Mapping magnetic sources at the millimeter to micrometer scale in dunite and serpentinite by high-resolution magnetic microscopy
Mapping magnetic sources at the millimeter to micrometer scale in dunite and serpentinite by high-resolution magnetic microscopy
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DOI：
10.1016/j.lithos.2018.09.018
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发表时间：
2018-12
期刊：
影响因子：
3.5
通讯作者：
Z. Pastore;S. McEnroe;G. W. Maat;H. Oda;N. Church;P. Fumagalli
中科院分区：
文献类型：
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作者：
Z. Pastore;S. McEnroe;G. W. Maat;H. Oda;N. Church;P. Fumagalli
岩石样品可具有广泛的磁性，这取决于组成、铁磁性矿物的量、粒度和微观结构。在这里，我们使用扫描磁显微镜，一种高灵敏度和高分辨率的磁力测量技术来映射岩石样品平面上的反射磁场。该技术允许调查离散的磁性矿物颗粒，或磁性纹理和结构与亚毫米尺度的分辨率。在这里，我们提出了一个案例研究的原始和蛇纹化纯橄榄岩薄切片的Reinfjord超镁铁质复合体，在北方挪威的磁扫描。磁性矿物学的特征是通过电子探针，扫描电子显微镜和光学显微镜，并与岩石磁性方法。在蛇纹岩样品中，磁性载体是端元磁铁矿，以大颗粒离散和微米级脉中的小颗粒形式出现。相比之下，原始纯橄榄岩样品包含大铬尖晶石颗粒，具有非常精细的等长出溶体，其组成范围从铁铬铁矿到端元磁铁矿。磁异常的正演和反演模型用于确定在扫描磁显微镜中观察到的离散磁源的逆磁化方向和强度。利用岩石样品的精细磁化强度研究了纯橄榄岩的磁性载体以及蛇纹石化作用对纯橄榄岩磁性的影响。模拟结果表明，扫描磁显微镜所绘制的偶极磁异常是由具有非均匀磁源的颗粒引起的。的磁化强度和磁性矿物的量是较高的蛇纹石化样品比原始纯橄榄岩样品，与测得的散装磁性。此外，蛇纹石化的样品显示出更大的可变性的磁化方向和更强的异质性相对于原始样品。将块体磁性的组分与单个矿物相严格关联的能力为岩石磁性、古磁性和勘探应用创造了新的可能性。
Rock samples can have wide range of magnetic properties depending on composition, amount of ferromagnetic minerals, grain sizes and microstructures. Here, we used scanning magnetic microscopy, a highly sensitive and high-resolution magnetometric technique to map remanent magnetic fields over a planar surface of a rock sample. The technique allows for the investigation of discrete magnetic mineral grains, or magnetic textures and structures with submillimeter scale resolution. Here, we present a case-study of magnetic scans of pristine and serpentinized dunite thin sections from the Reinfjord Ultramafic Complex, in northern Norway. The magnetic mineralogy is characterized by electron microprobe, scanning electron- and optical-microscopy, and with rock magnetic methods. In serpentinized samples the magnetic carrier is end-member magnetite occurring as large discrete grains and small grains in micron scale veins. By contrast, the pristine dunite sample contains large Cr-spinel grains with very fine equant exsolutions ranging in composition from ferrichromite to end-member magnetite. Forward and inverse modeling of the magnetic anomalies is used to determine the remanent magnetization directions and intensities of discrete magnetic sources observed in the scanning magnetic microscopy. The fine-scale magnetization of the rock sample is used to investigate the magnetic carriers and the effect of serpentinization on the magnetic properties of the dunite. Modeling shows that the dipolar magnetic anomalies that are mapped by scanning magnetic microscopy are caused by grains with heterogeneous magnetic sources. The intensity of the magnetization and the amount of magnetic minerals are higher in the serpentinized sample than the pristine dunite sample, consistent with the measured bulk magnetic properties. Furthermore, the serpentinized samples show a larger variability in the direction of the magnetization and a stronger heterogeneity with respect to the pristine sample. The ability to rigorously associate components of the bulk magnetic properties to individual mineral phases creates new possibilities for rock magnetic, paleomagnetic, and exploration applications.