Grocke, Stephanie B., Cottrell, Elizabeth, de Silva, Shanaka, and Kelley, Katherine A. 2016. "The role of crustal and eruptive processes versus source variations in controlling the oxidation state of iron in Central Andean magmas." Earth and Planetary Science Letters, 440 92–104. https://doi.org/10.1016/j.epsl.2016.01.026.
The composition of the continental crust is closely tied to subduction zone magmatism. Elevated oxygen fugacity ( f O 2 ) 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 f O 2 . The importance of the subducting slab and mantle wedge relative to the effects of this differentiation on the f O 2 recorded by continental arc magmas remains relatively unconstrained. Here, we focus on the effect of differentiation on magmatic f O 2 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 f O 2 recorded by lavas, pumice, and scoria: (1) whole rock Fe 3 / S Fe ratios, (2) Fe 3 / S Fe ratios in quartz-hosted melt inclusions, and (3) Fe–Ti oxide oxygen-barometry. Comparison of the f O 2 calculated from bulk Fe 3 / S Fe ratios (post-eruptive) with that derived from Fe–Ti oxides or melt inclusion Fe 3 / S Fe 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 f O 2 using a strict criterion developed here. 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 f O 2 , spanning the f O 2 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 f O 2 . We conclude that the slab/mantle source can exert greater control on magmatic f O 2 than processes occurring in even the thickest continental crust. Thus, the f O 2 of arc magmas, and hence their calc-alkaline nature, may be inherited from the mantle.
