The continuous ejecta deposit around the rim of Lonar impact crater, central India, contains angular basaltic boulders of size ≤5 m. These boulders experienced varying level of shock between 2–30 GPa due to impact, as indicated by the extreme fracturing of these basaltic boulders, fragmentation of plagioclase and titanomagnetite constituents of these ejected boulders, and the presence of maskelynite in them. We measure some rock magnetic properties, e.g., NRM/χ (natural remanent magnetization [NRM]/bulk magnetic susceptibility [χ]), REM (=NRM/saturation isothermal remanent magnetization [SIRM] ratio expressed in %), and anisotropy of magnetic susceptibility (AMS) on 53 subsamples from 18 oriented drill cores of the shocked ejected basaltic boulders from the eastern half of ejecta deposit in the present study. The measured data are similar in many respects to our previous observations on Lonar crater rim shocked basalts (Arif et al. 2012b). For example, a small population of the ejected basaltic boulder samples show very high NRM/χ (between 378 and 989 Am⁻¹; n = 7) and REM (between 1.5 and 7%; n = 4) and the AMS axes of these ejected basaltic boulders show triaxial distributions in stereographic projections. Moreover, some of the ejected basaltic boulders show higher values of squareness ratio (M_rs/M_s) and median destructive field (MDF) suggesting permanent changes in the intrinsic magnetic properties due to impact shock pressure. In stereographic plot, the high coercivity and high temperature (HC_HT) magnetization component of these ejected basaltic boulders are distributed in discrete clusters on the periphery of a small enveloping circle whose center (D = 108.0°, I = +69.2°) lies close to the HC_HT cluster of the crater rim shocked basalts. The center of this enveloping circle and the average HC_HT component of Lonar crater rim shocked basalts have the same statistical orientation, although the former has steeper dip. This distribution suggests the possibility that the ejected basaltic boulders, which were deposited during the modification stage of Lonar crater evolution, were magnetized in an impact-induced magnetic field that was rapidly decaying just after the impact. Our present study suggests that the ejected basaltic boulders and Lonar crater rim shocked basalts experienced high shock pressure (≥2 GPa) magnetization during impact.

中文翻译：

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从印度 Lonar 陨石坑喷出的玄武岩巨石的岩石磁性：对短寿命撞击产生的弱磁场存在的影响

印度中部 Lonar 撞击坑边缘周围的连续喷出物沉积包含大小≤5 m 的角状玄武岩巨石。由于撞击，这些巨石经历了 2-30 GPa 之间的不同程度的冲击，这些玄武岩巨石的极端破裂、这些喷出巨石的斜长石和钛磁铁矿成分的碎裂以及其中存在的掩长石说明了这一点。我们测量了一些岩石的磁性，例如 NRM/χ（自然剩磁 [NRM]/体磁化率 [χ]）、REM（=NRM/饱和等温剩磁 [SIRM] 比率，以 % 表示）和磁的各向异性在本研究中，来自喷射物沉积物东半部冲击喷射玄武岩巨石的 18 个定向钻芯的 53 个子样品的磁化率 (AMS)。测量数据在许多方面与我们之前对 Lonar 陨石坑边缘冲击玄武岩的观察结果相似（Arif 等人，2012b）。例如，少量喷出的玄武岩巨石样本显示出非常高的 NRM/χ（介于 378 和 989 Am^-1 ; n  = 7) 和 REM（介于 1.5% 和 7% 之间；n  = 4）以及这些喷出的玄武岩巨石的 AMS 轴在立体投影中显示出三轴分布。此外，一些喷出的玄武岩巨石显示出更高的矩形比 ( M _rs / M _s ) 和中值破坏场 (MDF)，表明由于冲击冲击压力导致固有磁特性发生永久性变化。在立体图中，这​​些喷出玄武岩巨石的高矫顽力和高温 (HC_HT) 磁化分量以离散的簇状分布在一个小包络圆的外围，其中心 ( D  = 108.0°, I = +69.2°) 位于火山口边缘冲击玄武岩的 HC_HT 群附近。该包络圆的中心与 Lonar 陨石坑边缘冲击玄武岩的平均 HC_HT 分量具有相同的统计取向，但前者的倾角更大。这种分布表明，在 Lonar 陨石坑演化的修改阶段沉积的喷出的玄武岩巨石可能在撞击后迅速衰减的撞击感应磁场中被磁化。我们目前的研究表明，喷出的玄武岩巨石和 Lonar 陨石坑边缘冲击的玄武岩在撞击过程中经历了高冲击压力 (≥2 GPa) 磁化。