Ion microprobe analysis of O-18/O-16 in authigenic and detrital quartz in the St Peter Sandstone, Michigan Basin and Wisconsin Arch, USA: Contrasting diagenetic histories

The oxygen isotopic compositions of authigenic quartz cements in sandstones provide a monitor of the temperatures, compositions, and origins of pore-occluding fluids during diagenesis, but quartz overgrowths are too fine-grained to be amenable to conventional isotopic analysis. We have used a Cameca ims-4f ion microprobe to determine oxygen isotopic variations in authigenic and detrital quartz in four samples of the Ordovician St. Peter Sandstone from the Michigan Basin and Wisconsin Arch, midwestern USA. Ion microprobe isotopic analyses have been successfully accomplished with an internal precision of +/-1 parts per thousand (1 sigma) and a spatial resolution of 20-30 mu m at low mass resolution using a high voltage offset technique. Repeated analyses of the quartz standard demonstrate a reproducibility of close to +/-1 parts per thousand (1 sd) in good agreement with that expected from counting statistics. The four ion probe samples were chosen as representative of thirteen samples that were mechanically and chemically disaggregated, sieved, and analysed as a function of sieve size by conventional bulk-mineral techniques. Conventional and ion microprobe analyses are mutually consistent, supporting the accuracy of the ion microprobe analyses. Within-sample isotopic variations of up to 13 parts per thousand and micro-scale isotopic variations of at least 4 parts per thousand over a distance of 100 mu m have been measured within quartz overgrowths in a sandstone from the Wisconsin Arch. Overgrowths are uniformly higher in delta(18)O than detrital grains, and gradients of up to 25% exist across a few microns. O-18-enriched quartz overgrowths in sandstones from the Wisconsin Arch show complex CL zonation and reflect one of two possible processes: (1) low-temperature quartz precipitation during mixing of meteoric waters with upwelling basinal fluids; (2) higher temperature quartz precipitation during episodic gravity-driven upwelling of warm basinal fluids (of comparable isotopic composition to Michigan Basin fluids) from the Illinois Basin, related to evolution of Mississippi Valley type Pb-Zn ore-forming fluids. Quartz overgrowths in Michigan Basin sandstones, which derived their silica locally by pressure solution, are thought to have precipitated continuously over a range of temperatures from hot basinal fluids of restricted isotopic composition and circulation. Detrital quartz shows significant intra-grain and intergrain isotopic variation (5-13 parts per thousand) but no significant inter-sample variation, consistent with a uniform source of granitoid and metamorphic quartz in space and time.