Manganese and chromium isotopes: Nebular processes and early solar system chronology

Stable isotopes and short-lived radionuclides are useful for examining nucleosynthetic origins of solar system material, studying mixing processes in the early solar nebula and dating nebular events and secondary processes. Chromium isotopes offer an excellent tool for these studies as chromium has four stable isotopes (50Cr, 52Cr, 53 Cr and 54Cr) of which one (53Cr) is the daughter of the short-lived radionuclide 53Mn (half-life of 3.7 Ma). Chromium isotopes can therefore be used both to examine mixing efficiencies in the early solar nebula using the stable isotope 54 Cr and to date secondary events (such as aqueous alteration in carbonaceous chondrites) using radiogenic 53Cr.;Isotopic anomalies of several neutron-rich nuclides observed in chondrites bear evidence of incomplete mixing in the early solar system. High-precision thermal ionization mass spectrometry (TIMS) can be used to determine 54Cr isotopic anomalies in the epsilon (parts per 104) range. Chromium isotopic analyses were performed using bulk samples of carbonaceous chondrites, ordinary chondrites and enstatite chondrites. In this study I report systematic deficits in ordinary chondrites (-0.66 to -1.80 epsilon 54Cr), excesses in carbonaceous chondrites (+0.29 to +1.66 epsilon 54Cr) and isotopic compositions in enstatite chondrites that overlap with terrestrial values. These observed differences in epsilon54Cr between different chondrite groups suggest that three or more Cr carriers were present in presolar matter and that the solar nebula that produced planetesimals between ∼1 and ∼3 AU was not isotopically uniform.;Radiogenic 53Cr can be used to date alteration products in carbonaceous chondrites. CM chondrite materials have been modified by several post-accretionary processes. These chondrites contain carbonates and other secondary minerals that are believed to have formed by alteration reactions on the CM parent body. The timescales for the formation of secondary minerals can be quantified using the radioactive decay of 53Mn to 53Cr. Here I report measurements of excess 53Cr in carbonates from the highly aqueously altered CM2.1 chondrites QUE 93005 and ALH 83100. These excesses are correlated with the 53Mn/ 55Mn ratio and result from the in situ decay of 53Mn, a short-lived radioisotope with a half-life of 3.7 Ma. Results from CM chondrites imply that the degree of aqueous alteration is roughly correlated with the age of carbonate formation in CM chondrites of different subtypes and that alteration started contemporaneously or shortly after CAI formation and lasted at least 4 Ma on the CM parent body.