Oxygen and Hydrogen Isotope Ratios in Sedimentary Rocks and Minerals

Abstract

O18/O16 and D/H measurements were made on a variety of sedimentary rocks and minerals, with special emphasis on the fine-grained, predominantly clay mineral fraction of the sediments. The interlayer water was removed from clay minerals prior to the isotopic analyses. Precision of the measurements was generally ± 0.2 permil for oxygen and ± 0.3 percent for hydrogen. In the case of zeolites the reproducibility was less constant because of difficulty encountered in removing zeolitic water. Listed in order of decreasing tendency to concentrate O18, the sedimentary minerals for which such data are available and their approximate isotopic fractionation factors relative to water at sedimentary temperatures are: [See abstract for table]. Mineral - water fractionations are related to the types of chemical bonds in the minerals. Clay minerals which form under sedimentary conditions appear to form in isotopic equilibrium with their environments. Subsequent isotopic exchange may occur easily at elevated temperatures, but only extremely slowly at low temperatures. Ocean sediments which are detrital in origin have isotopic compositions reflecting the fresh water origin of their clay minerals. There is no evidence for any isotopic exchange of detrital clay minerals with the marine environment in any of the ocean cores studied. (The sediments could be as old as 250,000 years as determined by ionium-thorium method.) Marine sediments which have large authigenic components are generally enriched in O18, reflecting the presence of phillipsite (δ O18 = +34 permil and montmorillonite δ O18 = +29 permil)e However, those containing a high concentration of iron and manganese oxide may be isotopically light. (A manganese nodule had δ O18 = +15 permil.) The isotopic compositions of the clay minerals of ocean sediments are compatible with a weathering origin for a kaolinite and a metamorphic or diagenetic origin for chlorite. Montmorillonite and illite are primarily of weathering origin but both of these minerals may have diagenetic components. There is no evidence for any isotopic exchange of sand-sized quartz or feldspar in the sedimentary environment and these minerals may be used as indicators of provenance in sediments. The presence of a large authigenic feldspar or quartz component may be readily detected by oxygen isotopic analysis. An authigenic feldspar had an O18/O16 ratio ten permil greater than igneous feldspars. Calculations have been performed to determine the effect of weathering and the formation of sediments on the isotopic composition of the hydrosphere. It is concluded that those processes could have depleted the hydrosphere 3 permil in O18 and enriched it 0.03 percent in deuterium through the course of geologic time.