Name: metal-silicate partitioning at high pressure and temperature: experimental methods and a protocol to suppress highly siderophile element inclusions
Uploaded: 2015-06-13T14:00:00
Description: Watch this Scientific Journal Video about Metal-silicate Partitioning at High Pressure and Temperature: Experimental Methods and a Protocol to Suppress Highly Siderophile Element Inclusions at JoVE.co

This work was supported by the Natural Sciences and Engineering Research Council of Canada Equipment, Discovery and Discovery Accelerator Grants awarded to J.M.B. N.R.B acknowledges support from the Carnegie Institution of Washington post-doctoral fellowship program. Stephen Elardo is also thanked for his assistance prior to filming with the piston-cylinder press at the Geophysical Lab.

1) Preparation of Starting Material
<ol>
	<li>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.
	<ol>
		<li>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 mi...

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...

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 formation1,2. 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. Further chemical communication between metal and silicate phases as metallic melt traverses the solid portion of the mantle is thought to be precluded due to the large size and rapid descent of metal diapirs3. This primary differentiation of the Earth into a metallic core and silicate mantle is revealed today by both geophysical and geochemical observations4&#8211;6. Interpreting these observations to yield plausible conditions for metal-silicate equilibrium at the base of a magma ocean, however, requires an appropriate database of experimental results.
The primitive upper mantle (PUM) is a hypothetical reservoir comprising the silicate residue of core formation and its composition therefore reflects the behaviour of trace elements during metal-silicate equilibrium. Trace elements are distributed between metal and silicate melts during core segregation on the basis of their geochemical affinity. The magnitude of an elements preference for the metal phase can be described by the metal-silicate partition coefficient <img alt="Equation 1" src="/files/ftp_upload/52725/52725eq1.jpg" />
<img alt="Equation 2" src="/files/ftp_upload/52725/52725eq2.jpg" />&#160; &#160; &#160; &#160; &#160;(1)
Where <img alt="Equation 3" src="/files/ftp_upload/52725/52725eq3.jpg" /> and <img alt="Equation 4" src="/files/ftp_upload/52725/52725eq4.jpg" /> denote the concentration of element i in metal and silicate melt respectively. Values of <img alt="Equation 5" src="/files/ftp_upload/52725/52725eq5.jpg" /> &#62;1 indicate siderophile (iron-loving) behaviour and those &#60;1 lithophile (rock-loving) behavior. Estimates of the PUM composition show that siderophile elements are depleted relative to chondrites7, typically considered as representative of Earth&#8217;s bulk composition6,8. This depletion is due to sequestration of siderophile elements by the core, and for refractory elements its magnitude should directly reflect values of <img alt="Equation 5" src="/files/ftp_upload/52725/52725eq5.jpg" /> . Lab experiments therefore seek to determine values of <img alt="Equation 5" src="/files/ftp_upload/52725/52725eq5.jpg" /> over a range of pressure (P), temperature (T) and oxygen fugacity (fO2) conditions that are relevant to metal segregation from the base of a magma ocean. The results of these experiments may then be used to delineate regions of P-T-fO2 space that are compatible with the PUM abundance of multiple siderophile elements (e.g.,9&#8211;11).
The high pressures and temperatures relevant to a magma ocean scenario can be recreated in the laboratory using either a piston-cylinder or multi-anvil press. The piston-cylinder apparatus provides access to moderate pressure (~2 GPa) and high temperature (~2,573 K) conditions, but enables large sample volumes and a variety of capsule materials to be easily used. The rapid cooling rate also permits quenching of a range of silicate melt compositions to a glass, thus simplifying textural interpretation of the run-products. The multi-anvil apparatus typically employs smaller sample volumes but with suitable assembly designs can achieve pressures up to ~27 GPa and temperatures of ~3,000 K. The use of these methods has allowed partitioning data for many of the moderately and slightly siderophile elements to be gathered over a large range of P-T conditions. Predictions of the PUM composition based on these data suggest metal-silicate equilibrium occurred at average pressure and temperature conditions in excess of ~29 GPa and 3,000 K respectively, although the exact values are model dependent. In order to account for the PUM abundance of certain redox sensitive elements (e.g.,&#160;V, Cr) the fO2 is also thought to evolve during accretion from ~4 to 2 log units below that imposed by co-existing iron and w&#252;stite (FeO) at equivalent P-T conditions (the iron-w&#252;stite buffer)12.
