Processes and time scales of magmatic evolution as revealed by Fe–Mg chemical and isotopic zoning in natural olivines
Processes and time scales of magmatic evolution as revealed by Fe–Mg chemical and isotopic zoning in natural olivines
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DOI：
10.1016/j.gca.2015.01.025
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发表时间：
2015-04
影响因子：
5
通讯作者：
Martin Oeser;R. Dohmen;I. Horn;S. Schuth;S. Weyer
中科院分区：
文献类型：
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作者：
Martin Oeser;R. Dohmen;I. Horn;S. Schuth;S. Weyer
在本研究中，我们通过飞秒激光烧蚀 (fs-LA) MC-ICP-MS 对板内火山区域化学分区的橄榄石异种晶和斑晶进行高精度原位铁和镁同位素分析，以研究铁和镁同位素分馏的程度及其是否适合获取岩浆演化信息。我们的结果表明，岩浆橄榄石中的化学分带（即 Mg#）通常与 δ56Fe 和 δ26Mg 的显着分带（分别高达 1.7‰ 和 0.7‰）相关。我们通过模拟熔体或橄榄石中的扩散以及同时生长或溶解，探索了铁和镁同位素动力学分馏的不同情况。结合橄榄石中的化学和同位素分带信息，可以区分岩浆演化过程中可能发生的各种过程，即橄榄石和熔体之间的扩散铁镁交换、快速晶体生长以及与晶体溶解或生长同时发生的铁镁相互扩散。橄榄石中的化学扩散似乎是驱动岩浆橄榄石中同位素分馏的主要过程。 Fe 和 Mg 扩散的简化模型适用于重现大多数所研究的橄榄石中的化学和同位素分带，此外还提供有关岩浆过程的时间信息。对于法国中央高原，对巴桑岩中地幔橄榄石扩散再平衡的模拟揭示了板内玄武质岩浆中地幔捕虏体的夹带与岩浆喷发之间的时间跨度很短（<2 年）。此外，我们还确定了中央高原单个大型露头的红硅石样本的高冷却速率（每年几十到几百摄氏度），这可能反映了喷发后大量熔岩流的冷却。橄榄石中 Fe 和 Mg 同位素分馏模型的结果表明 βFe 和 βMg 之间存在系统差异（即 βFe/βMg≈2），这意味着 54Fe 和 56Fe 的扩散率比（即 D54Fe/D56Fe）与 24Mg 和 26Mg 的扩散率非常相似，尽管 54Fe-56Fe 对的相对质量差异较小。这项研究表明，对橄榄石中铁镁化学和同位素分带的联合研究比单独的化学分带提供了关于岩浆演化的更多且更可靠的信息。
In this study, we applied high-precisionin situFe and Mg isotope analyses by femtosecond laser ablation (fs-LA) MC-ICP-MS on chemically zoned olivine xeno- and phenocrysts from intra-plate volcanic regions in order to investigate the magnitude of Fe and Mg isotope fractionation and its suitability to gain information on magma evolution. Our results show that chemical zoning (i.e., Mg#) in magmatic olivines is commonly associated with significant zoning in δ56Fe and δ26Mg (up to 1.7‰ and 0.7‰, respectively). We explored different cases of kinetic fractionation of Fe and Mg isotopes by modeling diffusion in the melt or olivine and simultaneous growth or dissolution. Combining the information of chemicalandisotopic zoning in olivine allows to distinguish between various processes that may occur during magma evolution, namely diffusive Fe–Mg exchange between olivine and melt, rapid crystal growth, and Fe–Mg inter-diffusion simultaneous to crystal dissolution or growth. Chemical diffusion in olivine appears to be the dominant process that drives isotope fractionation in magmatic olivine. Simplified modeling of Fe and Mg diffusion is suitable to reproduce both the chemical and the isotopic zoning in most of the investigated olivines and, additionally, provides time information about magmatic processes. For the Massif Central (France), modeling of diffusive re-equilibration of mantle olivines in basanites revealed a short time span (<2 years) between the entrainment of a mantle xenolith in an intra-plate basaltic magma and the eruption of the magma. Furthermore, we determined high cooling rates (on the order of a few tens to hundreds of °C per year) for basanite samples from a single large outcrop in the Massif Central, which probably reflects the cooling of a massive lava flow after eruption. Results from the modeling of Fe and Mg isotope fractionation in olivine point to a systematic difference betweenβFeandβMg(i.e.,βFe/βMg≈ 2), implying that the diffusivity ratio of54Fe and56Fe (i.e.,D54Fe/D56Fe) is very similar to that of24Mg and26Mg, despite the smaller relative mass difference for the54Fe–56Fe pair. This study demonstrates that a combined investigation of Fe–Mg chemicalandisotopic zoning in olivine provides additional and more reliable information on magma evolution than chemical zoning alone.