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Abstract
We derive a novel method for determining the oxidation state of a magma as zircon crystallised, with a standard error of ±0.6 log unit ƒO2, using ratios of Ce, U, and Ti in zircon, without explicit determination of the ionic charge of any of them, and without independent determination of crystallisation temperature or pressure or parental melt composition. It yields results in good agreement with oxybarometry on FeTi oxide phenocrysts and hornblende phenocrysts quenched in eruptive I and Atype dacites and rhyolites, but our zircon oxybarometer is also applicable to slowly cooled plutonic rocks and applicable to detrital and xenocrystic zircons. Zircon/melt partition coefficients of Ce and U vary oppositely with ƒO2 variation in the silicate melt. The Ce/U ratio in zircon also varies with the Ce/U element ratio in the silicate melt. During mafictofelsic magmatic differentiation, Ce and U are incorporated mainly in calciumdominated lattice sites of clinopyroxene, hornblende, apatite, and occasionally titanite and/or allanite, all of which have a similar degree of preference for Ce over U. We employ the U/Ti ratio in zircon and in silicate melt as a magmatic differentiation index. Convergent and divergentplatemargin differentiation series consistently follow the relation log (Ce/U) ≈ –0.5 log (U/Ti) + C´ in silicate melts of basaltic to rhyolitic composition. That correlation permits thermodynamic derivation of the oxybarometry relation among those elements in zircon: log ƒO■( @2(sample)) –log ƒO■( @2(FMQ))≈4/(2n+1) log [(Ce/√(U■( @i)×Ti)■(z@ )]+C, wherein Ui denotes agecorrected initial U content, FMQ represents the reference buffer fayalite + magnetite + quartz, and n varies with the average valence of uranium in the zircon’s parental silicate melt. We empirically calibrate this relation, using 1042 analysed zircons in 85 natural populations having independently constrained log ƒO2 in the range FMQ–4.9 to FMQ+2.9, to obtain the equation: log ƒO2(sample)) –log ƒO2(FMQ))=3.99 (±0.12)×log [(Ce/√(U×Ti))]+2.28 (±0.10■( @1)) with a correlation coefficient R = 0.963 and standard error of 0.6 log unit ƒO2 in calcalkalic, tholeiitic, adakitic, and shoshonitic, metaluminous to mildly peraluminous and mildly peralkaline melts in the composition range from kimberlite to rhyolite. Thermodynamic assessment and empirical tests indicate that our formulation is insensitive to varying crystallisation temperature and pressure at lithospheric conditions. We present a revised equation for Tiinzircon thermometry that accounts appropriately for pressure as well as reduced activity of TiO2 and SiO2 in rutile and quartzundersaturated melts. It can be used to retrieve absolute values of ƒO2 from values of ∆FMQ obtained from a zircon analysis.
Original language  English 

Article number  egaa034 
Number of pages  30 
Journal  Journal of Petrology 
Volume  61 
Issue number  3 
DOIs  
Publication status  Published  1 Mar 2020 
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Dive into the research topics of 'New Magmatic Oxybarometer Using Trace Elements in Zircon'. Together they form a unique fingerprint.Projects
 2 Finished

Hydrothermal Remobilisation of Base Metals & Platinum Group Elements in Magmatic Nickel Deposits
Fiorentini, M., Brugger, M., Perring, C., Liu, W. & Barnes, S.
1/01/12 → 31/12/13
Project: Research

From Core to Ore: Emplacement Dynamics of DeepSeated Nickel Sulphide Systems
1/01/11 → 31/05/16
Project: Research