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Cumberland Falls

Aubrite (main-group)
standby for cumberland falls photo
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).

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. standby for f chondrite plot
Diagram credit: Kuehner et al., 78th MetSoc, #5238 (2015)
standby for o-isotopic diagram
Diagram credit: Kuehner et al., 47th LPSC, #2304 (2016)
standby for o-isotopic diagram
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’
(https://doi.org/10.1016/j.gca.2017.09.013)
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 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 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.

standby for cumberland falls photo
On display at Smithsonian National Museum of Natural History
Photo courtesy of Martin Horejsi

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


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Shallowater

Enstatite achondrite, ungrouped
(impact melt rock, anomalous aubrite)
standby for shallowater photo
Found July, 1936
33° 42′ N., 101° 56′ W.

A single stone weighing 4.65 kg was found in Lubbock County, Texas. Despite its anomalous characteristics, Shallowater was classified as an igneous aubrite. Shallowater is a rare unbrecciated aubrite, and is the only aubrite composed primarily of coarse-grained orthoenstatite (ordered orthopyroxene) crystals (~80 vol%) rather than disordered enstatite common to the typical brecciated aubrites. It contains a second component of xenolithic material (~20 vol%) present in interstices and as inclusions in the orthoenstatite, which represents the melt-entrapped remnants of a solid impactor. This xenolithic material comprises EH-like minerals including weathered opaques (8 vol%), FeNi-metal (3.3 vol%), troilite (2.9 vol%), forsterite (2.9 vol%), plagioclase (2.5 vol%), low-Ca clinoenstatite (1 vol%), and schreibersite (0.4 vol%), along with traces of niningerite (instead of the typical alabandite) and oldhamite (Keil, et al., 1989). The plagioclase in Shallowater comprises two types: the majority is oligoclase (An17.6, Ab79.8, Or2.6), and the minor constituent is Ab-rich anorthoclase (An0.4, Ab88.2, Or11.4).

The metallic component in Shallowater is much larger (up to 9 vol%; Keil et al., 1989) than that of any other aubrite with the exception of Mt. Egerton and the probable aubrite-related Horse Creek, and consists primarily of homogeneous troilite (van Acken et al., 2010). It follows that Shallowater (and Mt. Egerton) contains the highest concentrations of HSE among aubrites (van Acken et al., 2012). Compared to typical aubrites which contain ~1–3 vol% diopside, Shallowater contains none, possibly reflecting low O and high S fugacities during crystallization from a different source magma (Fogel, 1997). The moderately volatile element Zn in Shallowater is isotopically heavier than it is in all other aubrites (Moynier et al., 2010), possibly reflecting evaporation of the lighter isotopes during a severe impact event. Contrariwise, aubrites are both highly depleted in Zn and enriched in the light isotopes of Zn, a circumstance inconsistent with an evaporation scenario. The mineralogy of Shallowater is inconsistent with an origin from either of the enstatite-chondrite parent bodies (EH and EL), or from the aubrite parent body, and it is considered to represent a fourth enstatite parent body. Studies of Shallowater have revealed a unique and complex cooling history, described in three stages by Keil, 1989:

  1. The first stage reflects rapid cooling (supercooling) from a melt temperature of ~1580°C to ~712°C. This is consistent with the collisional breakup of a differentiated body, consisting primarily of an enstatite melt phase, as a result of an impact with a solid enstatite-like object. The disruption and incorporation of cold impactor material into enstatite melt resulted in very rapid quenching. This was a low-velocity collision that permitted the gravitational reassembly of the disrupted body and the incorporation of xenolithic material from the solid body.
  2. Cooling rate data indicate that a long period of very slow cooling was then initiated which lasted for several million years, until temperatures reached ~680°C. This reflects the deep burial (e.g. 40 km deep on a 100 km-diameter body) of the Shallowater rock within the rubble-pile.
  3. A final stage of fast cooling to ~300°C commenced following another impact event. This incident excavated the Shallowater rock to within ~5 m of the asteroid surface, possibly through a second breakup and reassembly event, or to within ~5 m of the surface of an ejected fragment.

