Connectivity of molten Fe alloy in peridotite based on in situ electrical conductivity measurements: implications for core formation in terrestrial planets
Connectivity of molten Fe alloy in peridotite based on in situ electrical conductivity measurements: implications for core formation in terrestrial planets
复制标题
DOI：
10.1016/j.epsl.2004.03.010
复制
发表时间：
2004-05
影响因子：
5.3
通讯作者：
T. Yoshino;M. Walter;T. Katsura
中科院分区：
文献类型：
--
作者：
T. Yoshino;M. Walter;T. Katsura
用高温、1 Gpa的原位电导率测量方法，对橄榄岩中熔融的铁-S连通性进行了实验研究。起始原料是在Fe-FeS二元系中含有不同量(0，3，6，13，19，24体积%)的1 Gpa共晶成分的橄榄岩KLB-1的粉末混合物。在Fe-FeS系共晶点以上(∼980°C)和KLB1固相线以下(∼1200°C)，当体积分数超过∼5%时，熔融在固体硅酸盐中的Fe-S相互连接。电导率-温度路径表明，在部分硅酸盐熔融存在的情况下，熔融的Fe-S在橄榄岩基质中的连通性被抑制。根据对恢复样品的观察，估算出在低到中等硅酸盐熔体存在时，Fe-S熔体的渗流阈值为13±2vol.%。这些结果表明，如果小行星中Fe合金的体积分数最初大于5%，如果放射性核素的早期衰变加热使内部温度高于Fe合金熔点，则初始金属偏析是由铁合金在固体硅酸盐基质中的渗透流动控制的。早期太阳星云中的许多地物很可能满足了这些条件。要有效地去除硅酸盐中残留的铁合金(5%体积分数)，需要高度熔化硅酸盐，使金属能够以液滴的形式分离。在大型行星天体吸积的最后阶段，巨大的撞击可以提供高度融化所需的能量。或者，如果最初的金属偏析被推迟，直到行星物体增长到大尺寸(∼直径1000公里)，由于金属偏析而释放的引力势能可能会贡献足够的热量来形成岩浆海洋。
The connectivity of molten Fe–S in peridotite has been experimentally investigated by means of in situ electrical conductivity measurements at high temperatures and 1 GPa. Starting materials were powdered mixtures of peridotite KLB-1 with various amounts (0, 3, 6, 13, 19, 24 vol.%) of the 1 GPa eutectic composition in the Fe–FeS binary system. At temperatures above the eutectic point in the Fe–FeS system (∼980 °C) and below the solidus of KLB1 (∼1200 °C), molten Fe–S in a solid silicate matrix interconnects when the volume fraction is over ∼5%. Conductivity–temperature paths indicate that in the presence of partial silicate melting the connectivity of molten Fe–S in a peridotite matrix is inhibited. Based on observations of retrieved samples, the percolation threshold of Fe–S melts in the presence of low to moderate degrees of silicate melt is estimated at 13±2 vol.%. These results indicate that if the volume fraction of Fe-alloy in a planetesimal was initially greater than 5%, and if early heating by decay of radionuclides raised the temperature of the interior above the Fe-alloy melting point, initial metal segregation was controlled by permeable flow of molten iron alloy in a solid silicate matrix. These conditions were likely met by many terrestrial objects in the early solar nebula. Efficient removal of residual Fe-alloy (5 vol.%) from silicate requires high-degree melting of silicate so that metal can segregate as droplets. Giant impacts during the final stage of accretion of large planetary objects could supply the energy required for high-degrees of melting. Alternatively, if initial metal segregation were delayed until a planetary object grew to large size (∼1000 km in diameter), release of gravitational potential energy due to metal segregation could contribute enough heat to form a magma ocean.