High magnetic susceptibility produced by thermal decomposition of core samples from the Chelungpu fault in Taiwan
High magnetic susceptibility produced by thermal decomposition of core samples from the Chelungpu fault in Taiwan
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DOI：
10.1016/j.epsl.2008.05.002
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发表时间：
2008-07
影响因子：
5.3
通讯作者：
W. Tanikawa;T. Mishima;T. Hirono;W. Soh;Sheng-rong Song
中科院分区：
文献类型：
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作者：
W. Tanikawa;T. Mishima;T. Hirono;W. Soh;Sheng-rong Song
我们对台湾车龙埔断层钻探计划（TCDP）B孔岩心样品的断层岩石进行了热磁化率分析，以探讨断层岩心高磁化率的原因。通过加热至高达 900 °C 的不同最高温度以及在磁性分析之前进行高速摩擦测试，对测试样品进行了热处理和机械处理。天然断层岩石的热磁化率分析表明，在加热循环中，磁化强度在最高加热温度高于 400 °C 时增加，并在冷却循环中的 600 至 550 °C 和 300 °C 处呈现三步增加。这些行为与黄铁矿、菱铁矿和绿泥石的存在一致，表明TCDP泥岩最初包含这些矿物，它们有助于通过热机械反应产生磁化率。菱铁矿加热引起的磁化率变化是黄铁矿和绿泥石加热引起的磁化率变化的20倍，因此仅一小部分菱铁矿分解就足以引起在断层核中观察到的磁化率的轻微增加。颜色测量结果表明，在深部低氧条件下，摩擦加热引起的热分解发生，这阻止了矿物氧化成微红色的赤铁矿。这一发现支持了机械驱动的化学反应部分解释高磁化率的推论。动力学模型分析证实摩擦加热会引起菱铁矿和黄铁矿的热分解。我们的研究结果表明，黄铁矿分解为磁黄铁矿、菱铁矿以及在某种程度上绿泥石分解为磁铁矿是解释车笼埔断裂带内磁异常的可能机制。
We carried out thermomagnetic susceptibility analyses of fault rocks from core samples from Hole B of the Taiwan Chelungpu Fault Drilling Project (TCDP) to investigate the cause of high magnetic susceptibilities in the fault core. Test samples were thermally and mechanically treated by heating to different maximum temperatures of up to 900 °C and by high-velocity frictional tests before magnetic analyses. Thermomagnetic susceptibility analyses of natural fault rocks revealed that magnetization increased at maximum heating temperatures above 400 °C in the heating cycle, and showed three step increases, at 600 to 550 °C and at 300 °C during the cooling cycle. These behaviors are consistent with the presence of pyrite, siderite and chlorite, suggesting that TCDP gouge originally included these minerals, which contributed to the generation the magnetic susceptibility by thermomechanical reactions. The change in magnetic susceptibility due to heating of siderite was 20 times that obtained by heating pyrite and chlorite, so that only a small fraction of siderite decomposition is enough to cause the slight increase of the susceptibility observed in the fault core. Color measurement results indicate that thermal decomposition by frictional heating took place under low-oxygen conditions at depth, which prevented the minerals from oxidizing to reddish hematite. This finding supports the inference that a mechanically driven chemical reaction partly accounts for the high magnetic susceptibility. A kinetic model analysis confirmed that frictional heating can cause thermal decomposition of siderite and pyrite. Our results show that decomposition of pyrite to pyrrhotite, siderite and, to some extent, chlorite to magnetite is the probable mechanism explaining the magnetic anomaly within the Chelungpu fault zone.