Mn-cr Isotope Systematics in the Ll Type Ordinary Chondrite St. Séverin

Introduction: The Mn-Cr analyses of several H chondrites including Forest Vale, Dimmitt, Ste Mar-guerite and Richardton [1-3], have all shown reasonable agreement between the obtained 'Mn-Cr ages' and the known Pb-Pb ages determined on phosphates. These studies imply that the best fit lines in the 53 Mn-53 Cr evolution plots for these meteorites are isochrons rather than mixing lines and that the slope of the lines correspond to the Mn-Cr closure times for these ordinary chondrites. Furthermore, the fact that a correlation exists between the degree of metamorphism and the Mn-Cr closing times for H chondrites (H chon-drites with lower petrographic grade have older Mn-Cr ages) is only explainable if the Mn-Cr ages date meta-morphic events on the H chondrite parent body. In order to extend this work to include LL type ordinary chondrites we investigated the Mn-Cr isotope systematics of St. Séverin (SS). There are two essential conditions we have considered for selecting meteorites for Mn-Cr analysis: (i) a reliable and accurate U-Pb age must be available for the meteorite and (ii) the U-Pb age must be > 4550 Ma, because Mn-Cr is a relative chronometer and for closure times younger than 4550 Ma, the Mn-Cr age cannot be determined since the short lived