黔北三星洞石笋记录的岩石磁学特征分析

本文对发育于我国贵州省遵义市三星洞的一支石笋SX3-b进行了岩石磁学分析，测试了磁化率、等温剩磁等磁学参数，结合热磁实验，分析SX3-b石笋的基本磁性特征。通过对比洞穴上覆土壤及洞穴基岩的磁学参数，进一步探讨SX3-b石笋磁性矿物来源。研究结果表明：石笋SX3-b的磁化率均为负值，平均值为-1.657×10-8m3/kg，表明样品中抗磁性物质方解石晶体主导了磁化率的特征。石笋样品饱和等温剩磁（SIRM）值较低，平均仅为0.52×10-5Am2/kg，表明样品中含有数量极少的亚铁磁性矿物。石笋纯净部分热磁κ-T曲线没有显示出有特征性的转折点，说明在石笋较纯净部分，其磁性矿物含量极其有限；石笋沉积间断面处样品热磁κ-T曲线中加热曲线则在300℃时出现磁化率峰，揭示出样品中含有纤铁矿。洞穴外部土壤可能是石笋沉积间断面处磁性矿物颗粒的一个来源。

Rock magnetism of stalagmite in Sanxing Cave,northern Guizhou Province

Cave stalagmite is an effective archive in high resolution palaeoclimate research due to the facts that stalagmite consolidate quickly and is seldom modified by post-depositional,and stalagmite can be dated precisely by U-Th dating and layer counting.Rock magnetism palys an important role in the studying of palaeoenvironmental and palaeoclimatic changes by loess and lake sediments.Despite the great potential of speleothems for palaenvironmental studies,the typically very low concentration of ferromagnetic minerals resulting in low Nature Remanent Magnetisation has limited the usage.By combining the richness of rock magnetic parameters with the accuracy of the stalagmites sample can reflect paleoclimatic changer better.Stalagmite SX3-b was collected from Sanxing Cave (27°22'N,107°11'E).Sanxing Cave,about 2km in length and 720m in elevation,is located at Tiechang Town,70km southeast of Zunyi City,Guizhou Province,southwest China.The annual average precipitation is about 1000mm and the mean temperature is 15.4℃;mean cave temperature is approximately constant at 14.5℃;the well-developed vegetation consists mainly of subtropical broadleaf evergreen and deciduous mixed forests.The dimensions of stalagmite SX3-b is 150mm in length and 70mm in diameter.This article does some reach of rock magnetism about the SX3-b,and these studies include the magnetic susceptibility,the magnetic parameters of the isothermal remanent magnetization.By comparing the overlying soil and cave bedrock magnetic parameters to explore mineral resource and possible environment signficance.The measured results:the average magnetic susceptibility of SX3-b stalagmite is-1.657×10-8m3/kg and magnetic susceptibility were negative,which indicates that the sample diamagnetic material in calcite crystal led the characteristics of magnetic susceptibility.The SIRM value of the stalagmite is low,and the value of average only 0.52×10-5Am2/kg,which show that the sample contains few ferrous magnetic.The thermomagnetic κ-T curve of the pure part not show a characteristic of the turning point,it maybe indicate that in the pure part of the stalagmite contain few mineral,but in the part of stalagmite discontinuity,the thermomagnetic κ-T curve appears a 300℃ peak in heating curve,it may mean that contain lepidocrocite.Thermomagnetic curves in the overlying soil which contain lepidocrocite,indicating the overlying soil may be a source of mineral grains about the stalagmite discontinuity.