Iron-titanium oxides and oxygen fugacities in volcanic rocks

Abstract

It is shown that in silicate liquids the ferric-ferrous equilibrium is controlled by temperature, oxygen fugacity, and the composition of the liquid, particularly its alkali content. Thus, if the iron-titanium oxide minerals that precipitate from a silicate liquid reflect the ferric-ferrous equilibrium, the oxygen geobarometer of Buddington and Lindsley will have to be calibrated, especially for such volcanics as phonolites and pantellerites, which are rich in alkali. Estimates of oxygen fugacities for average basalts show that they probably crystallize through a more restricted range of oxygen fugacity than acid volcanics. Alkali gabbros with pseudobrookite, for which analytical data is given, have crystallized at relatively high oxygen fugacities, as have the volcanics (and kimberlites) that contain perovskite and an oxide phase. The calculated oxygen fugacities of sphene-bearing acid rocks approximate the values derived from the coexisting oxide equilibration data and indicate higher values than pyroxeneor olivine-bearing acid volcanics. The data on oxygen fugacity and crystallization temperature for basaltic liquids have been used to predict the composition of their coexisting oxide minerals; accordingly, basalts with Curie temperatures significantly in excess of approximately 200°C are considered to have been subsequently oxidized. Unless the temperature at which this later oxidation occurs is known, thermoremanent magnetization (TRM) cannot be demonstrated to exist in basalts. TRM can only be mineralogically demonstrated in the basaltic rocks that contain an orthorhombic oxide phase or its breakdown products.