Vanadium magnetite–melt oxybarometry of natural, silicic magmas: a comparison of various oxybarometers and thermometers

Abstract

To test a recently developed oxybarometer for silicic magmas based on partitioning of vanadium between magnetite and silicate melt, a comprehensive oxybarometry and thermometry study on 22 natural rhyolites to dacites was conducted. Investigated samples were either vitrophyres or holocrystalline rocks in which part of the mineral and melt assemblage was preserved only as inclusions within phenocrysts. Utilized methods include vanadium magnetite–melt oxybarometry, Fe–Ti oxide thermometry and -oxybarometry, zircon saturation thermometry, and two-feldspar thermometry, with all analyses conducted by laser-ablation ICP–MS. Based on the number of analyses, the reproducibility of the results and the certainty of contemporaneity of the analyzed minerals and silicate melts the samples were grouped into three classes of reliability. In the most reliable (n = 5) and medium reliable (n = 10) samples, all fO2 values determined via vanadium magnetite–melt oxybarometry agree within 0.5 log units with the fO2 values determined via Fe–Ti oxide oxybarometry, except for two samples of the medium reliable group. In the least reliable samples (n = 7), most of which show evidence for magma mixing, calculated fO2 values agree within 0.75 log units. Comparison of three different thermometers reveals that temperatures obtained via zircon saturation thermometry agree within the limits of uncertainty with those obtained via two-feldspar thermometry in most cases, whereas temperatures obtained via Fe–Ti oxide thermometry commonly deviate by ≥50 °C due to large uncertainties associated with the Fe–Ti oxide model at T-fO2 conditions typical of most silicic magmas. Another outcome of this study is that magma mixing is a common but easily overlooked phenomenon in silicic volcanic rocks, which means that great care has to be taken in the application and interpretation of thermometers and oxybarometers.