Apatite: A new redox proxy for silicic magmas?

Abstract

granites where redox conditions have been shown to change from close to the magnetite-28 hematite buffer to close to the nickel-nickel oxide buffer respectively (Stephens et al., 1985). Variations in the Mn concentration of apatites appear to be largely independent of differences 36 in the Mn concentration of the melt. Apatite Mn concentrations may therefore provide an 37 independent oxybarometer that is amenable to experimental calibration, with major relevance 38 to studies on detrital mineral suites, particularly those containing a record of early Earth 39 redox conditions, and on the climatic impact of historic volcanic eruptions. 40

Generation of Late Mesozoic Qianlishan A 2 -type granite in Nanling Range, South China: Implications for Shizhuyuan W–Sn mineralization and tectonic evolution

“…zircon, apatite and biotite) in equilibrium with a magma can be used to estimate the redox state of the magma (Ballard et al, 2002;Cao et al, 2012;Liang et al, 2006;Miles et al, 2014;Wones and Eugster, 1965 /Ce 3+ ratio (N 300) reflects oxidized magma (Ballard et al, 2002;C.-Y. Li et al, 2012;Liang et al, 2006).…”

Reply to comment by Marks et al. (2016) on “Apatite: A new redox proxy for silicic magmas?” [Geochimica et Cosmochimica Acta 132 (2014) 101–119]

“…2 between metaluminous and weakly peraluminous zones of the Criffell pluton (Miles et al, 2014). However, this could not explain the c. 27 fold increase observed in the Mn content of some apatites between these samples.…”

“…The empirical negative relationship reported by Miles et al (2014) between Mn-in-apatite and The influence of melt composition on Mn partition coefficients is poorly constrained.…”

“…Mn 2+ is more easily incorporated into the apatite structure, probably in place of Ca 2+ (Sha and Chappell., 1999;Miles et al, 2014). It remain less clear how more oxidised forms of Mn such as Mn 3+ are structurally bound, but this presumably that involves either coupled substitutions with other ions or the existence of cation vacancies (or both).…”

“…investigate the applicability of the Mn-in-apatite oxybarometer proposed by Miles et al (2014) across a range of magma compositions using published data on wellcharacterised samples. The authors show that for magma compositions outside of the calcalkaline and intermediate to silicic range used in the preliminary calibration, fO 2 values calculated from Mn-in-apatite vary significantly from independently constrained estimates.…”