Chemical projectile–target interaction and liquid immiscibility in impact glass from the Wabar craters, Saudi Arabia
Chemical projectile–target interaction and liquid immiscibility in impact glass from the Wabar craters, Saudi Arabia
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沙特阿拉伯瓦巴尔陨石坑的冲击玻璃中的化学射弹-目标相互作用和液体不混溶性
DOI：
10.1016/j.gca.2013.07.030
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发表时间：
2012
影响因子：
5
通讯作者：
R. Wirth
中科院分区：
文献类型：
--
作者：
C. Hamann;L. Hecht;M. Ebert;R. Wirth
冲击玻璃通常受到次生蚀变和化学风化的强烈影响。因此，为了了解相关的形成过程中，详细的岩相学研究未风化的影响玻璃是必要的保存淬火的影响玻璃的不均匀性可以作为一种工具，以更好地了解其成因。在这里，我们报告的岩相学和微观化学的影响玻璃从瓦巴尔撞击坑（沙特阿拉伯），与年龄为1300年，是最年轻的陆地撞击坑。IIIAB铁陨石的一部分在撞击和随后的风化中幸存下来，这一事实使Wabar在目前已确认的184个陆地撞击结构中发挥了特殊作用。在Wabar对黑色冲击熔体/玻璃品种进行的电子探针分析（EMPA）和透射电子显微镜（TEM）表明，由于选择性氧化，陨石Fe在高温下选择性地与高二氧化硅靶熔体混合，导致黑色熔体的高Fe/Ni比（平均37，个别值范围从13到449）和低的铁/镍比的弹丸液滴（“铁镍球”与铁/镍比平均为3;铁/镍比的陨石是1012）。黑色熔体显示出乳液纹理，这是硅酸盐液体不可溶解的结果。液-液相分离导致形成聚合不良的、富含二价阳离子如Fe 2+、Ca 2+或Mg 2+的超碱性熔体（Lfe），其分散在高度聚合的、高二氧化硅熔体（Lsi）基质中。典型的Wabar黑色熔体乳液显示出10- 20%Lfe的基体中的球织构，其以两组球和液滴（直径10-30 nm和0.1-0.4 μm）的形式均匀分散在10 - 80- 90%Lsi基体中，偶尔还有弥散的FeNi球。而在大于10 μm的FeNi球周围，典型的乳状液织构转变为分散在~ 79%Lfe中的~ 21%Lsi。这种纹理的变化被解释为陨石Fe从陨石FeNi球到目标熔体由于选择性氧化的Fe Ni和Co的陨石Fe的转移的证据。在华巴尔黑熔体的体积组成的变化在很大程度上取决于不混溶的ultrabasicLfe，felsicLSi，和陨石FeNi球的遗骸之间的体积比。基于自然发生的相分离玻璃（这项工作和文献）和淬火实验（文献），有越来越多的证据表明，液体的不可渗透性是一个主要的过程中形成的玻璃撞击岩。
Impact glasses are usually strongly affected by secondary alteration and chemical weathering. Thus, in order to understand relevant formation processes, detailed petrographic studies on unweathered impact glasses are necessary as preserved heterogeneities in quenched impact glasses may serve as a tool to better understand their genesis. Here, we report on petrography and microchemistry of impact glasses from the Wabar impact craters (Saudi Arabia) that, with an age of ∼300 years, are among the youngest terrestrial impact craters. The fact that parts of the IIIAB iron meteorite have survived impact and subsequent weathering is granting Wabar a special role among the presently 184 confirmed terrestrial impact structures. Electron microprobe analysis (EMPA) and transmission electron microscopy (TEM) obtained on the black impact melt/glass variety at Wabar suggest that meteoritic Fe was selectively mixed with high-silica target melt at high temperatures due to selective oxidation, resulting in high Fe/Ni ratios for the black melt (37 on average, individual values range from 13 to 449) and low Fe/Ni ratios for projectile droplets (“FeNi spheres” with a Fe/Ni ratio of 3 on average; Fe/Ni ratio for the meteorite is ∼12). The black melt shows emulsion textures that are the result of silicate liquid immiscibility. Liquid–liquid phase-separation resulted in the formation of a poorly polymerized, ultrabasic melt (Lfe) rich in divalent cations like Fe2+, Ca2+, or Mg2+, that is dispersed in a highly polymerized, high-silica melt (Lsi) matrix. The typical Wabar black melt emulsion displays a spheres-in-a-matrix texture of ∼10–20%Lfehomogeneously dispersed in the form of two sets of spheres and droplets (10–30 nm and 0.1–0.4 μm in diameter) in ∼80–90%Lsimatrix, plus occasionally disseminated FeNi spheres. Around large (>10 μm) FeNi spheres, however, the typical emulsion texture changes to ∼21%Lsidispersed in ∼79%Lfe. This change of texture is interpreted as evidence for the transfer of meteoritic Fe from the meteoritic FeNi spheres into the target melt due to selective oxidation of Fe over Ni and Co. Variations in the bulk composition of Wabar black melt largely depend on the volume ratios between immiscible ultrabasicLfe, felsicLsi, and remains of meteoritic FeNi spheres. Based on natural occurrences of phase-separated glasses (this work and literature) and quenching experiments (literature), there is growing evidence that liquid immiscibility is a major process in the formation of glassy impactites.