The sizes of the differentiated Shallowater and aubrite parent bodies are constrained by those processes which led to their melting. Arguments suggesting that the heat source was the decay of short-lived radionuclides like 26Al have not been reconciled with the apparent low Al and plagioclase contents in Shallowater and the aubrites. In a similar manner, John T. Wasson (2016) presented evidence that the slow heating generated entirely by the decay of 26Al is insufficient to melt asteroids, and that an additional heat source would have been required; e.g., the rapid heating incurred from major impact events. He determined that the canonical 26Al/27Al ratio of 0.000052 is much too low to cause any significant melting, and that a minimum ratio of 0.00001 would be required to produce a 20% melt fraction on a well-insulated body having a significant concentration of 26Al. For example, the initial ratio of 0.0000004–0.0000005 calculated for the angrites Sah 99555 and D’Orbigny based on their 26Al–26Mg isochrons is too low to have generated any significant melting without an additional heat source. Some have suggested that relatively small planetesimals might have been just the required size to allow heating by induction in the plasma environment of the T Tauri Sun.

In studies of the I–Xe system of the Shallowater and EL parent bodies, evidence was obtained that both bodies experienced a similar period of heavy bombardment that was concordant in time, suggesting that they formed in a similar region of the Solar System 4.566 (±0.002) b.y. ago (Brazzle et al., 1999); this precise I–Xe closure isochron of Shallowater enstatite was subsequently adopted as a reference standard for calibation of the I–Xe technique against the Pb–Pb chronometer. Since then, a new absolute age for the closure of Shallowater enstatite was calculated by Gilmour et al. (2006) based on the I–Xe and Pb–Pb systems to be 4.5633 (±0.0004) b.y., and this was further refined by Gilmour et al. (2009) to be 4.5623 (±0.0004) b.y., which is 1 m.y. earlier than previously determined. A succeeding high precision isotopic study conducted by Pravdivtseva et al. (2016) led them to suggest a further refinement of the absolute I–Xe age to 4.5624 (±0.0002) b.y. Additional data points based on I–Xe studies of Ibitira and NWA 7325 have now been merged into the Shallowater calibration, resulting in a new absolute closure age of 4.5627 (±0.0003) b.y., which is 0.3 m.y. older than the previous calculation (Gilmour and Crowther, 2016 and references therein).

Radiometric dating techniques utilizing 39Ar–40Ar data have determined an average degassing age of 4.53 b.y, likely representing an intense impact event. This is similar to Ar–Ar ages determined for some EH and EL meteorites such as Happy Canyon, but still older than for others, which suggests that the complex cooling history of Shallowater occurred very early after its formation. The CRE age of Shallowater is 28 (±4) m.y., which forms a cluster with the anomalous aubrites Mt. Egerton and Happy Canyon, as well as the normal aubrite ALHA78113 (and pairings).

A study of H–W systematics in various aubrites was conducted by Petitat et al. (2008). They found that in contrast to all other aubrites studied, metal in Shallowater has an unradiogenic W isotopic composition more similar to that in ordinary chondrites. They argue that Shallowater therefore could not have experienced the late heating event(s) which caused radiogenic W diffusion into metal.

On an oxygen 3-isotope diagram, the O-isotopic composition of Shallowater is indistinguishable from that of the aubrites and the EL chondrites. Interestingly, the EH chondrites define a slightly steeper slope, an anomaly that may reflect parent body metamorphism (Newton, 2000). Since the REE abundances measured in plagioclase were too low to account for the bulk REE content of Shallowater, a better candidate for the REE carrier was sought. The discovery of trace amounts of oldhamite provided the answer; it was proposed that the weathering products of oldhamite were now the carriers of much of the bulk REE (Heavilon, 1989).

