Single-crystal elasticity of martian mantle minerals and a flexible CO2 laser heating system

对火星内部地震波速度结构的观测越来越多地来自美国宇航局洞察号着陆器上的SEIS地震仪。对这些数据的解释主要依赖于对火星内部的矿物学和地震速度进行建模的能力，以便测试合理的成分和温度梯度。然而，迄今为止，火星地幔的这种模型是使用热力学参数构建的，这些参数要么是估计的，要么不是根据最近的相平衡和弹性数据确定的，要么不适合确定火星成分。在同时的高压和高温条件下，很少有弹性测量，大多数矿物的数据需要在一定程度上外推，这带来了很大的不确定性。在这个项目的第一个阶段，开发了一个新的系统来测量与整个火星地幔相对应的压力和温度下的声波速度。在该系统中，布里渊光谱测量与二氧化碳激光加热在金刚石砧池中同时进行，已成功地通过对pyrope单晶进行测量进行基准测试。通过将这些数据与富铁环伍德岩的进一步测量结果和文献中的最新数据相结合，获得了火星地幔底部的最新矿物物理模型。与以往基于陆地物质性质的模型存在显著差异。利用新模型来解释在1140千米处提出的火星地幔不连续，意味着该深度的温度在1870-1970 K之间。在更新阶段，还将同时进行单晶x射线衍射测量，以获得一个真正独特的系统，能够在任何类地行星的整个条件下确定任何矿物的全弹性张量。利用这一系统，在火星地幔稳定的压力和温度下，通过检查多数石榴石、橄榄石甚至低对称性矿物斜辉石的富铁单晶，可以完成对火星主要矿物的全弹性张量的测定。这些数据将用于开发一个内部一致的热力学模型，以预测火星地幔的矿物学和地震波速度，大大减少了不确定性。该模型不仅将用于解释火星地震结构的新观测结果并评估这些解释中的不确定性，而且还将首次评估地震各向异性如何有可能影响火星内部的观测结果。此外，通过研究由不同固溶体组分组成的矿物，我们将解决矿物物理学中的一个核心问题，即中间组分的性质是否可以用端元性质的线性组合来有效地描述。

Observations of the seismic wave velocity structure of the Martian interior are becoming increasingly available from the SEIS seismometer on the NASA InSight lander. The interpretation of such data relies crucially on the ability to model the mineralogy and seismic velocities of the Martian interior in order to test plausible compositions and temperature gradients. To date, however, such models for the Martian mantle are constructed using thermodynamic parameters that are either estimated, not determined from the most recent phase equilibria and elasticity data or are not suitable for determining Martian compositions. Very few elasticity measurements exist at simultaneous high pressure and temperature conditions, requiring data for most minerals to be extrapolated to some extent, which introduces significant uncertainties.In the first period of this project a new system was developed to measure acoustic wave velocities at pressures and temperatures corresponding to the entire Martian mantle. The system, where Brillouin spectroscopy measurements are performed simultaneously with CO2-laser heating in a diamond anvil cell, has been successfully benchmarked by performing measurements on single crystals of pyrope. By combining these data with further measurements on Fe-rich ringwoodite and recent data from the literature, an updated mineral-physics model for the base of the Martian mantle has been obtained. Significant differences exist with previous models based on properties of terrestrial materials. Using the new model to interpret a proposed Martian mantle discontinuity at 1140 km, implies a temperature at this depth in the range 1870-1970 K. In the renewal phase, simultaneous single crystal X-ray diffraction measurements will be also implemented, to obtain a truly unique system capable of determining the full elastic tensor of any mineral throughout the conditions of any terrestrial planet. Using this system, the determination of the full elastic tensors of the main Martian minerals will be completed by examining Fe-rich single crystals of majoritic garnet, olivine and even the low symmetry mineral clinopyroxene, at pressures and temperatures of their stability in the Martian mantle. These data will be used to develop a an internally consistent thermodynamic model to predict the mineralogy and seismic wave velocities of the Martian mantle with vastly reduced uncertainties. This model will not only be used to interpret the emerging observations of Martian seismic structure and assess the uncertainties in these interpretations, but will also provide a first assessment of how seismic anisotropy has the potential to influence observations of the Martian interior. Moreover, by studying minerals comprised of different solid solution components, we will address a central issue in mineral physics as to whether the properties of intermediate compositions can be effectively described using linear combinations of end member properties.