ION MICROPROBE STUDY OF OXYGEN ISOTOPIC COMPOSITIONS OF STRUCTURALLY NONEQUIVALENT GROWTH SURFACES ON SYNTHETIC CALCITE

A synthetic calcite crystal that exhibits surface-structure-controlled , differential incorporation of a trace element was examined by ion mi croprobe to determine whether the controls causing differential elemen t incorporation have a corresponding influence on incorporation of oxy gen isotopes. In contrast both to the behavior of trace elements and t o several previous studies claiming surface structural control on isot opic fractionation, the synthetic calcite fails to show any such surfa ce effect. These ion probe experiments also represent the first isotop ic analyses of symmetrically nonequivalent vicinal faces that compose the flanks of growth spirals. Hence we establish that structurally non equivalent growth steps, which also differ in growth step velocity, an d occurring on a single crystal face, show no measurable difference in fractionation during low-temperature solution growth. Although our fi ndings for a synthetic crystal differ from other reports for natural c rystals that were based on different techniques and larger sample size , our results are consistent with the view that surface-site preferenc es during growth are not significantly sensitive to the slight mass an d vibrational differences among light stable isotopes. Copyright (C) 1 997 Elsevier Science Ltd.