Abstract Investigations of trapped melt inclusions in minerals can yield insights into the compositions and conditions of parent magmas. These insights are particularly important for detrital grains like many of the lunar zircons found in samples returned by the Apollo missions. However, unlike their terrestrial counterparts, lunar zircons have potentially been exposed to billions of years of impact bombardment. Samples from terrestrial impact structures and impact shock experiments have revealed that deformation during an impact event produces melt and glass blebs that can mimic igneous melt inclusions in both morphology and composition. We have undertaken a geochemical and textural investigation of zircons from Apollo impact melt breccia 14311 to assess their formation mechanisms. The association of trapped melts with shock microtwins and monomineralic melt compositions suggests some inclusions formed as a result of the high pressures and temperatures of impact shock. All other inclusions in this study are associated with curviplanar features, planar features, crystal plastic deformation, or embayments (large regions in contact with adjacent melts or minerals) suggesting that they are not igneous melt inclusions. While these textures can be produced in tectonic environments, impacts are a likely formation mechanism since impacts are the main driver of tectonics on the Moon. The results of this study demonstrate that a combination of textural and compositional analyses can be employed distinguish between igneous melt inclusions and melt blebs in zircons from impact environments.
Abstract Detailed textural and geochemical analyses of the carbonaceous achondrites Northwest Africa (NWA) 7680 and NWA 6962 support a rapid progression of thermal events, by similar processes, on the same parent body. The achondrites have olivine compositions of Fa 44.8 and Fa 47.4 for NWA 7680 and NWA 6962, respectively. Replicate oxygen isotope analyses of grains and bulk powders from NWA 7680 yielded average Δ 17 O values of −1.04 ± 0.03‰ and −1.00 ± 0.05‰, respectively, which is identical to that reported for NWA 6962. The whole rock ɛ 54 Cr compositions are also equivalent for NWA 7680 and NWA 6962 (1.36 ± 0.05 and 1.30 ± 0.05, respectively). Both meteorites are plagioclase‐rich, and NWA 7680 is also Fe‐metal‐rich, suggesting they both formed via differentiation processes that resulted in the pooling of partial melt products. Major element geochemical trends show that both rocks could be formed through the melting of chondritic material on a CR chondrite‐like parent body. This is consistent with oxygen isotope and chromium isotope compositions. Intrusion of a late‐stage melt is evident in both meteorites and the crystallization products include silica‐rich, alkali‐deficient nepheline. The late‐stage liquid has partially melted and mixed with primary plagioclase in NWA 6962. In contrast, the late‐stage liquid was often restricted to grain boundaries in NWA 7680, leaving some of the primary plagioclase crystals intact. In situ dating of NWA 7680 phosphate minerals (merrillite and fluorapatite) reveals that it has not experienced long duration thermal metamorphism, or impact‐related Pb loss and age resetting since 4578 ± 17 Ma ( 207 Pb/ 206 Pb age ± 2σ, within error of solar system age). Phosphates associated with the late‐stage melt in NWA 6962 yield a 207 Pb/ 206 Pb age of 4556.6 ± 8.0 Ma (2σ) within 2σ of the NWA 7680 age. These early dates indicate that the observed chromium isotope signatures in these meteorites were not introduced by a later high‐temperature event, such as late impact accretion processes. These data are consistent with a rapid separation of inner and outer solar system chemical reservoirs, planetesimal melting, differentiation, and cooling, all within several million years of calcium‐aluminum‐rich inclusion formation.
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