New Magmatic Oxybarometer Using Trace Elements in Zircon
New Magmatic Oxybarometer Using Trace Elements in Zircon
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DOI：
10.1093/petrology/egaa034
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发表时间：
2020-10
影响因子：
3.9
通讯作者：
R. Loucks;M. Fiorentini;G. Henríquez
中科院分区：
文献类型：
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作者：
R. Loucks;M. Fiorentini;G. Henríquez
我们推导出一种新的方法来确定岩浆的氧化态作为锆石结晶，标准误差为± 0.6 log单位的氧化铈，铀，钛在锆石中的比例，没有明确的确定任何的离子电荷，没有独立的确定结晶温度或压力或母体熔体组成。它产生的结果与氧压法在喷发的I型和A型英安岩和流纹岩淬火的Fe-Ti氧化物斑晶和角闪石斑晶，但我们的锆石氧压计也适用于缓慢冷却的深成岩石和适用于碎屑和捕虏晶锆石。在硅酸盐熔体中，Ce和U的锆石/熔体分配系数随Al_2O_2的变化而变化。锆石中的Ce/U比值也随硅酸盐熔体中Ce/U元素比值的变化而变化。在镁铁质-长英质岩浆分异过程中，Ce和U主要以钙为主的单斜辉石、角闪石、磷灰石的晶格位置结合，偶尔也会结合到钛铁矿和/或褐帘石中，它们对Ce的偏好程度与U相似。我们采用锆石和硅酸盐熔体中的U/Ti比值作为岩浆分异的指标。在玄武质到流纹岩成分的硅酸盐熔体中，会聚和发散板块边缘分异系列一致地遵循log（Ce/U）-0统计5 log（U/Ti）+ C'的关系。这种相关性允许热力学推导锆石中这些元素之间的氧压关系：log fO 2（样品）−log fO 2（FMQ）<$42 n +1 log [Ce/（Ui×Ti）z]+C，其中Ui表示年龄校正的初始U含量，FMQ表示参考缓冲铁橄榄石+磁铁矿+石英，上标z表示锆石，n随锆石母体硅酸盐熔体中铀的平均价态而变化。我们使用85个自然群体中的1042个分析的锆石来经验性地校准这种关系，这些自然群体具有在FMQ - 4·9至FMQ + 2·9范围内的独立约束的log O2，得到方程log fO 2（样品）-log fO 2（FMQ）=3·998（±0·124）log[Ce/（Ui×Ti）z]+2 statist 284（±0·101），相关系数R = 0·963，标准误差为0·6 log单位的CO2在钙碱性，拉斑玄武岩，埃达克质，钾玄质，偏铝质轻度过铝质和轻度过碱性熔体的成分范围从金伯利岩流纹岩。热力学评估和经验测试表明，我们的配方是不敏感的结晶温度和压力在岩石圈条件。我们提出了一个修订后的钛在锆石测温方程，占适当的压力，以及减少活动的TiO 2和SiO 2在金红石和石英不饱和熔体。它可用于从锆石分析获得的ΔFMQ值中检索Δ O2的绝对值。
We derive a novel method for determining the oxidation state of a magma as zircon crystallized, 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 crystallization 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) + C' in silicate melts of basaltic to rhyolitic composition. That correlation permits thermodynamic derivation of the oxybarometry relation among those elements in zircon: log fO2(sample)−log fO2(FMQ)≈42n+1log[Ce/(Ui×Ti)z]+C, wherein Ui denotes age-corrected initial U content, FMQ represents the reference buffer fayalite + magnetite + quartz, superscript z denotes zircon, 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 fO2(sample)−log fO2(FMQ)=3·998(±0·124) log[Ce/(Ui×Ti)z]+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 crystallization 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.