Evidence for Fractional Condensation and Flash Reheating from Volatile Rich Objects in Ch-chondrites

Introduction: CH-carbonaceous chondrites have excess FeNi-metal and are strongly depleted in moderately volatile and volatile elements (Mn, Na, Zn, Se), but have CI refractory element/Mg ratios [1]. In contrast to other carbonaceous chondrites, CHs have high modal abundances of chondrules (~70 vol.%) with a mean diameter of 70-90 μm, FeNi-metals (~20 vol.%) and very low fractions of matrix (~5 vol.%), heavily altered by terrestrial weathering. Chondrules, however, show no signs of weathering [1]. The low FeO-content in silicates and the high Cr-concentrations in metal reflect reducing conditions during formation. Rare SiO2-rich components contain abundant free silica [2, 3]. Some zoned metal grains have preserved the signatures of nebular condensation [4, 5]. Despite the low volatile element contents of CHchondrites, two chondrules with very high concentrations of Mn, Na and K were discovered in Acfer 182. These unusual objects provide insight into the conditions of formation of these unusual meteorites.