³⁹Ar‐⁴⁰Ar ages of eucrites and thermal history of asteroid 4 Vesta

Abstract

available online at http://meteoritics.org 669 Abstract-Eucrite meteorites are igneous rocks that derived from a large asteroid, probably 4 Vesta. Past studies have shown that after most eucrites formed, they underwent metamorphism in temperatures up to ≥800°C. Much later, many were brecciated and heated by large impacts into the parent body surface. The less common basaltic, unbrecciated eucrites also formed near the surface but, presumably, escaped later brecciation, while the cumulate eucrites formed at depths where metamorphism may have persisted for a considerable period. To further understand the complex HED parent body thermal history, we determined new 39 Ar-40 Ar ages for 9 eucrites classified as basaltic but unbrecciated, 6 eucrites classified as cumulate, and several basaltic-brecciated eucrites. Precise Ar-Ar ages of 2 cumulate eucrites (Moama and EET 87520) and 4 unbrecciated eucrites give a tight cluster at 4.48 ± 0.02 Gyr (not including any uncertainties in the flux monitor age). Ar-Ar ages of 6 additional unbrecciated eucrites are consistent with this age within their relatively larger age uncertainties. By contrast, available literature data on Pb-Pb isochron ages of 4 cumulate eucrites and 1 unbrecciated eucrite vary over 4.4-4.515 Gyr, and 147 Sm-143 Nd isochron ages of 4 cumulate and 3 unbrecciated eucrites vary over 4.41-4.55 Gyr. Similar Ar-Ar ages for cumulate and unbrecciated eucrites imply that cumulate eucrites do not have a younger formation age than basaltic eucrites, as was previously proposed. We suggest that these cumulate and unbrecciated eucrites resided at a depth where parent body temperatures were sufficiently high to cause the K-Ar and some other chronometers to remain as open diffusion systems. From the strong clustering of Ar-Ar ages at ~4.48 Gyr, we propose that these meteorites were excavated from depth in a single large impact event ~4.48 Gyr ago, which quickly cooled the samples and started the K-Ar chronometer. A large (~460 km) crater postulated to exist on Vesta may be the source of these eucrites and of many smaller asteroids thought to be spectrally or physically associated with Vesta. Some Pb-Pb and Sm-Nd ages of cumulate and unbrecciated eucrites are consistent with the Ar-Ar age of 4.48 Gyr, and the few older Pb-Pb and Sm-Nd ages may reflect an isotopic closure before the large cratering event. One cumulate eucrite gives an Ar-Ar age of 4.25 Gyr; 3 additional cumulate eucrites give Ar-Ar ages of 3.4-3.7 Gyr; and 2 unbrecciated eucrites give Ar-Ar ages of ~3.55 Gyr. We attribute these younger ages to a later impact heating. Furthermore, the Ar-Ar impact-reset ages of several brecciated eucrites and eucritic clasts in howardites fall within the range of 3.5-4.

Impactor flux and cratering on Ceres and Vesta: implications for the early solar system

“…-We suggest that, although the impact that formed the D = 460 km crater on Vesta cannot be discarded in the present collisional evolution model, it is more likely to have occurred in the early massive phase of main belt while it was being excited and depleted or during the LHB. This conclusion may become consistent with studies from Bogard & Garrison (2003) who propose that such an impact occurred 4.48 Gyr ago. -We believe that if significant discrepancies between our results about the cratering on Ceres and Vesta and data obtained from the Dawn Mission were found, they should be linked to a higher degree of collisional evolution during the early massive phase of the main belt and/or the existence of the LHB.…”

“…However, a ∼30% probability of occurrence does not allow us to discard the production of this crater in the present collisional evolution model. If the D = 460 km crater on Vesta formed during the early massive phase of the main belt, this would be consistent with studies developed by Bogard & Garrison (2003), who propose that Vesta suffered such huge impact 4.48 Gyr ago.…”

On the Puzzle of Space Weathering Alteration of Basaltic Asteroids

“…However, since large fragments are expected to originate in the near-surface spall region, the second hypothesis seems more probable. This may also be confirmed by the variety 39 Ar shock ages of HEDs (Bogard & Garrison 2003). Furthermore, the spectral data of V-types seem to indicate that both mineralogies are present , but the limited sample prevents a firm conclusion.…”

40Ar‐39Ar studies of whole rock nakhlites: Evidence for the timing of formation and aqueous alteration on Mars

“…Another possibility is recoil, which was the preferred interpretation of Bogard and Garrison (2003) for patterns of steadily decreasing age with increasing extraction temperature in eucrites (e.g., their Figs. 1a and 4c).…”