Catastrophic fragmentation and reassembly of the parent body of the IVA iron and stony-iron meteorites

Details Catastrophic fragmentation and reassembly of the parent body of the IVA iron and stony-iron meteorites Catastrophic fragmentation and reassembly of the parent body of the IVA iron and stony-iron meteorites

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29s.530s&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 530-531

Astronomy and Astrophysics

Astrophysics

2

Fragmentation, Iron Meteorites, Meteoritic Microstructures, Order-Disorder Transformations, Stony Meteorites, Temperature Effects, Asteroids, Cooling, Kamacite, Lamella (Metallurgy), Transmission Electron Microscopy

Scientific paper

The IVA iron meteorites crystallized in an asteroid core in a manner analogous to the archetypal IIIAB irons. The core-mantle zone of the IVA asteroid was not made of pallasitic material, as in the IIIAB asteroid, but a bizarre coarse-grained mixture of pyroxene-tridymite and metal, represented by the Steinbach and Sao Joao Nepomuceno stony-irons. We have investigated the cooling history of the IVA stony irons and irons and conclude that their parent asteroid was catastrophically fragmented while the core was cooling between 1400 and 1200 C, and that fragments of core and mantle were reassembled. Transmission electron microscopy of the disordered clinobronzite in Steinbach using electron diffraction and high-resolution imaging techniques confirms that this meteorite was rapidly cooled in the temperature range 1200-700. Measurements of the widths of individual lamellae of clino- and orthopyroxene that constitute the disordered intergrowth show that the intergrowth did not form by shock from orthopyroxene; orthopyroxene lamellae greater than 20 unit cells wide are absent; and orthopyroxene lamellae account for approximately 40 vol% of the disordered intergrowth. The IVA stony irons have the characteristic metal microstructures and small kamacite bandwidths of IVA irons. Thus it is probable that the cooling history of IVA irons was perturbed when Steinbach was quenched. Slow cooling of IVA irons and stony-irons must have recommenced before temperatures fell below 600-550 C. These constraints require that the IVA body was catastrophically fragmented, forming meter-sized fragments of stony-iron and core fragments up to 100 m or more across. Reassembly of fragments began less than 6 hr later. This metal-silicate mixing did not form the IVA stony-irons Steinbach and Sao Joao Nepomuceno.

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