Petrogenesis of lunar meteorite Northwest Africa 2977: Constraints from in situ microprobe results |
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| Authors: | Ai‐Cheng ZHANG Wei‐Biao HSU Christine FLOSS Xian‐Hua LI Qiu‐Li LI Yang LIU Lawrence A TAYLOR |
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| Institution: | 1. State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China;2. Laboratory for Astrochemistry and Planetary Sciences, Purple Mountain Observatory, Nanjing 210008, China;3. Department of Earth and Planetary Sciences, Planetary Geosciences Institute, University of Tennessee, Knoxville, Tennessee 37996, USA;4. Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China;5. Laboratory for Space Sciences and Physics Department, Washington University, Saint Louis, Missouri 63130, USA;6. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China |
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| Abstract: | Abstract– Northwest Africa (NWA) 2977 is an olivine‐gabbro lunar meteorite that has a distinctly different petrographic texture from other lunar basalts. We studied this rock with a series of in situ analytical methods. NWA 2977 consists mainly of olivine and pyroxene with minor plagioclase. It shows evidence of intense shock metamorphism, locally as high as shock‐stage S6. Olivine adjacent to a melt vein has been partially transformed into ringwoodite and Al,Ti‐rich chromite grains have partially transformed into their high‐pressure polymorph (possibly CaTi2O4‐structure). Olivine in NWA 2977 contains two types of lithic inclusions. One type is present as Si,Al‐rich melt inclusions that are composed of glass and, in most cases, dendritic pyroxene. The other type is mafic and composed of relatively coarse‐grained augite with accessory chromite, RE‐merrillite, and baddeleyite. Two Si,Al‐rich melt inclusions are heavy rare earth elements (REE) enriched, whereas the mafic inclusion has high REE concentrations and a KREEP‐like pattern. The mafic inclusion could be a relict fragment captured during the ascent of the parent magma of NWA 2977, whereas the Si,Al‐rich inclusions may represent the original NWA 2977 melt. The calculated whole‐rock composition has a KREEP‐like REE pattern, suggesting that NWA 2977 has an affinity to KREEP rocks. Baddeleyite has recorded a young crystallization age of 3123 ± 7 Ma (2σ), which is consistent with results from previous whole‐rock and mineral Sm‐Nd and Rb‐Sr studies. The petrography, mineralogy, trace element geochemistry, and young crystallization age of NWA 2977 support the possibility of pairing between NWA 2977 and the olivine‐gabbro portion of NWA 773. |
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