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- Title
Triple silicon isotopic fractionation between silicates and metal in Enstatite Chondrites.
- Authors
Sun, H. X.; Chaussidon, M.; Moynier, F.
- Abstract
Introduction: Enstatite chondrites (EC) is a group of meteorites with unique mineralogy and petrology dominated by enstatite, various sulfides and metals. The high abundance of sulfides and nearly ~2% of Si in metal indicate that EC were formed under reducing conditions. Most isotopic evidence, except silicon, suggest that the Earh and Enstatite chondrites share a common precursor component. The slightly heavier Si isotopic composition of bulk silicate Earth (d30SiBSE ≈ -0.3‰) relative to EC (d30SiEC ≈ -0.5‰) could be partly due to Si isotopic fractionation during core-mantle differentiation. However, the Si content calculated for the Earth's core, using equilibrium fractionation factors and data from meteorites and experiments, cannot explain the Earth's d30SiBSE. In this study, we use high precision measurements of triple silicon isotopes to further study the mechanism of Si silicate-metal fractionation in EC. Sample and analytical Methods: Itqiy (EH7) is a coarse-grained meteorite composed of enstatite (Mg# > 98), metal and sulfides. The metal contains 1~3 wt% of Si while there is only <0.1 wt% Si in sulfide [1]. Enstatite and metal grains were separated and handpicked under the microscope after having been coarsely crushed in a metal motar. The enstatite grains were fused in 10 aliquots were fused in with NaOH flux at 730°C in Ag crucibles. The fusion cakes were dissolved by MQ and acidified to pH=1 by HNO3. Eight aliquots of metals were dissolved in 0.5 N HNO3 followed by dilution to reach a HNO3 concentration of 0.1N. Each aliquot contains 2 or 3 enstatite or metal grains. The solution was loaded in AG50-X12 resin to remove the matrix and the Si isotopic ratio were measured with a Neptune-Plus MC-ICP-MS at IPGP. The chemistry and tuning on the Neptune-Plus were modified to decrease the interference effect [2] and improve the analytical precision, which allows to identify small deviations between equilibrium and kinetic mass-dependent fractionation. Result: The d30SiEnstatite values of Itqiy range from -0.253 ± 0.028 to -0.877 ± 0.082 ‰ (2se) with an average of - 0.578 ± 0.119 ‰ (2se, n = 10). The d30SiMetal is lighter than d30SiEnstatite and ranges from -3.385 ± 0.065 to -3.905 ± 0.044 ‰ (2se) with an average of -3.603 ± 0.121 ‰ (2se, n = 8). The fractionation between the silicate and metal with propagated 2se is 3.025 ± 0.170. These isotopic compositions are consistent with a previous study [3]. Discussion: The linear regression between d29Si and d30Si values of silicate and metal gives a slope of 0.5114 ± 0.0091 (2se). This slope is lower than the theoretical ßeq (0.5178) for equilibrium isotopic fractionation at high temperature [4] and, similarly with what is observed for O isotopes, this might be due to a lower equilibrium temperature. However, the presence of ~0.65‰ variation in d30SiEnstatite implies an isotopic heterogeneity within silicates, which is unexpected considering petrology [1]. Furthermore, the triple Si isotope slopes for silicates only (0.6813 ± 0.0605, 2se) is distinct from what is expected for any mass-dependent isotopic fractionation law. The reason for that is not yet well understood but it could reflect the presence in the matrix of presolar materials [5] with variable Si isotopic anomalies and having undergone partial isotopic exchange with silicates and metal during partial melting and impact metamorphism of Itqiy.
- Subjects
SILICON isotopes; ISOTOPIC fractionation; ENSTATITE; CHONDRITES; EARTH'S core
- Publication
Meteoritics & Planetary Science, 2022, Vol 57, p6324
- ISSN
1086-9379
- Publication type
Article