Time of proto-Earth reservoir formation and volatile element depletion from Mn-Cr chronometry.

The Mn-Cr chronometry of Solar System materials constrains the early chemical evolution of the protoplanetary disk, which is critical for planet formation. Mn/Cr ratios in carbonaceous chondrites and the bulk silicate Earth indicate that meteorite parent bodies and Earth have variable depletions in volatile elements compared to the bulk Solar composition. This depletion is a consequence of the local temperature decreasing as a function of heliocentric distance before planetesimal accretion. Back-tracking the present-day εCr composition of the hypothetical proto-Earth fraction shows that the cessation of Mn-Cr fractionation from the bulk Solar composition occurred no later than ~3 Ma after CAI formation, similar to disk regions of carbonaceous chondrites at greater heliocentric distances. The timing of limited solid-gas interaction due to the dissipation of gas from the protoplanetary disk caused the cessation of Mn-Cr fractionation and provides a lower limit on its lifetime.