A prolonged bombardment history of asteroid 4 Vesta: insights from U–Pb chronology of eucrite phosphates and zirconsOPEN ACCESS
M. Koike, Y. Sumiya, R. Kanemaru, B.G. Rider-Stokes, R. Sakai, N. Takahata, K. Sasaki, H. Asanuma, Y. Sano
Geochimica et Cosmochimica Acta, In Press, Journal Pre-proof, Available online 6 March 2026
“Asteroid 4 Vesta is an ancient, differentiated rocky body, whose lithologies are represented on Earth as howardite–eucrite–diogenite meteorites (HEDs). Chronological studies of HEDs provide key constraints on the formation and evolution process of Vesta and offer insights into early Solar System history. Here, we present new U–Pb dataset for Ca-phosphate minerals (apatite and merrillite) and zircon in four eucrite-melt breccias, Northwest Africa (NWA) 13,166, NWA 10,602, NWA 10,603, and NWA 11,576. Zircon grains from the four samples yield a weighted mean 207Pb/206Pb age of 4556 ± 13 Ma (all uncertainties are reported at 2-sigma level), consistent with previously reported ages of old zircons in eucrites (a weighted mean 207Pb/206Pb age of 4552.6 ± 0.9 Ma; n = 25). Total U–Pb discordia are obtained from co-existing phosphate grains in NWA 13,166 (upper intercept at 4169 + 82/-83 Ma and lower intercept of 72 ± 26 Ma) and NWA 10,603 (4234 + 62/-63 Ma and 46 ± 74 Ma). Model 207Pb/206Pb ages of individual phosphate grains range from ∼ 4370 to ∼ 4110 Ma across the four studied samples. When considered with published data, phosphate 207Pb/206Pb ages in basaltic eucrites define two distinct populations: an older population with a weighted mean age of 4520.6 ± 5.0 Ma (n = 7), and a younger population with a weighted mean age of 4206 ± 27 Ma (n = 9). The older phosphates likely record prolonged cooling of Vesta’s crust and/or impact-related reheating at ∼ 4520 Ma. In contrast, the younger phosphates reflect multiple bombardment events at ∼ 4200 Ma. Microstructural observations of phosphate crystals reveal extensive deformation and recrystallization, consistent with impact-resetting of their U–Pb systems. Reported 40Ar/39Ar ages of HEDs span a broader range, clustering at ∼ 4490 Ma for unbrecciated eucrites, varying between ∼ 3900–2500 Ma for brecciated eucrites and howardites, and ≤ 2400 Ma for diogenites. These chronological constraints suggest that Vesta experienced an extended period of bombardment from ∼ 4200 Ma to ∼ 2500 Ma. As suggested by previous studies, the meteoritic record is more consistent with a prolonged and monotonic decline in impact rates beginning in the early stages of Solar System formation, rather than a discrete collisional spike envisioned in the Late Heavy Bombardment hypothesis.”
