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

地震观测表明，660公里长的不连续面厚度不到2公里，对应的压力小于0.1 GPa。这种尖锐程度与410公里的不连续性形成对比，后者的厚度估计为7公里。660 km的不连续性通常被解释为（Mg，Fe）2SiO 4的后尖晶石转变，其中（Mg，Fe）2SiO 4环伍德石分解为（Mg，Fe）SiO 3钙钛矿和方镁石（Mg，Fe）O。由于这三种矿物都有Mg-Fe固溶体，所以后尖晶石转变应该有一个有限的时间间隔，因此我们需要一个特殊的解释来解释660 km不连续面的极端尖锐性。因此，必须确定后尖晶石转变的转变间隔。如果转换的压力间隔大于0.1GPa，我们将不得不重新考虑深部地幔的结构和动力学。首先，可能上地幔和下地幔的化学成分不同，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后尖晶石转变的开始，并且一旦成核发生，转变进行得非常迅速。如果证明后尖晶石过渡的间隔非常薄，则可以通过660 km不连续面厚度的全球变化来评估垂直地幔流的存在。以前的实验研究没有成功地确定的压力区间的postpinite转变，因为他们缺乏足够的精度，在压力测定和遭受的影响，缓慢的动力学。以往研究中压力测定的精度不优于0.3 GPa，这太大，无法确定过渡间隔，过渡间隔可以小于0.1 GPa。相比之下，申请人已经建立了实验技术，以0.04 GPa的精度确定样品压力，这对于本项目来说应该足够了。他还建立了一种实验技术，通过使用助熔剂获得矿物的平衡组成。结合这两种技术，在2000 K恒温条件下，在压力-成分空间中确定了林伍德石+钙钛矿+方镁石的整个三相区。根据三相区的几何形状，考虑到镁铁交换，估算了真实的地幔中后尖晶石转变的压力区间。

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.