Structure-related geochemical (REE) and isotopic (K-Ar, Rb-Sr, delta O-18)characteristics of clay minerals from Rotliegend sandstone reservoirs (Permian, northern Germany)

Euhedral illite cementing the gas-bearing sandstone reservoirs of the Rotli egend in the Niedersachsische rift system (northern Germany) was studied al ong a horst-to-graben cross-section to examine its chemical and isotopic ch aracteristics. The data show that differentiated illite particles grew duri ng a tectono-thermal event marked by distinct episodic hydrothermal activit ies along fault drains acid in the poral space of sandstones at 210 Ma and at 195 to 190 Ma in horst positions, at 185 to 175 Ma in the nearby graben, and at 170 to 165 Ma in both the horst and graben. Varied REE distribution patterns and initial Sr-87/Sr-86 ratios (from 0.7124 to 0.7142) relative t o illite-particle size outline a high chemical variability of the fluids du ring illite growth. The delta(18)O values of illite range from +20.0 to +12 .4 parts per thousand and those of chlorite from +12.0 to +6.9 parts per th ousand. The changing delta(18)O values of the minerals most likely relate t ovaried delta(18)O values of the fluids, or to different water/rock ratios during crystallization. Differences among the REE and Sr-87/Sr-86 tracers a nd the delta(18)O values of different generations of illite also suggest a changing fluid chemistry relative to time. Significant positive Eu anomalie s and negative Ce anomalies in the REE distributionsof illite suggest felds par alteration by the migrating fluids in oxidizing environments. (C) 1999 Elsevier Science Ltd.