The effect of cooling rate on the intensity of thermoremanent magnetization (TRM) acquired by assemblages of pseudo-single domain, multidomain and interacting single-domain grains
The effect of cooling rate on the intensity of thermoremanent magnetization (TRM) acquired by assemblages of pseudo-single domain, multidomain and interacting single-domain grains
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DOI：
10.1093/gji/ggt078
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发表时间：
2013-06-01
影响因子：
2.8
通讯作者：
Dekkers, M. J.
中科院分区：
文献类型：
--
作者：
Biggin, A. J.;Badejo, S.;Dekkers, M. J.
用来测量古地磁场绝对强度的实验通常是通过比较火成岩保留的古老热剩余磁化强度(TRM)和实验室提供的新的TRM来实现的。这一过程的一个问题是，古代和实验室TRMS的相对大小可能会受到影响，不仅受到两次冷却发生时的外场强度的影响，而且还受到冷却本身发生的速度的影响。在这里，我们提出了一种新的测量方法，这种“降温速度效应”是通过实验室中不同降温速度的处理获得的，降温速度相差类似于200倍。实验中使用了含有尺寸大小的亚铁磁性颗粒的合成样品。理论上的考虑和以前的实验表明，降温速率效应依赖于磁畴状态。据报道，对于非相互作用的单区(SD)颗粒组合，其TRM量级增加了7%以上，降温速度降低了一个数量级。在这里，我们集中在较少研究的伪单磁区(P-SD)和多磁区(MD)状态下的磁铁矿颗粒，使用一系列的外加磁场强度来提供TRMS。我们还首次测量了氧化分解后的钛磁铁矿颗粒中的冷却速度效应，使其含有强烈相互作用的SD片层。在所有情况下，测量的降温速率效应与在理想的无磁相互作用的SD颗粒中观察到的相同，但要弱得多。平均而言，冷却速度每下降一个数量级，冷却效果不超过3%的TRM增加，在一些含有较粗颗粒的样品中，也没有表现出任何对外加磁场强度的系统依赖，冷却速度效应与零没有区别。由于不同的研究产生了不同的结果，所以在真正的MD谷物中，降温速度效应的意义和大小仍然不确定。然而，对于更实际相关的PSD和相互作用的SD颗粒的情况，这通常是火成岩中TRM的主要成分，似乎我们可以更有信心地断言。这种材料的降温速度效应与非相互作用的SD颗粒的降温速度效应相同，但更小：这是长程有序化的结果。在熔岩和含有这些物质的小型侵入岩中，这一比例不太可能超过10%。尽管应该始终尝试进行校正，但基于这类样品的古强度研究结果通常不会有严重的偏差。
Experiments designed to measure the absolute palaeointensity of the geomagnetic field generally do so by comparing the ancient thermoremanent magnetization (TRM) retained by an igneous rock with a new TRM imparted in the laboratory. One problem with this procedure is that the relative magnitudes of the ancient and laboratory TRMs may be influenced, not only by the external field intensities at the time the two coolings took place, but also by the rate at which the coolings themselves occurred. Here, we present new measurements of this 'cooling rate effect' obtained from treatments in the laboratory differing in cooling rate by a factor of similar to 200. Synthetic samples containing sized ferrimagnetic grains were used in the experiments. Theoretical considerations and previous experiments have indicated the cooling rate effect to be dependent on domain state. Increases in TRM magnitude of more than 7 per cent per order of magnitude decrease in cooling rate have been reported for assemblages of non-interacting single-domain (SD) grains. Here, we focus on magnetite grains in the less well-studied pseudo-single domain (P SD) and multidomain (MD) states using a range of applied field intensities to impart the TRMs. For the first time, we also measure the cooling rate effect in grains of titanomagnetite that have been oxyexsolved so that they contain strongly interacting SD lamellae. In all cases, the cooling rate effect measured was in the same sense as already observed in ideal magnetically non-interacting SD grains but was considerably weaker. On average, the effect did not exceed 3 per cent increase in TRM per order of magnitude decrease in cooling rate and did not show any systematic dependence on applied field intensity In some samples containing coarser grains, the cooling rate effect was not distinguishable from zero. The sense and magnitude of the cooling rate effect remain uncertain in truly MD grains as different studies have produced discrepant results. For the more practically relevant case of PSD and interacting SD grains, which commonly dominate the TRM in igneous rocks, however, it appears that we can be more confident in our assertions. The cooling rate effect in such materials is in the same sense as in non-interacting SD grains but smaller: a consequence of long-range ordering. In lavas and small intrusions containing these, it is unlikely to exceed 10 per cent. Although a correction should always be attempted, the results of palaeointensity studies based upon such samples will generally not be severely biased.