Petrologic, mineralogic, and geochemical investigations relating to Shallowater have ruled out an igneous or impact-melt origin on the aubrite parent body, as well as an impact-melt origin on the EH or EL parent bodies, and it is presumed that it represents a fourth enstatite parent body. The Kr and Xe noble gas isotopic compositions are Q-like (see the Yilmia page for further details about Q-gases), and are considered to be primordial due to the unbrecciated nature of the meteorite (Miura et al., 2006). The photo shown above is a 0.574 g specimen of Shallowater, which was sectioned from the 50.54 g partial slice shown below. standby for shallowater photo
Photo courtesy of the Macovich Collection The two photos shown below provide high resolution close-up views of both a prepared and an unprepared face of a Shallowater section (note the reflective free-metal inclusions conspicuous in the prepared face). shallowater
shallowater
click on image for a magnified view

Photography by R. Elliott—Fernlea Meteorites UK


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Peña Blanca Spring

Aubrite (main-group)
standby for peña blanca spring photo
Fell August 2, 1946
30° 7.5′ N., 103° 7.1′ W. After sonic booms and a distinct flight noise were heard, this brecciated aubrite fell into a small natural pond on the Gage Ranch at 1:20 in the afternoon, located 9.5 miles southeast of Marathon, Texas. It was reported (J. Lonsdale, University of Texas in Austin) that twenty-four people observed some aspect of the fall of the Peña Blanca (or White Bluff) Spring meteorite. Two ranch workers who were driving less than 50 feet from the pond heard loud explosions and their truck was suddenly splashed by water and plant debris. A group of grazing horses was frightened away from the pond. The cook at the ranch who was standing on the back porch at the time of the fall became an eyewitness to the meteors plunge into the pond, breaking branches of a willow tree on its way down. At their respective ranch houses which surround the pond, members of the Forker and Catto families were having lunch when they heard the tremendous explosion and impact. Upon seeing the disturbed muddy water in the spring-fed pond, they speculated that a meteorite had hit.

 

After lowering the water level and searching the pond bottom, fragments having a combined weight of 70.37 kg were recovered. The largest recovered mass of the aubrite, weighing 47.2 kg, was found in the pond about 8 feet from a 2-foot-diameter impact hole. Another large fragment weighing 13 kg was found within 1 foot of the impact hole, while another 444.2 g fragment was found outside the pond. Numerous other pieces having sizes ranging to that of a single grain were recovered. While the recovery was primarily conducted by Oscar Monnig and Harrison Morse, attempts by Oscar Monnig to obtain the largest piece was unsuccessful. This piece was kept by the ranch families until its purchase from the Forker estate by R. Haag in the mid 1980’s.

 

Current spectral studies link the aubrites to a few near-Earth Apollo asteroids, specifically 3103 Eger and 434 Hungaria (Kelley and Gaffey, 2002). These two high-albedo, iron-free asteroids are composed of an enstatite-like silicate, and are of the appropriate size to make them primary candidates for the aubrite source body. Further evidence has been compiled which is consistent with 3103 Eger being the aubrite source body. For example, the time of day in which aubrites have fallen constrains the orbit to one similar to that of Eger. In addition, the long cosmic-ray exposure age of aubrites is consistent with a stable residence on a near-Earth asteroid that has a long-lived orbit, similar to that of Eger. Moreover, the orbital parameters derived for Norton County match those of Eger better than all other orbits. Asteroid 3103 Eger was probably once a member of the Hungaria family of asteroids in the innermost asteroid belt, which was ejected into an Earth-crossing orbit. Notably, the asteroid 2867 Steins has recently been studied by the Rosetta spacecraft, and it was found to have an albedo and spectral properties consistent with those of an aubrite (with an abundance of CaS, or oldhamite) (Abell et al., 2008).

 

In their study of the Hf–W system, Schulz et al. (2010) determined that aubrites fall into two age clusters, probably reflecting impact resetting events, of ~8 and ~20 m.y. after Solar System formation. Peña Blanca Spring has a Mn–Cr age (and similar I–Xe age) of 4.563 (±0.003) b.y., which is consistent with that of Khor Temiki (Petitat et al, 2008). It has a cosmic-ray exposure age of 75 (±11) m.y., which, along with LAP 02233 (78 ±12 m.y.), lies just outside of an apparent cluster of several aubrites including Cumberland Falls (49 ±10 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.), which suggests a common ejection event for these meteorites. However, pre-irradiation within the regolith of the parent body accounts for a component of these exposure ages, and this raises doubts about the likelihood of their common ejection (Lorenzetti et al., 2003).

