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. 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 specimen of Happy Canyon shown above is a 0.2 g end piece containing rare free metal.