ORIGIN AND HISTORY OF IMPACT-MELT ROCKS OF ENSTATITE CHONDRITE PARENTAGE

We have conducted petrologic, chemical, and isotopic studies of two im pact-produced rocks of enstatite chondrite parentage. Ilafegh 009 is a total impact-melt rock with no residual lithic clasts. Formation on t he EL chondrite parent body is suggested by its mineralogy and mineral compositions. Cooling of the impact melt was rapid at melt temperatur es and decreased at subsolidus temperatures. In contrast to previous s tudies, we show that Happy Canyon is not a new enstatite achondrite bu t an impact-melt breccia of enstatite chondrite (and not aubrite) pare ntage. This rock formed by impact melting and incorporation into the m elt of clastic material (which resulted in relatively rapid cooling at all temperatures). Mineralogical and bulk compositional data (probabl y biased by the heterogeneous nature of this rock) do not allow unequi vocal determination of its parent body (i.e., EL vs. EH), although som e data such as bulk total Fe content seem to favor EL parentage. Both rocks were subjected to post-solidification shock, which was more seve re for Ilafegh 009 than for Happy Canyon. It appears that both impact melt rocks could have formed by impact melting approximately 4.57 Ga a go, as is indicated by the nearly identical I-Xe closure ages of 1.6 a nd 1.4 Ma before Bjurbole for Ilafegh 009 and Happy Canyon, respective ly. An apparently younger Ar-39-Ar-40 age of 4.53 Ga for Happy Canyon may be due to small biases in the intercalibration of the I-Xe and Ar- 39-Ar-40 chronometers, whereas the much younger 4.34-4.44 Ga age for I lafegh 009 reflects thermal resetting during shock metamorphism. Shall owater, which was impact-derived from a different enstatite achondrite parent body, has an I-Xe closure age 0.4 Ma younger than that for Ila fegh 009 and an Ar-39-Ar-40 age of 4.53 Ga. The ancient ages of these three rocks attest to the intense, early bombardment in this region of the solar system.