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首页> 外文期刊>Contributions to Mineralogy and Petrology >Reconciling Li and O diffusion in zircon with protracted magmatic crystal residence
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Reconciling Li and O diffusion in zircon with protracted magmatic crystal residence

机译：Zircon中的Zircon中的扩散和延伸的岩浆晶体住宅

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摘要

Lithium and O in zircon have strong potential as diffusion chronometers for assessing thermal histories of magmatic and metamorphic systems, but their diffusion mechanisms remain controversial. Zircon populationserupted from El Chichon volcano, Chiapas, Mexico, often contain xenocrysts of detrital origin with juvenile overgrowth forming a natural diffusion boundary within individual crystals. High-spatial resolution O and Li isotope analyses, U-Pb and U-Th geochronology, and Li (along with Al and Y) scanning ion imaging were acquired to compare diffusion model timescales with those derived from radiometric dating. Oxygen isotopes in El Chichon zircon xenocrysts preserve source heterogeneities, with xenocrystic and juvenile domains of zircon being in O-isotopic disequilibrium with the melt at the time of eruption. Preservation of these heterogeneities is consistent with recently determined O-isotope diffusion parameters when crystal residence at magmatic temperatures is assumed over the 10(3)-10(4) year timescales indicated by U-Th geochronology. Lithium concentration gradients across the same boundaries are sharp within the similar to 2-5 mu m lateral resolution of the ion beam, whereas Li isotopic compositions for xenocrystic cores are homogeneous at Li-7=+7.8 +/- 2.3 parts per thousand (1 standard error) and different from their putative source. Diffusive equilibration of Li isotopes in zircon is consistent with protracted magmatic crystal residence, but preservation of Li abundance heterogeneities is difficult to reconcile with evidence for recurrent high-temperature exposure of zircon in a highly active magmatic system. We speculate that Li partitioning in zircon is in part coupled to slow-diffusing trivalent cations, which can cause severe underestimation of diffusion timescales extracted from Li diffusion modeling.
机译：锆石中的锂和o具有强大的潜力，作为评估岩浆和变质系统的热历史的扩散计量计，但它们的扩散机制仍然存在争议。 Zircon人群从埃尔Chichon Volcano，墨西哥恰帕斯群体植物，通常含有邪恶来源的杂种晶体，少年过度生长形成单个晶体内的自然扩散边界。获取高空间分辨率O和Li同位素分析，U-Pb和U-Th地理学，以及Li（以及Al和Y）扫描离子成像，以将扩散模型时间数与来自辐射测定的那些进行比较。 El Chichon锆氧化锆中的氧同位素保持源异质性，Zircon的XenoCrystic和少年域在爆发时与熔体熔体在O-同位素不平衡中。这些异质性的保存与最近确定的O-同位素扩散参数一致，当岩浆温度下的晶体居住在由第10（3）-10（4）年的时间尺度上，是由U-Th地球化学表明的。相同边界的锂浓度梯度在类似于2-5μm的离子束的横向分辨率内锐利，而烯晶核的锂同位素组合物在Li-7 = + 7.8 + -2.3份均匀（1标准误差）与其推定源不同。锆石中锂同位素的扩散平衡与延伸的岩浆晶体住宅一致，但锂丰度异质性的保存难以与锆的复发性高温暴露在高活跃的岩浆系统中的证据中难以调和。我们推测Zircon中的Li分区部分耦合到缓慢扩散的三价阳离子，这可能导致从LI扩散建模中提取的扩散时间尺度严重低估。

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