Smith Lecture: Mass-dependent Triple Oxygen Isotope Variations in Sedimentary Rocks.
The recognition of 17O anomalies in the geologic record has been used to identify mass-independent mechanisms of isotopic fractionation associated with atmospheric processes such as photochemical reactions and to reach conclusions regarding changes in the partial pressures of O2 and CO2 in Earth’s atmosphere through time. Recent oxygen isotope measurements of terrestrial rocks and minerals reveal small but statistically significant deviations from a single mass-dependent fractionation line. These subtle anomalies may be explicable through differences in mass laws of isotopic fractionations among various equilibrium and kinetic mechanisms of isotopic fractionation. Here I present high-precision Delta17O measurements for sedimentary silica and oxides, including Archean and Phanerozoic cherts and banded iron formations. Negative Delta17O values from Phanerozoic cherts can be explained as a consequence of the mass laws of low-temperature precipitation reactions during growth of marine authigenic minerals. I hypothesize that subtly low Delta17O values may be common in terrestrial silicate minerals as a result of the mass laws of mass-dependent fractionations associated with mineral precipitation from seawater and possibly subsequent diagenetic and metamorphic isotopic exchange between minerals and fluids. If so, the Delta17O values of ancient rocks may be used to recognize materials that have remain closed with respect to oxygen isotopes since forming as low-temperature authigenic precipitates, meaning that Delta17O values may be used to recognize sedimentary silica and perhaps other minerals in the ancient geological record. More generally, this phenomenon will be important to the interpretation of high-precision measurements of Delta17O in carbonates and other minerals, which have been suggested as a target for studying the earth’s past water cycle.