Permian Rotliegende sandstone cores were collected from an area of about 14,800 km² (5710 mi²) mostly to the east-southeast of the city of Bremen in northern Germany, at depths between 4596 and 5330 m (15,079–17,487 ft). The separated size fractions (0.2, 0.2–1.0, and 1.0–2.0 m) consist of illite (90–100%) with small to minute amounts of chlorite and detectable quartz and feldspars in the coarser fractions. Scanning electron microscopic and transmission electron microscopic observations showed two types of illite morphologies: flakes coating detrital framework minerals and laths and fibers invading the pore space.

The data points of most size fractions fit two isochrons, with slopes providing ages of 191 8 and 178 1 Ma, with initial ⁴⁰Ar/³⁶Ar ratios reasonably close to the atmospheric value. Microthermometric fluid-inclusion determinations in quartz and calcite characterize two types of percolating fluids: a highly saline (19% NaCl equivalent) fluid at variable temperatures depending on the reservoirs (185 to 150C) and a slightly saline one (2.6% NaCl equivalent) again at varied temperatures (170 to 145C), also depending on the locations. These temperatures are higher than paleotemperatures calculated on the basis of a present-day burial gradient of 30.5C/km, therefore favoring hydrothermal illitization, with the oldest illite crystallizing at a generally higher temperature than that of the younger illite.

However, if illite coincidently precipitated with quartz, which is supported by petrographic observation, then illite with ages of about 200 Ma in the eastern to central Rotliegende sandstones of the area formed at a lower temperature but in more saline fluids than that of about 155 Ma. Alternatively, illite crystallization temperature and salinity of the interacting fluids could have been activated by the same hydrothermal activity at different locations in the studied area and could have decreased and changed, respectively, when the fluids moved in the sandstone reservoirs.

The potassium-argon data fit a regional model of recurrent thermal activity, with the oldest occurrence of authigenic illite in the eastern Imbrock-Ganderkesee area and a time-dependent trend of the hydrothermal activity toward the west. Previous results from northern Germany, central Netherlands, and the southern North Sea complete the model with an initial activity at the same 200-Ma time in the western offshore and central Netherlands and an activity trend toward the east.