High resolution ion microprobe investigation of the delta 18O of carbonate cements (Jurassic, Paris Basin, France): New insights and pending questions

The scope of this work is to investigate, at a high resolution, the oxygen isotope composition (delta O-18(carb)) of diagenetic products (synsedimentary and burial calcite cements) in shallow-marine carbonates. SIMS (Secondary Ion Mass Spectrometry) microprobe analyses were performed on thin sections from Oxfordian and Kimmeridgian Formations of the eastern Paris Basin and compared to data obtained on the same diagenetic products by conventional mass spectrometry (acid digestion). Hereby obtained, delta O-18 are similar, but the SIMS dataset displays a larger range of values. The isotopic zonation obtained by SIMS transects through sequences of cements filling pores, reveals an (expected) isotopic depletion from older stage synsedimentary calcites to younger stage blocky calcites and that follows the CL (cathodoluminescence) zonation. SIMS analyses however show that synsedimentary cements precipitated in intra-skeletal pores, have heavier delta O-18 than their inter-particle counterparts, with an offset of +4 parts per thousand-poe, despite similar petrographical characteristics. This difference is maintained in the delta O-18 of the first stages of blocky calcite cements, intra-skeletal blocky calcites showing heavier delta O-18 than the time equivalent and petrographically identical inter-particle calcites, with an offset of +5%(av-pDB) These offsets are tentatively explained by the precipitation of cements under non-equilibrium conditions in intra-skeletal pores, where organic matter decay may have played a key role, acting notably on the pH. The occurrence of isolated micro-diagenetic environments, co-existing at the thin section scale, is tentatively proposed as an explanation to these small scale and high amplitude delta O-18 heterogeneities. These results may question the sampling strategy for future works. Microdrilling may miss the observed range of variation, but averaging the values may not necessarily lead to real misinterpretations if a critical selection of samples is performed, targeting potentially similar micro-diagenetic environments and avoiding potentially specific ones, i.e. closed intra-skeletal pores. These results also definitely underscore the need for additional experiments to improve the reliability of SIMS, in order to develop the use of this very high resolution technique for carbonate diagenesis studies. (C) 2017 Elsevier B.V. All rights reserved.