Comments on a Paper by Jon. D. Woodhead Titled 'The Origin of Geochemical Variations in Mariana Lavas: a General Model for Petrogenesis in Intra-oceanic Island Arcs?'

A general model for andesite genesis in intra-oceanic arcs involvingmagnetite fractionation has been proposed. Such a model is notconsistent with the low abundance of cumulate magnetite in gabbroicxenoliths in the Marianas and other arcs, and with the generallylow normative magnetite contents of Marianas lavas. Fractionationmodels involving magnetite consistently overestimate incompatibleelement abundances, and underestimate compatible Cr abundance.Magnetite fraction ation models offer no ready means of explainingeither the ubiquitous evidence for mineralogic disequilibriain arc rocks of all types, or the consistent correlation ofFeO*/MgO ratio and K₂O content in Marianas andesites.