Redox in Magmas: Comment on a Recent Treatment of the Kaiserstuhl Volcanics (Braunger et al., Journal of Petrology, 59, 1731-1762, 2018) and Some Other Misconceptions

Michael Anenburg, Hugh St C. O'Neill

Research output: Contribution to journalComment / DebateOtherpeer-review

15 Citations (Scopus)

Abstract

The recent contribution by Braunger et al. (2018) provides a thorough petrographic and geochemical description of peralkaline and carbonatitic igneous rocks from the Kaiserstuhl Volcanic Complex, SW Germany (KVC). These data are used to calculate several intensive variables: temperature (T), pressure (P), silica activity (aSiO2), and oxygen fugacity (fO2), which are then applied to constrain the petrogenesis of the different KVC magma series, including identifying features of their source with an emphasis on redox conditions. Using a variety of thermodynamic equilibria, Braunger et al. (2018) showed that the redox states of the various silicate KVC magmas in intensive-variable space follow T–fO2 paths that are 1 to 3 log-bar units more oxidised than the fayalite–magnetite–quartz oxygen buffer (FMQ), denoted as ΔFMQ +1 to +3. These intensive-variable redox paths are consistent with previous work on other examples of the peralkaline-carbonatitic association (e.g. Marks et al., 2008, Zaitsev et al., 2012). However, Braunger et al. (2018) then introduce a novel variation to the treatment of relative oxygen fugacities in magmas, in which they present their results ‘relative to aSiO2-corrected buffer curves, expressed as ΔFMQ*’. The seemingly high values of this latter variable are caused by low aSiO2 rather than unusually high fO2, which invalidates their inference that ‘oxidizing and carbonated fluids/melts interacting with the mantle lithosphere is a major prerequesite for the subsequent genesis of mixed alkaline silicate rock-carbonatite complexes’. The carbonatites themselves are identified by Braunger et al. (2018) as even more oxidised, because they have ΔFMQ* ≈ 6, even though they equilibrated at an entirely unexceptional fO2 for igneous rocks (ΔFMQ = ±2). Braunger et al. (2018) interpret the strongly positive ΔFMQ* as evidence for a particularly oxidized mantle source. Here we argue that the concept of ΔFMQ* does not support any of these inferences; indeed it is unlikely ever to support anything, being physically meaningless. We also take the opportunity to reiterate that to relate a chemical potential in a magma such as fO2 to its value in the source is not a simple exercise, but requires a great deal of petrological modelling of the relevant compositional variables. Even where such modelling is feasible, it rests on assumptions that present petrologic knowledge may be insufficient to validate.
Original languageEnglish
Pages (from-to)1825-1832
Number of pages8
JournalJournal of Petrology
Volume60
Issue number9
DOIs
Publication statusPublished - 1 Sept 2019
Externally publishedYes

Cite this