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 sands tones provide a monitor of the temperatures, compositions, and origins of pore-occluding fluids during diagenesis, but quartz overgrowths ar e too fine-grained to be amenable to conventional isotopic analysis. W e have used a Cameca ims-4f ion microprobe to determine oxygen isotopi c variations in authigenic and detrital quartz in four samples of the Ordovician St. Peter Sandstone from the Michigan Basin and Wisconsin A rch, midwestern USA. Ion microprobe isotopic analyses have been succes sfully accomplished with an internal precision of +/-1 parts per thous and (1 sigma) and a spatial resolution of 20-30 mu m at low mass resol ution using a high voltage offset technique. Repeated analyses of the quartz standard demonstrate a reproducibility of close to +/-1 parts p er thousand (1 sd) in good agreement with that expected from counting statistics. The four ion probe samples were chosen as representative o f thirteen samples that were mechanically and chemically disaggregated , sieved, and analysed as a function of sieve size by conventional bul k-mineral techniques. Conventional and ion microprobe analyses are mut ually consistent, supporting the accuracy of the ion microprobe analys es. Within-sample isotopic variations of up to 13 parts per thousand a nd micro-scale isotopic variations of at least 4 parts per thousand ov er a distance of 100 mu m have been measured within quartz overgrowths in a sandstone from the Wisconsin Arch. Overgrowths are uniformly hig her in delta(18)O than detrital grains, and gradients of up to 25% exi st across a few microns. O-18-enriched quartz overgrowths in sandstone s from the Wisconsin Arch show complex CL zonation and reflect one of two possible processes: (1) low-temperature quartz precipitation durin g mixing of meteoric waters with upwelling basinal fluids; (2) higher temperature quartz precipitation during episodic gravity-driven upwell ing of warm basinal fluids (of comparable isotopic composition to Mich igan Basin fluids) from the Illinois Basin, related to evolution of Mi ssissippi Valley type Pb-Zn ore-forming fluids. Quartz overgrowths in Michigan Basin sandstones, which derived their silica locally by press ure solution, are thought to have precipitated continuously over a ran ge of temperatures from hot basinal fluids of restricted isotopic comp osition and circulation. Detrital quartz shows significant intra-grain and intergrain isotopic variation (5-13 parts per thousand) but no si gnificant inter-sample variation, consistent with a uniform source of granitoid and metamorphic quartz in space and time.