Simulation of the Planetary Interior Differentiation Processes in the Laboratory

Summary

The high-pressure and high-temperature experiments described here mimic planet interior differentiation processes. The processes are visualized and better understood by high-resolution 3D imaging and quantitative chemical analysis.

Abstract

A planetary interior is under high-pressure and high-temperature conditions and it has a layered structure. There are two important processes that led to that layered structure, (1) percolation of liquid metal in a solid silicate matrix by planet differentiation, and (2) inner core crystallization by subsequent planet cooling. We conduct high-pressure and high-temperature experiments to simulate both processes in the laboratory. Formation of percolative planetary core depends on the efficiency of melt percolation, which is controlled by the dihedral (wetting) angle. The percolation simulation includes heating the sample at high pressure to a target temperature at which iron-sulfur alloy is molten while the silicate remains solid, and then determining the true dihedral angle to evaluate the style of liquid migration in a crystalline matrix by 3D visualization. The 3D volume rendering is achieved by slicing the recovered sample with a focused ion beam (FIB) and taking SEM image of each slice with a FIB/SEM crossbeam instrument. The second set of experiments is designed to understand the inner core crystallization and element distribution between the liquid outer core and solid inner core by determining the melting temperature and element partitioning at high pressure. The melting experiments are conducted in the multi-anvil apparatus up to 27 GPa and extended to higher pressure in the diamond-anvil cell with laser-heating. We have developed techniques to recover small heated samples by precision FIB milling and obtain high-resolution images of the laser-heated spot that show melting texture at high pressure. By analyzing the chemical compositions of the coexisting liquid and solid phases, we precisely determine the liquidus curve, providing necessary data to understand the inner core crystallization process.

Introduction

Terrestrial planets such as the Earth, Venus, Mars, and Mercury are differentiated planetary bodies consisting of a silicate mantle and a metallic core. The modern planet formation model suggests that the terrestrial planets were formed from collisions of Moon-to-Mars-sized planetary embryos grown from km-sized or larger planetesimals through gravitational interactions^1-2. The planetesimals were likely differentiated already once the metallic iron alloys reached melting temperature due to heating from sources such as radioactive decay of short-lived isotopes such as ²⁶Al and ⁶⁰Fe, impact, and release of potential energy³. It....

Protocol

1. Prepare Starting Materials and Sample Chambers

1. Prepare two types of starting materials, (1) a mixture of natural silicate olivine and metallic iron powder with 10 wt% sulfur (metal/silicate ratios ranging from 4 to 30 wt%) for simulating percolation of liquid iron alloy in a solid silicate matrix during the initial core formation of a small planetary body, and (2) a homogeneous mixture of finely-grounded pure iron and iron sulfide for determining the planetary inner core crystallization.
2. Grind the starting materials to fine mixed powder under ethanol in an agate mortar for one hour and dried at 100 °C.
3. Load the starting material....

Results

We have conducted a series of experiments using mixtures of San Carlos olivine and Fe-FeS metal alloy with different metal-silicate ratios, as the starting materials. The S content of the metal is 10 weight % S. Here we show some representative results from high-pressure experiments performed at 6 GPa and 1,800 °C, using well-calibrated multi-anvil assemblies¹⁵. Under the experimental conditions, the Fe-FeS metal alloy is completely molten and the silicate (San Carlos olivine) remains crystalline. The purpose .......

Discussion

The techniques for the multi-anvil experiments are well established, generating stable pressure and temperature for an extended period of run time and producing relatively large sample volume. It is a powerful tool to simulate the interior processes of planets, especially for experiments, such as melt percolation, that require certain sample volume. The limitation is the maximum achievable pressure, up to 27 GPa with tungsten carbide (WC) anvils, reaching the core pressures of Mars and Mercury, but far too low pressure t.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Multi-anvil apparatus	Geophysical Lab	Home Builder
Diamond-anvil cell	Geophysical Lab	Home Builder
Laser-heating system	APS GSECARS	Designed by beamline staff	Public beamline
FIB/SEM Crossbeam	Carl Zeiss Ltd.	Auriga
Avizo 3D software	VSG	Fire for materials science