Paleomagnetic field reconstruction from mixtures of titanomagnetites
Paleomagnetic field reconstruction from mixtures of titanomagnetites
复制标题
DOI：
10.1016/j.epsl.2017.02.033
复制
发表时间：
2017-05
影响因子：
5.3
通讯作者：
T. Berndt;R. Ramalho;Miguel A. Valdez-Grijalva;A. Muxworthy
中科院分区：
文献类型：
--
作者：
T. Berndt;R. Ramalho;Miguel A. Valdez-Grijalva;A. Muxworthy
逐步热退磁和交变磁场退磁是古地磁研究中常用的分离不同来源剩磁成分的方法。磁性最硬的，即最高的解封温度/峰值磁场分量通常被解释为初级磁化，磁性较软的分量通常被解释为由于岩石形成后的地质事件（如再加热或新磁性矿物的形成）而导致的后续再磁化。从古地磁资料中正确解释构造旋转等地质事件序列，往往依赖于将观测到的磁向正确地归属于剩磁载体及其获取机制。利用数值模型模拟剩余物采集和逐步热退磁和AF退磁实验，我们发现不同磁性矿物（如不同钛含量的磁铁矿和钛磁铁矿）的混合存在对Zijderveld地块有非常显著的影响。在热退磁实验中，由于初级剩余物与随后由小晶粒富铁磁铁矿和大晶粒富钛磁铁矿进行的热或粘性再磁化的重叠，在中等温度或连续曲率下可能出现虚假的第三组分。磁性混合物的AF退磁图更为复杂：在Zijderveld图中，不同矿物携带的原生剩余物和次生剩余物可能以三组分或四组分的形式出现。交变退磁过程中，最高矫顽力分量不一定等于初级剩磁，也不一定对应类似热退磁实验中最高温度分量，即初级剩磁方向无法恢复。结果表明，这种影响是由于磁铁矿和钛磁铁矿对粘性或热粘性剩磁采集的不同响应性所致：低钛矿物更有效地记录了长采集时间的剩磁，而富钛矿物则同样记录了短采集时间的剩磁。在实验室时间尺度上的消磁实验中，两种矿物对Zijderveld地块的相对贡献与地质时间尺度上剩余物获取的相对贡献不同，导致Zijderveld地块的成分重叠。该模型还用于模拟古磁场强度实验，发现颗粒分布对Arai地块的坡度有影响，但与NRM采集冷却速率的影响相比可以忽略不计。模拟表明，对于缓慢冷却的岩石，根据矿物学，可能需要高达1.5至1.6的冷却速率校正。
Stepwise thermal demagnetization and alternating field (AF) demagnetization are commonly used in paleomagnetic studies to isolate remanent magnetic components of different origins. The magnetically hardest, i.e. highest unblocking temperature/peak field component is often interpreted as the primary magnetization and magnetically softer components as subsequent remagnetizations due to geological events posterior to the formation of the rock, such as reheating or formation of new magnetic minerals. The correct interpretation of the sequence of the geological events such as tectonic rotations from paleomagnetic data often relies on correctly attributing the observed magnetic directions to the remanence carriers and acquisition mechanisms. Using a numerical model to simulate remanence acquisition and stepwise thermal and AF demagnetization experiments, we show that the presence of mixtures of different magnetic minerals, such as magnetite and titanomagnetites of varying titanium-content can have very significant effects on Zijderveld plots. In thermal demagnetization experiments a spurious third component at intermediate temperatures or a continuous curvature may arise from an overlap of the primary remanence with a subsequent thermal or viscous remagnetization carried by small-grained iron-rich magnetite and large-grained titanium-rich titanomagnetite. AF demagnetization plots of magnetic mixtures are even more complex: primary and secondary remanences carried by different minerals may appear as either three or four components in Zijderveld plots. During alternating field demagnetization the highest coercivity component is not necessarily equivalent to the primary remanence and does not necessarily correspond to the highest temperature component in an analogous thermal demagnetization experiment, i.e., the primary remanence direction cannot be recovered. The effects are shown to be due to the different responsiveness of magnetite and titanomagnetites towards viscous or thermoviscous remanence acquisition: remanent magnetizations with long acquisition times are more effectively recorded by titanium-poor minerals, while short acquisition times are equally well recorded by titanium-rich minerals. In demagnetization experiments on laboratory timescales, the relative contribution of two minerals to Zijderveld plots differs to the relative contribution of remanence acquisition over geological timescales, leading to overlapping components in Zijderveld plots. The model was also used to simulate paleointensity (ancient magnetic field intensity) experiments and it was found that the grain distribution affects the slope of Arai plots, but is negligible compared to the effect of the cooling rate of NRM acquisition. The simulations suggest that for slowly cooled rocks a cooling rate correction of up to 1.5 to 1.6 may be required depending on the mineralogy.