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

Abstract

Abstract-Electron microprobe studies of several H5 and H6 chondrites reveal that olivine crystals exhibit systematic Fe-Mg zoning near olivine-metal interfaces. Olivine Fa concentrations decrease by up to 2 mol% toward zoned taenite + kamacite particles (formed after relatively small amounts of taenite undercooling) and increase by up to 2 mol% toward zoneless plessite particles (formed after 200 °C of taenite undercooling). The olivine zoning can be understood in terms of localized olivine-orthopyroxene-metal reactions during cooling from the peak metamorphic temperature. The silicate-metal reactions were influenced by solid-state metal phase transformations, and the two types of olivine zoning profiles resulted from variable amounts of taenite undercooling at temperatures <700 °C. The relevant silicate-metal reactions are modeled using chemical thermodynamics.Systematic olivine Fe-Mg zoning adjacent to metal is an expected consequence of retrograde silicate-metal reactions, and the presence of such zoning provides strong evidence that the silicate and metallic minerals evolved in situ during cooling from the peak metamorphic temperature.

The thermal history of equilibrated ordinary chondrites and the relationship between textural maturity and temperature

“…In type 4 chondrites, values of Mg# olivine are identical throughout the grain. In the type 5-6 chondrites, Mg# olivine increases by up to $0.02 as the interface with spinel is approached, which is consistent with results of Reisener et al (2001) for H5-6 chondrites. We did not attempt a careful characterization of the zoning.…”

“…Modeling of cooling rates for olivine-chromite pairs in terrestrial peridotites (Wilson, 1982;Ozawa, 1983Ozawa, , 1984 also suggests that chromite is the faster diffusing phase, and the observation that olivine in some equilibrated ordinary chondrites show zoning [ Fig. 2; also Reisener et al (2001)], while the coexisting spinel doesn't, is consistent with olivine being the slower diffusing phase. Finally, while the f O 2 dependence of Mg-Fe interdiffusion in chromite has not been determined experimentally, tracer diffusion data for Fe, Co, and Mn for spinels on the magnetite-chromite join at 1200°C (Lu et al, 1993;Tö pfer et al, 1995) all show an increase in diffusivity with decreasing f O 2 under reducing conditions, in contrast to olivine, for which the corresponding diffusion coefficients decrease with decreasing oxygen fugacity.…”

Mineral and chemical composition of theJezersko meteorite—A new chondrite fromSlovenia

“…The approximate final equilibration temperature of the Jezersko meteorite, estimated from compositions of coexisting olivine and orthopyroxene plotted in a diagram of olivine‐orthopyroxene composition between 900 and 500 °C constructed by Reisener et al. (), is about 750 °C, which roughly corresponds to the corrected equilibration temperature obtained from olivine‐chromite pairs. The cooling rate of meteorite Jezersko in the temperature range between 450 and 600 °C was estimated from metallographic data based on the correlation between the central Ni content and the half‐width of taenite (Willis and Goldstein ).…”

In situ analysis of platinum group elements in equilibrated ordinary chondrite kamacite and taenite

“…The exact mechanism of Fe and Ni diffusion through the silicates is still debated; however, grain boundary diffusion (Wood ), vapor transport (Wood ; Reisener and Goldstein ), and metal‐silicate equilibration (McSween and Labotka ; Reisener et al. ) have all been proposed. Our observations of intergrain variability present in type 6 metal grains suggests that these processes—if at work—were inefficient at best; diffusion of the PGEs between metal grains via the silicates is expected to have been a very slow process.…”