Mudrock-hosted carbonate concretions: a review of growth mechanisms and their influence on chemical and isotopic composition

Existing interpretations of cement textures and isotopic compositions may significantly under-estimate the depth and duration of concretionary growth. Minus-cement porosities can commonly under-estimate depths of concretionary growth for some, or all, of the following reasons; (i) cements might not passively replace host sediment porosity, (ii) non-cement carbonate phases (such as replaced bioclastic carbonate) can be significant, (iii) sediment compaction models over-estimate rates of porosity loss at shallow (<500 m) depths and (iv) cementation can create a framework that prevents compaction and preserves porosity. Cement textures can be used to distinguish two modes of growth; concentric growth, where successive layers of cement are added to the outer surface (radius increases with time), and pervasive growth, where cement crystals grow simultaneously throughout the concretion volume (little or no radius increase with time). Cement textures of siderite concretions are mostly consistent with pervasive growth, but many calcite microsparite concretions show no diagnostic textural features and could grow either concentrically or pervasively. Concretionary cementation, whether concentric or pervasive, occurred such that there was accessible porosity which could be filled by later cements. Pervasive growth in particular is associated with the retention of substantial amounts of porosity which may be filled by chemically and isotopically distinct phases. The resulting chemical gradients across concretions may then reflect variations in the relative proportions of early and later cements more than variations in porewater composition. Carbon isotope data from modern sediments show that dissolved carbonate in the methanogenic zone has a continuum of values from -30% to +15%, and thus overlaps C-13-depleted values normally considered characteristic of sulphate reduction. Many concretions previously thought to have grown entirely during sulphate reduction may therefore have continued cementation during methanogenesis, indicating a deeper and more prolonged cementation history. The necessary carbonate supersaturation for concretionary growth could either occur throughout the porewaters (the equilibrium model), or be generated in situ by organic matter decay (the local-equilibrium model), or created where external fluids are introduced (the fluid-mixing model).