K. Kehm

Origin and history of impact-melt rocks of enstatite chondrite parentage

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 IXe closure ages of 1.6 and 1.4 Ma before Bjurbole for Ilafegh 009 and Happy Canyon, respectively. An apparently younger 39Ar40Ar age of 4.53 Ga for Happy Canyon may be due to small biases in the intercalibration of the IXe and 39Ar40Ar 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 IXe closure age 0.4 Ma younger than that for Ilafegh 009 and an 39Ar40Ar 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.

I-Xe studies of the Acapulco meteorite: Absolute I-Xe ages of individual phosphate grains and the Bjurböle standard

We have measured Kr and Xe by laser volatilization of seventeen individual neutron-irradiated phosphate grains (nine apatite and eight merrillite) separated from the (reclassified) A-chondrite Acapulco. Radiogenic 129Xe from now-extinct 129I is only observed in the apatites, which all formed simultaneously (8.1 ± 1.2 Ma) after Xe closure in the Bjurbole (L4) standard. This relative closure time, when coupled with the Pb-Pb age of Acapulco phosphates (4.557 ± 0.002 Ga) provides an absolute I-Xe age for these apatites, for Bjurbole and for all samples previously referenced to the Bjurbole standard, subject to the assumptions implicit in I-Xe dating. We have also measured Kr and Xe by stepwise heating of irradiated and unirradiated whole-rock samples of Acapulco and of unirradiated phosphate concentrates. Iodine-derived xenon in the irradiated whole-rock sample does not yield an I-Xe isochron, consistent with the presence of multiple iodine host phases previously observed. Xenon from the spontaneous fission of now-extinct 244Pu is observed in the single phosphate grains and the inferred initial (244Pu/238U)0 ratios reflect fractionation of the actinides favoring plutonium in the merrillites and uranium in the apatites.