The effects of explosive-driven shocks on the natural remanent magnetization and the magnetic properties of rocks
The effects of explosive-driven shocks on the natural remanent magnetization and the magnetic properties of rocks
复制标题
DOI：
10.1016/j.pepi.2007.03.006
复制
发表时间：
2007-06-15
影响因子：
2.3
通讯作者：
Berthe, L.
中科院分区：
文献类型：
--
作者：
Gattacceca, J.;Lamali, A.;Berthe, L.
冲击波对天然岩石的磁化强度和地质材料的内禀磁性的影响仍然知之甚少。然而，超高速撞击现象在许多地外天体和地球的演化过程中至关重要。我们提出了新的实验中，四种岩石具有不同的岩性和磁性矿物（钛磁铁矿，磁铁矿，单斜磁黄铁矿，钛赤铁矿）使用高阶炸药（五矿），提供约30 GPa的最大压力的冲击。本文对冲击波进行了数值模拟，研究了冲击波对岩石的自然回复磁化强度和内禀磁性随爆炸距离的变化规律，结果表明，冲击波对岩石的内禀磁性具有永久的改变作用。冲击波能够将新的组构（具有平行于冲击方向的最小磁化率轴）与岩石的原始磁组构结合。当压力超过10 GPa时，磁铁矿、钛磁铁矿和磁黄铁矿岩石的磁化率明显增加。这些变化归因于铁磁颗粒的断裂和/或位错。这些结果表明，通常在10 GPa以上的压力下受到冲击的陨石（如火星陨石）的磁性可能并不能代表其母体的磁性，自然回复磁化似乎比等温回复磁化更能抵抗冲击，这可能是因为它由具有更高回复磁化率的颗粒携带。对于含钛磁铁矿的岩石，我们观察到的原始热还原磁化的部分冲击退磁和冲击磁化的外观。冲击波的退磁效应与携带原始剩磁的晶粒的矫顽力谱密切相关，而冲击重磁效应则取决于低矫顽力磁性晶粒的存在。当撞击发生在与原始磁化场强度相似的环境磁场中时，撞击后的磁化强度可能高于或低于撞击前的磁化强度，这取决于这两个因素。在撞击盆地上方观察到的磁异常通常被用作撞击时是否存在活动发电机的替代。我们的研究结果表明，与冲击岩石相关的磁异常可能会产生误导，因此，在撞击时发电机存在与否的唯一决定性线索是撞击过程中加热到阻塞温度以上的岩石体积可能携带的热剩磁。(c)2007 Elsevier B. V.保留所有权利。
The effects of shock waves on the natural remanent magnetization and the intrinsic magnetic properties of geological materials remain poorly known. Still, hypervelocity impact phenomena are of primary importance in the evolution of many extraterrestrial bodies and of Earth. We present new experiments in which four rocks with different lithology and magnetic mineralogy (titanomagnetite, magnetite, monoclinic pyrrhotite, titanohematite) were shocked using a high-order explosive (penthrite) that provided maximum pressure of about 30 GPa. The shock wave was modelled numerically and we studied the effects on the natural remanent magnetization as well as on the intrinsic magnetic properties of the shocked rocks as a function of the distance to the explosion.The intrinsic magnetic properties of the rock are permanently modified by the shock wave. The shock wave was able to superimpose a new fabric (with a minimum susceptibility axis parallel to the direction of shock) to the original magnetic fabric of the rock. Magnetite-, titanomagnetite- and pyrrhotite-bearing rocks show a noticeable increase of coercivity for pressure above 10GPa. These changes are attributed to fracturing and/or dislocations of the ferromagnetic grains. These results show that the magnetic properties of meteorites, which are commonly shocked to pressures well above 10GPa (e.g. Martian meteorites), may not be representative of the magnetic properties of their parent body.Natural remanent magnetization appears to be much more resistant to shock than isothermal remanent magnetization, probably because it is carried by grains with higher coercivity. For titanomagnetite-bearing rocks, we observe both a partial shock demagnetization of the original thermoremanent magnetization and the appearance of a shock magnetization. The demagnetizing effect of the shock wave depends closely on the coercivity spectrum of the grains carrying the original remanence, and the shock-remagnetizing effect depends on the presence of low-coercivity magnetic grains. With an impact occurring in an ambient magnetic field of similar intensity to the original magnetizing field, the post-shock magnetization may be higher or lower than the pre-shock magnetization depending on these two factors.The magnetic anomalies observed above impact basins are often used as a proxy to the presence or absence of an active dynamo at the time of impact. Our results show that the magnetic anomalies associated to shocked rocks can be misleading, so that the only decisive clue to the presence or absence of a dynamo at the time of impact is the possible thermoremanence carried by the volume of rock heated above blocking temperatures during the impact. (c) 2007 Elsevier B.V. All rights reserved.