Magnesian Calcite Cements and their Diagenesis: Dissolution and Dolomitization, Mururoa Atoll
- Maurice E. Tucker2 and
- Robin G. C. Bathurst3
Published Online: 29 APR 2009
Copyright © 1990 The International Association of Sedimentologists
How to Cite
Aissaoui, D. M. (1990) Magnesian Calcite Cements and their Diagenesis: Dissolution and Dolomitization, Mururoa Atoll, in Carbonate Diagenesis (eds M. E. Tucker and R. G. C. Bathurst), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304510.ch12
Department of Geological Sciences, University of Durham, UK
Derwen Deg Fawr, Llanfair DC, Ruthin, Clwyd, North Wales, UK
- Published Online: 29 APR 2009
- Published Print: 21 AUG 1990
Print ISBN: 9780632029389
Online ISBN: 9781444304510
- magnesian calcite cements and their diagenesis: dissolution and dolomitization, Mururoa Atoll;
- syn-sedimentary cements, affecting Mururoa atoll;
- Mg-calcite dissolution, relating to meteoric diagenesis;
- dolomitization of Mururoa cements involves alternations of calcite and dolomite;
- advanced dissolution, including disappearance of cement portions
The most ubiquitous syn-sedimentary cements affecting Mururoa atoll are composed of magnesian calcite. Three main types are distinguished: fibrous, bladed and sparitic on the basis of petrography, morphology and MgCO3 concentration of the constituting crystals, while peloid infills, a particular form of HMC chemical precipitation, also exist. Petrographic evidence and isotopic signatures are compatible with marine precipitation.
Mururoa atoll was exposed several times to meteoric diagenesis resulting in varied diagenetic alterations including selective dissolution and partial dolomitization of Mg-calcite cements. These alterations are responsible for substantial modifications of the initial cement fabrics and may introduce unconformities in the diagenetic chronology. The first stage of the partial dissolution of Mg-calcite induces the development of chalky, white friable zones within the initially crystalline, hard cement layers. At ultrascale, this is due to the creation of micro-voids along the elongate cement fibres. Advanced dissolution includes total disappearance of cement portions as attested to by large voids within the cement crust and/or between superposed cement layers. Mg-calcite dissolution is related to meteoric diagenesis during periods of Quaternary exposure. The creation of voids within Mg-calcite layers is due to the mechanical removal of previously altered calcium carbonate, a process suggesting marine or non-marine water flow, probably in the vadose environment.
Selective dolomitization of Mururoa cements involves alternations of calcite and dolomite which form successive cement-like rinds within primary cavities. At Mururoa, these alternations are the result of selective dolomitization of the pre-existing Mg-calcite cements rather than successive precipitation of calcite and dolomite. Selective dolomitization of Mg-calcite cements at Mururoa indicates that a given cement succession is not necessarily a simple chronological sequence. Oxygen isotope values of dolomites are enriched in δ18O by about 3‰ PDB within calcite-dolomite pseudo‰alternations. The dolomitizing fluid at Mururoa seems similar to present marine water although some mixture with meteoric water is probable to favour dissolution associated with dolomitization.