An isotopic, elemental and structural study of silicon nitride from enstatite chondrites
An isotopic, elemental and structural study of silicon nitride from enstatite chondrites
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DOI：
10.1016/j.gca.2018.05.025
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发表时间：
2018-08
影响因子：
5
通讯作者：
J. Leitner;C. Vollmer;T. Henkel;U. Ott;P. Hoppe
中科院分区：
文献类型：
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作者：
J. Leitner;C. Vollmer;T. Henkel;U. Ott;P. Hoppe
本文报道了几种低岩石型顽火辉石球粒陨石(EL3、EH3和EH4)中氮化硅(Si3N4)颗粒的原位研究。这些颗粒赋存于不同的主相中，包括Fe、Ni金属、蛇纹石、硫化物以及硅酸盐组分，它们来自太阳系。能量色散X射线能谱(EDS)表明，所有被研究的颗粒中都存在碳和氧。用NanoSIMS测量了288个颗粒的碳、氮同位素组成。与陆地空气相比，氮的同位素很轻，平均δ15N=−60 ± 1‰。平均碳同位素组成与陆源PDB标准相差不大。对一个较大的Si3N4颗粒(10 µm × 2µm)的TOF-SIMS研究表明，O位于颗粒内部，而不在相邻的颗粒中，同时也揭示了铬的存在。透射电子显微镜分析表明，铬以钙铝榴石(CrN)包裹体的形式存在。三个Si3N4颗粒的电子显微镜研究表明，它们是多晶的，与主体材料没有一致的结晶学关系。四种金属-硫化物组合的Si3N4丰度估算表明，氮化物中结合的氮量超过了固溶态Fe、Ni金属中可储存的N的最大浓度。因此，即使金属中的所有氮都被析出，也不足以形成观察到的Si3N4颗粒，这为反对出溶形成提供了进一步的证据。这清楚地表明，它们不是像早期研究中所说的那样，通过从宿主材料中逸出而形成的。相反，Si3N4必须是在进入顽火辉石球粒陨石母体之前形成的，要么是冲击波诱导的冷凝过程，要么是在NH3存在的情况下从主体相沉淀形成的。
We report an in-situ investigation of silicon nitride (Si3N4) grains from several enstatite chondrites of low petrologic types (EL3, EH3, and EH4). The grains occurred in various host phases, including Fe,Ni metal, schreibersite, sulfides, and also in the silicate fraction, and are of Solar System origin. Energy-dispersive X-ray spectroscopy (EDS) showed that carbon and oxygen are present in all investigated grains. Carbon- and N-isotopic compositions of 288 grains were measured by NanoSIMS. Nitrogen is isotopically light compared to terrestrial air, with an average δ15N = −60 ± 1‰. The average carbon isotopic composition does not deviate significantly from the terrestrial PDB standard. TOF-SIMS investigation of one particularly large Si3N4grain (10 µm × 2 µm) showed that the O is located within the grain and not in adjacent particles, and also revealed the presence of chromium. Transmission electron microscopy (TEM) analysis showed that the Cr is present as carlsbergite (CrN) inclusions. TEM investigation of three Si3N4grains showed them to be polycrystalline, with no consistent crystallographic relationship with the host material. Estimated Si3N4abundances for four metal-sulfide assemblages demonstrate that the amount of nitrogen bound in the nitrides exceeds the maximum concentration of N that can be stored in Fe,Ni metal in solid solution. Thus, even ifallof the N in the metal would have been exsolved, it would not have been enough to form the observed amounts of Si3N4grains, giving further evidence against formation by exsolution. This clearly shows that they did not form by exsolution from the host materials, as has been suggested in earlier studies. Instead, the Si3N4must have formed prior to incorporation into the enstatite chondrite parent bodies, either by shock wave-induced condensation processes, or by precipitation from the host phases in the presence of NH3.