ISOTOPIC COMPOSITION OF XENON AND KRYPTON IN SILICATE-GRAPHITE INCLUSIONS OF THE EL TACO, CAMPO DEL CIELO, IAB IRON METEORITE

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 He-3 during, or towards the end of, the exposure of El Taco to the cosmic radiation. In olivine the He-3/Ne-21 and Ne-22/Ne-21 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 less than or equal to Ar-36/Xe-132 less than or equal to 700 and 1.7 less than or equal to Kr-84/ Xe-132 less than or equal to 2.2. Xenon released from the silicates at extraction temperatures > 1000 degrees C is isotopically close to ureilite Xe, while the high-ir Xe from graphite and schreibersitz 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 Pu-244-Xe. The fission component, 10% in the case of Xe-136 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 I-127(n, gamma)-produced Xe-128* and radiogenic Xe-129* are released together, with the same Xe-128*/Xe-129* ratio in nominal olivine, pyroxene, and feldspar although the maximum release from pyroxene occurs about 300 degrees C higher than from feldspar. From this the cooling rate of silicates for the temperature range 1500-1200 degrees C is deduced to be greater than or equal to 100 degrees C per million years. The Kr-80/Kr-82 ratio of Br-79,Br-82(n, gamma)-produced Kr-80,Kr-82 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.