中文摘要：

前人研究表明410km和660km地震不连续面分别由橄榄石向瓦兹利石相变和后尖晶石相变引起。但对520km地震不连续面（简称D520）的研究相对较少,对其成因的解释还存在很大争议。橄榄石中瓦兹利石-林伍德石相变以及CaSiO_3钙钛矿的出溶反应被广泛认为是D520的相变成因。辉石相变为尖晶石＋斯石英组合也曾被认为是D520的可能成因。1 400℃条件下对MgSiO_3辉石相变的实验研究,结合前人对橄榄石相变的研究成果,计算了方辉橄榄岩在1 400℃、18GPa条件下因辉石-瓦兹利石＋斯石英和瓦兹利石-林伍德石相变引起的密度增量和波速增量,发现俯冲方辉橄榄岩中辉石的相变对520km深度的密度增量和波速增量有很大的贡献,有助于形成D520。此外,在探讨D520的成因时应综合考虑地幔转换带中温度,水以及矿物间Fe、Mg、Ca等主量元素分配等因素的影响。

英文摘要：

Previous studies have shown that the causes for the 410 km and 660 km seismic discontinuities are olivine-wadsleyite in （Mg, Fe） 2 SiO_4 and the post-spinel transition of ringwoodite,respectively.How- ever, studies on the 520 km seismic discontinuity （D520） are relatively inadequate and controversial. Based on seismic observations and the latest high P-T experimental studies, this paper attempts to analyze and review the previous studies on the causes of D520. The factors affecting the position and width of the D520 and phase transition of related mantle minerals are numerous. It is generally believed that the transforma- tion from β to 7 is the first approximation for the D520, although it can＇t explain the high velocities con- tracts and splitting near 520 km in transition zone at some regions. Recent experimental research shows the importance of the phase transition of non-olivine normative minerals to the formation of the D520. Except the exsolution of CaSiO_3-perovskite from majorite, this paper suggests the dissociation of MgSiO_3 pyrox- ene in harzburgite may be one cause of D520 as well. The bulk rock with 82% olivine and 18% pyroxene in stagnant harzburgite at 520 km depth can cause p≈6.6%, vp≈2.6%, and vs≈4.4%, comparable to that observed seismically at 520 km. Therefore, phase transformation in a stagnant subuducted harzburgite may provide constraint on formation of the 520 km discontinuity. The interaction between olivine-and pyroxene- normative components is another factor to be investigated. Meanwhile, the temperature and water in tran- sition zone and partitioning of major elements between minerals may play important roles in the formation of D520.