production of these lithologies. Previous studies of lunar impact-melt rocks have identified possible elevated Ru and Pd in lunar crustal target rocks. The new results provide no supporting evidence for such enrichments.If maximum estimates for HSE in the lunar mantle are compared with FAN and MGS averages, crust–mantle concentration ratios (D-values) must be ≤ 0.3. Such D-values are broadly similar to those estimated for partitioning between the terrestrial crust and upper mantle, with the notable exception of Re. Given the presumably completely different mode of origin for the primary lunar floatation crust and tertiary terrestrial continental crust, the potential similarities in crust–mantle HSE partitioning for the Earth and Moon are somewhat surprising. Low HSE abundances in the lunar crust, coupled with estimates of HSE concentrations in the lunar mantle implies there may be a ‘missing component’ of late-accreted materials (as much as 95%) to the Moon if the Earth/Moon mass-flux estimates are correct and terrestrial mantle HSE abundances were established by late accretion. |
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