A database of magnetic susceptibility measurements of stony achondrites (acapulcoitelodranite clan, winonaites, ureilites, angrites, aubrites, brachinites, howardite-eucrite-diogenite (HED) clan, and Martian meteorites, except lunar meteorites) is presented and compared to our previous work on chondrites. This database provides an exhaustive study of the amount of iron-nickel magnetic phases (essentially metal and more rarely pyrrhotite and titanomagnetite) in these meteorites. Except for ureilites, achondrites appear much more heterogeneous than chondrites in metal content, both at the meteorite scale and at the parent body scale. We propose a model to explain the lack of or inefficient metal segregation in a low gravity context. The relationship between grain density and magnetic susceptibility is discussed. Saturation remanence appears quite weak in most metal-bearing achondrites (HED and aubrites) compared to Martian meteorites. Ureilites are a notable exception and can carry a strong remanence, similar to most chondrites.

The Obolon impact structure, 18 km in diameter, is situated at the northeastern slope of the Ukrainian Shield near its margin with the Dnieper-Donets Depression. The crater was formed in crystalline rocks of the Precambrian basement that are overlain by marine Carboniferous and continental Lower Triassic deposits. The post-impact sediments comprise marine Middle Jurassic (Bajocian and Bathonian) and younger Mesozoic and Cenozoic deposits. Today the impact structure is buried beneath an about 300-meter-thick sedimentary rock sequence. Most information on the Obolon structure is derived from two boreholes in the western part of the crater. The lowest part of the section in the deepest borehole is composed by allogenic breccia of crystalline basement rocks overlain by clast-rich impact melt rocks and suevites. Abundant shock metamorphic effects are planar deformation features (PDFs) in quartz and feldspars, kink bands in biotite, etc. Coesite and impact diamonds were found in clast-rich impact melt rocks. Crater-fill deposits are a series of sandstones and breccias with blocks of sedimentary rocks that are covered by a layer of crystalline rock breccia. Crystalline rock breccias, conglomeratic breccias, and sandstones with crystalline rock debris have been found in some boreholes around the Obolon impact structure to a distance of about 50 km from its center. Those deposits are always underlain by Lower Triassic continental red clay and overlain by Middle Jurassic marine clay. The K-Ar age of impact melt glasses is 169 Ma, which corresponds to the Middle Jurassic (Bajocian) age. The composition of crater-fill rocks within the crater and sediments outside the Obolon structure testify to its formation under submarine conditions.

We have measured the size of the high-Ni particles in the cloudy zone and the width of the outer taenite rim in eight low shocked and eight moderately to heavily shocked IVA irons using a transmission electron microscope (TEM). Thin sections for TEM analysis were produced by a focused ion beam instrument. Use of the TEM allowed us to avoid potential artifacts which may be introduced during specimen preparation for SEM analysis of high Ni particles <30 nm in size and to identify microchemical and microstructural changes due to the effects of shock induced reheating. No cloudy zone was observed in five of the eight moderately to highly shocked >13 GPa) IVA irons that were examined in the TEM. Shock induced reheating has allowed for diffusion from 20 nm to 400 nm across kamacite/taenite boundaries, recrystallization of kamacite, and the formation, in Jamestown, of taenite grain boundaries. In the eleven IVA irons with cloudy zone microstructures, the size of the high-Ni particles in the cloudy zone increases directly with increasing bulk Ni content. Our data and the inverse correlation between cooling rate and high-Ni particle size for irons and stony-irons show that IVA cooling rates at 350-200 degrees C are inversely correlated with bulk Ni concentration and vary by a factor of about 15. This cooling rate variation is incompatible with cooling in a metallic core that was insulated with a silicate mantle, but is compatible with cooling in a metallic body of radius 150 +/- 50 km. The widths of the tetrataenite regions next to the cloudy zone correlate directly with high-Ni particle size providing another method to measure low temperature cooling rates.

The primordial asteroid belt contained at least several hundred and possibly as many as 10,000 bodies with diameters of 1000 km or larger. Following the formation of Jupiter, nebular gas drag combined with passage of such bodies through Jovian resonances produced high eccentricities (e = 0.3-0.5), low inclinations (i < 0.5 degrees), and, therefore, high velocities (3-10 km/s) for resonant bodies relative to both nebular gas and non-resonant planetesimals. These high velocities would have produced shock waves in the nebular gas through two mechanisms. First, bow shocks would be produced by supersonic motion of resonant bodies relative to the nebula. Second, high-velocity collisions of resonant bodies with non-resonant bodies would have generated impact vapor plume shocks near the collision sites. Both types of shocks would be sufficient to melt chondrule precursors in the nebula, and both are consistent with isotopic evidence for a time delay of ~1-1.5 Myr between the formation of CAIs and most chondrules. Here initial simulations are first reported of impact shock wave generation in the nebula and of the local nebular volumes that would be processed by these shocks as a function of impactor size and relative velocity. Second, the approximate maximum chondrule mass production is estimated for both bow shocks and impact-generated shocks assuming a simplified planetesimal population and a rate of inward migration into resonances consistent with previous simulations. Based on these initial first-order calculations, impact-generated shocks can explain only a small fraction of the minimum likely mass of chondrules in the primordial asteroid belt (~10^24-10^25 g). However, bow shocks are potentially a more efficient source of chondrule production and can explain up to 10-100 times the estimated minimum chondrule mass.