Theory of CRM acquired by grain growth, and its implications for TRM discrimination and palaeointensity determination in igneous rocks
Theory of CRM acquired by grain growth, and its implications for TRM discrimination and palaeointensity determination in igneous rocks
复制标题
DOI：
10.1111/j.1365-246x.1996.tb05285.x
复制
发表时间：
1996-07-01
影响因子：
2.8
通讯作者：
McClelland, E
中科院分区：
文献类型：
--
作者：
McClelland, E
SD晶粒中晶粒生长CRM的行为可以用Neel（1949）的TRM获取理论来预测。这种理论方法表明，CRM TRM的比例将不会是恒定的整个粒度范围内的磁铁矿或赤铁矿。因此，在同一组磁性颗粒中，CRM和TRM的阻塞温度谱将是不同的，并且颗粒生长CRM可以通过在一定温度间隔上的非线性古强度图来识别。结果表明，在热退磁CRM和TRM应解除封锁在相同的温度，T-b，但它们的幅度将是不同的，因为CRM的净分数排列是由阻塞体积和反应温度，而TRM的控制的最终体积和阻塞温度控制。CRM/TRM比磁铁矿和赤铁矿的计算使用标准的弛豫时间方程，和实验值的自发磁化强度作为温度的函数。计算CRM/TRM比值的实际例子Tb光谱表明，晶粒生长CRM磁铁矿和赤铁矿可以区分TRM的基础上的Thellier-Thellier palaeotensity实验的数据，跨越一个温度区间从室温到至少450摄氏度，或更小的，高温的间隔。然而，如果pTRM检查在低于约400摄氏度时失败，则晶粒生长CRM不能与TRM区分开，因为CRM/TRM比率在低于该温度时接近1。根据这个模型，在实验室时间尺度上生长的单域CRM应该总是小于实验室TRM，而在比实验室中可用的时间长得多的时间内形成的自然CRM可能是TRM的两倍。多畴晶粒生长CRM可能总是大于TRM，由于在低温晶体生长过程中的成核畴壁的困难。
The behaviour of grain-growth CRM in SD grains can be predicted using Neel's (1949) theory for the acquisition of TRM. This theoretical approach suggests that the ratio of CRM to TRM will not be constant throughout the grain-size range for either magnetite or haematite. Hence the blocking-temperature spectra for CRM and TRM in an identical set of magnetic grains will be different, and grain-growth CRM can be identified by non-linear palaeointensity plots over certain temperature intervals. It is shown that on thermal demagnetization both CRM and TRM should unblock at the same temperature, T-b, but their magnitudes will be different, because the net fractional alignment for CRM is controlled by the blocking volume and the reaction temperature, while that for TRM is controlled by the final volume and the blocking temperature. CRM/TRM ratios for magnetite and haematite are calculated using the standard relaxation-time equation, and experimental values of spontaneous magnetization as a function of temperature. Calculation of CRM/TRM ratios for actual examples of Tb spectra suggest that grain-growth CRM in both magnetite and haematite can be distinguished from TRM on the basis of a Thellier-Thellier palaeointensity experiment for data that span a temperature interval from room temperature up to at least 450 degrees C, or for smaller, high-temperature intervals. However, grain-growth CRM cannot be distinguished from TRM if pTRM checks fail below about 400 degrees C, as the CRM/TRM ratio is close to 1 below this temperature. Single-domain CRM grown over laboratory time-scales should always be smaller than a laboratory TRM according to this model, while natural CRM formed over much longer times than available in the laboratory may be as much as twice as strong as TRM. Multidomain grain-growth CRM may always be larger than TRM, due to the difficulty of nucleating domain walls during low-temperature crystal growth.