A NON-MAGMATIC IRON METEORITE AS PROJECTILE FOR THE GARDNOS CRATER

The late Precambrian Gardnos impact structure (60°40'N, 9°00'E) is situated 125 km to the NW of Oslo, Norway. The structure is deeply eroded and comprises a circular area with a diameter of ~5 km, composed of impact-breccias. To identify the type of projectile responsible for its formation, 16 impactite samples (80 g) were analyzed for PGE and Au using the Ni-sulfide fire assay ICP-MS. For these samples, the detection limit of the method is ~ 0.055 ng/g Ir, 0.098 ng/g Ru, 0.067 ng/g Pt, 0.022 ng/g Rh, 0.194 ng/g Pd and 0.13 ng/g Au, Os was not measured. Major and trace elements (including Ni and Cr) were measured in the same samples using XRF. Samples with the highest PGE values occur primarily at the suevite – sedimentary infill contact. Normalized to CI chondrites, the pattern of the PGE, plotted in order of decreasing condensation temperatures, is characterized by Ru and Rh enrichments. It shows no similarity with that obtained for different types of chondrites. Such Ru and Rh enrichments are common in differentiated iron meteorites. The PGE elements plotted against each other do show a good correlation (R > 0.94). If the slope of the regression line is used to represent the inter-element ratios in the Gardnos impactor, these ratios resemble those measured on the metal phase of non-magmatic iron meteorites. In the Gardnos samples, the Ir is depleted relative to Ni compared to undifferentiated meteorites. This observation in combination with non-chondritic PGE element ratios indicate that the impactor probably came from a large differentiated asteroid. However, the Gardnos impactite Ni/Cr ratio (between 2 and 7) is in the range of that of chondrites. The Gardnos impactor thus exhibits characteristics of both chondrites and iron meteorites. A non-magmatic iron meteorite seems to qualify as a plausible projectile, perhaps an IA or IIIC. These meteorites have a complex formation history (e.g. partial fractionation) and contain high amounts of silicate inclusions. Some of these silicate inclusions found in the metal phase are almost chondritic in composition. The projectile that formed the Gardnos crater probably came from a differentiated asteroid composed of a combination of a metallic phase and Cr-rich silicate phase.