SIDERITE CONCRETIONS FROM NONMARINE SHALES (WESTPHALIAN-A) OF THE PENNINES, ENGLAND - CONTROLS ON THEIR GROWTH AND COMPOSITION

Back-scattered electron microscopy has been used to examine the micros tructure of nonmarine-shale-hosted siderite concretions. The concretio ns are composed of 50-100 mu m, zoned crystallites, which exhibit no n oticeable center-to-edge variation within any individual concretion. T his indicates that siderite crystallites nucleated at virtually the sa me time across the entire concretion and that the concretions did not grow by radial addition of siderite layers around a central nucleus. F urther siderite precipitation took place by crystal growth onto the nu clei. The total proportion of siderite in any part of the concretion b ears 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-c hemical and stable-isotope data suggests that the siderite crystallite s are composed of an Fe-Mn-rich end member with a delta(13)C value of similar to +10 parts per thousand and a Mg-Ca-rich end member with a d elta(13)C value of similar to 0 parts per thousand to -5 parts per tho usand. The mineral-chemical and stable-isotope compositions of these c oncretions resulted from microbially mediated processes operating clos e (< 10 m) to the sediment-water interface, during methanogenesis. Met hanogenesis can generate low-delta(13)C as well as high-delta(13)C car bonate cements, hence deep-burial diagenetic reactions, such as decarb oxylation of organic matter, need not be invoked to generate solutes f or siderite precipitation.