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Core-Mantle Partitioning of Volatile Siderophile Elements and the Origin of Volatile Elements in the Earth
| Content Provider | Semantic Scholar |
|---|---|
| Author | Nickodem, Kyle Righter, Kevin Danielson, Lisa R. Pando, Kellye Lee, Christopher T. |
| Copyright Year | 2012 |
| Abstract | Introduction: There are currently several hypotheses on the origin of volatile siderophile elements in the Earth. One hypothesis is that they were added during Earth’s accretion and core formation and mobilized into the metallic core [1], others claim multiple stage origin [2], while some hypothesize that volatiles were added after the core already formed [3]. Several volatile siderophile elements are depleted in Earth’s mantle relative to the chondrites, something which continues to puzzle many scientists. This depletion is likely due to a combination of volatility and core formation. The Earth’s core is composed of Fe and some lighter constituents, although the abundances of these lighter elements are unknown [4]. Si is one of these potential light elements [5] although few studies have analyzed the effect of Si on metal-silicate partitioning, in particular the volatile elements. As, In, Ge, and Sb are trace volatile siderophile elements which are depleted in the mantle but have yet to be extensively studied. The metal-silicate partition coefficients of these elements will be measured to determine the effect of Si. Partition coefficients depend on temperature, pressure, oxygen fugacity, and metal and silicate composition and can constrain the concentrations of volatile, siderophile elements found in the mantle. Reported here are the results from 13 experiments examining the partitioning of As, In, Ge, and Sb between metallic and silicate liquid. These experiments will examine the effect of temperature, and metalcomposition (i.e., Si content) on these elements in order to gain a greater understanding of the core-mantle separation which occurred during the Earth’s early stages. The data can then be applied to the origin of volatile elements in the Earth. Procedures: The samples used for the series of experiments were powders composed of 70 wt.% Knippa Basalt, composition described in Lewis et al. [6], 30 wt.% metal mixture, and varying amounts of Si ranging from 0 to 10 wt.% Si. The metal mixture contained 80.9 wt.% Fe, 8.2 wt.% FeS, 2.4 wt.% Ge, 3.2 wt.% As2O3, 2.9 wt.% Sb2O3, and 2.4 wt.% In. These were ground into a powder and mechanically mixed. All experiments were run using graphite capsules. The runs were conducted using a piston cylinder apparatus at constant pressure of 1.0 GPa with various times and temperatures. Once at constant pressure, samples were heated to high enough temperatures to melt and attain equilibrium for durations based on diffusion time across the capsule [7]. The temperature was measured using a type C thermocouple (W-Re) wires with an accuracy of ±2° C. Samples were then quenched to a silicate glass with large metallic spheres by turning off the power and keeping constant pressure until the temperatures reached 100° C. Three series were performed (Table 1): two metalsilicate composition series with varying amounts of silicon performed at 1600°C and 1800°C, and an Si free temperature series ranging between 1500°C and 1800°C. Analysis: Samples were analyzed for major element composition using a Cameca SX100 for electron microprobe analysis at NASA-JSC. A 1 μm beam was used at 20kV and 10nA. A variety of natural and synthetic standards were used. All samples used graphite capsules and were carbon saturated, but carbon was not analyzed yet. However, based on previous studies, the carbon solubility within the metal can be up to 5.5 wt.% C between 1300°C and 1800°C [8]. For all samples the In, As, Sb, and Ge content of the glass was lower than the detection limit of the EMPA; therefore, the samples were analyzed for trace element composition using Laser Ablation Inductively Coupled Mass Spectrometer (LA-ICP-MS) at Rice University. Analysis was performed at Low Resolution (LR) and normalized to 43 Ca isotope. Isotopes 75 As, 115 In, 73 Ge, 74 Ge, and 121 Sb were the only trace elements specifically studied for this research. Results: Oxygen fugacity was calculated relative to the Iron-Wustite (IW) buffer using ∆IW= 2log[XFe/XFeO]. The ∆IW values ranged from ~-1.3 to -1.37 for Si free runs, compared to Si bearing runs which produced ∆IW values from -4.9 to -7.5. The partition coefficient is strongly dependent on oxygen fugacity (fO2). An increase in Si content will cause a decrease in fO2. The range of ΔIW values for these experiments falls in the range considered during Earth’s core formation (-1 to -5) [1,2]. Partitioning behavior of As, Sb, Ge, and In can be calculated in a differnent way using an exchange reaction of element i with valence n: iOn/2 silicate + (n/2)Fe metal = i metal + (n/2)FeO silicate |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120001844.pdf |
| Alternate Webpage(s) | https://www.lpi.usra.edu/meetings/lpsc2012/pdf/2295.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
