Experimentally determined Si isotope fractionation between zircon and quartz
Experimentally determined Si isotope fractionation between zircon and quartz
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DOI：
10.1016/j.gca.2019.06.035
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发表时间：
2019-09
影响因子：
5
通讯作者：
D. Trail;P. Savage;F. Moynier
中科院分区：
文献类型：
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作者：
D. Trail;P. Savage;F. Moynier
碎屑石英和锆石的硅同位素组成有可能让我们了解地球上二氧化硅循环和风化反应的长期变化。然而，迄今为止，矿物的硅同位素平衡分馏数据有限，这一事实阻碍了从背景外矿物推断源熔体硅同位素组成。在这里，我们报告了实验数据，以限制锆石和石英之间的硅同位素平衡分馏，使用两种根本不同的策略，但具有相同的实验设计。首先，在1.5 GPa、725、800和900°C的温度下，由Zr(OH) 4 和SiO 2 水热合成锆石和石英。第二个实验策略利用三同位素方法；起始材料由天然锆石和同位素标记的SiO 2 组成。在与直接合成实验相同的压力和温度下进行了三组水热时间序列实验。对于所有实验，石英和锆石被分离，并且通过溶液多接收器电感耦合等离子体质谱法测量30 Si/28 Si和29 Si/28 Si比率。三同位素法提供了平衡分馏的最佳指标，得出以下关系： Δ 30 Si (q t z-z r c)=(0.53±0.14)× 10 6/T 2 其中 Δ 30 Si (qtz-zrc) 是石英和锆石之间 30 Si/28 Si 的相对差值（单位：permil），T 是温度（单位：K），误差为 2 se 这种关系可以可用于计算锆石和其他相之间的分馏，并估计锆石结晶的熔体的 Si 同位素组成。结果可用于评估石英和锆石之间的平衡-不平衡同位素分馏以及火成岩中的共存相。这些数据还可以应用于脱离背景的锆石（和石英），以估计母岩的同位素组成。从纯火成岩来源的熔体中结晶出来的锆石（即不涉及“风化”材料）预计将显示 δ 30 Si NBS-28（30 Si/28 Si 与 NBS-28 标准的千分之一偏差）范围为− 0.7 至− 0.35 ‰。偏离该范围表明熔体源中非火成（即沉积）材料的同化。
The silicon isotope composition of detrital quartz and zircon have the potential to inform us about secular changes to the silica cycle and weathering reactions on Earth. However, inferring source melt Si isotope composition from out-of-context minerals is hampered by the fact that, to-date, there is limited Si isotope equilibrium fractionation data for minerals. Here, we report experimental data to constrain Si isotope equilibrium fractionation between zircon and quartz, using two fundamentally different strategies, but with the same experimental design. First, zircon and quartz were hydrothermally synthesized from Zr (OH) 4 and SiO 2 at 1.5 GPa and temperatures of 725, 800, and 900° C. The second experimental strategy utilized the three-isotope method; the starting materials consisted of natural zircon and isotopically-labelled SiO 2. Three sets of hydrothermal time-series experiments were conducted at the same pressure and temperatures as the direct synthesis experiments. For all experiments, quartz and zircon were separated and 30 Si/28 Si and 29 Si/28 Si ratios were measured by solution multi-collector inductively coupled plasma mass spectrometry. The three-isotope method, which provides the best indicator of equilibrium fractionations, yields the following relationship: Δ 30 S i (q t z-z r c)=(0.53±0.14)× 10 6/T 2 where Δ 30 Si (qtz-zrc) is the relative difference in 30 Si/28 Si between quartz and zircon in permil, T is temperature in K, and the error is 2 se This relationship can be used to calculate the fractionation between zircon and other phases, and to estimate the Si isotope composition of the melt from which a zircon crystallized. The results may be used to assess equilibrium-disequilibrium isotope fractionations between quartz and zircon and co-existing phases in igneous rocks. These data can also be applied to out-of-context zircon (and quartz) to estimate the isotope composition of the host rock. Zircons crystallizing from a melt derived from purely igneous sources–ie, without the involvement of “weathered” material–are expected to display a δ 30 Si NBS-28 (permil deviation of the 30 Si/28 Si from the NBS-28 standard) range from− 0.7 to− 0.35‰. Deviations from this range indicate assimilation of non-igneous (ie, sedimentary) material in the melt source.