Integrating μm-scale δ18O and δ13C SIMS Data Into Sediment Burial Histories: Diagenetic Records of Temperature and Fluid Composition in Illinois Basin Sandstones

Abstract

Advances in the analysis of zoned dolomite-ankerites by Secondary Ion Mass Spectrometry (SIMS) allow in situ measurements of δ18O and δ13C in carbonate from areas ≤10 μm in diameter, with attainable precisions of ±0.3‰ (2SD) for a 10 μm δ18O spot, and ±0.7‰ (2SD) for a 5 μm δ13C spot. The ability to correlate in situ SIMS data with imaging and micro-textures provides a unique tool for investigating the fluid and thermal histories recorded in diagenetic cements, where traditional analysis methods would homogenize signals recorded on such a fine scale. Improving our understanding of the conditions that lead to diagenesis and resultant porosity changes remains a priority for predicting the quality of hydrocarbon and groundwater reservoirs, interpreting basin history, and optimizing carbon sequestration. Diagenetic quartz and carbonate cements were analyzed by SIMS for δ18O and δ13C in a suite of samples collected from the Ordovician St. Peter and Cambrian Mt. Simon sandstones of the Illinois Basin in the central Midwest (U.S.). These cements are interpreted to contain isotopic records from initial depositional environment on through deep burial and heating, and preserve swings of up to 10‰ across distances of 10 μm or smaller. Early dolomite cores contain δ18O values ranging from 22 to 27‰ VSMOW (-8 to -3‰ VPDB), a range likely reflecting cementation in the meteoric-seawater interface. Subsequent dolomite growth was slowed or halted completely as sediments entered the burial compaction regime and were isolated from seawater ion sources; quartz overgrowths dominate this portion of the diagenetic history, with much of the silica derived from local pressure solution. Consistent zonation of δ18Oqtz cement values are interpreted to primarily record the geothermal gradient during burial, with connate fluids seeing only minimal isotopic change throughout this period. Towards the end of quartz growth, at around 270 Ma, dolomite-ankerite precipitation experienced a resurgence with the up dip migration of hot, tectonically driven brines associated with the Mississippi Valley-Type (MVT) deposits of the Upper Mississippi Valley. MVT-related carbonates are characterized by abrupt Fe enrichment during growth and a shift in δ13C values; the direction of the δ13C shift reflects the degree to which hot fluids were buffered by external carbonate (values shift towards ∼0‰ VPDB) or exposed to thermal maturation of organic matter (values shift down, going as low as -13‰ VPDB).

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016