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

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
standby for e achondrite cr isotope diagram

standby for e achondrite cr isotope diagram
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'

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.