The role of crustal and eruptive processes versus source variations in controlling the oxidation state of iron in Central Andean magmas

To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/earth-and-planetary-science-lettersThe composition of the continental crust is closely tied to subduction zone magmatism. Elevated oxygen fugacity (fO₂) plays a central role in fostering crystallization of oxide minerals and thereby aids in generating the calc-alkaline trend of iron depletion that characterizes the continents. Along continental margins, arc magmas erupt through continental crust and often undergo extensive differentiation that may modify magmatic fO₂. The importance of the subducting slab and mantle wedge relative to the effects of this differentiation on the fO₂ recorded by continental arc magmas remains relatively unconstrained. Here, we focus on the effect of differentiation on magmatic fO₂ using a suite of 14 samples from the Central Volcanic Zone (CVZ) of the Andes where the continental crust is atypically thick (60–80 km). The samples range in composition from ∼55 to 74 wt% SiO₂ and represent the Neogene history of the arc. Samples are basaltic andesite to rhyolite and span a range of radiogenic isotopic compositions (⁸⁷Sr/⁸⁶Sr = ∼0.705–0.712) that represent 30 to 100% crustal assimilation. We use several proxies to estimate the fO₂ recorded by lavas, pumice, and scoria: (1) whole rock Fe³⁺/ΣFe ratios, (2) Fe³⁺+/ΣFe ratios in quartz-hosted melt inclusions, and (3) Fe–Ti oxide oxygen-barometry. Comparison of the fO₂ calculated from bulk Fe³⁺/ΣFe ratios (post-eruptive) with that derived from Fe–Ti oxides or melt inclusion Fe³⁺/ΣFe ratios (pre-eruptive), enables us to quantify the effect of syn- or post-eruptive alteration, and to select rocks for bulk analysis appropriate for the determination of pre-eruptive magmatic fO₂ using a strict criterion developed here.\ud \ud Across our sample suite, and in context with samples from the literature, we do not find evidence for systematic oxidation due to crystal fractionation or crustal contamination. Less evolved samples, ranging from 55 to 61 wt% SiO₂, record a range of >3 orders of magnitude in fO₂, spanning the fO₂ range recorded by all samples in our suite. Among these less evolved magmas, we find that those erupted from volcanic centers located closer to the trench, closer to the Benioff Zone, and with more geochemical evidence of subducted components in the mantle source (elevated La/Nb) result in magmas that record systematically higher fO₂. We conclude that the slab/mantle source can exert greater control on magmatic fO₂ than processes occurring in even the thickest continental crust. Thus, the fO₂ of arc magmas, and hence their calc-alkaline nature, may be inherited from the mantle