Widespread54Cr Heterogeneity in the Inner Solar System

Abstract

Short-lived radionuclides can be used as high-resolution chronometers for establishing timescales of planetary formation provided that they were homogeneously distributed in the accretion disk. However, isotopic anomalies observed in meteorites bear evidence of incomplete mixing in the early solar system. High-precision thermal ionization mass spectrometry (TIMS) now enables the determination of isotopic anomalies as small as 12 parts per million in the neutron-rich isotope 54 Cr. Here, we report systematic deficits in 54 Cr relative to Earth in differentiated molten planetesimals (the parent bodies of eucrites, diogenites, mesosiderites, pallasites, angrites, and Mars) and even in some chondritic material (ordinary chondrites). In combination with variable 54 Cr excesses in the carbonaceous chondrites, this implies that at least two nebular reservoirs coexisted for differentiated and chondritic bodies. Preservation of the 54 Cr heterogeneity in space and time (several million years) motivates us to speculate that late stellar input(s) could have been significant contributions to inner nebula Cr reservoirs or that the 54 Cr cosmic memory was well preserved by the mineralogy of the carriers. The consequent spatial, dynamical, and temporal implications regarding solar system formation and dating models are explored further.