Diagenetic calcites and dolomites of the Upper Devonian Nisku Formation in central Alberta, Canada, were analyzed for trace elements by ion microprobe. The ion microprobe permits analyses on sample spots comparable in size (< 20 mu m) to those of conventional electron probes. Detection limits on the ion microprobe are < 1 ppm for many trace elements of interest. Trace element data obtained by ion microprobe are comparable to those obtained by ICP. Trace element variations in and between Upper Devonian diagenetic calcites from the Nisku Formation aid in the interpretation of the fluid composition during precipitation and/or recrystallization, as well as in the interpretation of primary carbonate mineralogy (e.g., high-Mg calcite vs. aragonite). Using the ion probe, microenvironments as well as temporal variations in diagenetic fluid composition can be resolved. The presence of significant trace element variations in Nisku carbonates on a scale of < 100 mu m indicates that fluid-rock interaction during burial of up to 4000 m and during more than 300 m.y. was very limited. Although several of the measured trends probably reflect incomplete retention of primary compositional variations, trace element variations in some Paleozoic limestones and dolostones reflect variations in fluid composition during "primary" precipitation (as opposed to fluid compositions during recrystallization). These findings encourage the use of trace element compositions for diagenetic interpretations even in very old and deeply buried carbonates.