Mathematics – Logic
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59..161m&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 59, Issue 1, pp.161-175
Mathematics
Logic
31
Scientific paper
We have conducted petrologic, chemical, and isotopic studies of two impact-produced rocks of enstatite chondrite parentage. Ilafegh 009 is a total impact-melt rock with no residual lithic clasts. Formation on the EL chondrite parent body is suggested by its mineralogy and mineral compositions. Cooling of the impact melt was rapid at melt temperatures and decreased at subsolidus temperatures. In contrast to previous studies, we show that Happy Canyon is not a new enstatite achondrite but an impactmelt breccia of enstatite chondrite (and not aubrite) parentage. This rock formed by impact melting and incorporation into the melt of clastic material (which resulted in relatively rapid cooling at all temperatures). Mineralogical and bulk compositional data (probably biased by the heterogeneous nature of this rock) do not allow unequivocal determination of its parent body (i.e., EL vs. EH), although some data such as bulk total Fe content seem to favor EL parentage. Both rocks were subjected to post-solidification shock, which was more severe for Ilafegh 009 than for Happy Canyon. It appears that both impact melt rocks could have formed by impact melting ~4.57 Ga ago, as is indicated by the nearly identical I---Xe closure ages of 1.6 and 1.4 Ma before Bjurböle for Ilafegh 009 and Happy Canyon, respectively. An apparently younger 39 Ar--- 40 Ar age of 4.53 Ga for Happy Canyon may be due to small biases in the intercalibration of the I---Xe and 39 Ar--- 40 Ar chronometers, whereas the much younger 4.34-4.44 Ga age for Ilafegh 009 reflects thermal resetting during shock metamorphism. Shallowater, which was impact-derived from a different enstatite achondrite parent body, has an I---Xe closure age 0.4 Ma younger than that for Ilafegh 009 and an 39 Ar--- 40 Ar 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.
Bogard Donald D.
Brearley Adrian J.
Casanova Ignacio
Garrison Daniel H.
Hohenberg Charles M.
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