NEW INVESTIGATIONS OF THE MANSON IMPACT STRUCTURE, IOWA

We are carrying out geochemical and mineralogical studies of the Manson Impact structure, Iowa, in order to better understand the nature of the impactor and the impact structure, using material from the drill cores taken with during the early 1990's with the Geological Survey and the Iowa Geological Survey Bureau. Platinum group elements (PGEs) and Os isotopes have been used in several impact sites to confirm the presence of meteoritic matter and can potentially help to determine the type of the impactor involved. Previous studies on the Manson Impact structure (Perknicka et. al 1996; Koeberl and Shirey, 1996) found evidence of extraterrestrial material (elevated Ir concentrations and low 187Os/188Os) in one sample of the impact melt breccia (M1 429.0). For our study, we have carefully handpicked melt material from six samples of melt-rich rock. These samples are free of clasts and rock fragments, and so potentially will have a more robust signature of the meteoritic material. We aim to have new PGE and Os isotope data for these samples before the meeting, and we will use the data to see if we can constrain the type of meteoritic material involved. We are also carrying out a search for impact diamond and zircons in the Manson rocks. We have selected two samples of suevite from core M11 (243.0-243.4 and 403-403.5). We used acid digestion techniques to dissolve and remove most of the silicate material, and studied the acid-residue with a SEM to identify the residual phases. We will present our preliminary results at the meeting.