Rock magnetism of remagnetized carbonate rocks: another look
Rock magnetism of remagnetized carbonate rocks: another look
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DOI：
10.1144/sp371.3
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发表时间：
2012-01-01
期刊：
影响因子：
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通讯作者：
Swanson-Hysell, Nicholas L.
中科院分区：
文献类型：
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作者：
Jackson, Mike;Swanson-Hysell, Nicholas L.
细粒磁铁矿的自生形成是许多碳酸盐岩普遍发生化学再磁化的原因。自生磁铁矿颗粒，占主导地位的超顺磁性和稳定的单畴尺寸范围内，也引起了独特的岩石磁性，现在通常被用作一个“指纹”的再磁化。我们重新审视的基础上，这种协会的磁性矿物学和粒度分布在重磁化碳酸盐具有这些特征的岩石磁性，包括“蜂腰”磁滞回线，高比例的非磁滞剩磁饱和剩磁和频率依赖的磁化率。对Helderberg组样品的新测量使我们能够量化超顺磁性，稳定的单畴和较大晶粒的比例，并评估剩磁载体的矿物学组成。主要的磁性相是磁铁矿状的，具有足够的杂质以完全抑制Verwey转变。粒度非常细：在室温下，大约75%的磁铁矿总含量是超顺磁性的，几乎所有其余的都是稳定的单畴。虽然有人提出，这些再磁化碳酸盐中的单畴磁铁矿缺乏形状各向异性（因此控制立方磁晶各向异性），我们已经发现了强有力的实验证据，立方各向异性不是化学再磁化的岩石磁性特征的重要基础因素。
Authigenic formation of fine-grained magnetite is responsible for widespread chemical remagnetization of many carbonate rocks. Authigenic magnetite grains, dominantly in the superparamagnetic and stable single-domain size range, also give rise to distinctive rock-magnetic properties, now commonly used as a 'fingerprint' of remagnetization. We re-examine the basis of this association in terms of magnetic mineralogy and particle-size distribution in remagnetized carbonates having these characteristic rock-magnetic properties, including 'wasp-waisted' hysteresis loops, high ratios of anhysteretic remanence to saturation remanence and frequency-dependent susceptibility. New measurements on samples from the Helderberg Group allow us to quantify the proportions of superparamagnetic, stable single-domain and larger grains, and to evaluate the mineralogical composition of the remanence carriers. The dominant magnetic phase is magnetite-like, with sufficient impurity to completely suppress the Verwey transition. Particle sizes are extremely fine: approximately 75% of the total magnetite content is superparamagnetic at room temperature and almost all of the rest is stable single-domain. Although it has been proposed that the single-domain magnetite in these remagnetized carbonates lacks shape anisotropy (and is therefore controlled by cubic magnetocrystalline anisotropy), we have found strong experimental evidence that cubic anisotropy is not an important underlying factor in the rock-magnetic signature of chemical remagnetization.