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首页> 外文期刊>Pure and Applied Geophysics >Induced magnetization of magnetite-titanomagnetite in alternating fields ranging from 400 A/m to 80,000 A/m; Low-field susceptibility (100-400 A/m) and beyond
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Induced magnetization of magnetite-titanomagnetite in alternating fields ranging from 400 A/m to 80,000 A/m; Low-field susceptibility (100-400 A/m) and beyond

机译：在范围从400 A / m到80,000 A / m的交变场中感应磁铁矿-钛磁铁矿的磁化;低场磁化率（100-400 A / m）及以上

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摘要

For remanence-bearing minerals (RBM) such as magnetite-titanomagnetite, susceptibility to induced magnetism (M) measured in alternating fields (H-AC) is field-dependent. However, for fields <= 400 A/m, measured in an AC induction coil instrument (at 19,100 Hz), susceptibility k(0) = M/H-AC is sufficiently linear to provide a reproducible rock (or mineral) magnetic characteristic and its anisotropy may be related to arrangements of minerals in rock, or for single mineral grains to their crystalline or shape anisotropy. For any remanence-bearing mineral at higher fields k(HF) (= M/H-AC) is not constant and the term susceptibility is not normally used. This study bridges the responses between traditional low-field susceptibility measurements and those due to high applied fields, for example when studying hysteresis or saturation magnetization of RBM. Where vertical bar k(HF)vertical bar is measured in alternating fields that peak significantly above 400 A/m the M(H-AC ) relation is forced to follow a hysteresis loop in which vertical bar k(HF)vertical bar > k(0) for small vertical bar H-AC vertical bar and where vertical bar k(HF)vertical bar decreases to zero for very large fields that achieve saturation magnetization. Hysteresis nonlinearity is due to remanence acquired with one field direction requiring a reverse field for its cancellation. We investigate the transition from initial, traditional "low-field" susceptibility (k(0)) measurements at 60 A/m, through 24 different fields from 400 A/m to 40,000 A/m (for very high k(0) to 80,000 A/m). This reveals M(H-AC ) dependence beyond from conventional k(0) through the range of hysteresis behavior in fields equal to and exceeding that required to achieve saturation magnetization (M-S). We show k(HF) increases with peak H-AC until the peak field is slightly less than saturation magnetization in natural rock samples rich in magnetite (TM0 = Fe3O4) and TM60 (Fe2.4Ti0.6O4). All sample suites predominantly contain multidomain grains with subordinate pseudo-single domain and single-domain grains. k/k(0) increases by <= 5% for fields up to 2 kA/m. Above 4 kA/m k/k(0) increases steeply and peaks, usually between 24 kA/m and 30 kA/m where all grains magnetic moments are activated by H-AC since this exceeds the coercive force of most grains. For higher peak H-AC , k/k(0) declines sharply as increased H-AC values more effectively flip M with each field-direction switch, leading to the low gradient at distal portions of the hysteresis loop. For M-0-TM60 bearing rocks, susceptibility peaks for fields similar to 12 kA/m and for magnetite rich rocks up to 24 kA/m. These values are approximately 10% of saturation magnetizations (M (S)) reported for the pure minerals from hysteresis DC field measurements. Both the field at peak k/k(0) and the peak k/k(0) value appear to be controlled by the dominant domain structure; multidomain behavior has larger k/k(0) peaks at lower H-AC . Stacked k/k(0) versus H-AC curves for each sample suite (n = 12 to n = 39) were successfully characterized at the 95% level by a polynomial fit that requires the cubic form k/k(0) = a + bH + cH(2) + dH(3). Thus, for most M-TM bearing rocks, susceptibility andanisotropy of susceptibility (AMS) measurements made on different instruments would be sufficiently precise for most geological applications, if peak alternating fields are <= 700 A/m.
机译：对于诸如磁铁矿-钛磁铁矿这样的剩磁矿物（RBM），在交变磁场（H-AC）中测得的感应磁化率（M）取决于磁场。但是，对于在AC感应线圈仪器（19,100 Hz）下测量的<= 400 A / m的磁场，磁化率k（0）= M / H-AC具有足够的线性，可提供可再现的岩石（或矿物）磁特性，它的各向异性可能与岩石中矿物的排列有关，或者与单个矿物晶粒的晶体或形状各向异性有关。对于高场中任何残留的矿物，k（HF）（= M / H-AC）并不是恒定的，因此通常不使用“磁化率”一词。这项研究在传统的低场磁化率测量与由于高应用磁场引起的响应之间架起了桥梁，例如在研究RBM的磁滞或饱和磁化强度时。如果在交变场中测量的垂直线k（HF）垂直线的峰值明显高于400 A / m，则M（H-AC）关系被迫遵循磁滞回线，其中垂直线k（HF）垂直线> k（ 0）对于小的垂直线H-AC垂直线，而对于达到饱和磁化强度的非常大的场，垂直线k（HF）垂直线减小到零。磁滞非线性是由于在一个磁场方向上需要剩磁而需要消除磁场而产生的。我们调查了从最初的传统“低场”磁化率（k（0））测量以60 A / m到24 A不同场从400 A / m到40,000 A / m（对于非常高的k（0）到40,000 A / m）的过渡80,000 A / m）。这揭示了M（H-AC）依赖关系超出了传统的k（0），并且在磁滞行为范围内等于或超过了达到饱和磁化强度（M-S）所需的场。我们显示k（HF）随着峰值H-AC的增加而增加，直到峰值场略小于富含磁铁矿（TM0 = Fe3O4）和TM60（Fe2.4Ti0.6O4）的天然岩石样品中的饱和磁化强度。所有样本套件主要包含具有从属伪单域和单域晶粒的多域晶粒。对于2 kA / m以下的磁场，k / k（0）增加<= 5％。高于4 kA / m k / k（0）急剧增加并达到峰值，通常在24 kA / m和30 kA / m之间，其中所有晶粒的磁矩均由H-AC激活，因为这超过了大多数晶粒的矫顽力。对于较高的H-AC峰值，k / k（0）急剧下降，因为增加的H-AC值可通过每个场方向开关更有效地翻转M，从而导致磁滞回线远端的梯度较低。对于M-0-TM60轴承岩石，磁场的磁化率峰值接近12 kA / m，而富含磁铁矿的岩石的磁化率峰值则高达24 kA / m。这些值大约是磁滞DC场测量报告的纯矿物饱和磁化强度（M（S））的10％。峰值k / k（0）和峰值k / k（0）处的场似乎都由主导域结构控制;多域行为在较低的H-AC处具有较大的k / k（0）峰。通过多项式拟合（要求三次形式k / k（0）= a）成功地在95％的水平上对每个样品套件（n = 12至n = 39）的堆叠k / k（0）与H-AC曲线进行了表征。 + bH + cH（2）+ dH（3）。因此，对于大多数M-TM轴承岩石，如果峰值交变场<= 700 A / m，则对于大多数地质应用，在不同仪器上进行的磁化率和磁化率各向异性（AMS）测量将足够精确。

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