Ordinary chondrite metallography: Part 2. Formation of zoned and unzoned metal particles in relatively unshocked H, L, and LL chondrites

Abstract

Abstract-We studied the metallography of Fe-Ni metal particles in 17 relatively unshocked ordinary chondrites and interpreted their microstructures using the results of P-free, Fe-Ni alloy cooling experiments (described in Reisener and Goldstein 2003). Two types of Fe-Ni metal particles were observed in the chondrites: zoned taenite + kamacite particles and zoneless plessite particles, which lack systematic Ni zoning and consist of tetrataenite in a kamacite matrix. Both types of metal particles formed during metamorphism in a parent body from homogeneous, P-poor taenite grains. The phase transformations during cooling from peak metamorphic temperatures were controlled by the presence or absence of grain boundaries in the taenite particles. Polycrystalline taenite particles transformed to zoned taenite + kamacite particles by kamacite nucleation at taenite/taenite grain boundaries during cooling. Monocrystalline taenite particles transformed to zoneless plessite particles by martensite formation and subsequent martensite decomposition to tetrataenite and kamacite during the same cooling process. The varying proportions of zoned taenite + kamacite particles and zoneless plessite particles in types 4-6 ordinary chondrites can be attributed to the conversion of polycrystalline taenite to monocrystalline taenite during metamorphism. Type 4 chondrites have no zoneless plessite particles because metamorphism was not intense enough to form monocrystalline taenite particles. Type 6 chondrites have larger and more abundant zoneless plessite particles than type 5 chondrites because intense metamorphism in type 6 chondrites generated more monocrystalline taenite particles. The distribution of zoneless plessite particles in ordinary chondrites is entirely consistent with our understanding of Fe-Ni alloy phase transformations during cooling. The distribution cannot be explained by hot accretion-autometamorphism, post-metamorphic brecciation, or shock processing.

Olivine zoning and retrograde olivine-orthopyroxene-metal equilibration in H5 and H6 chondrites

“…The composition of the metastable taenite (γ M ) at the taenite/silicate interface will remain constant at ~10 wt% Ni throughout cooling (Fig. 1b) (Reisener and Goldstein 2003b). The compositions of γ E and γ M at taenite/silicate interfaces will diverge along the γ E and γ M paths throughout cooling (Fig.…”

“…1b, left) Goldstein 2003a, 2003b). The grain boundary kamacite precipitate will grow by consuming one of the adjacent taenite grains (Reisener and Goldstein 2003b).…”

“…These metal particles are named "zoneless plessite particles" because they lack systematic rim-to-core Ni zoning (Fig. 2b) (Sears and Axon 1975;Reisener and Goldstein 2003b).…”

Advances in magneto‐optical imaging applied to rock magnetism and paleomagnetism

“…These discontinuities probably correspond to the distribution of ferromagnetic minerals with higher magnetization at the rims of the grains. In fact, metallographic studies report that most metamorphic (type 4-6) ordinary chondrites contain zoned Fe-Ni grains, which have a central region with low-Ni content (about 30 ∼ 40 wt % Ni, paramagnetic taenite) and an outer region with high Ni content [Reisener and Goldstein, 2003]. Since the Ni-rich region contains tetrataenite (ordered Fe 50 Ni 50 ) with high coercivity [Nagata and Funaki, 1982;Nagata et al, 1986;Gattacceca et al, 2003], the rim MOI pattern may have some relationships with this tetrataenite rim.…”

Spectroscopy and Mineralogy of a New Meteorite Kamargaon (L6) Chondrite

“…Kamacite Co concentrations vary systematically in various chondrite chemical groups; kamacite in H, L, LL group chondrites contain 0.4-0.5, 0.7-1.0 and 1.5-3 wt. % Co, respectively (Rubin, 1990;Reisener and Goldstein, 2003). The kamacite Co content (0.811-0.827 wt.…”