断裂熔融作用中单质铁的形成及其指示的孕震环境

Metallic iron formed by melting and its seismogenic setting indication

目前缺乏来自于地震断裂带的假玄武玻璃的原始特征研究,制约着我们对地震孕震环境和地震发生机制的认识。本文以龙门山断裂带汶川科钻2号钻孔岩心的碎裂岩及花岗闪长岩为研究对象,通过氩气环境下的高温加热实验(最高温度达1750℃)、显微结构观察、地球化学分析和岩石磁学测试,探讨断裂熔融作用中单质铁的形成及其指示意义。花岗闪长岩和碎裂岩在1100℃时发生部分熔融作用,形成了非晶质和微晶;在≥1300℃时形成了大量单质铁组成的微球粒,可能是含铁矿物发生了以碳物质为还原剂的高温还原反应。高温实验后样品的磁化率值高于碎裂岩的磁化率值,新生成的磁铁矿是≤1100℃的样品高磁化率值的主要原因;熔融作用中形成的单质铁和少量的磁铁矿是≥1300℃样品高磁化率值的重要原因。结合前人对于快速摩擦实验熔体、地幔物质及陨石的研究,我们认为粘土矿物、硅酸盐矿物含量较多的断层泥在同震滑移中因热增压机制很难形成单质铁;而花岗岩、安山岩、辉长岩、闪长岩等岩浆岩和糜棱岩在流体作用弱和硫化物含量低的环境中易在大地震中形成单质铁。龙门山断裂带的假玄武玻璃的熔融温度≥1300℃,硫元素含量较低并发现有过剩铁元素。因此,表明龙门山断裂带的假玄武玻璃中可能形成了单质铁,其和磁铁矿是假玄武玻璃高磁化率值异常的重要原因,并指示了深部流体作用弱、硫化物含量低、还原性的孕震环境。

The natural property of pseudotachylyte from earthquake fault zones remains unknown,which limit our understanding of seismogenic setting and earthquake mechanism.To investigate the formation of metallic iron in pseudotachylyte and its implications,we carried out high temperature heating experiments(up to 1750℃)under an argon environment on cataclasite and granodiorite from the Wenchuan earthquake Fault Scientific Drilling Borehole 2(WFSD-2)cores.Microstructural,geochemical and rock-magnetic analyses were performed.The melting of cataclasite and granodiorite occurred at 1100℃yielded amorphous components and microlites.Above 1300℃,many circular metallic iron spherulites were formed by the reducing action of Fe-bearing minerals at elevated temperatures.The magnetic susceptibility(MS)values of the samples after the heating experiments are higher than prior to it.The newly-formed magnetite induces the high MS values of the samples below 1100℃.The metallic iron formed in the melting and few magnetite grains explain the high MS values of samples above 1300℃.Combined with previous researches of high-velocity friction experiments,mantle materials and chondrites,metallic iron may not form in the fault gouge having clay or silicate minerals because of the thermal pressurization during large earthquakes,while they can form in the granite,andesite,gabbro,diorite and mylonite with little fluid,few sulfides and amount of reducing materials during large earthquakes.The excess iron,few sulfides and the frictional melting temperature reached 1300℃during ancient earthquakes in the Longmen Shan thrust belt,indicating that metallic iron might be formed in the pseudotachylyte and might explain the high MS values of pseudotachylyte.The metallic iron formed within the pseudotachylyte in the Longmen Shan thrust belt might indicate a depth reducing seismogenic environment with weak fluid activity,few sulfides and amount of reducing materials.