Loading...
Please wait, while we are loading the content...
Similar Documents
In-Situ Analyses of Highly Siderophile Elements in Fe-Ni Metal of Ordinary Chondrites
| Content Provider | Semantic Scholar |
|---|---|
| Author | Okabayashi, Satoki Yokoyama, Tatsuhiro Nakanishi, Nao Iwamori, Hikaru |
| Copyright Year | 2017 |
| Abstract | Introduction: Ordinary chondrites (OCs) are by far the most abundant class of meteorites that have not been experienced extensive differentiation including coremantle segregation. They are characterized by relatively small but abundant Fe-Ni metal grains (<~100 μm). FeNi metal grains in OCs occur in chondrules (“interior grains”), at the surface of chondrules (“margin grains”), and in matrix (“isolated grains”). Although Fe-Ni metal is one of the major components in OCs, the formation processes of metal grains have been still veiled. Kong and Ebihara [1,2] reported that metal grains of some OCs did not possess the solar Co/Ni ratio. The authors thus ruled out direct condensation in the solar nebula for the sole or major origin of OC metals. Alternatively, they suggested that reduction of FeO in silicate during chondrule heating formed OC metals because apparent metal-silicate partition coefficients for some elements (e.g., W, Mo) were consistent with those obtained in melting experiments that simulated the chondrule formation [1,2]. Highly siderophile elements (HSE: Ru, Rh, Pd, Re, Os, Ir, Pt, Au) in Fe-Ni metal grains are key tracers for studying the origin of OC metals due to their significantly high affinity to metals relative to silicates. Previous works on in-situ analyses of HSEs for individual OC metal grains presented positive correlations between refractory HSEs (Ru, Re, Os, Ir, Pt), whereas these elements did not show any meaningful correlation with volatile HSEs (Pd, Au) [3]. Such discrepancy between refractory and volatile HSEs cannot be explained by the simple reduction of FeO in silicate suggested by [1, 2]. The goal of this study is to understand the processes that fractionated HSEs in OC metals. In this study, insitu analysis using LA-ICP-MS was performed for measuring HSE concentrations in Fe-Ni metal grains. One of the obstacles to illuminating the metal formation is thermal metamorphism on OC parent bodies. In this study, therefore, we specifically focused on the type-3 OCs to minimize the effect of parent body thermal processing. Experimental: Slabs of Richfield (LL3.7), NWA 6910 (L3.3) and Sahara 97210 (L/LL3.2) were cut into small pieces and embedded in petropoxy 154 resin. Their surface was polished, and then mapped by SEMEDS (S-3400N, Hitachi High-Technologies) to classify the observed Fe-Ni metals into interior, margin, and isolated grains. The areas of Fe-Ni metal grains were also measured and the grains with >20 μm diameter were dedicated for LA-ICP-MS analysis. We utilized a Ti:sapphire UV-fs laser (IFRIT, Cyberlaser) with quadrupole ICP-MS (XSERIES 2, Thermo Scientific) for LA-ICP-MS analysis. Fourteen siderophile elements including HSEs (Fe, Co, Ni, Cu, Mo, Ru, Rh, Pd, W, Re, Os, Ir, Pt, Au) were measured by spot analysis (~20–40 μm). During the analysis, Mg, Si, and S were also monitored to avoid beam overlaps from silicate and sulfide phases. The laser repetition rate was fixed at 10 Hz, and the fluence was varied (8–15 J cm-2) depending on the size of metal grains and concentration of elements. The pits produced after 60 s ablation were ~20–40 μm in diameter. An iron meteorite, Chinga (ung), the standard of our LA-ICP-MS analysis, was measured each after 10 spots of sample measurements. The data acquisition was made in the time-resolved analysis mode. The concentrations of siderophile elements were calculated by using the signal of 61Ni as for the internal standard. |
| Starting Page | 1211 |
| Ending Page | 1211 |
| Page Count | 1 |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://www.hou.usra.edu/meetings/lpsc2017/pdf/1211.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
