U-Th-Pb AGES OF HIGH-GRADE METAMORPHISM RECORDED IN POLYMICT EUCRITE LEWIS CLIFF (LEW) 85300
The inner solar system experienced an apparent increase in impact intensity almost a billion years after planet formation as predicted from Ar-Ar data of impact metamorphosed materials and crater density analyses. Evidence of this apparent increase in impact rate (the ‘late heavy bombardment’ or LHB) are well preserved in asteroidal materials such as the howardite-eucrite-diogenite (HED) suite of meteorites thought to originate from asteroid 4-Vesta or compositionally similar asteroidal material referred to as Vestoids. Previous studies of the Ar-Ar and K-Ar isotope systems applied to impact melt glass, rocks, and mineral fractions, have yielded ages that are interpreted to indicate that the LHB on the HED parent body occurred between about 3.3 and 3.8 Ga. The relatively low closure temperature of ~ 300 C for Ar diffusion in most materials opens two interpretations of the age distributions. 1) The Ar-Ar ages reflect a period of increased bombardment. Or, 2) the Ar-Ar ages reflect waning impact intensity, where the impacts earlier than ~4.1 were largely energetic enough to fully reset the Ar-Ar system and may also be recorded in isotope systems with higher closure temperature (e.g., ~ 550 C for U-Pb in phosphate). With waning overall impact intensity, the Ar-Ar system would still be reset during the period between 3.3 and 3.8, but the U-Pb in phosphate ages may not be fully reset. Thus, U-Pb in phosphate ages may record impact metamorphic events that are older than Ar-Ar ages of the same specimen. Polymict eucrite LEW 85300,46 was targeted for this analysis because the specimen is thoroughly metamorphosed, has been petrologically analyzed, and there are Ar-Ar data to compare with the phosphate age data. Three distinct phosphate ages were found in distinct clasts and matrix within LEW 85300,46 (4479 ± 71 Ma, 4218 ± 30 Ma, 4126 ± 18 Ma). These ages correspond to a period of time between 4.5 and 4.1 Ga which predates the hypothesized LHB. The U-Pb in phosphate ages likely record intense impact metamorphism that is not typically recorded by the Ar-Ar system. Therefore, it is possible that the inner solar system did not experience a LHB, but instead experienced progressively waning impact intensity.