Mn-Cr systematics of the early Solar System revisited

We have reinvestigated the Mn-Cr systematics in a number of primitive meteorites, differentiated planetesimals and terrestrial planets in order to address the chronology of the early stages of protoplanetary disk evolution and planetary formation. Our analytical procedure is based on the assumption of terrestrial abundances for ⁵⁰Cr and ⁵²Cr only; recognizing that a data reduction scheme based on Earth-like ⁵⁴Cr/⁵²Cr abundances in all meteorites is not tenable. Here we show that initial ε_⁵³Cr compositions of ⁵⁴Cr-rich and ⁵⁴Cr-poor acid leach fractions in the primitive carbonaceous chondrite Orgueil differ by 0.9ε, reflecting primordial mineral-scale heterogeneity. However, asteroidal processing effectively homogenized any ε_⁵³Cr variations on the planetesimal scale, providing a uniform present-day solar ε_⁵³Cr=0.20±0.10. Thus, our ⁵³Mn-⁵³Cr data argue against the previously suggested ⁵³Mn heliocentric gradient. Instead, we suggest that inner Solar System objects possessed an initially homogeneous ⁵³Mn/⁵⁵Mn composition, which determined by two independent means is estimated at (6.28 ± 0.66) × 10⁻⁶. Our revised Mn-Cr age for Ste. Marguerite (SM) metamorphism of 4562.9 ± 1.0 Ma is identical to the Pb-Pb age of SM phosphates. Using this age, we confirm that mantle differentiation of the eucrite parent body occurred 4564.9 ± 1.1 Ma ago, and revise the time interval between this event and CAI formation to 2.2 ± 1.1 Ma. We also constrain metamorphism in carbonaceous chondrites of type 2 and 3 to have occurred between 1 and 6 Ma after CAI formation. The ⁵³Mn-⁵³Cr correlation among chondrites, planetesimals and terrestrial planets (the eucrite parent body, Mars and Earth) provides evidence for Mn/Cr fractionation within the protoplanetary disk recorded by all precursor materials of the terrestrial planets and primitive asteroids. This fractionation appears to have occurred within 2 Ma of CAI formation.