Isotopically zoned carbonate cements in Early Paleozoic sandstones of the Illinois Basin: delta O-18 and delta C-13 records of burial and fluid flow

摘要

SEM/SIMS imaging and analysis of delta O-18 and delta C-13 in sandstones from a transect through the Illinois Basin (USA) show systematic lam-scale isotopic zonation of up to 10 parts per thousand in both carbonate and quartz cements of the middle Ordovician St. Peter and Cambrian Mt. Simon formations. Quartz delta O-18 values are broadly consistent with the model of Hyodo et al. (2014), wherein burial and heating in the Illinois Basin is recorded in systematically zoned quartz overgrowths. Observations of zoned dolomite/ankerite cements indicate that they preserve a more extended record of temperature and fluid compositions than quartz, including early diagenesis before or during shallow burial, and late carbonates formed after quartz overgrowths. Many carbonate cements show innermost dolomite with delta O-18 values (21-25 parts per thousand VSMOW) that are too low to have formed by deposition at low temperatures from ancient seawater (delta O-18 > -3 parts per thousand) and most likely reflect mixing with meteoric water. A sharp increase in Fe content is commonly observed in zoned carbonate cements to be associated with a drop in delta O-18 and an abrupt shift in delta C-13 to higher or lower values. These changes are interpreted to record the passage of hot metal-rich brines through sandstone aquifers, that was associated with Mississippi-Valley Type (MVT) Pb-Zn deposits (ca. 270 Ma) of the Upper Mississippi Valley. Local variability and individual trends in delta C-13 are likely controlled by the sources of carbon and the degree to which carbon is sourced from adjacent carbonate units or thermal maturation of organic matter. Quartz overgrowths in sandstones provide an excellent record of conditions during burial, heating, and pressure-solution, whereas carbonate cements in sandstones preserve a more-extended record including initial pre-burial conditions and punctuated fluid flow events. (C) 2017 Elsevier B.V. All rights reserved.