Aubrite (main-group)
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Fell April 9, 1919
36° 50' N., 84° 21' W.

Several stones fell in Whitley County, Kentucky at noon after the appearance of a fireball and sonic booms. The largest fragment recovered was estimated to weigh 31 pounds. Like most aubrites, Cumberland Falls is a polymict breccia composed of chalky-white enstatite fragments, along with accessory metal, iron sulfide, and graphite. Siderophile element patterns in Cumberland Falls metal provide evidence of a fractionation process. This process has been characterized as occurring either through quenching of an impact-produced metallic melt, or through crystallization within a magma chamber (van Acken et al., 2010).

Unlike typical aubrites, Cumberland Falls contains xenolithic inclusions of a unique type of forsteritic chondrule-bearing material having a bulk composition, mineralogy, O-isotopic affinity, chondrule size, and chondrule textural type similar to those of LL chondrites. Howver, the HSE depletion in these inclusions is not observed in LL chondrites (van Acken et al., 2012). The aubrite ALHA78113 (Verkouteren and Lipschutz, 1983), the ungrouped chondrite Acfer 370 (Moggi-Cecchiare et al., 2009), the ungrouped chondrite NWA 7135 (Irving et al., 2015), and the ungrouped chondrite El Médano 301 (Pourkhorsandi et al., 2016) are the only other meteorites that are characterized by this unique forsteritic (Fa0.7 in Cumberland Falls) silicate composition. In addition to these, Peña Blanca Spring reflects a chondritic noble gas signature during stepped heating (Miura et al., 2006), suggesting that microscopic chondritic inclusions are present. The nature of the association of the enstatite with the chondritic inclusions in Cumberland Falls suggests that the chondritic object was disrupted by collision with the enstatite parent body, during which it experienced severe shock, heat-generated reduction, and rapid cooling. This energetic event is attested by miniscule blebs of metallic FeNi and sulfide dispersed in the silicate grains producing silicate darkening, undulose to mosaic extinction with planar fractures in olivine, impact-melt clasts, and a shock stage of S2–S3 (A. Rubin, 2010). The forsterite fragments were incorporated in the regolith of the aubrite host, and annealed under pressure to form the polymict breccia that we see today. Shock-derived jadeitic pyroxene has not been found in any other meteorite.

In a study of the forsterite inclusions in Cumberland Falls, Kuehner et al. (2016) ascertained that they are petrologic type 6, compared to the F chondrites Acfer 370, NWA 7135, and El Médano 301 which are petrologic type 3/4. Results of O-isotope analyses for the Cumberland Falls inclusions, NWA 7135, and El Médano 301 show that the values overlap and establish a unique trend line between the ordinary chondrites and the TFL (see diagram below). See the NWA 7135 page for further information regarding these forsterite inclusions in Cumberland Falls and their association with this F chondrite grouplet.

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Diagram credit: Kuehner et al., 47th LPSC, #2304 (2016)

The pre-atmospheric diameter of Cumberland Falls, as calculated from cosmogenic production rates, was 160–200 cm—quite large by aubrite standards. Just as all aubrites exhibit complex irradiation histories, both on the parent body and in space, Cumberland Falls presents a cosmic-ray exposure age range of 49 (±10) m.y., based on 81Kr–Kr, to 60.9 m.y. This CRE age is consistent with a possible cluster that might include Pesynoe (~40 m.y.), (Bishopville (52 ±3 m.y.), Bustee (52.6 m.y.), Khor Temiki (53.9 m.y.), Y-793592 (55.0 m.y.), and LEW 87007 (58.5 m.y.), while Pena Blanca Spring (75 ±11 m.y.) and LAP 02233 (78 ±12 m.y.) are only slightly higher. However, since pre-irradiation within the regolith of the parent body likely accounts for a component of these exposure ages, doubts are raised concerning the likelihood of their common ejection (Lorenzetti et al., 2003). It has been suggested that a minimum of four, and as many as nine break-up events occurred on the aubrite parent body. In light of their higher than average neutron-capture-produced noble gas components, Cumberland Falls and Bishopville are considered to have resided near the surface of the aubrite parent body.

For additional information on the formation of the aubrite group visit the Mayo Belwa page. The photo above shows a 1.22 g partial slice sectioned through a black chondritic inclusion. The top photo below shows a closeup view of a chondritic inclusion in a large mass curated at the National Museum of Natural History, Smithsonian Institution. The bottom photo is an excellent petrographic thin section micrograph of Cumberland Falls, shown courtesy of Peter Marmet.

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On display at Smithsonian National Museum of Natural History
Photo courtesy of Martin Horejsi

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click on photo for a magnified view
Photo courtesy of Peter Marmet