Isotope systematics of the eucrite Jonzac: A look into the history of the eucrite parent body using the Lu-Hf, Sm-Nd, Pb-Pb, & U-Pb isotopic systems
Dietderich, Jesse 1980-
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The main objective of this research is to understand the history of the eucrite parent body through the isotopic and petrologic study of the monomict eucrite Jonzac. Jonzac is a type 7 eucrite defined by less prominent Ca-zoning than that of type 4 eucrites and that it contains partly inverted pigeonite, which is a type 6 feature. The metamorphic textures observed include aggregates of polygonal pyroxene, jagged-shaped plagioclase, and recrystallization of the clastic matrix. Four isotopic systems (Lu-Hf, Sm-Nd, Pb-Pb, and U-Pb) were used to determine ages reflecting igneous formation and subsequent thermal and impact metamorphism of this meteorite. The igneous crystallization age is 4.546 ± 0.050 Ga (2σ) defined by U-Pb in zircon which we interpret as being the least prone to thermal disturbances. The other ages are either much younger or show signs of disturbances. A 4-point Lu-Hf isochron of age 4.236 ± 0.026 Ga (2σ; MSWD = 2.4) and a step-leaching 3-point Pb-Pb inverse isochron age of 4.352 ± 0.018 Ga (2σ; MSWD = 436) are much younger than the generally accepted formation interval for non-cumulate eucrites. A 4-point Sm-Nd isochron produced an ‘age’ of 4.733 ± 0.087 Ga (2σ; MSWD = 0.38). The discordant set of isochron ages indicate decoupling between isotope systems and support the hypothesis that the Lu-Hf, step-leaching Pb-Pb, and Sm-Nd isotope systems are disturbed. The ages likely reflect disturbances related to recrystallization during a period(s) of thermal metamorphism likely related to impact processes. The Lu-Hf and step leaching Pb-Pb ages place constraints on the timing of impact processes on the HED parent body and these ages are substantially older than the generally accepted period of late-heavy-bombardment (4.1 – 3.8 Ga) inferred from lunar studies.