Widespread cementation induced by inflow of continental water in the eastern part of the Paris basin: O and C isotopic study of carbonate cements

Mesozoic limestones from the eastern part of the Paris Basin display, mostly in strata rich in bioherms, abundant macro-cavities (vugs) and some connected micro-fractures almost completely filled with euhedral calcite crystals (late calcite spar). The interparticle porosity of limestones is also filled with euhedral calcite spar cements similar to those observed in vugs and fractures, and rarely displays the preservation of the structures typical of early marine cements. These calcite spars (both from vugs/fractures infillings and from cemented porosity) were studied in drilled cores crosscutting the Kimmeridgian to Bathonian series with the main objective of determining the nature and flow regime of the fluids responsible for the strong porosity reduction observed in that part of the Paris Basin Mesozoic limestones. The approach combined mineralogical and geochemical studies (O and C stable isotopic compositions measured by conventional and SIMS methods) with temperature estimates from fluid inclusion analyses. The delta(13)O values of late calcite are close to those of the host limestones and correlate with the PC changes in host rocks, indicating a local source of C. By contrast, late calcites from vugs, fractures and also in the cemented interparticle porosity of limestones have delta(18)O values around +21parts per thousand/SMOW, values which are distinct from those of bulk host limestones (+25parts per thousand to +27parts per thousand). The delta(18)O value of the fluid is estimated to be -6.8 to -2.5parts per thousand for a crystallisation temperature between 32 and 42 C determined from fluid inclusion studies. Thus, the parent fluid of cemented interparticle porosity and vug- and fracture-infillings likely contained a component of meteoric water, in contrast to the previous belief that the reduction of porosity in those series was related to early marine diagenesis. Possible situations in which such meteoric fluid could have occurred are: (i) a stage when the limestone aquifers were directly connected to surface water, and/or (ii) channelling of water, from deeper zones in the basin, through regional fault systems. The O isotopic compositions of the calcite cements in the underlying 140 m - thick Callovo-Oxfordian argillites and those of the less recrystallised Bathonian to Kimmeridgian limestones are those expected for unmodified marine calcite: delta(18)O =ca. +28parts per thousand SMOW. Therefore, from the isotopic point of view, the Call ovo-Oxfordian argillites remained essentially unaffected by the paleo-fluid circulation documented in the adjacent limestones series. This indicates the argillites acted as an efficient permeability barrier in the past. (C) 2004 Elsevier Ltd. All rights reserved.