Origin of pore-lining chlorite in the aeolian Rotliegend of northern Germany

Pore-lining chlorite is common in Rotliegend lake shoreline aeolian sandstones from northern Germany and preserves abnormally high primary intergranular porosity. In the north of the study area, in a basinward direction, the chlorites are Mg-rich while towards the south they become Fe-rich over a distance of about 15 km. All are unusually rich in Mn. Magnesium-rich examples tend to be more abundant than illite, while Fe-rich examples cover framework grains less continuously and are admired with more abundant illite. Oxygen isotope analysis of 30 chlorites in the 2-6 mu m fraction gave delta(18)O (SMOW) values of 7 to 12 parts per thousand (mean 9.8 parts per thousand). These data show no obvious trend across the study area, nor in relation to changes in chlorite composition, or burial depth. The honeycomb arrangement of chlorite crystals suggests chlorite formation via the sequence smectite-corrensite-chlorite. Chlorite distribution and the systematic changes in its composition suggest that formation of a precursor was related to lateral migration of Mg-rich fluids from basinal shales and/or evaporites during shallow burial. Interaction of these fluids with early formed oxyhydroxide coatings on the aeolian sand grains provided a source of Fe and the Mn. The isotope data suggest that the eventual formation of chlorite during deep burial occurred from waters with positive delta(18)O values, comparable to those present during the deep burial formation of illite.