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In-Situ, Micron-Scale δ13C & δ18O Analyses (by SIMS) of Chemo-Isotopically Zoned Carbonate Cements of Diagenetic Origin — A Case Study on the Implications for the Thermal and Burial History of the Eau Claire Fm., Illinois Basin (USA)

Abstract

The δ13C & δ18O of zoned carbonate cements of diagenetic origin can be measured in-situ on the sub-10-μm scale using Secondary Ion Mass Spectrometry (SIMS). A critical aspect of analytical methods advancement is the continued development of standards for correcting systematic, compositionally-dependent - but generally highly non-linear - sample matrix effects that bias measured isotope ratios. Precision is a trade-off with beam size and, depending on the isotope system, varies between ±0.3‰ (2SD; 10μm beam, δ18O) and ±1.0‰ (2SD; 3μm beam for δ18O, 5–10μm beam for δ13C). Analyses on this scale enhance the spatial resolution in applied problems relating to carbonate diagenesis and carbonate cementation of sandstone-shale systems. For example, by preserving petrographic relations between two different authigenic cement phases exhibiting equilibrium growth textures (e.g. dolomite-quartz overgrowths), in-situ δ18O measurements provide a temperature constraint for a particular stage of cementation that is independent of assumptions about the δ18O of porewaters from which cements precipitated. Transects across early-to-late cement generations provide a clearly-resolved, zone-by-zone account of changes in δ13C & δ18O which elucidate the evolution of diagenetic conditions, including 1) temperature during progressive burial and heating, 2) isotopically-distinct fluid/brine migration events, 3) the progression of organic matter (OM) maturation and 4) the reaction of smectite⇒illite clay, which affects porewater δ18O. A regional case study of dolomite-ankerite cements in the sandy-shaly Late Cambrian Eau Claire Fm of the Illinois Basin reveals systematic depth-related C- and O-isotope zoning (early-to-late δ18O: -5⇒-16‰ VPDB, δ13C ~0⇒-10‰ VPDB). δ18O-based T-modeling - using both internal and external constraints on porewater δ18O - indicates heating to ~90°C during basin subsidence, and additional conductive heating to ~130°C during the basin-scale, late Paleozoic hot brine migrations through the underlying Mt. Simon sandstone aquifer that resulted in regional Mississippi Valley-type ore body formation. The δ13C of the latest cement generations indicates only a moderate contribution of OM-derived, isotopically-light C, reflecting the overall organic-leanness of Eau Claire sediments (TOC <0.5%). These results compliment SIMS studies of δ18O in quartz-overgrowths of the Eau Claire and Mt Simon Fms (Hyodo et al., 2014, Chem Geol; Pollington et al., 2011, Geology).