Trace-element source and mobility during limestone burial diagenesis—an example from the Middle Jurassic of eastern England

Abstract

Summary Two phases of burial calcite from the Lincolnshire Limestone (Bajocian) of central Lincolnshire have been analysed for their trace element (Fe, Mn, Mg, Sr) and strontium isotopic compositions. The earliest, non-ferroan, brightly luminescent phase has the following mean trace element concentrations: Fe- 919 ppm; Mn- 116 ppm: 3 1778 ppm; Sr- 327 ppm. The later, ferroan, dully luminescent phase has the following mean compositions: Fe- 3580 ppm; Mn- 225 ppm; Mg- 1400 ppm; Sr- 318 ppm. The strontium isotopic composition of the two phases is indistinguishable, with a mean 87 Sr/ 86 Sr ratio of 0.70820. This value is considerably more radiogenic than host Bajocian marine carbonate ( 87 Sr/ 86 Sr = 0.70725). The Fe and Mn of the burial calcites were sourced by iron and manganese oxyhydroxides associated with shales bounding (and internal to) the Lincolnshire Limestone. The relatively shallow maximum burial depths (550 m) and low temperatures (maximum of 42°C) attained by the formation in the area of study preclude major clay or sandstone diagenetic transformations as potential sources for Fe and Mn. These trace elements moved into the Limestone in solution via compaction driven cross-formational flow from bounding shale aquicludes. The Sr and Mg trace element components of the burial cements were derived from: (i) remobilized Bajocian marine carbonate, and (ii) remobilized Carboniferous marine carbonate. These sources satisfy the requirements of a linear Sr versus Mg relationship observed for the burial cements, that is, a constant Sr/Mg ratio with time, but highly variable absolute Sr and Mg contents. These sources also satisfy the near constant 87 Sr/ 86 Sr composition of the burial calcites. The burial hydrology required by the trace element budget above is a convective flow system with cross-formational elements as normal bleeder faults connecting the Carboniferous Limestone and the Lincolnshire Limestone. Additional compaction-driven flow occurred from bounding shale formations (chiefly the underlying Grantham Formation and to a lesser extent the Upper Estuarine ‘Series’) to the Lincolnshire Limestone. Up-dip flow within the fully confined Limestone provides an enformational limb to the convective system.