岫岩陨石坑撞击角砾岩的岩相学和冲击变质特征SCIEI北大核心CSCD

撞击角砾岩是陨石撞击过程形成的特有岩石种类,是研究撞击成坑过程、陨石坑定年、矿物岩石冲击变质的理想对象。岫岩陨石坑是一个直径1800m的简单陨石坑,坑内有大量松散堆积的撞击角砾岩。本研究通过光学显微镜、费氏台、电子探针、X射线荧光光谱仪、电感耦合等离子质谱仪等分析测试手段,主要研究了岫岩陨石坑撞击角砾岩的岩相学和冲击变质特征,并在此基础上讨论了撞击角砾岩的形成过程和陨石坑的形貌特征。岫岩陨石坑内产出有三种撞击角砾岩,分别是来自上部的玄武质角砾岩和复成分岩屑角砾岩,以及底部的含熔体角砾岩。组成玄武质角砾岩和复成分岩屑角砾岩的碎屑受到的冲击程度较低,仅有少量石英发育面状变形页理,指示不超过20GPa的冲击压力。而组成含熔体角砾岩的碎屑受到了很强的冲击,发育了熔融硅酸盐玻璃、石英面状变形页理、柯石英、二氧化硅玻璃、击变长石玻璃、莱氏石等冲击变质特征,指示的峰值压力超过50GPa。本研究证实了含熔体角砾岩通常产出在简单陨石坑底部,由瞬间坑的坑缘和坑壁垮塌的岩石碎屑与坑底的冲击熔体混合形成。岫岩坑的真实深度是495m,真实深度与直径的比值为0.275,符合简单陨石坑的尺寸特征。陨石坑内的撞击角砾岩中心厚度为188m,与直径之比为0.104,略低于其它简单坑,可能是受丘陵地貌影响导致改造阶段垮塌到坑内的岩石角砾偏少。     

世界陨石坑研究

本文综述了全球陨石坑研究的研究历史和最新成果、基本的概念、陨石坑的识别要点、世界著名的陨石坑、陨石撞击地球可能引起的岩浆活动、陨石撞击与生命演化等内容。确定一个陨石坑,要从有一定弧度的地貌开始,鉴别低平圆形地质体是陨石还是其它原因造成的,综合确定岩石的岩石学特征、岩石中是否有撞击变质矿物、残余陨石、重力异常。陨石撞击太阳系的所有行星。由于地球表面遭受严重的风化和侵蚀,地质学家很难发现陨石坑。截至2021年3月31日,全球陨石坑数据库中有190个经确认的陨石坑,但中国只有一个,中国地质学家在发现陨石坑方面应当积极努力。对一个陨石坑认识可能不很成熟,但往往能改变对一个地区的地质成因理论的认识,形成完整的陨石坑证据链可能需要几代科学家的不断努力。     

世界陨石坑研究

本文综述了全球陨石坑研究的研究历史和最新成果、基本的概念、陨石坑的识别要点、世界著名的陨石坑、陨石撞击地球可能引起的岩浆活动、陨石撞击与生命演化等内容。确定一个陨石坑,要从有一定弧度的地貌开始,鉴别低平圆形地质体是陨石还是其他原因造成的,综合确定岩石的岩石学特征、岩石中是否有撞击变质矿物、残余陨石、重力异常。陨石撞击太阳系的所有行星。由于地球表面遭受严重的风化和侵蚀,地质学家很难发现陨石坑。截至2021年3月31日,全球陨石坑数据库中有190个经确认的陨石坑,但中国只有一个,中国地质学家在发现陨石坑方面应当积极努力。对一个陨石坑认识可能不很成熟,但往往能改变对一个地区的地质成因理论的认识,形成完整的陨石坑证据链可能需要几代科学家的不断努力。     

岫岩陨石坑(Xiuyan Crater)简介EI北大核心CSCD

岫岩陨石坑位于辽宁省鞍山市岫岩满族自治县境内,是一个位于丘陵地区,保存状态良好的碗形陨石坑,在陨石坑分类中属于简单陨石坑。它系5万年前该区发生的一次小行星撞击事件中形成的一个撞击地质构造。陨石坑坐落在元古宇变质岩地层上,坑的底部堆积有厚度近百米的湖沼相沉积物和厚度188 m的撞击     

岫岩陨石坑(Xiuyan Crater)简介

<正>岫岩陨石坑位于辽宁省鞍山市岫岩满族自治县境内,是一个位于丘陵地区,保存状态良好的碗形陨石坑,在陨石坑分类中属于简单陨石坑。它系5万年前该区发生的一次小行星撞击事件中形成的一个撞击地质构造。陨石坑坐落在元古宇变质岩地层上,坑的底部堆积有厚度近百米的湖沼相沉积物和厚度188 m的撞击     

