I-Xe and 40Ar- 39Ar analyses of silicate from the Eagle Station pallasite and the anomalous iron meteorite Enon

Abstract

Silicate from two unusual iron-rich meteorites were analyzed by the I-Xe and 40Ar- 39Ar techniques, Enon, an anomalous iron meteorite with chondritic silicate, shows no loss of radiogenic 40Ar at low temperature, and gives a plateau age of 4.59 ± 0.03 Ga. Although the Xe data fail to define an I-Xe correlation (possibly due to a very low iodine content), the inferred Pu U ratio is more than 2σ above the chondritic value, and the Pu abundance derived from the concentration of Pu-fission Xe is 6 times greater than the abundance inferred for Cl meteorites. These findings for Enon, coupled with data for IAB iron meteorites, suggest that presence of chondritic silicate in an iron-rich meteorite is diagnostic of an old radiometric age with little subsequent thermal disturbance. The Eagle Station pallasite, the most 16O-rich meteorite known, gives a complex 40Ar- 39Ar age pattern which suggests a recent (≲0.85 Ga) severe thermal disturbance. The absence of excess 129Xe, and the low trapped Ar and Xe contents, are consistent with this interpretation. The similarity between 40Ar- 39Ar data for Eagle Station and for the olivine-rich meteorite Chassigny lends credence to the previous suggestion of a connection between Chassigny and pallasites, in the sense that similar processes operating at similar times on different parent bodies may have been involved in the formation of olivine in both types of meteorites.