Oxygen Isotope Behavior in the Sulfate-Water System [antikvár]

Journal of Geophysical Research Vol. 73, No. 18, September IS, 1968 Oxygen Isotope Behavior in the Sulfate-Water System' R. M. Lloyd Shell Development Company (A Division of Shell Oil Company) Exploration and Production Research Center, Houston, Texas 77001 Fundamental data on oxygen isotope behavior in the sulfate-water system are given. Oxygen exchange rates between sulfate ions and water are very slow and show a marked pH dependence. Of the order of 10° to 10® years are required to closely approach isotopic equilibrium at earth surface conditions. Oxygen isotope fractionation experiments yielded the following relationships : In a = 3878/(r2) _ 0.0034 In a = 3251/(i'2) - 0.0056 Anhydrite water Dissolved sulfate water During progressive evaporation of saturated CaSOi solution the gypsum formed tends to concentrate 0" relative to dissolved sulfate by 2%o. Sulfate reducing bacteria preferentially metabohze 0" by about 4.6%o in both pure and natural cultures. Oxidation of sulfide to sulfate in natural environments is a complex process making use of both water oxygen and molecular oxygen. Introduction The oxygen isotope method is proving to be one of the most useful tools for studying the enviromnents of interactions of rocks, minerals, and water over a wide range of geological conditions. The pioneer paleotemperature work of Urey et al. [1951], which considered primarily biological precipitation of calcium carbonate in oceanic environments, has been expanded over the years to include other mineral systems and to encompass a much broader range of environmental conditions. One of the common rock-forming mineral groups which has received surprisingly little attention in this expanded effort is the sulfate group. Until very recently the only geochemically oriented study of oxygen isotopes in sulfates was that of Teis [1957]. This study included a few experiments that demonstrated a very slow rate of exchange between sulfate oxygen and water plus a few isotopic analyses of some naturally occurring sulfates. Because sulfates are important primary and secondary minerals in sedimentary rocks and are also found in the igneous and metamorphic rock systems, it is appropriate that we should learn more of the fundamental oxygen isotope be- havior of these minerals. This report will present fundamental data on the rates of oxygen isotope exchange between sulfates and water and on the equilibrium fractionation factors for anhydrite water and dissolved sulfate water. In addition, we will describe some experiments and natural observations illustrating kinetic fractionation effects on oxygen isotopes in the sulfur cycle in nature. Method of Analysis ' EPR Publication No. 512. To acquire oxygen isotope data on sulfates comparable to those obtained on other minerals, it was necessary to devise a method that would convert the oxygen of the sulfate to carbon dioxide with a 100% yield. The method selected for this study was one adapted from the carbon combustion method described by Clayton and Epstein [1958]. Essentially the method consists of grinding the sulfate sample with a tenfold excess of spectrographic graphite, forming the mixture into a pellet, and heating the pellet to 1000°C in an evacuated system. The gases evolved consist of a mixture of carbon dioxide and carbon monoxide. The gas mixture is passed through a nickel catalyst furnance where the carbon monoxide is converted to carbon dioxide and the total carbon dioxide sample collected by freezing in liquid nitrogen. The heating of the 6099 ' I'