Geochemical identification of projectiles in impact rocks
Geochemical identification of projectiles in impact rocks
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DOI：
10.1111/j.1945-5100.2006.tb00448.x
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发表时间：
2006-11
影响因子：
2.2
通讯作者：
R. Tagle;L. Hecht
中科院分区：
文献类型：
--
作者：
R. Tagle;L. Hecht
摘要：本文评价了三种主要的地球化学方法在精确检测和鉴定撞击物中的陨石物质方面的潜力和局限性。通过确定受污染的撞击物中的某些同位素和元素比例，可以实现对撞击物中抛射物成分的识别。同位素方法是基于Os和Cr的同位素比率。锇同位素对探测撞击物中微量甚至<0.05 wt%的地外成分具有高度敏感性。然而，这只适用于目标岩性，几乎没有地幔物质的化学特征或年轻的幔源基性岩石。此外，该方法目前还不适合弹丸类型的精确识别。铬同位素方法需要相对较高的抛射污染（几wt%）来检测外星成分，但可能允许识别三种不同的外星物质，普通球粒陨石，顽火辉石球粒陨石和分化的无球粒陨石。该方法的一个显著优点是它不受目标岩性和冲击后蚀变的影响。元素比的使用，包括铂族元素（PGE: Os, Ir, Ru, Pt, Rh, Pd），结合Ni和Cr，代表了一种非常有效的方法，用于探测和识别地球和月球撞击中的弹丸。对于大多数弹丸类型，该方法几乎与目标成分无关，特别是考虑PGE比时。即使在上地幔物质成分较高的陆地目标岩性中也是如此。通过比较由混合线（靶-射）斜率得出的“射弹元素比”与不同类型可能的射弹（如球粒陨石）的元素比，可以识别出射弹。然而，这需要一组不同程度的弹丸污染的冲击样品。
Abstract— The three major geochemical methods for impactor identification are evaluated with respect to their potential and limitations with regards to the precise detection and identification of meteoritic material in impactites. The identification of a projectile component in impactites can be achieved by determining certain isotopic and elemental ratios in contaminated impactites. The isotopic methods are based on Os and Cr isotopic ratios. Osmium isotopes are highly sensitive for the detection of minute amounts of extraterrestrial components of even <<0.05 wt% in impactites. However, this only holds true for target lithologies with almost no chemical signature of mantle material or young mantle‐derived mafic rocks. Furthermore, this method is not currently suitable for the precise identification of the projectile type. The Cr‐isotopic method requires the relatively highest projectile contamination (several wt%) in order to detect an extraterrestrial component, but may allow the identification of three different groups of extraterrestrial materials, ordinary chondrites, an enstatite chondrites, and differentiated achondrites. A significant advantage of this method is its independence of the target lithology and post‐impact alteration. The use of elemental ratios, including platinum group elements (PGE: Os, Ir, Ru, Pt, Rh, Pd), in combination with Ni and Cr represents a very powerful method for the detection and identification of projectiles in terrestrial and lunar impactites. For most projectile types, this method is almost independent of the target composition, especially if PGE ratios are considered. This holds true even in cases of terrestrial target lithologies with a high component of upper mantle material. The identification of the projectile is achieved by comparison of the “projectile elemental ratio” derived from the slope of the mixing line (target‐projectile) with the elemental ratio in the different types of possible projectiles (e.g., chondrites). However, this requires a set of impactite samples of various degree of projectile contamination.