FeTiMM – A new oxybarometer for mafic to felsic magmas

doi: 10.7185/geochemlet.1740 The oxidation state of magmas is a key parameter that is notoriously difficult to reconstruct. The most common approach is via magnetite–ilmenite oxybarometry. However, many natural magmas do not contain ilmenite, preventing application of this technique. Here we present a new method that allows fO2 to be reconstructed based on the partitioning of Fe and Ti between magnetite and silicate melt. The new method, which we call FeTiMM, is applicable to both ilmenite-free and ilmenite-bearing samples, and even to slowly cooled intrusive rocks such as granites. FeTiMM was calibrated on 109 experiments covering a wide range of oxygen fugacities, temperatures, pressures and silicate melts ranging from basaltic to rhyolitic composition, and returned fO2 values that agree within 0.5 log units with independently constrained fO2 values in all but five cases. A first test on 19 natural samples of dacitic to rhyolitic compositions was equally successful. FeTiMM thus opens the door for numerous new applications in various disciplines of Earth Sciences, including the fields of volcanology, igneous petrology, experimental geochemistry and ore geology. Received 26 July 2017 | Accepted 18 September 2017 | Published 25 October 2017 1. Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany * Corresponding authors (email: robert.arato@uni-bayreuth.de; andreas.audetat@uni-bayreuth.de) Introduction Oxygen fugacity is an important thermodynamic parameter in magmatic systems because it exerts a first order control on phase equilibria as well as on mineral–melt and fluid–melt partition coefficients. The most commonly used and most reliable way to reconstruct magmatic fO2 is via magnetite–ilmenite oxybarometry (e.g., Buddington and Lindsley, 1964; Carmichael, 1967; Stormer, 1983; Andersen and Lindsley, 1988; Ghiorso and Sack, 1991; Lattard et al., 2005; Ghiorso and Evans, 2008). Alternative approaches are based on (i) the Fe2+/Fe3+ ratio of whole rocks (Kress and Carmichael, 1989; Putirka, 2016), (ii) mineral reactions involving olivine, pyroxene, and/or sphene (Frost and Lindsley, 1992; Lindsley and Frost, 1992; Andersen et al., 1993; Xirouchakis et al., 2001), (iii) biotite, K-feldspar and magnetite (Wones and Eugster, 1965; Wones, 1981) , (iv) zircon Ce (and Eu) anomalies (Ballard et al., 2002; Trail et al., 2012; Smythe and Brenan, 2016), and (v) single amphibole oxybarometry (Ridolfi et al., 2010). However, despite these various approaches, reconstruction of magmatic fO2 in igneous rocks remains difficult, particularly in the case of intrusives, because during slow cooling Fe-Ti-oxide minerals usually get either reset or altered. Furthermore, many magmas do not contain ilmenite but only magnetite (Frost and Lindsley, 1991), precluding the application of magnetite–ilmenite oxybarometry. Calibration of the FeTiMM Oxybarometer The aim of this study was to develop an oxybarometer that is based on element partitioning between a mineral and silicate melt, such that it can be applied to samples in which these phases occur as inclusions within phenocrysts and thus were protected from re-equilibration and alteration during slow cooling. Iron partitioning between magnetite and melt is a promising candidate because magnetite is a common mineral and because magnetite solubility in silicic melts has been shown to depend on fO2 (Gaillard et al., 2001). However, magnetite solubility (and thus Fe partitioning between magnetite and silicate melt) also depends strongly on temperature (T) and melt composition. At constant fO2 and T, magnetite solubility increases by a factor of up to 6 as the alumina saturation index (ASI; defined as the molar Al2O3/(Na2O + K2O + CaO) ratio) decreases from 1.0 to 0.6. Magnetite solubility is thus not suitable as an oxybarometer unless T and ASI can be extremely well constrained, which is commonly difficult in natural samples (Arató and Audétat, 2017a). However, we noticed that the effect of ASI on magnetite solubility is similar to that on TiO2 solubility (Kularatne and Audétat, 2014), with the latter being independent on fO2. Hence, the effect of melt composition may be diminished by dividing the mgt–melt partition coefficient of Fe (DFe) by that of Ti (DTi). We tested this idea first on a set of 50 of our own experiments conducted in the system magnetite–H2O–rhyolite melt at various oxygen fugacities, temperatures, pressures, melt ASIs, and magnetite compositions (Arató and Audétat, Geochemical Perspectives Letters Letter Geochem. Persp. Let. (2017) 5, 19-23 | doi: 10.7185/geochemlet.1740 2