On the Distribution and Role of Oxygen in the Geochemistry of the Earth's Crust

Abstract Literature relating to geochemistry of oxygen for the period of 1955 to 1966 is reviewed and recent progress in geochemical research in this field is discussed. The importance of oxygen as the major constituent of the Earth's siliceous crust and mantle is re-emphasized. Recent studies on the effect of the fugacity of this element in magmas and the isotopic fractionation of O 16 and O 18 in igneous rocks show that the path of fractional differentiation of a magma is greatly affected by varying influence of this element. Silica-rich residual melts of the calc-alkaline series type, common in orogenic belts, may result when relatively high oxygen fugacity causes magnetite to crystallize early with the ferromagnesian minerals. Iron-rich residual melts, of the Skaergaard type in Greenland, may form at lower oxygen activities through crystallization of Mg, Fe-olivine, which causes a relative increase of iron toward the residual end. Oxygen as the main anion or electron acceptor in the early stages of magmatic differentiation may be the cause of the enrichment of the light major elements in the Earth's crust. The concentration of odd-numbered major elements Al, Na and K is so explained. All the light major elements have a high affinity for oxygen. Isotope fractionation studies of oxygen-18 and -16 have greatly contributed to our ability to study and interpret the complex history of magmas and metamorphic rocks. It has been found that minerals group in relation to their tendency to retain and thus concentrate the heavy isotope O 18 . This tendency increases with the growing complexity of the silicate framework. Measured equilibria of oxygen isotopes vary according to the conditions during the formation of a rock, thus enabling the geochemist to indicate the processes through which the rock has been passing during its formation. In the study of granites, for example, the wide scatter of data measured may speak of a metamorphic origin, whereas equilibria corresponding to igneous rocks may indicate igneous derivation or a thorough homogenization due to remelting of a sediment. Paleotemperature studies become possible when we know the equilibria of O 18 /O 16 at certain temperatures. Oxygen fugacities in metamorphic processes and the formation of magnetite and hematite are of major importance when we try to explain the genesis of the Precambrian iron deposits. Due to a possible buffer-effect of Fe 2 O 3 present in these systems and sometimes due to the presence of C and S, no simple picture can be obtained, but a satisfactory explanation of the formation of iron oxides from siderite-bearing muds without the necessity of introducing oxygen is indicated. In the formation of planets through the accretion of solar nebula dust particles the relative abundance of oxygen and hydrogen may even influence the final structure of the planet and the spherical distribution of major elements, especially iron. This because the oxidation-reduction equilibria in metal-oxide-silicate mixtures are very sensitive over a relatively narrow temperature range of about 100° to 850°C. Reporting and calculation of rock analyses in cationic percentages is strongly recommended. This has become practically feasible due to the recent developments in fast-neutron activation analysis for oxygen in rocks, minerals and meteorites. This is due to methods of accurate oxygen analysis developed in the past 5 years. Studies of oxygen- and other elemental-gradients in rocks formed at different pressures are proposed.