Effects of shock pressure and temperature on titanomagnetite from ICDP cores and target rocks of the El’gygytgyn impact structure, Russia
Effects of shock pressure and temperature on titanomagnetite from ICDP cores and target rocks of the El’gygytgyn impact structure, Russia
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DOI：
10.1007/s11200-016-0819-3
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发表时间：
2016
影响因子：
0.9
通讯作者：
A. Kontny;Lea Grothaus
中科院分区：
文献类型：
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作者：
A. Kontny;Lea Grothaus
本研究的目的是调查陨石撞击对磁性的影响，包括磁化率和Verwey转变的贫钛磁铁矿的火山岩从3.6马老El 'gygytgyn的影响结构位于俄罗斯东北部的鄂霍次克-楚科奇火山带。该靶岩主要由流纹岩和少量安山岩组成，是地球上罕见的火山靶岩中的撞击构造。研究了来自火山口外部、火山口边缘和El 'gygytgyn ICDP钻探湖底（mblb）以下316至517米深度段的27个样品。地表长英质火山岩的平均比磁化率（18.1 × 10 - 6m3/kg）与火山口中央隆起附近的岩芯（1.9 × 10 - 6m3/kg）相比，显著降低了约90%。铁磁Fe-Ti氧化物组合（Verwey转变温度，TV：-161至-150 °C，居里温度，TC：451至581 ° C），发生在所有研究的样品中，差异显着。在表面，钛磁赤铁矿普遍与钛磁铁矿共生。岩芯中不含钛磁赤铁矿，但在-172 °C至-188 °C之间的第二次TV中，要么显示出向钛磁铁矿和钛铁矿的转化，要么显示出强烈的碎裂。suevite中磁化率温度依赖性的可逆曲线表明至少500 ° C的高沉积温度。在多矿物和单矿物撞击角砾岩中，根据温度相关的磁化率循环，建议钛磁铁矿的机械变形和与冲击有关的至少200 - 350 ° C的温度。低温氧化沿着强烈角砾化的颗粒表面钛磁铁矿建议，造成较低的电视，我们认为，这一现象与后冲击热液活动。El 'gygytgyn的磁化率下降主要是受到冲击的影响，而冲击后的热液作用导致了显著的额外损耗。这些观测结果解释了为什么磁低在撞击结构上是一种普遍存在的现象。
The aim of this study was to investigate the effect of meteorite impacts on magnetic properties including magnetic susceptibility and the Verwey transition of Ti-poor titanomagnetite of volcanic rocks from the 3.6 Ma old El’gygytgyn impact structure located in the Okhotsk-Chukotka volcanic belt in north-eastern Russia. The target rocks consist mainly of rhyolite with some andesites, and is a rare example of impact structures within volcanic target rocks on Earth. 27 samples from outside the crater, the crater rim and from the depth interval 316 to 517 m below lake bottom (mblb) of the El’gygytgyn ICDP drilling were studied. A significant decrease of the average specific magnetic susceptibility by around 90% was observed between felsic volcanic rocks from the surface (18.1 × 10-6m3/kg) and the drill cores from near the crater central uplift (1.9 × 10-6m3/kg). Ferrimagnetic Fe-Ti oxide assemblages (Verwey transition temperature, TV: -161 to -150°C, Curie temperature, TC: 451 to 581°C), occurring in all studied samples, differ significantly. At the surface titanomaghemite is ubiquitously associated with titanomagnetite. The drill cores lack titanomaghemite, but either show a transformation into titanomagnetite and ilmenite or a strong fragmentation associated with a second TVbetween -172 and -188°C. Reversible curves of temperature dependence of magnetic susceptibility in the suevite indicate high depositional temperatures of at least 500°C. In the polymict and monomict impact breccia mechanical deformation of titanomagnetite and temperatures of at least 200-350°C related to the shock are suggested from temperature dependent magnetic susceptibility cycling. Lowtemperature oxidation along strongly brecciated grain surfaces in titanomagnetite is suggested to cause the lower TV and we suggest that this phenomenon is related to postimpact hydrothermal activity. The strong magnetic susceptibility decrease at El’gygytgyn is mainly influenced by shock, and post-impact hydrothermalism causes a significant additional depletion. These observations explain why magnetic lows are a ubiquitous phenomenon over impact structures.