 

Peña Blanca Spring, along with Bustee, contains the lowest concentration of HSE among aubrites (van Acken et al., 2012). It is depleted in Zn and contains the isotopically lightest Zn known so far in the Solar System (Moynier et al., 2010). It is believed that this isotopically light Zn present in aubrites was the result of contamination through condensation of an isotopically light vapor associated with the Zn previously lost by the EL parent body during extended thermal metamorphism. Moynier et al. (2010) contend that the evidence is consistent with the evolution of aubrites from an EL parent body. For additional information on the formation of the aubrite group visit the Mayo Belwa page. The above specimen of Peña Blanca Spring is an 11.9 g fragment composed of large crystals of enstatite with a cataclastic texture.


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NWA 6350

Aubrite (main-group)
standby for nwa 6350 photo
click on photo for a magnefied view Found 2010
no coordinates recorded A small, relatively fresh stone weighing 50.5 g was found in the Sahara and designated NWA 6350. Analysis was conducted by the University of Washington in Seattle (A. Irving), and NWA 6350 was initially determined to be a likely pairing with the fusion crusted, 39.1 g aubrite, NWA 5217, found in 2007 in Morocco. Thereafter, it was considered likely that it was a member of a large pairing group, additionally comprising the NWA-series numbers 4537, 4799, 4832, 4871, 5217, 5419, 6193, 6675, and probably the largest and least weathered mass, 7214, all of which together weigh 5,047.6 g.

Northwest Africa 6350 is a rare unbrecciated (common only to the aubrites Mt. Egerton and Shallowater), cumulate-textured aubrite, formed through igneous processes and fractional crystallation. Northwest Africa 6350 consists of a fine- to medium-grained aggregate of mostly pure enstatite along with minor amounts of sodic plagioclase, daubreelite, Si-bearing kamacite, Cr-bearing troilite, oldhamite, alabandite, niningerite, caswellsilverite, graphite, and rare zincian brezinaite (Bunch and Wittke, NAU). The enstatite grains exhibit a preferred orientation.

According to the authoritative source, the Meteoritical Bulletin Database, out of a total of nearly 6,000 meteorites recovered thus far from the desert regions of Northwest Africa, only a small percentage are aubrites. Besides NWA 6350 and its large pairing group, the anomalous aubrite NWA 1235 was determined to be a unique reduced achondrite genetically related to the enstatite meteorite clan. Although NWA 2736 was initially classified as an aubrite, in-depth studies conducted by Bunch et al. (2006) examining numerous paired samples (with various NWA-series designations) revealed the presence of chondrules. Therefore, that pairing group has been reclassified as an EL3 chondrite.

In addition to these classified aubrites, three separate NWA stones have been classified as enstatite achondrites: 1) the 42.9 g NWA 2526 found in 2003 contains 10% metal; 2) the 132.8 g NWA 1840 found in 2003 has many features similar to Shallowater and might be related to that unique enstatite parent body, or it could represent a 5th enstatite parent body; 3) the 483 g NWA 4642 was found in 2007.

