Mineralogical study of brown olivine in Northwest Africa 1950 shergottite and implications for the formation mechanism of iron nanoparticles
Mineralogical study of brown olivine in Northwest Africa 1950 shergottite and implications for the formation mechanism of iron nanoparticles
复制标题
西北非 1950 年褐橄榄石的矿物学研究及其对铁纳米颗粒形成机制的影响
DOI：
10.1111/maps.12949
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发表时间：
2017
影响因子：
2.2
通讯作者：
and T. Kogure
中科院分区：
文献类型：
--
作者：
A. Takenouchi;T. Mikouchi;and T. Kogure
火星陨石，特别是六角陨石，含有深色橄榄石（所谓的“棕色橄榄石”），其颜色是由冲击事件期间橄榄石中形成的铁纳米颗粒引起的。讨论了西北非洲2737（NWA 2737）磷灰岩中棕色橄榄石的形成过程和条件。然而，NWA 2737 中棕色橄榄石的形成条件不能适用于钠橄榄石，因为 NWA 2737 具有与钠橄榄石不同的冲击历史。因此，本研究对NWA 1950 角橄榄石中的棕色橄榄石进行了观察，并探讨了角橄榄石的一般形成过程和条件。我们对 NWA 1950 的观察表明，与 NWA 2737 中的棕色橄榄石不同，橄榄石在颗粒之间和颗粒内呈不均匀变暗。XANES 分析表明棕色橄榄石含有少量 Fe3+，TEM/STEM 观察表明铁金属纳米颗粒周围不存在富含 SiO 的相。这些观察结果表明，铁纳米颗粒是通过橄榄石的歧化反应形成的（3Fe2+橄榄石→Fe0金属+2Fe3+橄榄石+沃利文，其中沃利文是指橄榄石中的空位）。除了橄榄石预期的情况外，棕色橄榄石的某些部分在 SEM 观察和拉曼峰中还显示出层状结构，这意味着棕色橄榄石经历了相变（例如，变成尖橄榄石）。为了引起异质变暗，需要约 1500–1700 K 的异质高温和至少 ~90 ms 的冲击持续时间。这种非均质高温导致震后温度较高（>900 K），从而导致大多数高压相发生反向转变。因此，尽管缺乏高压相，NWA 1950（=带有棕色橄榄石的火星陨石）与其他高度震动的陨石群相比经历了更高的压力和温度。
Martian meteorites, in particular shergottites, contain darkened olivine (so‐called “brown olivine”) whose color is induced by iron nanoparticles formed in olivine during a shock event. The formation process and conditions of brown olivine have been discussed in the Northwest Africa 2737 (NWA 2737) chassignite. However, formation conditions of brown olivine in NWA 2737 cannot be applied to shergottites because NWA 2737 has a different shock history from that of shergottites. Therefore, this study observed brown olivine in the NWA 1950 shergottite and discusses the general formation process and conditions of brown olivine in shergottites. Our observation of NWA 1950 revealed that olivine is heterogeneously darkened between and within grains different from brown olivine in NWA 2737. XANES analysis showed that brown olivine contains small amounts of Fe3+and TEM/STEM observation revealed that there is no SiO‐rich phase around iron metal nanoparticles. These observations indicate that iron nanoparticles were formed by a disproportionation reaction of olivine (3Fe2+olivine→ Fe0metal+ 2Fe3+olivine+ Volivine, where Volivinemeans a vacancy in olivine). Some parts of brown olivine show lamellar textures in SEM observation and Raman peaks in addition to those expected for olivine, implying that brown olivine experienced a phase transition (to e.g., ringwoodite). In order to induce heterogeneous darkening, heterogeneous high temperature of about 1500–1700 K and shock duration of at least ~90 ms are required. This heterogeneous high temperature resulted in high postshock temperature (>900 K) inducing back‐transformation of most high‐pressure phases. Therefore, in spite of lack of high‐pressure phases, NWA 1950 (= Martian meteorites with brown olivine) experienced higher pressure and temperature compared to other highly shocked meteorite groups.