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281.6 grams, 123 x 71 x 6 mm.   Iron, IIAB

TKW 270 kg. Fall not observed. Found 1933 in Oklahoma, USA.



Steve writes:

Sometime before 1930 in a gully on the Carter County, Oklahoma farm of J. C. Dodson Sr., a weathered 270 kg mass was found exposed among another 2 kg of oxidized iron and shale. The mass was embedded in the 110 million year old lower Cretaceous Antlers Sandstone, likely near the shore of what was a shallow sea at that time. But it was only in 1952, after several years of Dodson’s begging for it to be studied, that Allen A. Graffham – the Tucker Tower museum curator at the newly created Lake Murray State Park¹ – finally recognized it to be a meteorite.

Graffham wrote a letter to the University of New Mexico’s Dr. Lincoln La Paz describing the meteorite and the presence of its extremely thick rusted crust that to Graffham supported Lake Murray’s extreme age. La Paz had the meteorite excavated and shipped to Albuquerque. After its thick oxide and shale coating was removed, the remaining 60 x 40 x 23 cm unaltered mass – the oldest meteorite found to date² – was subsequently cut in half for further study³. La Paz initially classified Lake Murray as an octahedrite, but noted it could also be an example of a transitionary hexaoctahedrite. Today it is known to be a type IIAB iron, chemically related to such other irons as the more famous Sikhote-Alin.

While debris found around the main mass was initially assumed to be smaller pieces from the same fall, it’s more likely that Lake Murray’s single mass partially disintegrated from its long terrestrial exposure, and millions of years of water, snow, and wind subsequently scattered pieces of the oxidized material around it. Its weight before weathering is thought to have been at least 500 kg and might possibly have exceeded 900 kg, making Lake Murray the largest meteorite from Oklahoma.

Etched sections of Lake Murray display both granular areas and Widmanstӓtten structures whose orientations suggest the original mass was a single austenite crystal at one time; there are also indications of significant reheating before impact – likely from some extraterrestrial shock event. While the meteorite’s kamacite previously had numerous Neumann bands, they have now largely disappeared. Its troilite is shock-melted and composed of fine-grained iron-sulfide eutectics. Schreibersite inclusions take on distinctive shapes that have been described by some as "hieroglyphic".

This 281.6 gram slice exhibits some of the aforementioned characteristics. Photo 1 shows it framed with a smaller piece that I acquired a year or so earlier. Photos 2 and 3 provide better pictures of its front and rear sides. I’ve personally found irons difficult to photograph, so Photo 4 is an animated gif taken with light reflecting at different angles to better show some of the characteristics of its crystalline structure. Photo 5 provides a front and back side close-up of the “hieroglyphic” inclusion characteristic of Lake Murray. Finally, Photo 6 depicts a cut edge to help visualize how this inclusion permeates the meteorite.


FOOTNOTES:

¹ Lake Murray is a man-made earthen dam reservoir on a 10,000 acre park created in April of 1933 when Oklahoma purchased the land for $90,000. Dodson’s farm was part of that purchase.

² Some might argue that the Brunflow, Sweden chondrite is the oldest known meteorite (it was found in Ordovician limestone dated at 450 million years old), but unlike Lake Murray, Brunflow has been completely replaced by barite and calcite with no extant extra-terrestrial material remaining.

³ One half was returned to the Tucker Museum where it has been on display ever since.