Earth and Planetary Sciences ETDs

Publication Date

7-1-2014

Abstract

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 (>40%) 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 <<600°C, and Fe-Ni-S eutectic textures show reheating temperatures of >950°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.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Brearley, Adrian

Second Committee Member

Shearer, Charles

Third Committee Member

McCubbin, Francis

Language

English

Keywords

meteorites, chondrites, sulfides, metal, geochemistry, impact reheating

Document Type

Thesis

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