Microsegregation rates of liquid Fe-Ni-S metal in natural silicate-metal systems: A combined experimental and numerical study

We present results of an image-based numerical model aimed at quantifying the microsegregation and flow of liquid metal in meteorites prior to the onset of silicate melting. The sample material is the H6 chondrite Kernouve. The model utilizes the observed geometry of two distinct chondrite textures associated with grain-scale melt segregation in the following: (1) the undeformed (natural) state and (2) during deformation and partial melting under controlled (laboratory) conditions. The numerical simulations recover liquid metal segregation rates of similar to 10(-6) to 10(-4) m s(-1) for matrix permeabilities (k) of 10(-12) < k < 10(-10) m(2) and pressure gradients of similar to 1 and 10(4) Pa m(-1). The velocity flow field is position-dependent across the sample, reflecting initial grain-scale heterogeneity and anisotropy in the spatial distribution of metal prior to melting. In addition to porous flow, we use a coupled Brinkman-Navier-Stokes solution to quantify liquid metal segregation through deformation-induced microscale veins. Melt flow velocities in veins are several orders faster than matrix flow, implying that a combination of porous (grain-scale) flow feeding into a network of small-scale cracks and veins during the initial stages of partial melting may be an extremely efficient mechanism for segregating liquid metal from silicate matrix in planetesimals undergoing deformation. This mechanism may be temporary and confined only to the earliest stages of melt microsegregation because with increasing temperature, the onset of silicate melting shuts off liquid metal segregation by creeping matrix flow. The point at which this occurs marks an important transition in the mode and style of internal differentiation.