Disequilibrium carbon and oxygen isotope variations in natural calcite

Abstract

OXYGEN isotope fractionation in the calcite–water system is widely used to investigate the temperature, fluid composition and reaction rate of calcite precipitation, under the assumption that equilibrium is maintained during precipitation. Here I present analyses of coarsely crystalline, inorganic calcite displaying combined growth and sector zoning, which show this assumption to be untrue. Variations in the isotope composition of successive growth zones can be interpreted as due to changing conditions of precipitation, with equilibrium being maintained throughout. But variations across different sectors of the same synchronous growth surface (∼2‰ for δ¹³C and ∼0.9‰ for δ¹⁸O) record disequilibrium in the same crystal. This type of disequilibrium may be common, as calcite precipitates in a great variety of crystal form combinations; moreover, many noncarbonate minerals also grow as crystals with two or more crystallographic forms. Indeed, Boyd et al.¹ reported fractionation of nitrogen isotopes between cubic and octahedral sectors of a synthetic diamond, but I believe that crystallographi-cally controlled isotope fractionation of stoichiometric ions has not hitherto been recognized.