--> δ18O and δ13C-Zoned Carbonate Cements as Records of Porewater Temperature and Composition in Siliciclastics: In-Situ SIMS Analyses From Early Paleozoic Sandstones in the Illinois Basin, USA

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δ18O and δ13C-Zoned Carbonate Cements as Records of Porewater Temperature and Composition in Siliciclastics: In-Situ SIMS Analyses From Early Paleozoic Sandstones in the Illinois Basin, USA

Abstract

The collective suite of cementing minerals in a sandstone develops over the course of a sedimentary sequence's diagenetic history. Growth and dissolution patterns, combined with chemical and isotopic zoning within a single mineral overgrowth, can record chemical and morphological evidence of the initial burial conditions as well as a host of chemical and mechanical burial processes, including compaction, mineral alteration, hydrocarbon generation, and pulses of fluid flow—meteoric or otherwise. Previous studies attempting to isolate diagenetic information from carbonate cements have been limited by an inability to resolve the μm-scale isotopic heterogeneity that is commonly present. Recent advances in the analysis of carbonate minerals by Secondary Ion Mass Spectrometry (SIMS) allow in-situ measurements of δ18O and δ13C from areas ≤10 μm in diameter. Precision of ±0.3‰ (2SD) is attainable for δ18O with a 10 μm spot; for δ13C precision of ±0.7‰ (2SD) is attainable with a 5 μm spot. We used SIMS analysis of δ18O in carbonate and silicate cements to test a model wherein slow deposition and burial in the Illinois Basin is recorded in the diagenetic minerals of the middle-Ordovician St. Peter and Cambrian Mt. Simon sandstones (Hyodo et al., 2014 Chem Geol). δ18O and δ13C-zoned dolomite-ankerite cements in these formations preserve systematic variations consistent with chemical and thermal processes operating on a basin-wide scale.δ18O in early dolomites range from 23–27‰ VSMOW (−7 to −3‰ VPDB), but later zones show sequentially lower values associated with burial and heating, as was previously observed for δ18O values in zoned quartz overgrowths (Pollington et al., 2011 Geology). This long period with low fluid fluxes due to burial and compaction was punctuated by a brief period of metal-carrying brine migration, driven through the sandstone aquifers by uplift in the Ouachitas and associated with the Mississippi-Valley Type (MVT) ore deposits (ca. 270 Ma) of the Upper Mississippi Valley. Some late ankerites have δ18O values of 18‰ VSMOW (−12‰ VPDB) or lower, suggesting that temperatures were elevated above the geotherm by hot brines and/or δ18O of porewater was lowered. Either way, fluid flow is indicated and we conclude that some of these late periods of diagenetic growth are likely associated with MVT fluids. Values of δ13C in zoned dolomite-ankerites range from 0 to −7‰ VPDB and frequently vary independently of δ18O, also suggesting open-system behavior.