新铝相矿物在地幔底部熔化的计算模拟研究

On one hand, recent evidences from more and more mineral physics experiments and a few ultra-deep diamond inclusions indicate that the new aluminous (NAL) phase, which derived from the dissociation of majoritic garnets at pressures corresponding to the uppermost of the lower mantle, could possibly exist in the specific regions of the lower mantle, such as the locations where the subducted slabs piled, and may keep stable down to the lowermost part of the mantle. The most distinguished feature of NAL among other lower-mantle phases is that it has a very large quasi-circular tunnel in its crystal structure which provides the pleasant sites for relatively large-sized cations. This characteristic crystal structure of the NAL endows it with the potential to host alkali elements, e.g., K and Na. On the other hand, one of the most important discoveries in geophysics in recent years is that the ultra-low velocity zones (ULVZs) lie about 10-20 kilometers above the core-mantle boundary exhibiting seismic velocities that are lower than radial-Earth reference models by about 5-10 percent for compressional and 20-30 percent for shear waves. The composition and origin of the zones remain uncertain. One group of the explanations for ULVZs originates from iron enrichment. Several scenarios have been proposed, such as core-mantle reaction, remnants of a basal magma ocean, or subducted banded iron formation. Another plausible group explanation for ULVZs originates from partial melt, however, until now no explicit molten phase has been identified in the lowermost mantle. According to the experimental experiences at shallower depth, the incorporation of alkali elements, especially K, could extremely lower the melting temperatures of their hosting phase. Furthermore, a recent high-pressure experiment studied the stability of K-rich NAL shows that the crystallization of NAL became rather sluggish and the x-ray diffraction peaks also became weak at pressures greater than 120 GPa, which implies that NAL could melt at a relative low temperature in the lowermost mantle. Thus, we suppose that NAL would likely be the first phase to melt when the subducted materials arrived at the lowermost mantle and be responsible for the ULVZs above the core-mantle boundary. In order to test our supposition, we are going to perform first-principles molecular dynamics simulations of the melting process of NAL phase at the condition of lowermost mantle using state-of-the-art supercomputers. The results of the structural and dynamical properties, densities and melting points of this phase will be used to constrain the nature and discuss the origin of ULVZs.

高压实验发现洋脊玄武岩在上地幔及转换带内会逐渐转变为镁铝榴石成分，镁铝榴石在下地幔顶部会发生分解，分解产物中含有一类六方结构的独立富铝矿物，宫岛信义称其为新铝相矿物(new aluminous phase, NAL)。随着大洋板块的俯冲，NAL可能会在下地幔直至核幔边界局部地区发生聚集。NAL在c轴方向具有容积很大的近圆形孔道，独特的晶体结构使其可以赋存离子半径很大的碱金属元素。含有碱金属元素的矿物往往具有较低的熔点。俯冲板块中的NAL是否能在到达地幔底部后率先发生熔化，这对于解释近年来地震观测发现的位于核幔边界之上的超低速区具有重要的地球物理和地球动力学意义。目前还没有实验和计算给出该相矿物在地幔底部压力下的熔点。本项目拟通过量子分子动力学模拟的方法，考察新铝相矿物在地幔底部的熔化过程，目标是获得其结构、热力学、动力学、熔点等物理参数，并籍此界定和讨论地幔底部超低速区的属性和起源。

地震学观测发现地幔底部存在地震波传播速度剧烈下降的异常区域，弄清楚这些超低速区的成因对于理解地球内部的动力学过程十分重要。科学家猜想造成地震波波速降低的可能原因是地幔底部的岩石发生了部分熔融，然而是什么原因造成了岩石的熔化尚无定论。一方面通过实验手段测量超过百万大气压（>100 GPa）下物质的熔化温度极为困难；另一方面计算模拟中会出现晶体过热的现象从而高估熔点，同时现有的模拟方法需要的计算量巨大。为了克服这些困难，本项目的主要内容是通过开发新的理论手段预测下地幔主要矿物的熔化温度，以期为超低速区成因的部分熔融假说提供证据支持。我们开发了通过构建吉布斯曲面来获得物质熔化曲线的计算方法，通过量子分子动力学计算模拟获得了方镁石、布里奇曼石和富铝相等下地幔矿物的高压熔化曲线，同时我们也计算了固相矿物的热弹性性质、熔体的输运性质以及二者的热力学性质等。研究发现在核幔边界压力下富铝相的熔点比布里奇曼石低约1000 K，该结果有力地支持和完善了部分熔融假说。超低速区的形成很可能是俯冲大洋板片在地幔底部堆积后与高温的外核接触发生了部分熔融的结果，上述认识把板片俯冲运动和地幔对流联系起来，提高了人们对于地幔动力学过程的认识，对于理解固体地球内部的组成和运动具有重要意义。