New method to investigate interconnection of core-forming metallic melt in a solid silicate matrix

Abstract:

The segregation mechanism of liquid metal and solid silicate have an important effect on the geophysical and geochemical properties of planetary bodies. Determination of interconnectivity of the liquid metal in solid silicate is the first step to quantitatively understand the percolative core formation process. Previously, interconnectivity is investigated by measuring the apparent dihedral angels or the electrical conductivity, or by X-ray tomography. Here we report a new method by using tracer element migration as an indicator of melt interconnection. In this method, the silicate-liquid metal system is surrounded by Pt capsule, served as the tracer element. If the melt in the system forms a network, the tracer element is expected to migrate into the melt. Otherwise, it remains at the sample-capsule boundary. Compared to the conventional methods, this method can conclusively demonstrate the melt connectivity. In addition, it can distinguish the residual melt from melt which forming the network by measuring the melt composition.

As an example, we use this method to study the connectivity of Fe-S melts in a deformed and undeformed bridgmanite matrix at 25 GPa and 2100 K in multi-anvil experiments. Our results demonstrated that Fe-S melt formed an interconnected network in a bridgmanite matrix under deformation and left ~0.4 vol. % residual melts in the system, while it formed isolated pockets under hydrostatic conditions. The high segregation velocity of the melt in a deformed bridgmanite matrix makes the stress-induced percolation a viable core formation mechanism. Moreover, the melts left in the mantle after draining could explain the highly siderophile elements (HSE) chondritic abundance in the Earth’s mantle without late veneer.

Finally, we emphasize that this method can be used not only in a single-phase system but also in a complex system with multiple mantle phases. The connectivity threshold can also be determined. The planetary applications of the newly developed method will place important constraints on the planetary evolution.