Isotopic composition of xenon and krypton in silicate-graphite inclusions of the El Taco, Campo del Cielo, IAB iron meteorite

Abstract

Noble gases, extracted by temperature-stepped pyrolysis from acid-resistant silicate, schreibersite, and graphite residues of inclusions from the IAB iron meteorite El Taco (Campo del Cielo), have been analyzed for their isotopic composition. The concentrations of spallogenic (light) gases agree with previous data on adjacent specimens. Nominal feldspar and pyroxene fractions have lost up to 80% of their 3He during, or towards the end of, the exposure of El Taco to the cosmic radiation. In olivine the 3 Ne 21 Ne and NePNe ratios are as low as 2.18 ± 0.01 and 1.09 ± 0.03, respectively, presumably because the silicates were irradiated within a FeNi matrix under extremely heavy shielding. Argon, Kr, and Xe in the silicates are dominated by a trapped “planetary” component with 100 ≤ 36 Ar 132 Xe ≤ 700 and 1.7≤ 84 Kr 132 Xe ≤ 2.2 . Xenon released from the silicates at extraction temperatures > 1000°C is isotopically close to ureilite Xe, while the high- T Xe from graphite and schreibersite is unlike any of the presently well-established types of Xe. The isotopic composition of this Xe can be generated as a mixture of mass-fractionated UXe, with light isotopes depleted by (2.2 ± 0.1) % per atomic mass unit, and 244PuXe. The fission component, 10% in the case of 136Xe, is not due to in situ-production; it must have been added prior to incorporation of the Xe into the El Taco parent body and presumably underwent the mass fractionation process together with UXe. From the silicates 127I( n, γ)-produced 128Xe ∗ and radiogenic 129Xe ∗ are released together, with the same 128 Xe ∗ 129 Xe ∗ ratio in nominal olivine, pyroxene, and feldspar although the maximum release from pyroxene occurs about 300°C higher than from feldspar. From this the cooling rate of silicates for the temperature range 1500-1200°C is deduced to be ≥100°C per million years. The 80 Kr 82 Kr ratio of 79,81Br( n, γ)-produced 80,82Kr is 2.86 ± 0.04, in agreement with the value found in inclusions of the IAB Linwood but higher than observed in almost all stone meteorites.