Timing and Origin of the Angrite Parent Body Inferred from Cr Isotopes

Angrite meteorites are some of the oldest materials in the solar system. They provide important information on the earliest evolution of the solar system and accretion timescales of protoplanets. Here, we show that the ⁵⁴Cr/⁵²Cr ratio is homogeneously distributed among angrite meteorites within 13 parts per million, indicating that precursor materials must have experienced a global-scale melting such as a magma ocean. The ⁵³Cr/⁵²Cr and Mn/Cr ratios are correlated, which is evidence for an initial ⁵³Mn/⁵⁵Mn ratio of (3.16 - 0.11) - 10^-6. When anchored to the U-corrected Pb-Pb age for the D'Orbigny angrite, this initial ⁵³Mn/⁵⁵Mn corresponds to an absolute age of 4563.2 ±0.3 Ma, i.e., 4.1 ±0.3 Ma after Ca-Al-rich inclusion-formation. This age is distinct from that of the volatile depletion events dated by the ⁸⁷Sr/⁸⁶Sr initial ratio and therefore must correspond to the age of crystallization of the magma ocean and crust formation of the angrite parent body (APB), which can also constrain a slightly bigger size of APB than that of Vesta. Furthermore, this age is similar to those obtained from internal isochrons of the oldest volcanic angrites that cooled rapidly at the surface of the parent body (with ages of 4564 ∼ 4563 Ma), while older than those obtained from plutonic angrites (4561 ∼ 4556 Ma) that cooled down slowly, located deeper within the parent body. This implies that cooling of the APB took at least ∼8 Myr after its differentiation.