Impact controversies: Impact recognition criteria and related issues
Impact controversies: Impact recognition criteria and related issues
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影响力争议：影响力认定标准及相关问题
DOI：
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发表时间：
2014
期刊：
影响因子：
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通讯作者：
C. Koeberl
中科院分区：
文献类型：
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作者：
W. Reimold;L. Ferrière;A. Deutsch;C. Koeberl
我们惊讶地看到，2013年8月刊的《流星与行星科学》(MAPS)的封面上装饰着一张格陵兰混合岩的现场图像。该说明提到了格陵兰岛的Maniitsoq构造，并解释说，封面上显示的岩性“被解释为是由于现已深入挖掘的太古宙撞击结构中的地壳规模的热液对流单元”(Scherst en和Garde 2013)。我们发现这一说法令人惊讶，因为Maniitsoq结构(Garde等人)。2012、2013)没有被广泛接受，因为这些文件中报告的证据不符合关于影响的既定承认标准(例如，见法文和Koeberl[2010]及其参考文献；Reimold等人)。2013年)。加德和他的同事断言，当已建立的标准“不起作用”时，可以制定新的标准。在我们看来，提出这样一种与地图封面相关的声明--强调这一结构，是对(我们坚持称之为)拟议的但未经证实的影响结构的不必要的赞扬。本封面提到的Scherst en和Garde(2013)的文章包含了高质量的锆石U-Pb数据，这些数据被解释为撞击事件导致的同位素体系的热液重新平衡。Reimold等人。(2013)建议对Maniitsoq锆石颗粒进行详细调查，以调查可能存在的撞击证据，其形式为平面断裂、冲击诱导的颗粒结构或锆石中的孪生。然而，Scherst en和Garde(2013)中展示的大量锆石图像并没有说明冲击变形的任何纹理证据。虽然我们明确赞赏高质量的U-Pb年龄，但我们必须坚持认为，仍然没有确凿的证据表明马尼托克受到了影响。间接证据也不排除马尼托克迄今报告的观测和数据的替代方案，如岩浆/构造解释。让我们假设Maniitsoq可能是一个直径约150公里的严重侵蚀的撞击结构。这将把Maniitsoq置于“大型、陈旧、被侵蚀的撞击结构”的范围内，如加拿大的萨德伯里或南非的弗雷德福德。在这些情况下，特别是在长期存在争议的、被严重侵蚀的Vredefort结构中，撞击起源的最终证据来自于在石英中发现了破碎圆锥和平面变形特征(PDF)，以及撞击熔岩中存在陨石成分，所有这些都被撞击界接受为撞击起源的明确证据。Leroux等人展示了Vredefort石英中经常经过退火和/或装饰的平面变形特征。(1994)通过详细的透射电子显微镜分析，以巴西双胞胎和装饰的高阶PDF的形式表示真实的冲击变形。只有有了这一证据--进一步得到锆石冲击变形(平面断裂和粒状锆石结构)的明确证据的进一步支持--不同的作者才能够在后来的撞击背景下解释角砾岩体、构造观察和形态测量数据。平面是平面变形特征中的关键术语，因此Scherst en和Garde(2013)将“通常弯曲的、粗化的和部分退火的”特征描述为“平面特征”(p.1474)与已发表的定义不一致。它们的特征并没有被完美地保存下来，但从来都不是平面的。因此，我们认为它们不能提供震荡事件的证据。他们还指出，“已知撞击结构中经过验证但保存不完善的PDF在文献中很少被报道”，这代表了另一个模糊的发现--因为“不完美保存”(在他们的意义上指的是弯曲的)特征很少被识别为撞击诊断PDF。正如下面进一步强调的，“装饰的PDF”仍然是平面的，即使它们在薄片中的痕迹只代表直的流体包裹体痕迹。Maniitsoq只是最近几个提出地质结构撞击起源的例子之一，这些例子是基于与普遍接受的撞击标准无关的证据提出的。这些公认的标准包括存在冲击变质作用的证据(例如各种矿物中的破碎锥体--图1a和1b，或平面变形特征[PDF--见图2])，以及地外射弹的残留物或化学痕迹(例如，见French和Koeberl[2010]和Koeberl[2014]最近的评论)。在Maniitsoq的案例中，Garde和他的同事通过解释《流星学与行星科学》49，Nr5,723-731(2014年)DOI：10.1111/maps.12284发展了他们自己的冲击变质标准
We were surprised to see the cover of the August 2013 issue of Meteoritics & Planetary Science (MAPS) adorned by a field image of a Greenlandic migmatite. The caption refers to the Maniitsoq structure in Greenland and explains that the lithology shown on the cover “is interpreted as due to a crustal-scale hydrothermal convection cell in a now deeply exhumed Archean impact structure” (Scherst en and Garde 2013). We found this statement surprising, as the Maniitsoq structure (Garde et al. 2012, 2013) has not been widely accepted as an impact structure, because the evidence reported in these papers does not comply with established recognition criteria for impact (see, e.g., French and Koeberl [2010] and references therein; Reimold et al. 2013). Garde and colleagues assert that, when established criteria “do not work,” new ones can be devised. To present such a statement as that related to the MAPS cover—highlighting this structure, in our view gives unnecessary credit to a (as we insist on calling) proposed but unconfirmed impact structure. The article by Scherst en and Garde (2013), to which this cover refers, contains high-quality U-Pb data for zircon, which are interpreted as hydrothermal re-equilibration of the isotope systems as the result of an impact event. Reimold et al. (2013) recommended