Pergamon Geochimicaet Cosmochimica Acta,Vol. 61, No, 23, pp. 5057-5063, 1997 Copyright© 1997 Elsevier ScienceLtd Printed in the USA. All rights reserved 0016-7037/97 $17.00 + .00 PII S0016-7037(97) 00309-8 Ion microprobe study of oxygen isotopic compositions of structurally nonequivalent growth surfaces on synthetic calcite RICHARD J. REEDER,1 JOHN W. VALLEY, 2 COLIN M. GRAHAM, 3 and JOHN M. EILER 4 ~Department of Geosciences, State University of New York at Stony Brook, Stony Brook, New York 11794-2100, USA 2Departrnent of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA 3Departrnent of Geology and Geophysics, University of Edinburgh, Edinburgh EH9 3JW, UK 4Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA (Received June 14, 1997; accepted in revised form August 19, 1997) 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. Copy- right © 1997 Elsevier Science Ltd 1. INTRODUCTION It is now widely appreciated that mineral surfaces play a fundamental role in controlling the distribution of trace and minor elements between fluid and solid during crystalliza- tion. One of the best examples is the crystal face-specific incorporation of elements that results in sector zoning (Naka- mura,1973; Dowty, 1976), a distinctive feature found in crystals forming in igneous, metamorphic, and sedimentary environments alike. An important related example is the dif- ferential incorporation of trace elements between structurally nonequivalent growth steps on an individual face of a crystal, which results in so-called intrasectoral zoning (Paquette and Reeder, 1995 ). There has recently been much interest in the possibility that light stable isotope distributions might be similarly affected during growth, resulting in face-specific fractionation and isotopic sector zoning (e.g., Boyd et al., 1988; Dickson, 1991, 1996, 1997; Klemm et al., 1990, 1991, 1992; Onasch and Vennemann, 1995; Reeder, 1991; Klein and Lohmann, 1995, 1996). The implications of a surface- controlled influence on incorporation of different isotopes of an element would be profound and, in particular, would question the viability of some interpretations based on equi- librium fractionation. Several studies of light stable isotope distributions among growth sectors of minerals have raised suspicions that sur- face processes, which differ among nonequivalent faces of a crystal, may indeed cause differential incorporation of iso- topes. Boyd et al. (1988) examined the 615N isotopic compo- sitions in the growth sectors formed under the cubic and octahedral faces of a synthetic diamond crystal and showed an enrichment of ~45%o in the cubic sector relative to the octahedral sector. The nitrogen occurs as a trace element in the diamond, and the cubic and octahedral growth sectors 5057 were shown to contain different nitrogen concentrations (i.e., compositional sector zoning). An explanation for the isoto- pic differences between the sectors in that study remains uncertain and is partly complicated by the difference in nitro- gen contents in the different sectors and the lack of knowl- edge of the growth conditions. Klemm et al. ( 1990, 1991, 1992) examined ~sO/160 ratios in nonequivalent growth sectors of natural amethyst and smoky quartz crystals. They reported slight 61sO differences among the (0]11), (01]1), and (0110) sectors, ranging from 0.2 to 0.4%0. They considered the origin of the isotopic differences in relation to differing growth rates of the faces. Other observations that have generated much interest are those of Dickson (1991), who examined 6180 and 6~3C values of growth sectors and of faces of natural calcite crys- tals. Using conventional microsampling methods, including scraped material from crystal faces, Dickson ( 1991 ) reported differences in 613C values between nonequivalent faces/sec- tors ranging from 0.5 to 2%0 and 6~sO differences up to 0.9%0. Dickson (1991) argued that these differences re- flected face-dependent incorporation of isotopes and, there- fore, nonequilibrium fractionation. Although deviations from equilibrium fractionation due to kinetic influences have long been known (e.g., McCrea, 1950; Turner, 1982), such fac- tors are not expected to exert an influence under the growth conditions that typically result in well-formed single crystals, such as those studied by Dickson (1991). Consequently, the possibility of surface-controlled, nonequilibrium fraction- ation of carbon and oxygen isotopes into sedimentary car- bonate minerals, similar to the surface-controlled trace ele- ment incorporation, has raised concern about the many important applications that are based on equilibrium fraction- ation (cf. Reeder, 1991 ).