Iron -nickel alloy phase transformations and metal -silicate reactions in low shock, highly equilibrated ordinary chondrites
The iron-nickel alloy phase transformations and metal-silicate reactions that occurred in relatively unshocked, types 4–6 ordinary chondrites were investigated using several approaches.
Laboratory cooling experiments were used to study kamacite nucleation and growth during cooling. When polycrystalline taenite cools to temperatures of kamacite stability, kamacite allotriomorphs form at the taenite grain boundaries. If the taenite is phosphorus-saturated, intragranular (Widmanstatten) kamacite needles will form after small amounts of undercooling. However, phosphorus-free alloys will undercool more than 200°C without forming intragranular kamacite precipitates. A monocrystalline, phosphorus-free taenite particle will remain homogeneous as it cools through the taenite + kamacite field and will transform to martensite without a composition change.
Metallographic techniques were used to study the microstructures and phase compositions of metal particles in relatively unshocked ordinary chondrites. Chondritic metal particles had widely varying bulk compositions after chondrite aggregation, but experienced intergrain homogenization and taenite grain growth during prograde and peak temperature metamorphism. Taenite grain growth was extensive in types 5 and 6 ordinary chondrites, resulting in many monocrystalline taenite particles. The phase transformations that occurred during cooling depended on whether taenite (phosphorus-poor) was polycrystalline or monocrystalline. Polycrystalline taenite particles experienced small amounts of undercooling within the taenite + kamacite field and transformed to “zoned taenite + kamacite particles” by the diffusion-controlled reaction, taenite → taenite + kamacite. Monocrystalline taenite particles experienced more than 200°C of undercooling; they remained homogeneous and metastable (iron-supersaturated) during cooling. The monocrystalline taenite particles cooled below the martensite-start temperature, and transformed to “zoneless plessite particles” by the taenite → martensite → tetrataenite + kamacite reaction. The occurrence of zoned taenite + kamacite particles and zoneless plessite particles within the same ordinary chondrite is compatible with cold accretion followed by prograde and retrograde metamorphism.
The importance of metal-silicate reactions in ordinary chondrites was investigated by analyzing olivine crystals near olivine-metal interfaces. Olivine fayalite concentrations decrease by approximately 2 mole % near zoned taenite + kamacite particles, and increase by approximately 2 mole % near zoneless plessite particles. Chemical thermodynamic modeling shows that silicate-metal reactions occurred during slow cooling due to variable amounts of taenite undercooling.