a detailed investigation of Maniitsoq zircon grains to investigate the possible presence of impact evidence in the form of planar fractures, shock-induced granular texture, or twinning in zircon. The numerous zircon images shown in Scherst en and Garde (2013), however, fail to illustrate any textural evidence of shock deformation. While we explicitly appreciate the high-quality U-Pb ages, we must insist that there is still no tangible evidence for impact at Maniitsoq. Nor does the circumstantial evidence preclude alternatives, such as magmatic/tectonic explanations, for the observations and data reported so far from Maniitsoq. Let us assume that Maniitsoq could be a deeply eroded impact structure with a diameter of some 150 km. This would place Maniitsoq within the range of “large, old, eroded impact structures,” such as Sudbury (Canada) or Vredefort (South Africa). In these cases, and especially in that of the long controversial, deeply eroded Vredefort Structure, the ultimate proof of impact origin came from the discovery of shatter cones and planar deformation features (PDFs) in quartz, as well as the presence of a meteoritic component in impact melt rocks, all of which are accepted in the impact community as unambiguous proof of impact origin. The often annealed and/or decorated planar deformation features in Vredefort quartz were shown by Leroux et al. (1994) through detailed TEM analysis to represent bona fide shock deformation in the form of basal Brazil twins and decorated, higher order PDFs. Only with this proof in hand—further supported by unambiguous evidence for shock deformation in zircon (planar fractures and granular zircon texture)—were various authors able to later interpret breccia bodies, structural observations, and morphometric data within an impact context. Planar is the key term in planar deformation features, so that the description of “commonly curved, coarsened, and partially annealed” features as “planar features” (p. 1474) by Scherst en and Garde (2013) is not consistent with the published definitions. Their features are not imperfectly preserved, but have never been planar. Hence, we consider that they cannot provide evidence for a shock event. They also state “Proven but imperfectly preserved PDFs in known impact structures are only rarely reported in the literature,” which represents another obscure finding—as “imperfectly preserved” (in their sense referring to curved) features only rarely would be recognized as impact-diagnostic PDFs. As further emphasized below, “decorated PDFs” are still planar, even if their traces in thin section only represent straight fluid inclusion trails. Maniitsoq is just one of several recent instances where the impact origin of geological structures was proposed based on evidence unrelated to the commonly accepted criteria for impact. These recognized criteria include the presence of evidence of shock metamorphism (such as shatter cones—Figs. 1a and 1b, or planar deformation features [PDFs—see Fig. 2] in various minerals), and remnants or chemical traces of extraterrestrial projectiles (e.g., see the recent reviews by French and Koeberl [2010] and Koeberl [2014]). In the Maniitsoq case, Garde and coworkers developed their own criteria for shock metamorphism by interpreting Meteoritics & Planetary Science 49, Nr 5, 723–731 (2014) doi: 10.1111/maps.12284