Experimental constraints on melting temperatures in the MgO-SiO2 system at lower mantle pressures
Experimental constraints on melting temperatures in the MgO-SiO2 system at lower mantle pressures
复制标题
较低地幔压力下 MgO-SiO2 体系熔化温度的实验限制
DOI：
10.1016/j.epsl.2017.05.020
复制
发表时间：
2017
影响因子：
5.3
通讯作者：
Baron M
中科院分区：
文献类型：
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作者：
Baron M
利用激光加热金刚石砧细胞（LH-DAC）技术，实验测定了低地幔压力下mgo - sio2体系的共晶熔化曲线。我们研究了镁-镁sio3二元体系中桥辉石+方长石的共熔熔融，以及镁sio3 - sio2二元体系中桥辉石+辉长石的共熔熔融，它们分别与天然橄榄岩和玄武岩类似。模型玄武岩的熔融曲线发生在较低的温度下，d T/d P斜率较浅，曲率略小于模型橄榄岩的熔融曲线。总体而言，本研究中检测到的熔化温度与之前的实验和从头计算模拟在~ 25 GPa的温度下非常吻合（Liebske和Frost, 2012； de Koker等人，2013）。然而，在较高的压力下，测量到的共晶熔化曲线的温度比根据低压实验数据的热力学模型推断的曲线和由原子模拟计算的曲线要低。我们发现我们的数据与之前计算的熔融温度和sio2端元的熔体热力学性质不一致，并且表明在接近mg2sio4成分的mgo - sio2熔体中存在最大的短程有序。相对于mgsio3熔体绝热层的模式橄榄岩共熔曲率表明，全球岩浆海洋的结晶将开始于~ 100gpa，而不是地幔底部，从而允许早期的基底熔体层。模式橄榄岩熔融曲线位于地幔地温上方~ 500k处的核幔边界处，表明除非加入其他组分充分减少固相，否则橄榄岩不会熔融。模型玄武岩熔融曲线与地幔底部的地热相交，预计会发生洋壳的部分熔融。
Eutectic melting curves in the system MgO–SiO 2 have been experimentally determined at lower mantle pressures using laser-heated diamond anvil cell (LH-DAC) techniques. We investigated eutectic melting of bridgmanite plus periclase in the MgO–MgSiO 3 binary, and melting of bridgmanite plus stishovite in the MgSiO 3–SiO 2 binary, as analogues for natural peridotite and basalt, respectively. The melting curve of model basalt occurs at lower temperatures, has a shallower d T/d P slope and slightly less curvature than the model peridotitic melting curve. Overall, melting temperatures detected in this study are in good agreement with previous experiments and ab initio simulations at∼ 25 GPa (Liebske and Frost, 2012; de Koker et al., 2013). However, at higher pressures the measured eutectic melting curves are systematically lower in temperature than curves extrapolated on the basis of thermodynamic modelling of low-pressure experimental data, and those calculated from atomistic simulations. We find that our data are inconsistent with previously computed melting temperatures and melt thermodynamic properties of the SiO 2 endmember, and indicate a maximum in short-range ordering in MgO–SiO 2 melts close to Mg 2 SiO 4 composition. The curvature of the model peridotite eutectic relative to an MgSiO 3 melt adiabat indicates that crystallization in a global magma ocean would begin at∼ 100 GPa rather than at the bottom of the mantle, allowing for an early basal melt layer. The model peridotite melting curve lies∼ 500 K above the mantle geotherm at the core–mantle boundary, indicating that it will not be molten unless the addition of other components reduces the solidus sufficiently. The model basalt melting curve intersects the geotherm at the base of the mantle, and partial melting of subducted oceanic crust is expected.