Dating and Tracing the Origin of Enstatite Chondrite Chondrules with Cr Isotopes

Chondrules are major components of chondrites and are believed to drive the accretion of planetary embryos. As such, constraining the timing and origin of chondrules is central for understanding the early evolution of the solar system and the formation of planets. Enstatite chondrites (ECs) have isotope compositions for multiple elements that match that of the Earth and, thus, are considered to be good analogs of the precursor material from which the Earth formed. Here, we report the first high-precision mass-independent Cr isotope data of nine chondrules in one of the least-altered EH chondrites, Sahara 97096. Seven primitive chondrules show typical ⁵⁴Cr/⁵²Cr ratios of bulk ECs, whereas two chondrules have ratios similar to carbonaceous chondrites. The presence of two chondrules with a carbonaceous chondrite signature suggests early inward transport of material to the EC accretion region. The Mn/Cr ratios of the EC-like chondrules (except one with high Fe content) correlate with their ⁵³Cr/⁵²Cr isotope ratios, which we interpret as a fossil isochron, with a slope corresponding to a ⁵³Mn/⁵⁵Mn initial ratio of (5.01 0.59) 10^-6 (2σ). When anchored to the D'Orbigny angrite, this ⁵³Mn/⁵⁵Mn ratio returns an absolute age of 4565.7 0.7 Ma for EC chondrule formation (precursor age), 1.6 0.7 Ma after solar system formation. This protracted formation of EC chondrules may suggest that the mass transfer of outer solar system material started prior to the end of planetary embryo accretion, as chondrules could represent the main building blocks of terrestrial planets.