EL-chondrite, Impact-Melt Rock
(EL6/7 in MetBull 54)
(Primitive Enstatite Achondrite [Pilski et al., 2011])
34° 48.1' N., 101° 34' W.
A single mass of ~16.3 kg was plowed up on the R. L. Grigsby farm about 2 km northwest of Wayside, Texas. According to the paper 'Happy Canyon: A New Type of Enstatite Achondrite' by Olsen et al. published in Meteoritics, vol. 12, #2 (1977):
"The find appears to have a terrestrial age that precedes settlement in the area. Its exterior has lost a great deal of material by weathering. Large cracks penetrate the interior. Some of the cracks contain water-deposited carbonate minerals commonly found in the cracks of meteorites that have resided long in the soil. Based on these observations and the dryness of the region, the terrestrial age is estimated to be between 300 to 500 years before present."
Current chemical, petrologic, and isotopic studies have classified Happy Canyon as a member of the EL-chondrite parent body. It was impact-melted ~4.53 b.y. ago from a metamorphosed rock of petrologic type 6 or less, cooling rapidly from a temperature of 1425°C due to the incorporation of cold clastic material (fine-grained lithology). During this impact-melt event Ar and other volatile elements were degassed. The older IXe chronometer referencing an age of ~4.565 b.y. attests to the fact that this chronometer was not reset during metamorphism (Bogard et al., 2010). After solidification, the entire rock was impact-shocked to stage S2, causing undulatory extinction of enstatite.
Studies of Zn isotopes in enstatite chondrites by Moynier et al. (2010) led them to conclude that Happy Canyon was derived from either an EH-like, an EL3 chondrite, or possibly an aubrite parent body, and rejects an origin from EL6 material or Shallowater. Boesenberg et al. (2014) found that the feldspar compositions in Happy Canyon overlap those of other EL meteorites.
The convergence of the extremely old ages of both Happy Canyon and chondrules has given support to a new theory known as the molten-planetesimal model. This model involves the low-velocity collision of two molten planetesimals resulting in impact splashing and formation of chondrules and larger blobs of melt. Happy Canyon and other impact-melt rocks (e.g., Ilafegh 009) are said to represent the larger splash material from an impact in which small splash droplets produced chondrules and macrochondrules. High-pressure phases that were formed were likely transformed to low-pressure polymorphs through high-temperature annealing (Rubin and Wasson, 2011). Alternatively, Pilski et al. (2011) propose that this type of meteorite exhibiting relict chondrules and triple junctions should more properly be classified as a primitive enstatite achondrite representing the residue from the rapid partial melting on an enstatite chondrite parent body.
The CRE age for Happy Canyon determined by Patzer and Schultz (2001) based on cosmogenic 3He, 2lNe, and 38Ar is 26.8, 37.4, and 15.9 m.y., respectively. The comparatively lower CRE age based on 38Ar is attributed to loss of this noble gas by the dissolution and decomposition of Ca- and Fe-bearing minerals during an extended terrestrial residence. In addition, they have resolved three potential CRE age clusters for E chondrites at about 3.5, 8, and 25 m.y. The specimen of Happy Canyon shown above is a 0.2 g end piece containing rare free metal.