海南岛白沙陨击坑及其成坑陨石

海南岛白沙陨击坑是一个直径约３．５ｋｍ的环形镶边坳陷，组成陨击坑边缘的环形山连续性好，并具二元结构；下部是层理清晰的下白垩统紫红色砂岩，其中长石、石英等粒状矿物普遍受冲击破碎，发育有冲击微页理和击变玻璃，云母呈膝折状变形；上部是冲击角砾岩块杂乱堆垒成的溅射覆盖层，冲击角砾岩因冲击熔融结晶而貌似凝灰岩，但其中矿物成分十分复杂，含有镁橄榄石、镍纹石以及高密度石英等，岩石化学计算结果说明它是由砂岩变质而成的，与火成岩无关。坑内保留有回落角砾岩，常见到沿裂缝贯入的脉状角砾岩。在陨击坑内找到了重３．７５ｋｇ的石陨石碎块，其中含碱硅镁石、陨铁大隅石、四方镍纹石、陨硫钙石和陨硫铁等陨石标型矿物，但不具球粒结构，ＣａＯ含量为９．１９％，属富钙的无球粒陨石，认为是白沙陨石坑的成坑陨石。在陨击坑中找到富钙无球粒陨石，为陨击坑提供了最直接可靠的证据，也为石陨石撞击成坑提供了实例。     

岫岩陨石坑菱铁矿角砾岩的特征及成因

岫岩陨石坑直径1.8 km,是一个简单碗形坑.通过在陨石坑中心实施的岩芯钻探,在被厚达107 m第四系湖相沉积物覆盖的撞击角砾岩单元顶部位置,发现少量“菱铁矿角砾岩”.这种菱铁矿角砾岩由菱铁矿微晶和矿物岩石碎屑组成.全岩碳同位素分析显示出较高的δδ13C异常,平均高达+13.76‰(V-PDB标准).菱铁矿形成时间约为37 ka,晚于撞击成坑事件(50 ka),也晚于湖泊相沉积物的沉积年龄(39～50 ka).在还原环境下,细菌分解有机质形成甲烷引起的碳同位素分馏是造成菱铁矿δ13C显著正异常的主要原因.显然,这些菱铁矿属于沉积成因.沉淀的菱铁矿胶结岩石和矿物碎屑形成菱铁矿角砾岩.     

美国发现引起恐龙灭绝的陨石坑

据美国《科学》杂志报道,美国地质调查局的科学家在衣阿华州西北部曼松村附近发现一个直径35km的陨石坑,并认为它是引起恐龙等大量物种灭绝的陨石所留下的痕迹。据研究,坑中充满冲击形成的石英,陨石坑年龄为65±1Ma,正当K/T边界年龄。科学家推测,     

苏州西山震裂锥:太湖冲击坑又一新证据

震裂锥已被公认为陨石冲击地球表面遗留的标志。研究及统计资料表明,震裂锥与陨石冲击形成的中、大型冲击坑 有关。太湖西山震裂锥呈圆锥形,锥体表面有自锥顶向下辐射的锥纹,锥纹具有分叉的特征,这些特征与震裂锥的国际公 认的定义和标准相符。此外,西山震裂锥还具有其特有的其他特征：碎裂岩化显著;气化-熔融现象发育;锥体表面具网 状构造及波纹状、蜂窝状等多种气印。岩相学研究显示,震裂锥及含锥岩石中冲击变质现象明显,微页理（PDFs）、微裂 隙（PFs） 以及靶岩熔融现象发育。以上这些冲击变质的标志,可证明西山震裂锥是冲击成因,而非地表水风化淋溶石灰岩 的喀斯特或风蚀成因的凤稜石。西山震裂锥的发现、太湖湖底冲击击变角砾岩“太湖石”的确定,为太湖冲击坑的研究增 添了新的诊断性证据;加上早期研究确定的、冲击回落至太湖湖底淤泥层中的冲击溅射物,这些众多证据为确定“太湖冲 击坑”或“太湖冲击事件”展示了美好前景。但是,要确定太湖冲击坑的具体位置、大小及构造模式等,尚需更多的深入 研究。     

海南岛白沙陨石坑的遥感图像解译与验证

据著名的冲击构造学家E.M.Shoemaker估计,在地球演化史中产生的直径大于10km的陨石冲击构造不少于1500个,较小的陨石冲击构造数量更多,但因长期复杂的地质作用的破坏和改造,幸存者甚少.全球已发现的陨石冲击坑约130个,有的受剥蚀而残缺不全,另一些为后期沉积物掩埋,由钻探发现不能直接观测,只有少数保存和出露都较好,如美国亚里桑那州的梅特奥尔(Meteor)陨石坑和爱沙尼亚的卡利(Kaali)陨石坑等,其成坑时代很新、规模小、冲击能量不大、冲击形成物种类不丰富.     

