Morphological analysis of olivine grains annealed in an iron‐nickel matrix: Experimental constraints on the origin of pallasites and on the thermal history of their parent bodies

Abstract

Morphological analysis of olivine grains annealed in an iron-nickel matrix:Experimental constraints on the origin of pallasites and on the thermal history of their parent bodies Abstract-Two types of pallasites can be distinguished on the basis of the grain shape of olivine (rounded or angular). It has been suggested that these two types of textures resulted from different degrees of annealing at high temperature in the parent body. In order to characterize the kinetics of rounding of olivine grains in an Fe-Ni matrix, we carried out a series of annealing experiments using a mixture of olivine and Fe-Ni powder. We were able to reproduce, at a miniature scale, the range of textures in pallasites. The rate of rounding was rapid enough to be observed and measured at the scale of a few micrometers to 20 mm, even though the experiments were performed below the solidus of the Fe-Ni metal. For instance, grains »14 mm in diameter became nearly spherical within 7 days at 1400ºC. For the morphological analysis of olivine grains, we used two independent techniques: the "critical diameter method" and the "Gaussian diffusion-resample method," a new technique specially developed for our study. Both techniques indicate that the rounding time scale is proportional to the cube of the grain size and that morphological adjustments in our experiments occurred by volume diffusion in the olivine lattice, not by surface diffusion along the olivine-metal boundaries. We used our experimental data to estimate the time scales required for the development of olivine-metal textures in natural pallasites. We determined that small scale rounding of olivine grains in a solid metal matrix can be produced within relatively short time intervals: »100 years to produce rounded olivine grains 0

Diffusion kinetics of Cr in olivine and 53Mn–53Cr thermochronology of early solar system objects

“…The rounding of the olivine grains are thought to reflect prolonged annealing time. However, from experimental studies of the kinetics of rounding olivine grains in a Fe-Ni matrix, Saiki et al (2003) concluded that the large rounded grains in the Brenham-type pallasites were unlikely to have been produced by prolonged annealing of angular olivine grains in a Fe-Ni matrix. Instead, these olivine grains seem to be igneous cumulates.…”

“…Pallasites contain both rounded and angular olivine grains, and have been classified into different textural types according to the presence of the rounded vs angular grains, and the microscopically rounded corners of the angular grains (Scott, 1977;Ohtani, 1983). The presence of angular olivine grains has been interpreted to be the result of fragmentation and excavation of the silicate mantle of the parent body by a giant impact, followed by mixing with the metal core, and insufficient annealing time (Scott, 1977;Saiki et al, 2003). The rounding of the olivine grains are thought to reflect prolonged annealing time.…”

The Influence of Interfacial Energies on Igneous Microstructures

“…Detailed information is available only for olivine (Cabane et al 2005) and quartz (Manley 1996;Cabane et al 2001;Asada et al 2002;Saiki et al 2003).…”

Fast grain growth of olivine in liquid Fe–S and the formation of pallasites with rounded olivine grains

“…This hypothesis could be tested, if annealing of small fragments in the shallow mantle is shown to result in rounded grains with a grain size comparable to that observed in pallasite meteorites before temperatures decrease such that the annealing kinetics becomes insignificant. Saiki et al (2003) performed annealing experiments on a mixture of olivine and solid Fe-Ni, demonstrating that rounding of angular olivine fragments in hot solid metal is efficient for the smallest fragments, but inefficient over meaningful time spans for large fragments >1 mm. Saiki et al (2003) also pointed out that the absence of sulphur in their experiments might have slowed down the rate of textural annealing.…”

“…Saiki et al (2003) performed annealing experiments on a mixture of olivine and solid Fe-Ni, demonstrating that rounding of angular olivine fragments in hot solid metal is efficient for the smallest fragments, but inefficient over meaningful time spans for large fragments >1 mm. Saiki et al (2003) also pointed out that the absence of sulphur in their experiments might have slowed down the rate of textural annealing.…”