Ignacio Casanova

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.

Composition of metal in aubrites: Constraints on core formation

Abstract Aubrites are differentiated stone meteorites with small amounts of metal. Study of eight aubrites reveals that Fe,Ni grains occur in a variety of textures, from irregular μm-sized particles to rounded nodules of up to 1.5 cm in diameter. The pattern of siderophile element abundances in the silicates is characteristic of igneous metal segregation (the more siderophilic elements are more depleted). Despite large uncertainties in metal/silicate distribution coefficients, the calculated minimum metal content of the aubrite precursor is not unlike that of enstatite chondrites and suggests that metal segregation was an extensive process in the early magmatic evolution of the aubrite parent body. However, the lack of appreciable fractionation in the trace element signature of the metal suggests that the metal now observed in aubrites did not undergo fractional crystallization in a core. We argue instead that the analyzed metal nodules represent a fraction of the iron-nickel that, during partial melting, was not completely segregated from the silicates.