Noble Gas Measurements of the Grant Iiiab Iron Meteorite

Introduction: In contrast to stony meteorites, which have exposure ages usually not higher than a few tens of million years, most iron meteorites exhibit cosmic-ray exposures of hundreds of million years. Consequently, iron meteorites have collected information about the solar system, encoded in cosmogenic nuclides, for much longer than stony meteorites. Iron meteorites are therefore indispensable for long-term studies of solar system dynamics. For example, the still open question of whether there has been any long-term change in the galactic cosmic-ray (GCR) intensity is only accessible using iron meteorites. The study of long-term changes in the GCR is important for several reasons. First, unobserved variations in the GCR intensity compromise exposure age studies of the dynamics within the asteroid belt. Second, a possible variability of the GCR might give some information about the number of galactic spiral arms and, third, it has been argued that changes in the GCR intensity could affect the Earth's climate [1]. However, for such studies a consistent database of cosmogenic nuclides in iron meteorites together with physical model calculations to understand their production mechanisms are mandatory. In the noble gas laboratory of the University of Bern, we are now able to routinely measure the helium, neon, and argon isotopic concentrations in iron meteorites. In order to study how to handle iron meteorites, i.e. extract and clean the noble gases and choose appropriate sample weights, a variety of experiments were performed. The first experiment was a stepwise heating procedure using Grant (IIIAB) samples. There we determined what temperature is needed to degas iron meteorite samples. In the second experiment, the spatial distribution of noble gases in Grant has been tested in order to choose representative sample weights for further analyses. After having successfully demonstrated that we are able to accurately and precisely measure noble gases in iron meteorites, the next step was the recalibration of the preatmospheric center of Grant. This is of particular importance because several (empirical and semiempirical) models developed to calculate cosmogenic production rates in iron meteoroids, e.g., the Signer-Nier model [2] and the Voshage model [3], are based on Grant data. Methods: Two different experimental setups are used to degas and analyze the samples. One is a conductive heating system connected to a MAP 215-50 mass spectrometer. The other is a RF-heating system