Ferric/ferrous ratios in 1984 Mauna Loa lavas: a contribution to understanding the oxidation state of Hawaiian magmas

Abstract

Ferric/ferrous ratios have been used to estimate the oxygen fugacity of lavas erupted in 1984 on Mauna Loa Volcano, Hawaii. Rapidly quenched lavas erupted close to vents are less oxidized than rapidly quenched lavas scooped from lava flows several kilometers away from the vents. These results demonstrate that sampling is of critical importance in determining the oxidation state of lava. The oxidation state of the vent lavas, below or at magnetite–wustite (MW), is significantly lower than that previously reported for Hawaiian lavas (~FMQ). Similarly, rapidly quenched lavas from the ongoing Kilauea eruption and Loihi seamount, all have oxygen fugacities that are close to MW and on the low side of the range previously reported for Hawaiian lavas. From this we conclude that the initial oxygen fugacity of parental Hawaiian magmas is close to MW, not FMQ, and that previous estimates of the oxidation state of Hawaiian lavas may have been too high. This implies that the plume source of these magmas is also at or below MW, but not as reduced as the mantle source of mid-ocean ridge basalts. Additionally, Mauna Loa lavas appear to be slightly more reduced than Kilauea or Loihi lavas, perhaps indicating heterogeneous oxidation within the Hawaiian plume.