Marina A. Ivanova

The Absolute Chronology and Thermal Processing of Solids in the Solar Protoplanetary Disk

Dating the First Solids The solar systems first solids: calcium-aluminum–rich inclusions and chondrules are found in meteorites and provide a direct record of the dynamics of the solar protoplanetary disk that led to the formation of the solar system. Previous results indicate that chondrules formed 1 to 2 million years after the inclusions—an age difference that has been used in constructing models of chondrule formation. Based on uranium- and lead-isotope measurements of a collection of these primitive materials, Connelly et al. (p. 651) show that chondrules in fact started to form at the same time as the inclusions, 4.567 billion years ago, and that their formation took about 3 million years. Isotopic dating implies that, contrary to previous results, two types of primitive solar system solids formed coevally. Transient heating events that formed calcium-aluminum–rich inclusions (CAIs) and chondrules are fundamental processes in the evolution of the solar protoplanetary disk, but their chronology is not understood. Using U-corrected Pb-Pb dating, we determined absolute ages of individual CAIs and chondrules from primitive meteorites. CAIs define a brief formation interval corresponding to an age of 4567.30 ± 0.16 million years (My), whereas chondrule ages range from 4567.32 ± 0.42 to 4564.71 ± 0.30 My. These data refute the long-held view of an age gap between CAIs and chondrules and, instead, indicate that chondrule formation started contemporaneously with CAIs and lasted ~3 My. This time scale is similar to disk lifetimes inferred from astronomical observations, suggesting that the formation of CAIs and chondrules reflects a process intrinsically linked to the secular evolution of accretionary disks.

The origin of chromitic chondrules and the volatility of Cr under a range of nebular conditions

Abstract We characterize ten chromitic chondrules, two spinelian chondrules and one spinel-bearing chondrule and summarize data for 120 chromitic inclusions discovered in an extensive survey of ordinary chondrites. Compositional and petrographic evidence suggests that chromitic chondrules and inclusions are closely related. TheCr/(Cr + Al) ratios in the spinel of these objects range from 0.5 to 0.9 and bulk Al 2 O 3 contents are uniformly high ( > 10 wt%, except for one with 8 wt%). No other elements having comparable solar abundances are so strongly enriched, and alkali feldspar and merrillite are more common than in normal chondrules. The Cr/Mg ratios in chromitic chondrules are 180–750 × the ratios in the bulk chondrite. With the possible exception of magnetic clumping of chromite in the presolar cloud, mechanical processes cannot account for this enrichment. Examination of nebular equilibrium processes shows that 50%-condensation temperatures of Cr atpH 2 /pH 2 O of 1500 are several tens of degrees below those of Mg as Mg 2 SiO 4 ; the condensation of Cr is primarily as MgCr 2 O 4 dissolved in MgAl 2 O 4 at nebular pressures of 10 −4 atm or below. AtpH 2 = 10 −3 atm condensation as Cr in Fe-Ni is favored. Making the nebula much more oxidizing reduces the difference in condensation temperatures but Mg remains more refractory. We conclude that nebular equilibrium processes are not responsible for the enhanced Cr/Mg ratios. We propose that both Cr and Al became enriched in residues formed by incomplete evaporation of presolar lumps. We suggest that spinels remained as solid phases when the bulk of the silicates were incorporated into the evaporating melt; vaporization of Al and Cr were inhibited by the slow kinetics of diffusion. Subsequent melting and crystallization of these residues fractionated Cr from Al. The resulting materials constituted major components in the precursors of chromitic chondrules. Our model implies that chromitic chondrules and inclusions preserve the Cr isotopic record of presolar sources.

EVIDENCE FOR MULTIPLE SOURCES OF 10 Be IN THE EARLY SOLAR SYSTEM

Beryllium-10 is a short-lived radionuclide (t1/2 = 1.4 Myr) uniquely synthesized by spallation reactions and inferred to have been present when the solar system’s oldest solids (calcium–aluminum-rich inclusions, CAIs) formed. Yet, the astrophysical site of 10 Be nucleosynthesis is uncertain. We report Li–Be–B isotope measurements of CAIs from CV chondrites, including CAIs that formed with the canonical 26 Al/ 27 Al ratio of ∼5 × 10 −5 (canonical CAIs) and CAIs with Fractionation and Unidentified Nuclear isotope effects (FUN-CAIs) characterized by 26 Al/ 27 Al ratios much lower than the canonical value. Our measurements demonstrate the presence of four distinct fossil 10 Be/ 9 Be isochrons, lower in the FUN-CAIs than in the canonical CAIs, and variable within these classes. Given that FUN-CAI precursors escaped evaporation–recondensation prior to evaporative melting, we suggest that the 10 Be/ 9 Be ratio recorded by FUN-CAIs represents a baseline level present in presolar material inherited from the protosolar molecular cloud, generated via enhanced trapping of galactic cosmic rays. The higher and possibly variable apparent 10 Be/ 9 Be ratios of canonical CAIs reflect additional spallogenesis, either in the gaseous CAI-forming reservoir, or in the inclusions themselves: this indicates at least two nucleosynthetic sources of 10 Be in the early solar system. The most promising locale for 10 Be synthesis is close to the proto-Sun during its early mass-accreting stages, as these are thought to coincide with periods of intense particle irradiation occurring on timescales significantly shorter than the formation interval of canonical CAIs.

History of the meteorite collection of the Russian Academy of Sciences

Abstract The meteorite collection of the Russian Academy of Sciences is the largest and most unique collection of meteorites in Russia, and one of the famous meteorite collections in the world. The collection contains more than 1230 meteorites and approximately 25 000 individual samples. It also has samples of tektites and impactites, rocks from terrestrial impact craters. Practically all types of meteorites are represented in the collection, making it an excellent foundation for scientific investigations in Russia and worldwide. One hundred and ninety of the collection’s meteorites came from territory that was under Russian jurisdiction at the time of accession. The meteorites are mostly represented by main masses and most of them are of historical significance. The Academy of Sciences’ meteorite collection played a significant role in the formation of the science of meteoritics. As well as a scientific resource, the Academy of Sciences’ meteorite collection is a unique social phenomenon.