Reliability of paleointensity method using ARM from volcanic rocks

考察了确定火山岩古强度的肖氏方法的可靠性。本文介绍了一种新研制的具有高反辐射性能的AF消磁系统，并利用现代火山岩样品对肖氏法中的校正方法进行了试验。加热后，几乎所有样品的ARM容量都发生了高达20%的变化。这些样品在原始Shaw方法中被拒绝，其中只有ARM容量没有变化的样品被视为成功。然而，如果根据加热前后ARMs的比值对表观古强度进行校正，则本研究中的所有样品都给出了真实的“古强度”。这些结果表明，ARM比率的校正方法的工作，就在本研究中所使用的熔岩样品而言，Shaw方法被应用到39个样品采集自Ruapehu火山，新西兰安山岩熔岩序列。一些样品也进行了检查的Thellier方法，其中只涉及热退磁。绝大多数样品的ARM都有一定的变化，引入ARM变化的校正后，得到了较为可靠的结果。虽然没有绝对的方法来验证所获得的结果，但它们在熔岩中表现得非常好。通过对岩石磁背景的分析，可以看出Shaw方法中的ARM校正是成功的。一些岩石磁性参数，如初始磁化率与ARM强度进行了比较。加热后，磁化率的变化通常比ARM强度的变化更大。这是伴随着强烈的负相关性的比率的ARM和磁化率。简单的解释是，实验室加热只影响较大的磁性颗粒，而这些颗粒对剩磁没有贡献。

Reliability of the Shaw method of paleointensity determination from volcanic rocks was examined. An AF demagnetizing system with high performance of giving ARM was newly introduced.Samples from present-day lava were used to test correction methods involved in the Shaw method. Almost samples showed change in ARM capacity up to 20% after heating. Such samples were rejected in the orginal Shaw method in which only samples giving no change in ARM capacity were treated as successful. However, all the samples in this study gave true "paleointensities" if the apparent paleointensity was corrected according to the ratio of ARMs before and after heating. These results suggest that the correction methods using ARM ratio work as far as the lava samples used in this study are concerned.Shaw method was applied to 39 samples collected from andesite lava sequences at Ruapehu Volcano, New Zealand. Some samples were also examined by the Thellier method in which only thermal demagnetization was involved. Most samples showed some amount of change of ARM,and apparently reliable results were obtained after introducing the correction by ARM change. Although there is no absolute way to validate the obtained results, they showed very good within lava consistencies. Good agreement with the results from the Thelliers' method were also observed.Rock magnetic background was examined to understand this apparent success of ARM correction in the Shaw method. Some rock magnetic parameters such as initial magnetic susceptibility were compared with ARM intensities. More change in magnetic susceptibility than ARM intensity was usually observed after heating. This was accompanied by strong negative correlation of ratios of ARM and magnetic susceptibility. Simple interpretation is that the laboratory heating affects only larger magnetic grains which do not contribute to remanences.