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

Abstract

A synthetic calcite crystal that exhibits surface-structure-controlled, differential incorporation of a trace element was examined by ion microprobe to determine whether the controls causing differential element incorporation have a corresponding influence on incorporation of oxygen isotopes. In contrast both to the behavior of trace elements and to several previous studies claiming surface structural control on isotopic fractionation, the synthetic calcite fails to show any such surface effect. These ion probe experiments also represent the first isotopic analyses of symmetrically nonequivalent vicinal faces that compose the flanks of growth spirals. Hence we establish that structurally nonequivalent growth steps, which also differ in growth step velocity, and occurring on a single crystal face, show no measurable difference in fractionation during low-temperature solution growth. Although our findings for a synthetic crystal differ from other reports for natural crystals that were based on different techniques and larger sample size, our results are consistent with the view that surface-site preferences during growth are not significantly sensitive to the slight mass and vibrational differences among light stable isotopes.