¹A.A.Nemchin,²M.D.Norman,³M.L.Grange,⁴R.A.Zeigler,³M.J.Whitehouse,⁵J.R.Muhling,⁶R.Merle
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.12.013]
¹School of Earth and Planetary Sciences, Curtin University, Australia
²Research School of Earth Sciences, The Australian National University, Canberra ACT 2601, Australia
³Swedish Museum of Natural History, S-104 05 Stockholm, Sweden
⁴Astromaterials Acquisition and Curation Office, NASA Johnson Space Center, Houston Texas USA
⁵School of Earth Sciences, The University of Western Australia, 6009, Perth, Australia
⁶Department of Earth Sciences, Natural Resources and Sustainable Development, Uppsala University, Sweden
Copyright Elsevier

Glass beads formed by ejection of impact-melted lunar rocks and soils are an important component of lunar soils. These glasses range from 10’s of microns to up to a few cm in diameter and contain variable, but usually relatively low (several hundred ppb to a few ppm), quantities of U. Because Pb is a volatile element, it tends to be lost from the melts, so individual impact glasses can be dated by the U-Th-Pb isotopic systems. The presence of two additional Pb components in lunar glasses, likely linked to addition of lunar Pb to the beads during their residence on the lunar surface and from terrestrial laboratory contamination, require corrections to the data before accurate formation ages of the glasses can be determined. Here we report a U-Th-Pb isotopic and geochemical study of impact glasses from the Apollo 14 soil 14163, which documents multiple impacts into chemically diverse targets that can be linked to the main groups of rocks found on the Moon, i.e., mare basalts, highlands plagioclase-rich rocks, and KREEP (from high contents of K, REE and P) enriched rocks. The impact ages show a bimodal distribution with peaks at ∼3500-3700 Ma and <1000 Ma, similar to that obtained previously by 40Ar-39Ar dating of other suites of lunar regolith glasses. Our data suggest two predominant age peaks at ∼100 Ma and ∼500 Ma, with other statistically definable clusters of ages also possible. As Pb is relatively resistant to subsolidus diffusive loss in these glasses, the age clusters probably represent primary formation ages during impact events, although processes such as preferential preservation of young glasses and impact conditions necessary for production of regolith glasses need further quantification.

¹Jonathan A.Lewis,^1,2Rhian H.Jones
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.12.014]
¹Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131
²Department of Earth and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK
Copyright Elsevier

Thermal metamorphism in undisrupted ordinary chondrite (OC) parent bodies is thought to occur through the radioactive decay of 26Al, producing an onion-shell-like structure with higher peak metamorphic temperatures corresponding to increasing depth. During retrograde metamorphism, the onion-shell model predicts slower cooling rates with increasing petrologic type. However, cooling rates determined by pyroxene diffusion, metallographic, and other methods are inconsistent with onion-shell-like cooling, leading to a model of asteroid disruption and reaccretion into a rubble pile, after peak metamorphism. Potassium-feldspar exsolution in albite, in a perthite texture, has been noted in OCs and can be used as another method for determining cooling rates. We conducted a survey of K-feldspar occurrences and textures, within chondrules, in petrologic type 3.6-6 H, L, and LL OCs. Potassium-feldspar is present as a secondary feature, in primary and secondary albite, as fine-scale exsolution lamellae, 0.1-1.5 μm wide, as well as in larger patches up to 50 μm in size. Exsolution is present in all OC groups and is most common in petrologic type 4.

In the H4 chondrite Avanhandava, we estimate the cooling rate from perthite to be 3-17 °C/yr over a temperature interval of 765-670 °C. Peristerite is also present in Avanhandava for which we estimate a cooling rate of 0.2-2.4×10-3 °C/yr from 570-540 °C. In general, the relatively fast, high-temperature cooling rate determined by perthite is similar to cooling rates recovered from two-pyroxene speedometry. The peristerite cooling rate is closer to the slow, lower temperature metallographic cooling rates. Because K-feldspar exsolution is present in similar fine-scale lamellae in all OC groups, we suggest that all OC parent bodies experienced the same cooling history at high temperatures. These results are inconsistent with predictions of OC asteroid cooling from undisturbed onion-shell metamorphism but are consistent with models involving disruption after peak metamorphism followed by reassembly.