Comment on “On the recent bimodal magmatic processes and their rates in the Torfajökull–Veidivötn area, Iceland” by G.F. Zellmer, K.H. Rubin, K. Grönvold, and Z. Jurado-Chichay

Zellmer et al. [Zellmer, G.F., Rubin, K.H., Grönvold, K. and Jurado-Chichay, Z., 2008, On the recent bimodal magmatic processes and their rates in the Torfajökull–Veidivötn area, Iceland. Earth and Planetary Science Letters, 269: 388–39] use 238U–230Th–226Ra data for whole rocks and mineral separates from recent Icelandic eruptions to construct internal isochrons, from which they calculate mineral ages. However, in contrast to other recent work using 230Th–226Ra data to calculate mineral ages, they do not correct for differences in partitioning behavior of radium and barium. Differences in equilibrium partitioning of these elements are expected based on experimental data and on theoretical calculations for the phases analyzed by Zellmer and coauthors (plagioclase, pyroxene, and magnetite). Therefore, either (1) the crystals and liquids are in chemical equilibrium, in which case the correction for Ra/Ba fractionation must be applied, or (2) the crystals are not in equilibrium with the liquids, in which case more information is required in order to interpret the model ages. As a result, the mineral ages presented by Zellmer et al. are not accurate. Concentrations of trace elements in their mineral separates are not consistent with equilibrium partitioning, but they may be consistent with mixing between different phases in the rocks. Therefore, the linear 230Th–226Ra–Ba data arrays presented by Zellmer and coauthors likely represent mixing arrays rather than true isochrons.