AN ION MICROPROBE STUDY OF DIAGENETIC CARBONATES IN THE DEVONIAN NISKU FORMATION OF ALBERTA, CANADA

Diagenetic calcites and dolomites of the Upper Devonian Nisku Formatio n in central Alberta, Canada, were analyzed for trace elements by ion microprobe. The ion microprobe permits analyses on sample spots compar able in size (< 20 mum) to those of conventional electron probes. Dete ction limits on the ion microprobe are < 1 ppm for many trace elements of interest. Trace element data obtained by ion microprobe are compar able to those obtained by ICP. Trace element variations in and between Upper Devonian diagenetic calcites from the Nisku Formation aid in th e interpretation of the fluid composition during precipitation and/or recrystallization, as well as in the interpretation of primary carbona te mineralogy (e.g., high-Mg calcite vs. aragonite). Using the ion pro be, microenvironments as well as temporal variations in diagenetic flu id composition can be resolved. The presence of significant trace elem ent variations in Nisku carbonates on a scale of < 100 mum indicates t hat fluid-rock interaction during burial of up to 4000 m and during mo re than 300 m.y. was very limited. Although several of the measured tr ends probably reflect incomplete retention of primary compositional va riations, trace element variations in some Paleozoic limestones and do lostones reflect variations in fluid composition during ''primary'' pr ecipitation (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 carbonat es.