A large proportion (~40%) of aubrites are witnessed falls, which is thought to reflect the fact that these highly reduced meteorites are particularly susceptible to terrestrial weathering once they arrive on Earth. Although NWA 6350 is a comparatively fresh meteorite that has preserved its accessory minerals, the original FeNi-metal component in the form of kamacite has been converted to secondary weathering products manifest as limonite veinlets and orange staining along enstatite grain boundaries. Since all of the stones constituting the pairing group were recovered throughout the years 2005–2010, they have experienced a range of terrestrial weathering processes. They now exhibit weathering grades of W0/1–W5, yet mineral phases associated with the aubrite group are still prevalent in them all (Irving and Kuehner, UWS). The surprisingly rapid alteration processes that affect all aubrites in Earth’s oxidizing and wet environs is demonstrated visually in the following photos of stones from a common fall. On the left, shown courtesy of Darryl Pitt, is the fresh (W0/1) 2.2 kg NWA 7214 stone that was recovered in 2006, exhibiting a high content of FeNi-metal flakes throughout with virtually no visible oxidation. The two much smaller stones—the 510 g NWA 6675 in the middle and the 50.5 g NWA 6350 stone on the right—had both remained in the terrestrial environment until their recovery in 2010 and have sustained considerable weathering; the 10× larger NWA 6675 stone has experienced significantly less alteration than NWA 6350 probably due to their comparative sizes.

Keil (2010) suggests that the extremely reducing conditions under which aubrites formed is evidence for a location within 1 AU of the Sun, but on a parent body other than any of the known E chondrites or the Shallowater source object. The size of the differentiated aubrite planetesimal(s) is constrained by those processes which caused it to melt. Arguments suggesting that the heat source was the decay of short-lived radionuclides such as 26Al have not been reconciled with the apparent low Al and plagioclase contents in aubrites. In a similar manner, John T. Wasson (2016) presented evidence that the slow heating generated entirely by the decay of 26Al is insufficient to melt asteroids, and that an additional heat source would have been required; e.g., the rapid heating incurred from major impact events. He determined that the canonical 26Al/27Al ratio of 0.000052 is much too low to cause any significant melting, and that a minimum ratio of 0.00001 would be required to produce a 20% melt fraction on a well-insulated body having a significant concentration of 26Al. For example, the initial ratio of 0.0000004–0.0000005 calculated for the angrites Sah 99555 and D’Orbigny based on their 26Al–26Mg isochrons is too low to have generated any significant melting without an additional heat source. It has been suggested that relatively small planetesimals such as the aubrite planetesimal(s) might have been just the required size to allow heating by induction in the plasma environment of the T Tauri Sun.

Current spectral studies link the aubrites to a few near-Earth Apollo asteroids, specifically 3103 Eger and 434 Hungaria (Kelley and Gaffey, 2002). These two high-albedo, iron-free asteroids are composed of an enstatite-like silicate, and are of the appropriate size to make them primary candidates for the aubrite source body. Further evidence has been compiled which is consistent with 3103 Eger being the aubrite source body. For example, the time of day in which aubrites have fallen constrains the orbit to one similar to that of Eger. In addition, the long cosmic-ray exposure age of aubrites is consistent with a stable residence on a near-Earth asteroid that has a long-lived orbit similar to that of Eger. Moreover, the orbital parameters derived for Norton County match those of Eger better than all other orbits. Asteroid 3103 Eger was probably once a member of the Hungaria family of asteroids, located in the innermost asteroid belt at 1.9 AU. It was subsequently ejected into an Earth-crossing orbit. Notably, the asteroid 2867 Steins was recently studied by the Rosetta spacecraft and was found to have an albedo and spectral properties consistent with those of an aubrite (with an abundance of CaS or oldhamite) (Abell et al., 2008); however, the unique texture and mineralogy of NWA 5217/6350 indicate it derives from a significantly larger parent body.

For additional information on the formation of the aubrite group visit the Mayo Belwa page. The specimen of NWA 6350 shown above is a 1.03 g partial slice.


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NWA 6675

Aubrite (main-group)
Home Page
mouseover for alternate view angle

Found 2010
no coordinates recorded A single heavily weathered stone weighing 510 g was found in the Sahara and subsequently sold to G. Tomelleri in Erfoud, Morocco in 2010. Analyses were conducted at the Museo di Scienze Planetarie in Italy (Vanni Moggi Cecchi), and NWA 6675 was classified as an enstatite achondrite, or aubrite. Northwest Africa 6675 is a rare unbrecciated (common only to the aubrites Mt. Egerton and Shallowater), cumulate-textured aubrite, formed through igneous processes and fractional crystallation. It is considered likely that this meteorite is part of a large pairing group, additionally comprising the NWA-series numbers 4537, 4799, 4832, 4871, 5217, 5419, 5885, 6193, 6350, and probably the largest and least weathered mass, 7214, all of which together weigh 5,047.6 g.

