Metal-silicate Partitioning at High Pressure and Temperature: Experimental Methods and a Protocol to Suppress Highly Siderophile Element Inclusions

Summary

We present a procedure to determine the metal-silicate partitioning of siderophile elements, emphasizing techniques that suppress the formation of metal inclusions in experiments for the noble metals. The results of these experiments are used to demonstrate the effect of core-formation on the highly siderophile element composition of the mantle.

Abstract

Estimates of the primitive upper mantle (PUM) composition reveal a depletion in many of the siderophile (iron-loving) elements, thought to result from their extraction to the core during terrestrial accretion. Experiments to investigate the partitioning of these elements between metal and silicate melts suggest that the PUM composition is best matched if metal-silicate equilibrium occurred at high pressures and temperatures, in a deep magma ocean environment. The behavior of the most highly siderophile elements (HSEs) during this process however, has remained enigmatic. Silicate run-products from HSE solubility experiments are commonly contaminated by dispersed metal inclusions that hinder the measurement of element concentrations in the melt. The resulting uncertainty over the true solubility and metal-silicate partitioning of these elements has made it difficult to predict their expected depletion in PUM. Recently, several studies have employed changes to the experimental design used for high pressure and temperature solubility experiments in order to suppress the formation of metal inclusions. The addition of Au (Re, Os, Ir, Ru experiments) or elemental Si (Pt experiments) to the sample acts to alter either the geometry or rate of sample reduction respectively, in order to avoid transient metal oversaturation of the silicate melt. This contribution outlines procedures for using the piston-cylinder and multi-anvil apparatus to conduct solubility and metal-silicate partitioning experiments respectively. A protocol is also described for the synthesis of uncontaminated run-products from HSE solubility experiments in which the oxygen fugacity is similar to that during terrestrial core-formation. Time-resolved LA-ICP-MS spectra are presented as evidence for the absence of metal-inclusions in run-products from earlier studies, and also confirm that the technique may be extended to investigate Ru. Examples are also given of how these data may be applied.

Introduction

Terrestrial accretion is thought to have occurred as a series of collisions between planetesimals with a chondritic bulk composition, terminating in a giant-impact phase thought responsible for moon formation^1,2. Heating of the proto-earth by impacts and the decay of short-lived isotopes was sufficient to cause extensive melting and the formation of a magma ocean or ponds through which dense Fe-rich metallic melts could descend. Upon reaching the base of the magma ocean, metallic melts encounter a rheological boundary, stall, and undergo final metal-silicate equilibrium before eventually descending through the solid mantle to the growing core². F....

Protocol

1) Preparation of Starting Material

1. Synthetic Basalt
   Note: A basaltic composition is used as the silicate starting material as more depolymerized compositions, although more relevant to a magma ocean scenario, are difficult or impossible to quench to a glass in piston-cylinder and multi-anvil experiments.
   1. Weigh the desired amounts of component oxide or carbonate (Ca and Na) powders, with the exception of Fe, and add to an agate mortar (see example in Table 1). An Fe-free mixture weighing ~4 g should provide sufficient starting material for an extensive suite of experiments.
   2. Add ethanol to the agate mortar unti....

Results

The following examples and discussion focus on experiments to determine HSE solubility in silicate melts at low fO₂. For comprehensive examples of how MSE and SSE partitioning data from multi-anvil experiments may be used to constrain the P-T-fO₂ conditions of core metal segregation, the reader is referred to references^9–11. Figure 7B-D displays back scattered electron images from typical experimental run-products........

Discussion

The results of inclusion-free experiments performed using the protocols outlined here have previously been compared with literature data in references²⁹ (Os, Ir, Au), ³⁰ (Re, Au) and ³¹ (Pt). Pt is most instructive in demonstrating the usefulness of inclusion-free run-products. For experiments run at low fO₂, Ertel et al.⁴⁸ assigned inclusions to a stable origin and therefore restricted data reduction to the lowest counts-per-second region of time-r.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
G10 Epoxy/Fiberglass Sheet	Accurate plastics, Inc.	GEES.020N.3648
Powdered starting materials- -Oxides, metals, carbonates	Alfa Aesar	Specific to desired experiment
Castable 2-part MgO ceramic	Aremco	Ceramcast - 584
PTFE Dry Lubricant	Camie-Campbell	2000 TFE-Coat
Graphite resistance heaters	Carbone of America (Now owned by Mersen USA)	Custom Order
Barium Carbonate	Chemical Products Corporation	Custom Order	Calcined free-flowing (CFF) grade
C-Type Thermocouple Wire (W26%Re, W5%Re)	Concept Alloys		~0.25 mm diameter is suitable for most experiments
Zirconia Cement	Cotronics; Resbond 940 2-part cement		Use 100 parts powder for every 25 to 28 parts activator
Polyvinyl Acetate (PVA) Glue	e.g., Bostik		Often sold as 'white glue'
Cyanoacrylate Glue	e.g., Krazy Glue/Loctite
Piston cylinder pressure vessel and WC piston	Hi-Quality Carbide Tooling Inc.	Custom Order
Silica Glass Tubing	Quartz Plus	Custom Order
Crushable ZrO2 tubes	Saint-Gobain	Custom Order
Crushable MgO rods and tubes	Saint-Gobain	Custom Order
WC cubes for multi-anvil experiments	Tungaloy	Custom Order	Cubes are grade-F WC alloy
Single hole alumina tube for multi-anvil thermocouple	Vesuvius McDanel	AXS071730-04-06
4-hole alumina tube for piston cylinder thermocouple	Vesuvius McDanel	AXF1159--07-12
4-hole alumina tube for multi-anvil thermocouple	Vesuvius McDanel	AXF1159-04-06