Geologic and geotechnical effects of an impact caused by an airplane crash

A Turkish Airlines (THY) Boeing 737-400 plane crashed into alluvial soils creating an approximately 13 m deep and 30 m wide crater near the village of Adatepe, Ceyhan in southern Turkey. Effects of the impact on the soils in and around the crater were investigated from both the geological and soil mechanics point of view.

The results show that the impact caused severe deformations in the soils in and around the crater. The soils deformed similar to metamorphic rocks seen at many terrestrial hypervelocity impact craters around the world and became overconsolidated up to a distance of about 10 m from the crater wall as a result of the impact.

Also, the crash was recorded as a 2.7 magnitude earthquake by a nearby microtremor seismograph which provided both the location (epicenter) and time of the crash which was not known immediately after the crash.     

The convincing identification of terrestrial meteorite impact structures: What works,what doesn't,and why

In the geological sciences it has only recently been recognized how important the process of impact cratering is on a planetary scale, where it is commonly the most important surface-modifying process. On the Moon and other planetary bodies that lack an appreciable atmosphere, meteorite impact craters are well preserved, and they can commonly be recognized from morphological characteristics, but on Earth complications arise as a consequence of the weathering, obliteration, deformation, or burial of impact craters and the projectiles that formed them. These problems made it necessary to develop diagnostic criteria for the identification and confirmation of impact structures on Earth. Diagnostic evidence for impact events is often present in the target rocks that were affected by the impact. The conditions of impact produce an unusual group of melted, shocked, and brecciated rocks, some of which fill the resulting crater, and others which are transported, in some cases to considerable distances from the source crater. Only the presence of diagnostic shock-metamorphic effects and, in some cases, the discovery of meteorites, or traces thereof, is generally accepted as unambiguous evidence for an impact origin. Shock deformation can be expressed in macroscopic form (shatter cones) or in microscopic forms (e.g., distinctive planar deformation features [PDFs] in quartz). In nature, shock-metamorphic effects are uniquely characteristic of shock levels associated with hypervelocity impact. The same two criteria (shock-metamorphic effects or traces of the impacting meteorite) apply to distal impact ejecta layers, and their presence confirms that materials found in such layers originated in an impact event at a possibly still unknown location. As of 2009 about 175 impact structures have been identified on Earth based on these criteria. A wide variety of shock-metamorphic effects has been identified, with the best diagnostic indicators for shock metamorphism being features that can be studied easily by using the polarizing microscope. These include specific planar microdeformation features (planar fractures [PFs], PDFs), isotropization (e.g., formation of diaplectic glasses), and phase changes (high pressure phases; melting). The present review provides a detailed discussion of shock effects and geochemical tracers that can be used for the unambiguous identification of impact structures, as well as an overview of doubtful criteria or ambiguous lines of evidence that have erroneously been applied in the past.     

Volatile enhanced dispersal of high velocity impact melts and the origin of tektites

In hypervelocity meteorite impacts, shock energies produce temperatures well above the melting point of a wide area of the impacted target rocks. This produces impact melt during excavation and expansion of the transient crater cavity. The vast majority of this melt is retained in the crater-fill stratigraphy where it may form coherent melt units and/or be variably mixed with non-molten target rocks. A small portion (1–3%) of this melt is ejected from the crater at very high velocities – potentially faster than the impactor itself – forming impact glasses and, in rare cases, tektites. Why only some impacts form large volumes of high velocity impact glass and even fewer form tektites remains poorly understood. Many of the expected theoretical controls on the production and dispersal of high-velocity impact melt (target rock type, impact size, impact angle) do not seem to apply; comparison of the volume and nature of ejected melt around complex and simple craters on Earth reveals no systematic relationship to any of these parameters. The geologic evidence suggests that there is another controlling mechanism that promotes production of high velocity impact melt and tektite formation in some impacts. The Darwin impact event shows clearly that the presence of water rich surface layers in the target stratigraphy enhances by orders of magnitude the production of high velocity ejected melt; as hinted at by some numerical models. For tektites from all four strewn fields, the presence of water rich surface layers at the impact site can be inferred and it seems this is the missing feature of the target stratigraphy required to explain tektite origin.     

