OXYGEN-ISOTOPE MICROANALYSES OF DIAGENETIC QUARTZ - POSSIBLE LOW-TEMPERATURE OCCLUSION OF PORES

Secondary ion mass spectrometry (SIMS) was used to obtain delta(18)O m easurements on similar to 20 mu m spots in authigenic and detrital qua rtz from Mississippian sandstones of the western Canadian sedimentary basin. Detrital quartz shows a large range in delta(18)O (from 5 to 25 parts per thousand) which overlaps the analyses of authigenic cements (20-34 parts per thousand). Combining these data with the burial hist ory and precipitation sequence of minerals indicates that meteoric wat er may have precipitated quartz at temperatures between 15 and 35 degr ees C over a period of 180 MA (300-120Ma). This nearly isothermal prec ipitation may have occluded pores in samples from the central portion of the basin, but quartz precipitation continued up similar to 75 degr ees C at the basin margin. Conventional bulk analyses of these samples gave delta(18)O values of 13.4 parts per thousand for detrital quartz and 19.8 parts per thousand for authigenic quartz. Models based on bu lk isotopic analyses can bias the interpretation of the timing of diag enetic quartz precipitation toward much higher temperatures. The SIMS measurements in diagenetic silicates can identify discrete episodes of precipitation allowing a more accurate interpretation for the timing of diagenetic events.