Iron, IIAB, coarsest octahedrite
(Compositionally related to group IIG)
Fell February 12, 1947
46° 9' 36" N., 134° 39' 12" E.
At 10:38 A.M. in the Sikhote-Alin Mountains of eastern Siberia, people witnessed the largest single meteorite fall ever recorded. The brilliant bolide approached from the north accompanied by echoed detonations and ended in a huge roar as it exploded in the atmosphere, sending thousands of fragments to the frozen ground of the snow-covered taiga forest.
A subsequent search resulted in the discovery of over 100 impact pits and the recovery of over 30 tons of material within an ellipse having dimensions of 12 × 4 km. It is calculated that an initial mass of between 200 and 500 tons having a velocity of ~14.5 km/s experienced multiple fragmentations at low altitude from 28 to 16 km or less. Consequently, fragments fell at relatively low velocities, below 3.5 km/s, and developed both smooth, regmaglypted surfaces and ragged, twisted surfaces. The shock wave from the explosion was felt hundreds of miles away, while a dark funnel-shaped smoke trail visible from over 300 km away lingered in the sky until that evening. An artist by the name of Medvedev witnessed the dramatic event and captured the moment on canvas (see photo below). Through eyewitness reports, the orbital elements were calculated; the asteroid had a semi-major axis of 2.162 A.U.
Of the estimated 100 tons of material that reached the Earth, most was in the form of ablation dust. The largest single mass recovered weighs 1,745 kg and has an oriented, shield-like shape. Shrapnel-type fragments were found imbedded in nearby trees while regmaglypted individuals, some adorned with their own miniature impact craters, were found around many craters; the largest crater measuring 28 meters across and 6 meters deep.
Sikhote-Alin has a coarsest octahedrite Thomson (Widmanstätten) structure and is a typical member of the chemical class IIAB. At an earlier time, a division of the similar magmatic irons into the IIA and IIB groups was made based on this structure, but because there is no compositional hiatus between these groups, such a division is considered arbitrary and is no longer recognized (Wasson et al., 2007).
Based on the PbAg isochron, one group of IIAB irons with higher Ir contents gives a younger age of 8.9 m.y. after CAIs, while another group with lower Ir contents crystallized later at 10.9 m.y. after CAIs (Theis et al., 2011). The age of Sikhote Alin suggests an even later crystallization or perhaps that it experienced a resetting event. However, from results of age studies conducted by Kruijer et al. (2012, 2013), and utilizing noble gas and HfW chronometry for those IIAB irons that have the lowest CRE ages, it was determined that core formation occurred ~1.01.5 m.y. after CAIs; this predates the accretion of most chondrite parent bodies. They also determined that iron from groups IIIAB and IVA began segregating ~1.52.0 m.y. after CAIs, while segregation in groups IVB and IID occurred still later at ~2.03.0 m.y. after CAIs. In further studies of select IIAB irons, including Sikhote Alin, Carver, Richland, and Sandia Mountains, Mei et al. (2020) applied HfW chronometry employing more precise W isotope data corrected for neutron capture effects. By using this technique they calculated the core formation age for the IIAB parent body to be ~0.6 m.y. after CAIs.
It was argued by Wasson and Choe (2009) that group IIG irons are chemically similar to those of the IIAB iron group, forming extensions to IIAB trends on elementAu diagrams. They suggested that the formation of IIG irons occurred inside isolated P-rich cavities which remained after crystallization of an evolved IIAB magma (see the Tombigbee River page for more complete details). The specimen shown above is a baby thumbprinted individual weighing 17.5 g.