Reconstructing the Wolfe Creek meteorite impact: deep structure of the crater and effects on target rock

The Wolfe Creek Meteorite Crater is an impact structure 880 m in diameter, located in the Tanami Desert near Halls Creek, Western Australia. The crater formed?<?300 000 years ago, and is the second largest crater from which fragments of the impacting meteorite (a medium octahedrite) have been recovered. We present the results of new ground-based geophysical (magnetics and gravity) surveys conducted over the structure in July?–?August 2003. The results highlight the simple structure of the crater under the infilling sediments, and forward modelling is consistent with the true crater floor being 120 m beneath the present surface. The variations in the dip of the foliations around the crater rim confirm that the meteorite approached from the east-northeast, as is also deduced from the ejecta distribution. Crater scaling arguments suggest a projectile diameter of?>?12.0 m, a crater formation time of 3.34 s, and an energy of impact of ～0.235 Mt of TNT. We also use the distribution of shocked quartz in the target rock (Devonian sandstones) to reconstruct the shock loading conditions of the impact. The estimated maximum pressures at the crater rim were between 5.59 and 5.81 GPa. We also use a Simplified Arbitrary Langrangian–Eulerian hydrocode (SALE 2) to simulate the propagation of shock waves through a material described by a Tillotson equation of state. Using the deformational and PT constraints of the Wolfe Creek crater, we estimate the maximum pressures, and the shock-wave attenuation, of this medium-sized impact.     

Hickman Crater,Ophthalmia Range,Western Australia: evidence supporting a meteorite impact origin ∗

A newly discovered, morphologically well-preserved crater with a mean diameter of 260 m is reported from the Ophthalmia Range, Western Australia. The crater is located in hilly terrain ～36 km north of Newman, and is situated in the Paleoproterozoic Woongarra Rhyolite and the overlying Boolgeeda Iron Formation. The morphometry of the crater is consistent with features characteristic of small meteorite impact craters. The rhyolite of the crater's rim exhibits widespread shatter features injected by veins of goethite bound by sharply defined zones of hydrous alteration. The alteration zones contain micro-fractures injected by goethite, which also fills cavities in the rhyolite. The goethite veins are interpreted in terms of forceful injection of aqueous iron-rich solutions, probably reflecting high-pressure hydrothermal activity by heated iron-rich ground water. None of these features are present in the Woongarra Rhyolite outside the immediate area of the crater. Petrography of the rhyolite indicates possible incipient intracrystalline dislocations in quartz. The Boolgeeda Iron Formation, which crops out only on the southern rim of the crater, displays brecciation and mega-brecciation superposed on fold structures typical of the banded iron-formations in the region. Geochemical analysis of two goethite veins discloses no siderophile element (Ni and PGE) anomalies, negating any contribution of material from an exploding meteorite. Instead, the strong iron-enrichment of the fractured rhyolite is attributed to a hydrothermal system affecting both the Boolgeeda Iron Formation and the Woongarra Rhyolite, and localised to the area of the crater. An absence of young fragmental volcanic material younger than the Woongarra Rhyolite is inconsistent with an explosive diatreme, leading us to a preferred interpretation in terms of an original impact crater about 80 m deep excavated by a ～10 m-diameter projectile and accompanied by hydrothermal activity. A minor north–south asymmetry of the crater, and an abundance of ejecta north, up to about 300 m northwest and northeast of the crater, suggest high-angle impact from the south. A youthful age of the structure, probably Late Pleistocene (10⁴–10⁵ years old), is indicated by damming of the drainage of a south-southeast-flowing creek by the southern crater rim.     

The Araguainha impact: a South American Permo–Triassic catastrophic event

The Araguainha meteorite impact was certainly one of the most catastrophic events in the history of the South American continent. The impact occurred around 250 Ma ago, when the region was covered by the estuarine waters of the Parana Basin in central parts of Brazil. The impacting body of approximately 2–3 km in diameter was sufficiently large to excavate a 2 km-thick sedimentary sequence of the Parana Basin and to expose a 4 km-wide core of basement crystalline rocks in the central part of the crater. The huge scar left by the meteorite collision is 40 km in diameter, the largest and best preserved impact crater on the continent. Combined field observations and remote sensing analysis demonstrates that the Araguainha impact structure preserves all morphological/structural features of large lunar craters, being thus an important analogue to study large extraterrestrial craters. The catastrophic energy released upon impact, close to 10⁶ megatons of TNT, must have been disastrous for marine organisms living in the Parana Basin. Ongoing studies are currently evaluating the link between the Araguainha impact and the Permian–Triassic mass extinction, which is the greatest of the mass extinctions in Earth history.