A large proportion (~40%) of aubrites are witnessed falls, which is thought to reflect the fact that these highly reduced meteorites are particularly susceptible to terrestrial weathering once they arrive on Earth. The surprisingly rapid alteration processes that affect all aubrites in Earth’s oxidizing and wet environs is demonstrated visually in the following photos of stones from a common fall. On the left, shown courtesy of Darryl Pitt, is the fresh (W0/1) 2.2 kg NWA 7214 stone that was recovered in 2006, exhibiting a high content of FeNi-metal flakes throughout with virtually no visible oxidation. The two much smaller stones—the 510 g NWA 6675 in the middle and the 50.5 g NWA 6350 stone on the right—had both remained in the terrestrial environment until their recovery in 2010 and have sustained considerable weathering; the 10× larger NWA 6675 stone has experienced significantly less alteration than NWA 6350 probably due to their comparative sizes.

Keil (2010) suggests that the extremely reducing conditions under which aubrites formed is evidence for a location within 1 AU of the Sun, but on a parent body other than any of the known E chondrites or the Shallowater source object. The size of the differentiated aubrite planetesimal(s) is constrained by those processes which caused it to melt. Arguments suggesting that the heat source was the decay of short-lived radionuclides such as 26Al have not been reconciled with the apparent low Al and plagioclase contents in aubrites. In a similar manner, John T. Wasson (2016) presented evidence that the slow heating generated entirely by the decay of 26Al is insufficient to melt asteroids, and that an additional heat source would have been required; e.g., the rapid heating incurred from major impact events. He determined that the canonical 26Al/27Al ratio of 0.000052 is much too low to cause any significant melting, and that a minimum ratio of 0.00001 would be required to produce a 20% melt fraction on a well-insulated body having a significant concentration of 26Al. For example, the initial ratio of 0.0000004–0.0000005 calculated for the angrites Sah 99555 and D’Orbigny based on their 26Al–26Mg isochrons is too low to have generated any significant melting without an additional heat source. It has been suggested that relatively small planetesimals such as the aubrite planetesimal(s) might have been just the required size to allow heating by induction in the plasma environment of the T Tauri Sun.

The plot on an oxygen three-isotope diagram calculated for another member of this aubrite pairing group (NWA 4537) lies within the aubrite and E chondrite field near the TFL (Greenwood and Franchi, OU). Current spectral studies link the aubrites to a few near-Earth Apollo asteroids, specifically 3103 Eger and 434 Hungaria (Kelley and Gaffey, 2002). These two high-albedo, iron-free asteroids are composed of an enstatite-like silicate, and are of the appropriate size to make them primary candidates for the aubrite source body. Further evidence has been compiled which is consistent with 3103 Eger being the aubrite source body. For example, the time of day in which aubrites have fallen constrains the orbit to one similar to that of Eger. In addition, the long cosmic-ray exposure age of aubrites is consistent with a stable residence on a near-Earth asteroid that has a long-lived orbit similar to that of Eger. Moreover, the orbital parameters derived for Norton County match those of Eger better than all other orbits. Asteroid 3103 Eger was probably once a member of the Hungaria family of asteroids, located in the innermost asteroid belt at 1.9 AU. It was subsequently ejected into an Earth-crossing orbit. Notably, the asteroid 2867 Steins was recently studied by the Rosetta spacecraft and was found to have an albedo and spectral properties consistent with those of an aubrite (with an abundance of CaS or oldhamite) (Abell et al., 2008); however, the texture and mineralogy of aubrites indicate they derive from a significantly larger parent body.

For additional information on the formation of the aubrite group visit the Mayo Belwa page. Two photos of a 1.95 g partial slice of NWA 6675 are shown above comparing the effects of different angles of incident light.