ABSTRACT

Back-scattered electron microscopy has been used to examine the microstructure of nonmarine-shale-hosted siderite concretions. The concretions are composed of 50-100 m, zoned crystallites, which exhibit no noticeable center-to-edge variation within any individual concretion. This indicates that siderite crystallites nucleated at virtually the same time across the entire concretion and that the concretions did not grow by radial addition of siderite layers around a central nucleus. Further siderite precipitation took place by crystal growth onto the nuclei. The total proportion of siderite in any part of the concretion bears no simple relationship to the porosity of the enclosing shale at the time of precipitation, and growth by passive precipitation in pore space is unlikely. Integration of microprobe data with bulk mineral-chemical and stable-isotope data suggests that the siderite crystallites are composed of an Fe-Mn-rich end member with a ¹³C value of ~ +10o/oo and a Mg-Ca-rich end member with a ¹³C value of ~ 0o/oo to -5o/oo. The mineral-chemical and stable-isotope compositions of these concretions resulted from microbially mediated processes operating close (< 10 m) to the sediment-water interface, during methanogenesis. Methanogenesis can generate low-¹³C as well as high-¹³C carbonate cements, hence deep-burial diagenetic reactions, such as decarboxylation of organic matter, need not be invoked to generate solutes for siderite precipitation.