L.V. Forman

Defining the mechanism for compaction of the CV chondrite parent body

The Allende meteorite, a relatively unaltered member of the CV carbonaceous chondrite group, contains primitive crystallographic textures that can inform our understanding of early Solar System planetary compaction. To test between models of porosity reduction on the CV parent body, complex microstructures within ~0.5-mm-diameter chondrules and ~10-μm-long matrix olivine grains were analyzed by electron backscatter diffraction (EBSD) techniques. The large area map presented is one of the most extensive EBSD maps to have been collected in application to extraterrestrial materials. Chondrule margins preferentially exhibit limited intragrain crystallographic misorientation due to localized crystal-plastic deformation. Crystallographic preferred orientations (CPOs) preserved by matrix olivine grains are strongly coupled to grain shape, most pronounced in shortest dimension , yet are locally variable in orientation and strength. Lithostatic pressure within plausible chondritic model asteroids is not sufficient to drive compaction or create the observed microstructures if the aggregate was cold. Significant local variability in the orientation and intensity of compaction is also inconsistent with a global process. Detailed microstructures indicative of crystal-plastic deformation are consistent with brief heating events that were small in magnitude. When combined with a lack of sintered grains and the spatially heterogeneous CPO, ubiquitous hot isostatic pressing is unlikely to be responsible. Furthermore, Allende is the most metamorphosed CV chondrite, so if sintering occurred at all on the CV parent body it would be evident here. We conclude that the crystallographic textures observed reflect impact compaction and indicate shock-wave directionality. We therefore present some of the first significant evidence for shock compaction of the CV parent body.

Nebula sulfidation and evidence for migration of “free-floating” refractory metal nuggets revealed by atom probe microscopy

Disk models have been proposed that imply particles migrate rapidly in a protoplanetary disk. However, the only physical constraints on these processes from meteorites are observations of refractory inclusions in cometary material from the NASA Stardust mission. Atom probe microscopy (APM) of sub-micrometer refractory metal nuggets (RMNs) contained within a Sc-Zr–rich ultrarefractory inclusion (URI) from the ALH 77307 carbonaceous Ornans (CO) 3.0 meteorite revealed the presence of sulfur at 0.06–1.00 atomic percent (at%) abundances within RMNs. The mineralogical assemblage, petrographic texture, and flat chondrite-normalized highly siderophile element ratios indicate S exposure was unlikely to have occurred after the RMNs were incorporated into the URI. APM analyses suggest these RMNs were likely “free floating” when they were exposed to a S-condensing gas. This requires early, rapid migration of RMNs to cooler regions of the disk to incorporate S and then cycling back to the Ca-Al–rich inclusion (CAI)–forming region for incorporation in the URI, or conditions in the CAI-forming region that promote the incorporation of S into RMNs.