Rb Sr dating of fluid migration in hydrocarbon source rocks

Rb Sr isotopic analyses of pyrite and its gypsum alteration product in sandy permeable horizons within the Middle Triassic Botneheia Member, an immature hydrocarbon source rock shale sequence exposed at Kvitberget, Spitsbergen, showed that the pyrite was oxidized 214 ± 14 Ma ago, at a time of oxygenated water ( 87Sr/ 86Sr ratio ≑ 0.7102) influx during regional emergence above sea level. Three separated glauconite pellet fractions and calcite from the same sedimentary sequence form a Rb Sr isochron with an age of 191.0 ± 2.2 Ma, which corresponds with the time of an important marine transgression following the regressive period which began ∼214 Ma ago. The glauconite pellet Rb Sr systems were reset when the pore water was replaced by a new fluid with a different composition ( 87Sr/ 86Sr=0.7089) at the time of the transgression. These data show how the relationship between pore fluid composition, mineral alteration-recrystallization and resetting of Rb Sr mineral systems can be used to constrain the diagenetic and fluid migrational history of source rock shale sequences.