A. Trinquier

Widespread 54Cr Heterogeneity in the Inner Solar System

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 54Cr. Here, we report systematic deficits in 54Cr 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 54Cr excesses in the carbonaceous chondrites, this implies that at least two nebular reservoirs coexisted for differentiated and chondritic bodies. Preservation of the 54Cr 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 54Cr 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.

Evidence for a Late Supernova Injection of 60Fe into the Protoplanetary Disk

High-precision 60Fe-60Ni isotope data show that most meteorites originating from differentiated planetesimals that accreted within 1 million years of the solar systems formation have 60Ni/58Ni ratios that are ∼25 parts per million lower than samples from Earth, Mars, and chondrite parent bodies. This difference indicates that the oldest solar system planetesimals formed in the absence of 60Fe. Evidence for live 60Fe in younger objects suggests that 60Fe was injected into the protoplanetary disk ∼1 million years after solar system formation, when 26Al was already homogeneously distributed. Decoupling the first appearance of 26Al and 60Fe constrains the environment where the Suns formation could have taken place, indicating that it occurred in a dense stellar cluster in association with numerous massive stars.

High-precision analysis of chromium isotopes in terrestrial and meteorite samples by thermal ionization mass spectrometry

We present a method for the chemical separation of Cr from meteorite and terrestrial samples for isotopic analysis by thermal ionization mass spectrometry (TIMS). After sample digestion, separation of Cr(III) is achieved by means of a two-column cation-exchange chromatography procedure using AG 50W-x8 resin. In a first column, Cr(III) is isolated from major elements and the majority of trace elements. In a second column, trace amounts of Fe, Al and Ti are further removed. Total procedural yields are > 80%. Cr isotopes are measured by TIMS in the static multicollection mode. Mn/Cr ratios are obtained by multi-collector inductively coupled plasma source mass spectrometry (MC-ICPMS). The accuracy of our protocol was tested by reference to terrestrial analogs and comparison of Cr isotopic data for samples that underwent Cr purification following the cation-exchange chromatography described here and an alternative separation method employing both a cationic and an anionic chromatography step. Using our technique, Mn/Cr ratios reproduce to <2% (2σ) and 53Cr/52Cr and 54Cr/52Cr to 6 ppm and 12 ppm, respectively (2σ). This highly precise procedure allows the variability of Cr isotopes in the inner solar system objects to be addressed. Our method enabled us to document an initial homogeneity for 50,52,53Cr isotopes within 10 ppm, while 20–70 ppm deficits in 54Cr abundances have been resolved for a number of meteorite samples.