Primordial noble gases in a graphite-metal inclusion from the Canyon Diablo IAB iron meteorite and their implications

Abstract

We have carried out noble gas measurements on graphite from a large graphite-metal inclusion in Canyon Diablo. The Ne data of the low-temperature fractions lie on the mixing line between air and the spallogenic component, but those of high temperatures seem to lie on the mixing line between Ne-HL and the spallogenic component. The Ar isotope data indicate the presence of Q in addition to air, spallogenic component and Ar-HL. As the elemental concentration of Ne in Q is low, we could not detect the Ne-Q from the Ne data. On the other hand, we could not observe Xe-HL in our Xe data. As the Xe concentration and the Xe/Ne ratio in Q is much higher than that in the HL component, it is likely that only the contribution of Q is observed in the Xe data. Xenon isotopic data can be explained as a mixture of Q, air, and "El Taco Xe." The Canyon Diablo graphite contains both HL and Q, very much like carbonaceous chondrites, retaining the signatures of various primordial noble gas components. This indicates that the graphite was formed in a primitive nebular environment and was not heated to high, igneous temperatures. Furthermore, a large excess of 129Xe was observed, which indicates that the graphite was formed at a very early stage of the solar system when 129I was still present. The HL/Q ratios in the graphite in Canyon Diablo are lower than those in carbonaceous chondrites, indicating that some thermal metamorphism occurred on the former. We estimated the temperature of the thermal metamorphism to about 500600 C from the difference of thermal retentivities of HL and Q. It is also noted that El Taco Xe is commonly observed in many IAB iron meteorites, but its presence in carbonaceous chondrites has not yet been established.