Thermal histories of CB meteorites: Constraints from compositions and microstructures of sulfides

CBs are metal-rich carbonaceous chondrites, and are subdivided into type CBa and CBb, which are primarily distinguished by the size of particles. Although CBs are classified as chondrites, the high abundance of metal (\u3e40%) make them a distinct group of meteorites. The origin of CBs is highly debated, but these meteorites are thought to have formed in an impact-generated vapor plume 4.563 Ga ago [Amelin et al., 2005; Krot et al., 2005], rather than in the solar nebula. This study focuses on textures and compositions of homogeneous and exsolved sulfides in metal grains in CB meteorites to constrain secondary thermal histories. We use SEM, EPMA, FIB, and TEM techniques to examine metal and sulfides, and quantify modal abundances of metal particles and sulfide inclusions using high-resolution BSE images with ImageJ software. CB metal (Fe,Ni) particles contain different types of sulfide inclusions, which we categorize as: homogeneous low-Cr sulfide inclusions composed of MSS1, exsolved sulfide inclusions of MSS1 with high-Cr daubreelite, sulfide inclusions with Fe,Ni metal blebs, and arcuate sulfides. Some particles also contain Fe-Ni-S eutectic textures composed of a mixture of MSS1 and Fe,Ni metal. The four CB meteorites analyzed (CBa Gujba, CBa Weatherford, CBb HH 237, and CBb QUE 94411) all contain these sulfides within metal grains. Sulfide inclusions possibly formed as a result of precipitation of excess S from solid metal at low temperatures. The CB parent body was likely affected by late impacts, causing heterogeneous heating of surface material. This work provides evidence of heterogeneous reheating by impacts. Examination of exsolved sulfides shows that they record maximum reheating temperatures of \u3c\u3c600°C, and Fe-Ni-S eutectic textures show reheating temperatures of \u3e950°C. Observations of fine-grained textures and small size of particles indicates reheating must have been followed by very rapid cooling. Plastic deformation is also observed in some metal grains and lamellae of exsolved sulfide inclusions, indicating deformation post-dates formation of inclusions. Our study shows that CBa and CBb meteorites not only formed in a similar environment, but they also experienced similar secondary processing

Evidence for impact induced pressure gradients on the Allende CV3 parent body: Consequences for fluid and volatile transport

Carbonaceous chondrites, such as those associated with the Vigarano (CV) parent body, exhibit a diverse range of oxidative/reduced alteration mineralogy (McSween, 1977). Although fluids are often cited as the medium by which this occurs (Rubin, 2012), a mechanism to explain how this fluid migrates, and why some meteorite subtypes from the same planetary body are more oxidized than others remains elusive. In our study we examined a slab of the well-known Allende (CV3OxA) meteorite. Using several petrological techniques (e.g., Fry's and Flinn) and Computerized Tomography (CT) we discover it exhibits a strong penetrative planar fabric, resulting from strain partitioning among its major components: Calcium–Aluminum-rich Inclusions (CAIs) (64.5%CT) > matrix (21.5%Fry) > chondrules (17.6%CT). In addition to the planar fabric, we found a strong lineation defined by the alignment of the maximum elongation of flattened particles interpreted to have developed by an impact event. The existence of a lineation could either be non-coaxial deformation, or the result of a mechanically heterogeneous target material. In the later case it could have formed due to discontinuous patches of sub-surface ice and/or fabrics developed through prior impact compaction (MacPherson and Krot, 2014), which would have encouraged preferential flow within the target material immediately following the impact, compacting pore spaces. We suggest that structurally controlled movement of alteration fluids in the asteroid parent body along pressure gradients contributed to the formation of secondary minerals, which may have ultimately lead to the different oxidized subtypes

Particle size distributions in chondritic meteorites: Evidence for pre-planetesimal histories

Magnesium-rich silicate chondrules and calcium-, aluminum-rich refractory inclusions (CAIs) are fundamental components of primitive chondritic meteorites. It has been suggested that concentration of these early-formed particles by nebular sorting processes may lead to accretion of planetesimals, the planetary bodies that represent the building blocks of the terrestrial planets. In this case, the size distributions of the particles may constrain the accretion process. Here we present new particle size distribution data for Northwest Africa 5717, a primitive ordinary chondrite (ungrouped 3.05) and the well-known carbonaceous chondrite Allende (CV3). Instead of the relatively narrow size distributions obtained in previous studies (Ebel et al., 2016, Friedrich et al., 2015, Paque and Cuzzi, 1997, and references therein), we observed broad size distributions for all particle types in both meteorites. Detailed microscopic image analysis of Allende shows differences in the size distributions of chondrule subtypes, but collectively these subpopulations comprise a composite “chondrule” size distribution that is similar to the broad size distribution found for CAIs. Also, we find accretionary ‘dust’ rims on only a subset (∼15–20%) of the chondrules contained in Allende, which indicates that subpopulations of chondrules experienced distinct histories prior to planetary accretion. For the rimmed subset, we find positive correlation between rim thickness and chondrule size. The remarkable similarity between the size distributions of various subgroups of particles, both with and without fine grained rims, implies a common size sorting process. Chondrite classification schemes, astrophysical disk models that predict a narrow chondrule size population and/or a common localized formation event, and conventional particle analysis methods must all be critically reevaluated. We support the idea that distinct “lithologies” in NWA 5717 are nebular aggregates of chondrules. If ≥cm-sized aggregates of chondrules can form it will have implications for planet formation and suggests the sticking stage is where the preferential size physics is operating