NOBLE-GASES IN METAL AND SCHREIBERSITE OF THE ACUNA (IIIAB) IRON METEORITE

We measured abundances and isotopic compositions of noble gases in met al and schreibersite of the Acuna (IIIAB) iron meteorite. The concentr ations of noble gases in Acuna metal are very low compared to those re ported so far for other iron meteorites. The isotopic ratios of He, Ne and Ar indicate that they are mostly of cosmogenic origin. Cosmogenic components are even present in Kr and Xe, which could not have been p roduced from Fe, Ni and P and are probably due to the spallation of tr ace elements of higher masses. The high He-4/Ne-21 ratio of 420 in Acu na metal indicates that the samples were at a deep position within a v ery large meteoroid. The exposure ages of Acuna were estimated to be 5 0-200 Ma from He-3, Ne-21 and Ar-38 abundances and by utilizing the di agrams of production rates vs. the He-4/Ne-21 ratio based on the Signe r-Nier model. The low exposure age of Acuna may indicate a history dif ferent from that of other IIIAB irons whose exposure ages cluster at s imilar to 670 Ma. Otherwise, Acuna may be one of the samples with the low production rate, which can not be estimated from the diagrams of t he Signer-Nier model.