Carbonate cements in late Dinantian (Asbian and Brigantian) limestones of the Derbyshire carbonate platform record a diagenetic history starting with early vadose meteoric cementation and finishing with burial and localized mineral and oil emplacement. The sequence is documented using cement petrography, cathodoluminescence, trace element geochemistry and C and O isotopes. The earliest cements (Pre-Zone 1) are locally developed non-luminescent brown sparry calcite below intrastratal palaeokarsts and calcretes. They contain negligible Fe, Mn and Sr but up to 1000 ppm Mg. Their isotopic compositions centre around delta-O-18 = -8.5 parts-per-thousand, delta-C-13 = -5.0 parts-per-thousand. Calcretes contain less C-13. Subsequent cements are widespread as inclusion-free, low-Mg, low-Fe crinoid overgrowths and are described as having a 'dead-bright-dull' cathodoluminescence. The 'dead' cements (Zone 1) are mostly non-luminescent but contain dissolution hiatuses overlain by finely detailed bright subzones that correlate over several kilometres. Across 'dead'/bright subzones there is a clear trend in Mg (500-900 ppm), Mn (100-450 ppm) and Fe (80-230 ppm). Zone 1 cements have isotopic compositions centred around delta-O-18 = -8.0 parts-per-thousand and delta-C-13 = -2.5 parts-per-thousand. Zone 2 cement is bright, thin and complexly subzoned. It is geochemically similar to bright subzones of Zone 1 cements. Dull Zone 3 cement pre-dates pressure dissolution and fills 70% or more of the pore space. It generally contains little Mn, Fe and Sr but can have more than 1000 ppm Mg, increasing stratigraphically upwards. The delta-O-18 compositions range from -5.5 to -15 parts-per-thousand and the delta-C-13 range is -1 to +3.2 parts-per-thousand. Zone 4 fills veins and stylolite seams in addition to pores. It is synchronous with Pb, Ba, F ore mineralization and oil migration. Zone 4 is ferroan with around 500 ppm Fe, up to 2500 ppm Mg and up to 1500 ppm Mn. Isotopic compositions range widely; delta-O-18 = -2.7 to -9 parts-per-thousand and delta-C-13 = -3.8 to +2.5 parts-per-thousand. Unaltered marine brachiopods suggest a Dinantian seawater composition around delta-O-18 = 0 parts-per-thousand (SMOW), but vital isotopic effects probably mask the original delta-C-13 (PDB) value. Pre-Zone 1 calcites are meteoric vadose cements with light soil-derived delta-C-13 and light meteoric delta-O-18. An unusually fractionated 'pluvial' delta-O-18 (SMOW) value of around -6 parts-per-thousand is indicated for local Dinantian meteoric water. Calcrete delta-O-18 values are heavier through evaporation. Zone 1 textures and geochemistry indicate a meteoric phreatic environment. Fe and Mn trends in the bright subzones indicate stagnation, and precipitation occurred in increments from widespread cyclically developed shallow meteoric water bodies. Meteoric alteration of the rock body was pervasive by the end of Zone 1 with a general resetting of isotopic values. Zone 3 is volumetrically important and external sources of water and carbonate are required. Emplacement was during the Namurian-early Westphalian by meteoric water sourced at a karst landscape on the uplifted eastern edge of the Derbyshire-East Midland shelf. The light delta-O-18 values mainly reflect burial temperatures and an unusually high local heat flow, but an input of highly fractionated hinterland-derived meteoric water at the unconformity is also likely. Relatively heavy delta-C-13 values reflect the less-altered state of the source carbonate and aquifer. Zone 4 is partly vein fed and spans burial down to 2000 m and the onset of tectonism. Light organic-matter-derived delta-C-13 and heavy delta-O-18 values suggest basin-derived formation water. Combined with textural evidence of geopressures, this relates to local high-temperature ore mineralization and oil migration. Low water-to-rock ratios with host-rock buffering probably affected the final isotopic compositions of Zone 4, masking extremes both of temperature and organic-matter-derived CO2. This cement sequence is typical of Dinantian shelves and platforms throughout England and Wales. We consider that the introduction of unconformity-derived Zone 3 cement into Dinantian carbonates may have been widespread in the late Carboniferous resulting in an early and comprehensive reduction in porosity. This has significant implications for the hydrocarbon reservoir potential of these rocks and helps to explain the generally non-prospective nature of the platform facies.
