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.

地震观测表明，660 km的不连续层厚度小于2 km，对应的压力小于0.1 GPa。这种清晰度与厚度估计为7公里的410公里不连续层形成鲜明对比。660公里的间断通常可以用（Mg,Fe）2SiO4中的尖晶石后转变来解释，其中（Mg,Fe）2SiO4环woodite解离成（Mg,Fe）SiO3钙钛矿和（Mg,Fe）O方长石。由于这三种矿物具有Mg-Fe固溶体，尖晶石后转变应该有一个有限的间隔，因此我们需要一个特殊的解释来解释660公里不连续的极端尖锐。因此，必须确定尖晶石后过渡的过渡间隔。如果转换的压力区间为> - 0.1 GPa，我们将不得不重新考虑深部地幔的结构和动力学。首先，上地幔和下地幔的化学成分可能不同，地幔对流至少部分是分层的。另一种解释是，缓慢的成核动力学阻止了尖晶石后转变的开始，一旦成核发生，转变进行得非常迅速。如果证明尖晶石后转变的间隔非常薄，则可以通过660-km不连续层厚度的全球变化来评估垂直地幔流的存在。由于以往的实验研究在压力测定方面缺乏足够的精度，并且受到动力学缓慢的影响，因此未能成功地确定尖晶石后转变的压力区间。以往研究的压力测定精度不超过0.3 GPa，过大，无法确定过渡区间，过渡区间可以小于0.1 GPa。相比之下，申请人已经建立了实验技术来确定样品压力，精度为0.04 GPa，这对于目前的项目来说应该是足够的。他还建立了一种利用助熔剂获得矿物平衡成分的实验技术。结合这两种技术，在2000 K的恒定温度下，在压力-组成空间中确定了环伍德矿+钙钛矿+方长石的整个三相区域。根据三相区几何形状，考虑镁铁与镁铁的预期交换，估算真实地幔中尖晶石后转变的压力区间。