The University of Arizona

"New" lunar meteorites: Impact melt and regolith breccias and large-scale heterogeneities of the upper lunar crust

P. H. Warren, F. Ulff-Mller, G. W. Kallemeyn

Abstract


We have analyzed nine highland lunar meteorites (lunaites) using mainly INAA. Several of these rocks are difficult to classify. Dhofar 081 is basically a fragmental breccia, but much of its groundmass features a glassy-fluidized texture that is indicative of localized shock melting. Also, much of the matrix glass is swirly-brown, suggesting a possible regolith derivation. We interpret Dar al Gani (DaG) 400 as an extremely immature regolith breccia consisting mainly of impact-melt breccia clasts; we interpret Dhofar 026 as an unusually complex anorthositic impact-melt breccia with scattered ovoid globules that formed as clasts of mafic, subophitic impact melt. The presence of mafic crystalline globules in a lunar material, even one so clearly impact-heated, suggests that it may have originated as a regolith. Our new data and a synthesis of literature data suggest a contrast in Al2O3- incompatible element systematics between impact melts from the central nearside highlands, where Apollo sampling occurred, and those from the general highland surface of the Moon. Impact melts from the general highland surface tend to have systematically lower incompatible element concentration at any given Al2O3 concentration than those from Apollo 16. In the case of Dhofar 026, both the bulk rock and a comparatively Al-poor composition (14 wt% Al2O3, 7 μg/g Sm) extrapolated for the globules, manifest incompatible element contents well below the Apollo 16 trend. Impact melts from Luna 20 (57E) distribute more along the general highland trend than along the Apollo 16 trend. Siderophile elements also show a distinctive composition for Apollo 16 impact melts: Ni/Ir averaging ~1.8x chondritic. In contrast, lunaite impact-melt breccias have consistently chondritic Ni/ Ir. Impact melts from Luna 20 and other Apollo sites show average Ni/Ir almost as high as those from Apollo 16. The prevalence of this distinctive Ni/Ir ratio at such widely separated nearside sites suggests that debris from one extraordinarily large impact may dominate the megaregolith siderophile component of a nearside region 2300 km or more across. Highland polymict breccia lunaites and other KREEP-poor highland regolith samples manifest a strong anticorrelation between Al2O3 and mg. The magnesian component probably represents the chemical signature of the Mg-suite of pristine nonmare rocks in its most pure form, unaltered by the major KREEP-assimilation that is so common among Apollo Mg-suite samples. The average composition of the ferroan anorthositic component is now well constrained at Al2O3 ~29-30 wt(implying about 17-19 wt% modal mafic silicates), in good agreement with the composition predicted for flotation crust over a ferroan magma ocean (Warren 1990).

Keywords


Impact melt breccias;KREEP;Lunaites;Lunar Siderophile elements

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