Aubrite (main-group)
54Cr = 0.06 [±0.12]; δ53Cr = 0.24 [±0.03] ‰)
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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. As with 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).

High-precision Cr isotope analyses were conducted by Zhu et al. (2021) to investigate both the origin (54Cr systematics) and the timing (53Mn–53Cr chronometry) of the enstatite achondrites. As shown in the two diagrams below, these E achondrite samples represent three distinct groups or parent bodies. Although the high δ53Cr values of aubrites distinguishes them from E chondrites, Zhu et al. (2021) surmise that this is the result of "isotope fractionation of isotopically light Cr-sulphide during core formation, resulting in an isotopically heavy mantle".

  1. Itqiy (ε54Cr = –0.26 [±0.03]; δ53Cr = –0.03 [±0.03] ‰) and possibly MS-MU-019, MS-MU-036, and NWA 2526
  2. Shallowater (ε54Cr = –0.12 [±0.04]; δ53Cr = 0.10 [±0.03] ‰)
  3. Main-group aubrites (ε54Cr = 0.06 [±0.12]; δ53Cr = 0.24 [±0.03] ‰)

ε54Cr vs. Δ17O Diagram for Enstatite Meteorites
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Diagrams credit: Zhu et al., Goldschmidt Conference, #5519 (2021)
'Tracing the origin and differentiation of the enstatite achondrite parent bodies using Cr isotopes'
See also article by Zhu et al. in GCA, vol. 308 (2021)
'Tracing the origin and core formation of the enstatite achondrite parent bodies using Cr isotopes'

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. However, 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 (see photo; Pourkhorsandi et al., 2016, 2017) 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.

Through studies of the chondritic inclusions in Cumberland Falls, the petrologic type was ascertained by Binns (1969) to be mostly type 3 and 4, and by Kuehner et al. (2016) to be type 6. By comparison, the F chondrites Acfer 370, NWA 7135 are petrologic type 3/4, and El Médano 301 is petrologic type 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 oxygen isotope diagrams 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., 78th MetSoc, #5238 (2015)
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Diagram credit: Kuehner et al., 47th LPSC, #2304 (2016)
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Diagram credit: Pourkhorsandi et al., GCA, vol. 218, p. 109 (2017)
'The ungrouped chondrite El Médano 301 and its comparison with other reduced ordinary chondrites'

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, because 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 breakup 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 Smithsonian Institution, the middle photo shows a specimen in the collection of Mendy Ouzillou with a large chondritic inclusion, and the bottom photo is an excellent petrographic thin section micrograph of Cumberland Falls shown courtesy of Peter Marmet.

Display at Smithsonian National Museum of Natural History
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click on image for a magnified view
Photo courtesy of John Divelbiss

Cumberland Falls Chondritic Inclusion Close-up
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click on image for a magnified view
Photo courtesy of Mendy Ouzillou

Cumberland Falls This Section
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click on image for a magnified view
Photo courtesy of Peter Marmet