The effect of fluorine on the stability of wadsleyite: Implications for the nature and depths of the transition zone in the Earth's mantle
The effect of fluorine on the stability of wadsleyite: Implications for the nature and depths of the transition zone in the Earth's mantle
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DOI：
10.1016/j.epsl.2017.11.011
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发表时间：
2018-01
影响因子：
5.3
通讯作者：
T. Grützner;S. Klemme;A. Rohrbach;F. Gervasoni;J. Berndt
中科院分区：
文献类型：
--
作者：
T. Grützner;S. Klemme;A. Rohrbach;F. Gervasoni;J. Berndt
地幔含有大量的挥发性元素，如氢（H），碳（C）和卤素氟（F），氯（Cl）和溴（Br）和碘（I）。关于氢和碳的全球循环有丰富的知识，但关于不同地球水库中卤素的浓度以及俯冲带中卤素再循环过程中的行为的数据很少。在这里，我们专注于存储潜力的F在更深的部分地球的地幔。过渡带是地球地幔中的一个区域（410-660公里），以其高水储存能力而闻名，因为橄榄石、华兹利石和林伍德石的高压多晶型物已知能够结合百分之几的水。为了评估潜在的分馏水和F之间的过渡区的地球地幔，我们着手调查的卤素F在wadsleyite和橄榄石在过渡区条件下的存储容量。实验在简化的地幔组成中在1400° C至1900° C的温度和17至21 GPa的压力下在多砧装置中进行。结果表明，氟能使橄榄石-华氏体转变向高压方向移动。我们发现，F有一个相反的效果，水，后者的过渡区向低压延伸。此外，硅镁石的氟存储容量显着低于之前的预期。氟含量在1470±60 μg/g ~ 2110±600 μg/g之间，与温度和压力无关。在过渡带条件下，硅镁橄榄石的贮F量甚至低于镁橄榄石，后者可掺入3930±140 μg/g F。根据我们的数据，我们发现，过渡区不可能是一个水库的F，因为它被假定为水。此外，我们认为，在俯冲过程中的挥发分轴承板，分馏水从F将发生，水优先进入过渡带和F仍然在最低的上地幔橄榄岩。
The Earth's mantle contains significant amounts of volatile elements, such as hydrogen (H), carbon (C) and the halogens fluorine (F), chlorine (Cl) and bromine (Br) and iodine (I). There is a wealth of knowledge about the global cycling of H and C, but there is only scant data on the concentrations of halogens in different Earth reservoirs and on the behavior of halogens during recycling in subduction zones. Here we focus on the storage potential of F in deeper parts of the Earth's mantle. The transition zone is a region in the Earth's mantle (410–660 km) known for its high water storage capacity, as the high pressure polymorphs of olivine, wadsleyite and ringwoodite are known to be able to incorporate several per-cent of water. In order to assess potential fractionation between water and F in the transition zone of the Earth's mantle, we set out to investigate the storage capacity of the halogen F in wadsleyite and olivine at transition zone conditions. Experiments were performed in a simplified mantle composition at temperatures from 1400° C to 1900° C and pressures from 17 up to 21 GPa in a multi anvil apparatus. The results show that F can shift the olivine–wadsleyite transition towards higher pressure. We find that F has an opposing effect to water, the latter of which extends the transition zone towards lower pressure. Moreover, the F storage capacity of wadsleyite is significantly lower than previously anticipated. F concentrations in wadsleyite range from 1470±60 μg/g to 2110±600 μg/g independent of temperature or pressure. The F storage capacity in wadsleyite is even lower than the F storage capacity of forsterite under transition zone conditions, and the latter can incorporate 3930±140 μg/g F under these conditions. Based on our data we find that the transition zone cannot be a reservoir for F as it is assumed to be for water. Furthermore, we argue that during subduction of a volatile-bearing slab, fractionation of water from F will occur, where water enters preferentially the transition zone and F remains in the peridotite of the lowermost upper mantle.