Determination of the three phase region of the post-spinel transition in (Mg,Fe)2SiO4: explanation of the extreme sharpness of the 660-km discontinuity and implication for chemical structure and dynamics of the deep mantle

(Mg,Fe)2SiO4 后尖晶石相变三相区的确定：解释 660 公里不连续性的极端锐度及其对深部地幔化学结构和动力学的影响

• 批准号: 257122795
• 负责人: Professor Dr. Tomoo Katsura
• 金额: --
• 依托单位: Deutsches Elektronen-Synchrotron (DESY)
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2014
• 资助国家: 德国
• 起止时间: 2013-12-31 至 2019-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/257122795?language=en
• 关键词: Determination; three; phase; region; post

Seismic observations indicate that the 660-km discontinuity is less than 2 km thick, corresponding to <0.1 GPa in pressure. Such sharpness is in contrast with that of the 410-km discontinuity, whose thickness is estimated to be 7 km. The 660-km discontinuity is usually explained by the postspinel transition in (Mg,Fe)2SiO4, in which (Mg,Fe)2SiO4 ringwoodite dissociates into (Mg,Fe)SiO3 perovskite plus periclase (Mg,Fe)O. Because these three minerals have Mg-Fe solid solutions, the postspinel transition should have a finite interval, and therefore we need a special explanation to account for the extreme sharpness of the 660-km discontinuity. For this reason, the transition interval of the postspinel transition must be determined. If the pressure interval of the transition is >0.1 GPa, we will have to reconsider the structure and dynamics of the deep mantle. Firstly, it is possible that the chemical compositions are different between the upper and lower mantles, and that mantle convection is at least partially layered. An alternative explanation is that the sluggish kinetics of nucleation prevents initiation of the postspinel transition, and once nucleation occurs, the transition proceeds very rapidly. If it is proved that the interval of the postspinel transition is extremely thin, the presence of vertical mantle flow could be assessed by global variation of the thickness of the 660-km discontinuity. Previous experimental studies have not successfully determined the pressure interval of the postspinel transition because they have lacked sufficient precision in pressure determination and suffered from the effects of sluggish kinetics. The precision of pressure determination in previous studies was no better than 0.3 GPa, which was too large to determine the transition interval, which could be less than 0.1 GPa. In contrast, the applicant has already established experimental techniques to determine sample pressure with a precision of 0.04 GPa, which should be sufficient for the present project. He has also established an experimental technique to obtain the equilibrium compositions of minerals by using a flux. Combining these two techniques, the whole three phase region of ringwoodite+perovskite+periclase will be determined in pressure-composition space at a constant temperature of 2000 K. The pressure interval of the postspinel transition in the real mantle will be estimated based on the geometry of the three phase region by considering the expected Mg-Fe exchange with majorite.

地震观察结果表明，660公里的不连续性小于2 km，对应于压力中<0.1 GPA。这种清晰度与410公里的不连续性相反，估计其厚度为7 km。 660公里的不连续性通常由（mg，fe）2SIO4中的后跨度转变来解释，其中（mg，fe）2SiO4 ringwoodwoodite将分离为（mg，fe）sio3 perovskite plus contovskite plus conticlase plus conticlase（mg，fe）o。由于这三个矿物具有MG-FE实心溶液，因此螺旋后过渡应该具有有限的间隔，因此我们需要一个特殊的解释来说明660公里的不连续性的极端清晰度。因此，必须确定后跨度过渡的过渡间隔。如果过渡的压力间隔> 0.1 GPA，我们将不得不重新考虑深幔的结构和动力学。首先，上层和下层之间的化学成分可能不同，并且地幔对流至少部分分层。另一种解释是，成核动力学的动力学阻止了螺旋后转变的引发，一旦成核发生，过渡就会很快进行。如果证明后跨度过渡的间隔非常薄，则可以通过660公里不连续性的厚度的全球变化来评估垂直地幔流的存在。先前的实验研究尚未成功确定螺旋后过渡的压力间隔，因为它们在压力确定方面缺乏足够的精度，并且遭受了缓慢动力学的影响。先前研究中压力确定的精度不超过0.3 GPA，这太大而无法确定过渡间隔，该间隔可能小于0.1 GPA。相比之下，申请人已经建立了实验技术，以确定样品压力的精度为0.04 GPA，这对于本项目应该足够。他还建立了一种实验技术，以使用通量获得矿物质的平衡组成。结合了这两种技术，将在2000 K的恒定温度下在压力组合空间中确定Ringwoodite+Perovskite+Ceniclase的整个三相区域。将根据预期的MG-FE与主要地点的预期MG-FE交换来估计，将根据三相区域的几何形状来估算固定后旋转的压力间隔。