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Terrestrial Oxygen Isotope Variations and Their Implications for Planetary Lithospheres

Washington University St. Louis, Missouri 63130, U.S.A., criss{at}wustl.edu

Oxygen isotopes provide key data on the formation and evolution of the Solar System, particularly for Earth and other rocky bodies that are principally constituted of oxygen-rich phases. The dynamic interactions between Earth’s lithosphere, hydrosphere, biosphere and atmosphere are elucidated by a wealth of oxygen isotope data, that in turn provide a template for predicting the types of variations that occur on other planets. These isotopic variations are basically controlled by two master effects. First, isotopic fractionations among coexisting phases tend to be greatest at low temperatures, and are also large when they involve interactions between solids or liquids and gaseous phases. Second, material balance effects require the ¹⁸O contents of the most abundant phases to be close to the planetary average, and restrict extreme isotopic enrichments or depletions to minor phases. These dual controls of fractionation and material balance work in concert with practically any other dynamic process to foster the upward enrichment and increased heterogeneity of ¹⁸O contents on Earth, and, by implication, in planetary lithospheres in general. Upward ¹⁸O enrichment in planetary lithospheres is promoted by the tendency for the least dense silicate and oxide minerals to concentrate ¹⁸O relative to more dense minerals, and also by convective transfer, fractional crystallization, hydrothermal alteration, and other processes. Earth’s continents and the uppermost parts of its ocean floor consequently have higher ¹⁸O contents than average upper mantle material, and normal sediments, carbon dioxide and oxygen gas are all very rich in ¹⁸O. Heterogeneity is evident in the large ranges of ¹⁸O values observed in sediments (at least +8 to +38) and terrestrial igneous rocks (at least –10.5 to +16), and in the even larger ranges in natural waters and atmospheric gases, all representing materials that constitute, or interacted with, oxygen reservoirs near Earth’s surface. Key identified processes include weathering, precipitation from water, phase changes, exchange with infiltrating hydrothermal fluids, and assimilation of wallrocks having disparate compositions. All such processes that promote the upward enrichment and heterogeneity of ¹⁸O complicate the deduction of the bulk planetary contents of this isotope.