Oxygen Isotope Ratios of Syntaxial Quartz Overgrowths In Cambrian Sandstones, Central Wisconsin

An ion microprobe study of oxygen isotope ratios of quartz overgrowths in Cambrian quartz arenites from central Wisconsin results in d¹⁸O values indicative of low-temperature overgrowth formation.  The samples analyzed in the study are basal Upper Cambrian sandstones from the Elk Mound Group, never buried to depths over one kilometer and composed of detrital quartz grains with authigenic quartz overgrowths.  All cements are optically continuous with the detrital quartz grains they surround.  Overgrowths present in these samples were analyzed in thin section with an ims-1280 ion microprobe using a 10 μm analysis spot.  Analytical precision of the UWQ-1 standard using the 10 μm spot averages ±0.26‰ (2SD).  The small analysis spot allows multiple analyses per overgrowth and avoids mixing of zoned samples, which cannot be avoided with bulk analytical techniques.  Results of the in-situ analysis show a bimodal distribution for overgrowth data; sample 07WI-1 from Wood County has an average d¹⁸O value of 20.3±2.4‰ (n=33, 2SD) while samples 07WI-2 from Lincoln County and 07WI-4 from Marathon County have average d¹⁸O values of 31.1±4.2‰ (n=23, 2SD) and 31.0±3.1‰ (n=38, 2SD), respectively.  Detrital cores of sandstone grains in all samples have an average d¹⁸O of 10.4±4.1‰ (n=29, 2SD).

The overgrowths exhibit complex concentric zonation under cathodoluminescence; however, each sample is homogeneous and shows no systematic change in d¹⁸O across growth bands.  Sample 07WI-1 exhibits oxygen isotope ratios lower than previous ion microprobe studies of Lower Paleozoic sandstones in Wisconsin (Kelly et al. 2007 GCA).  Samples 07WI-2 and 07WI-4 occur further north on the Wisconsin Dome.  The higher d¹⁸O in these samples suggest overgrowth formation at temperatures lower than observed in overgrowths formed by hydrothermal activity or deep burial; these d¹⁸O values suggest overgrowths are meteoric cements.  Overgrowths in sample 07WI-1 formed at a higher temperature or from lower d¹⁸O water.