Noble gases in metal and schreibersite of the Acuña (IIIAB) iron meteorite

Abstract

Abstract— We measured abundances and isotopic compositions of noble gases in metal and schreibersite of the Acuña (IIIAB) iron meteorite. The concentrations of noble gases in Acuña metal are very low compared to those reported 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 produced from Fe, Ni and P and are probably due to the spallation of trace elements of higher masses. The high 4He/21Ne ratio of 420 in Acuña metal indicates that the samples were at a deep position within a very large meteoroid. The exposure ages of Acuña were estimated to be 50–200 Ma from 3He, 21Ne and 38Ar abundances and by utilizing the diagrams of production rates vs. the 4He/21Ne ratio based on the Signer‐Nier model. The low exposure age of Acuña may indicate a history different from that of other IIIAB irons whose exposure ages cluster at ∼670 Ma. Otherwise, Acuña may be one of the samples with the low production rate, which can not be estimated from the diagrams of the Signer‐Nier model.