THE ROLE OF OXYGEN FUGACITY IN THE PRECIPITATION OF FE-OXIDE IN AN ALKALI BASALT SYSTEM

The scarcity of compositions that fall between alkali basalt and trachyte on oceanic islands is known as the Daly gap. Mafic alkali basalt and felsic trachyte are abundant, while there is a near absence of intermediate compositions. Clague (1978), offered a fractionation model that presents an explanation for why oceanic island volcanic rocks should be bimodally distributed. Clague suggested that SiO2 does not vary directly as a function of increasing fractional crystallization. Instead, approximately halfway through the crystallization history, these systems undergo a rapid increase in SiO2 from 50 to 57% with only a small change in the total fractionation, around 15%. This rapid SiO2 increase could transform a fractionating alkali basalt into a trachyte so rapidly that the intermediate composition magmas (55 to 57% SiO2) never have an opportunity to erupt on the surface. It was also shown that the rapid increase of SiO2 corresponded to a rapid decrease in FeO. The onset of the FeO fractionation is most likely controlled by the crystallization of Fe-oxide which, in turn, is controlled by oxygen fugacity. The goal of this research is to experimentally test Clague’s model by melting and crystallizing charges of alkali basalt composition in a 1 atm furnace at consistently lower temperatures (increments of 20 C) keeping the oxygen fugacity fixed along Ni/NiO oxygen buffer. The composition of the glass and mineral phases in each experiment were analyzed using electron microprobe analysis. These data were used to quantify oxide percentages as a function of percent fractional crystallization in order to track the liquid line of descent of an oceanic island system under 1 atmosphere and Ni/NiO conditions. These results were then compared with those of a similar study conducted along the QFM oxygen buffer (Biesiada and Brophy, 2015) to see if oxygen fugacity significantly exerts a control on when Fe-oxide will start to precipitate. Within the constraints of the experiment (20 C intervals), we could not demonstrate that oxygen fugacity exerts a control on when Fe-oxide precipitates. This study does reconfirm that Fe-oxide precipitation at around 50% crystallization, as required by the model of Clague, is inconsistent with the suggested oxygen fugacities for oceanic island magmas and their mantle source.