Although the PUM abundance of many siderophile elements can be accounted for by metal-silicate equilibrium at the base of a deep magma ocean, it has proved difficult to assess if this situation also applies to the most highly siderophile elements (HSEs). The extreme affinity of the HSEs for iron-metal indicated by low pressure (P ~0.1 MPa) and temperature (T &#60;1,673 K) experiments suggests the silicate earth should be strongly depleted in these elements. Estimates of the HSE content for PUM, however, indicate only a moderate depletion relative to chondrite (Figure 1). A commonly posited solution to the apparent HSE excess is that Earth experienced a late-accretion of chondritic material subsequent to core-formation13. This late-accreted material would have mixed with the PUM and elevated HSE concentrations but had a negligible effect on more abundant elements. Alternatively, it has been suggested that the extremely siderophile nature of HSEs indicated by low P-T experiments does not persist to the high P-T conditions present during core-formation14,15. In order to test these hypotheses, experiments must be carried out to determine the solubility and metal-silicate partitioning of HSEs at appropriate conditions. Contamination of the silicate portion of quenched run-products in many previous studies however, has complicated run-product analysis and obscured the true partition coefficients for HSEs between metal and silicate melts.
In partitioning experiments where the HSEs are present at concentration levels appropriate to nature, the extreme preference of these elements for Fe-metal prevents their measurement in the silicate melt. To circumvent this problem, solubility measurements are made in which the silicate melt is saturated in the HSE of interest and values of <img alt="Equation 5" src="/files/ftp_upload/52725/52725eq5.jpg" /> are calculated using the formalism of Borisov et al.16. Quenched silicate run-products from HSE solubility experiments performed at reducing conditions, however, often display evidence for contamination by dispersed HSE&#177;Fe inclusions17. Despite the near ubiquity of these inclusions in low fO2 experiments containing Pt, Ir, Os, Re and Ru, (e.g., 18&#8211;27), there is notable variability between studies in their textural presentation; compare for example references 22 and 26. Although it has been demonstrated that inclusions can form which are a stable phase at the run conditions of an experiment28, this does not preclude the formation of inclusions as the sample is quenched. Uncertainty surrounding the origin of inclusions makes the treatment of analytical results difficult, and has led to ambiguity over the true solubility of HSEs in reduced silicate melts. Inclusion-free run-products are required to assess which studies have adopted an analytical approach that yields accurate dissolved HSE concentrations. Considerable progress in suppressing the formation of metal-inclusions at reducing conditions has now been demonstrated in experiments using a piston-cylinder apparatus, in which the sample design was amended from previous studies by adding either Au or Si to the starting materials29&#8211;31. The addition of Au or elemental Si to the starting materials alters the sample geometry or fO2 evolution of the experiment respectively. These methods are intended to suppress metal inclusion formation by altering the timing of HSE in-diffusion versus sample reduction, and are discussed in Bennett et al.31. Unlike some previous attempts to cleanse the silicate melt of inclusions, such as mechanically assisted equilibration and the centrifuging piston-cylinder, the present protocol can be implemented without specialized apparatus and is suitable for high P-T experiments.
Described in detail here is a piston-cylinder based approach to determine the solubility of Re, Os, Ir, Ru, Pt and Au in silicate melt at high temperature (&#62;1,873 K), 2 GPa and an fO2 similar to that of the iron-w&#252;stite buffer. Application of a similar experimental design may also prove successful in HSE experiments at other pressures, providing the required phase relations, wetting properties and kinetic relationships persist to the chosen conditions. Existing data however, are insufficient to predict whether our sample design will be successful at pressures corresponding to a deep magma ocean. Also outlined is a general approach used to determine moderately and slightly siderophile element (MSE and SSE respectively) partitioning using a multi-anvil device. Extension of the inclusion-free dataset for HSEs to high pressure is likely to employ similar multi-anvil methods. Together, these procedures provide a means to constrain both the conditions of core-segregation and the stages of terrestrial accretion.

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metal-silicate partitioning at high pressure and temperature: experimental methods and a protocol to suppress highly siderophile element inclusions

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.

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 references9&#8211;11. Figure 7B-D&#160;displays back scattered electron images from typical experimental ru...

Watch this Scientific Journal Video about Metal-silicate Partitioning at High Pressure and Temperature: Experimental Methods and a Protocol to Suppress Highly Siderophile Element Inclusions at JoVE.co

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

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.

Estimates of the primitive upper mantle (PUM) composition reveal a depletion in many of the siderophile (iron-loving) elements, thought to result from ...

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