Relationship Between O¹⁸/O¹⁶ Ratios in Coexisting Carbonates, Cherts, and Diatomites

Abstract

The O¹⁸/O¹⁶ ratios in sedimentary carbonates have been studied by several authors. The effect of temperature and environment on the isotopic composition of oxygen is relatively well known (Urey et al., 1951; Baertschi, 1951; Urey, 1947; Clayton and Degens, 1958). Information is scarce regarding the behavior of oxygen isotopes in silicates, chemically precipitated in the process of sedimentation or supergene diagenesis. Two samples are reported by Silverman (1951) and two by Clayton and Epstein (1958) which indicate an enrichment of O¹⁸ in cherts in marine beds and in marine diatomites relative to quartz of igneous or metamorphic origin. An understanding of the processes which result in certain isotopic compositions of coexisting cherts and carbonates should contribute to the clarification of the genesis and history of cherts. This approach was deemed desirable since considerable information is available about the O¹⁸/O¹⁶ and C¹³/C¹² ratios in both organically precipitated and consolidated limestones (Craig, 1953; Lowenstam and Epstein, 1957), and such data about the limestones should serve as a basis for comparison of the O¹⁸/O¹⁶ ratio of coexisting cherts as they respond to similar environment. Previously it was shown that the O¹⁸/O¹⁶ ratios of quartz in hydrothermal zones reflect environmental conditions of the formation of the quartz (Clayton and Epstein, 1958). Silica precipitated in the oceans results in more complicated products. The relation between the hydrated form of SiO₂ and subsequently recrystallized quartz is a problem of interest in many aspects of sedimentary petrology. Among these is the possibility of using the O¹⁸/O¹⁶ ratio of siliceous fossils as a possible temperature indicator very much like the O¹⁸/O¹⁶ ratios of carbonate fossils have been used (Urey et al., 1951; Epstein et al., 1951). A series of 35 samples of coexisting limestones and cherts or diatomites of different environments and ages was analyzed for the O¹⁸/O¹⁶ ratios. The factors associated with the precipitation of carbonates are relatively well known. We know less about the chemistry associated with the precipitation of silica. Silica is present today in normal marine and fresh-water environments in true solution and below saturation for amorphous silica but supersaturated with respect to quartz (Krauskopf, 1956; Siever, 1957; Iler, 1955). Nevertheless a direct precipitation of quartz in the immediate process of sedimentation is nowhere observed and time is probably a critical factor. The bulk of silica, which is continuously liberated from the rocks during weathering processes and transported into rivers, lakes, and oceans is removed from the water system partly by the action of organisms, which lay down a skeleton of amorphous silica (i.e., diatomites, radiolaria, sponges), and partly by formation of authigenic silicates such as clay minerals or feldspars. There are several hypotheses about the origin of cherts. The most likely one involves amorphous silica, which is extracted from the water system by silica-secreting organisms, eventually becomes deposited, and is diagenetically mobilized, migrating in soluble form in the direction of a concentration gradient to reprecipitate as quartz and accumulate at points of least solubility (Bramlette, 1946). This reorganization may occur shortly after sedimentation.