¹Dennis Marcel Vanderliek,¹Harry Becker,²Alexander Rocholl
Earth and Planetary Science Letters 576, 117216 Link to Article [https://doi.org/10.1016/j.epsl.2021.117216]
¹Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstraße 74-100, 12249 Berlin, Germany
²Deutsches GeoForschungsZentrum GFZn, Telegrafenberg, D-14473 Potsdam, Germany
Copyright Elsevier

Because of their robustness against resetting, in situ U-Pb ages of zircons in lunar impactites have the potential to provide constraints on the lunar bombardment history that may complement the more common K-Ar ages. Most previous work has focused on relatively large zircons that show growth zoning and ages were mostly interpreted as early igneous crystallization ages. Here we combine high-resolution mineralogical imaging and in situ U-Pb dating by ion microprobe to identify, characterize and date <20 μm size zircons in thin sections of lunar impact breccias. Several tens of grains of zircons of this size range were identified in thin sections of impactites from the Apollo 15 and 16 landing sites. Small zircons are more abundant in both noritic and evolved clinopyroxene, SiO2 or K-feldspar bearing lithologies compared to anorthositic bulk compositions. Both granular zircon aggregates and overgrowth on existing zircon or baddeleyite (in breccias 15455 and 67915) are interpreted to reflect high-temperature recrystallization of zircons or its high-temperature-pressure precursor phases, following shock heating events by impact. In contrast, conchoidal or poikilitic zircons <10 μm in Fe-Ni metal bearing noritic clasts or matrix (67915, 67955) crystallized in situ from impact melt. Most U-Pb ages of the 24 analyzed grains are either concordant or reverse discordant with 207Pb-206Pb ages ranging from 4.15 to 4.25 Ga. The small age range, combined with a large textural spectrum and the frequent presence of Fe-Ni metal suggest zircon crystallization from impact melt and recrystallization of pre-existing zirconium-bearing minerals by impact heating. Such ‘impact’ zircons with 4.2 Ga ages have now been reported from most Apollo landing sites, suggesting widespread formation and modification of zircons by basin-forming impacts at this time. The contrast between U-Pb zircon (predominantly 4.2 Ga) and K-Ar feldspar ages (predominantly 3.9 Ga) likely reflects resetting of the latter chronometer by impact heating.

¹Jonathan A.Lewis,^1,2Rhian H.Jones,¹Adrian J.Brearley
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.10.004]
¹Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA
²Department of Earth and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK
Copyright Elsevier

In ordinary chondrites (OCs), feldspar is present both in chondrules as a primary, igneous phase, and as a secondary phase that results from crystallization of chondrule mesostasis glass during thermal metamorphism. To further understand the chemical and physical conditions prevailing during thermal metamorphism in OCs, we conducted a study of feldspar microtextures and compositions within chondrules, focusing on alteration and equilibration features. We included OCs representing the full metamorphic sequence (petrologic types 3-6) and all OC groups (H, L and LL). Our observations show that primary calcic plagioclase alters to sodalite, scapolite, and nepheline in petrologic types 3.2-3.9, and to albite in types 3.6-5. Plagioclase also develops alteration features such as zoning, micropores, and alteration lamellae in types 3-4. Sodic plagioclase is present in minor amounts as a primary phase, but also forms from the crystallization of chondrule mesostasis glass (types 3.2-3.9), and predominantly through albitization reactions in calcic plagioclase (types 3.6-5). K-feldspar occurs in albite in types 3.6-6 as fine-scale exsolution lamellae and as larger patches.

We combine these observations into an overall model of metasomatism during thermal metamorphism in OCs. Hydrous alteration during prograde metamorphism results in most of the alteration and equilibration features we observe in plagioclase. During retrograde metamorphism, high temperature, short duration infiltration of anhydrous, alkali- and halogen-bearing fluids causes incorporation of K into plagioclase that subsequently exsolves. Overall, we show that evidence of metasomatism is present throughout the metamorphic sequence in all the OCs, and thus was a ubiquitous process on all OC parent bodies. Recognition of the effects of fluid activity in OCs of even the lowest petrologic subtypes has important consequences for radioisotope chronometers, such as Al-Mg and I-Xe, that rely on the integrity of phases such as plagioclase and feldspathic mesostasis glass which are highly susceptible to alteration. Furthermore, the presence of aqueous alteration features in OCs implies that the non-carbonaceous chondrite isotopic reservoir must have also had ices and that models of protoplanetary disk evolution must also include the presence of ices in the inner solar system.