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 Fe-Ti oxide phenocrysts and hornblende phenocrysts quenched in eruptive I- and A-type 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 mafic-to-felsic magmatic differentiation, Ce and U are incorporated mainly in calcium-dominated 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 divergent-plate-margin differentiation series consistently follow the relation log (Ce/U) ≈ –0.5 log (U/Ti) + Ć 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)] + C, wherein U_i denotes age-corrected 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 ƒO_2(sample) – log ƒO_2(FMQ) = 3.99₈ (±0.12₄) × log [Ce/√(U_i×Ti)] + 2.28₄ (±0.10₁), with a correlation coefficient R = 0.96₃ and standard error of 0.6 log unit ƒO₂ in calc-alkalic, 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 Ti-in-zircon thermometry that accounts appropriately for pressure as well as reduced activity of TiO₂ and SiO₂ in rutile- and quartz-undersaturated melts. It can be used to retrieve absolute values of ƒO₂ from values of ΔFMQ obtained from a zircon analysis.

中文翻译：

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使用锆石中微量元素的新型岩浆气压计

我们推导出一种新颖的方法，用于确定氧化态的岩浆的锆石结晶，具有±0.6对数单位的标准误差ƒO ₂，使用的Ce，U，和Ti的比率在锆石，无离子电荷的任何明确的判定它们，并且没有独立确定结晶温度或压力或母体熔体组成。它的结果与在喷发性I型和A型dacites和流纹岩上淬火的Fe-Ti氧化物隐晶和角闪石隐晶的氧压测量结果非常吻合，但我们的锆石氧饱和度计也适用于缓慢冷却的深成岩，适用于碎屑和异晶锆石。锆石/熔体的Ce和U的分配系数与ƒO相对变化₂硅酸盐熔体的变化。锆石中的Ce / U比也随硅酸盐熔体中的Ce / U元素比而变化。在从镁铁质到长英质的岩浆分化过程中，Ce和U主要掺入斜方晶烯，角闪石，磷灰石的钙占主导的晶格位点，偶尔还掺入钛铁矿和/或尿铝石，所有这些都对Ce的优先级高于U.我们采用锆石和硅酸盐熔体中的U / Ti比作为岩浆分化指数。在玄武质到流纹岩成分的硅酸盐熔体中，收敛板和扩散板边缘的微分级数序列始终遵循关系log（Ce / U）≈–0.5 log（U / Ti）+Ć。这种相关性允许对锆石中那些元素之间的大气压力关系进行热力学推导：logƒO2 _（样品） – logƒO2 _（FMQ） =（⁴ / _{2n + 1}）log [（Ce /√（U _i ×Ti）] + C，其中U _i代表年龄校正后的初始U含量，FMQ代表参考缓冲区铁橄榄石+磁铁矿+石英，n随。在锆石的亲代硅酸盐熔体铀的平均价数，我们凭经验校准该关系，使用在85个自然种群1042个分析锆石具有独立地约束日志ƒO ₂在FMQ-4.9至FMQ + 2.9的范围内，以获得等式：日志ƒO _{2（样本）} – logƒO2 _（FMQ） = 3.99 ₈（±0.12 ₄）×log [Ce /√（U _i ×Ti）] + 2.28 ₄（±0.10 ₁），相关系数R = 0.96 ₃和0.6个对数单位ƒO的标准误差₂在钙碱性，拉斑，埃达克，和钾玄，铝质在从金伯利岩的组成范围轻度过铝和轻度过碱性熔体流纹岩。热力学评估和经验测试表明，我们的配方对岩石圈条件下变化的结晶温度和压力不敏感。我们提出了锆石钛测温法的修正公式，该公式适当地考虑了压力以及金红石和石英欠饱和熔体中TiO ₂和SiO _2的活性降低。它可以用来检索ƒO的绝对值₂ 从锆石分析获得的ΔFMQ值。