引起昆明及其周边地区石灰岩高磁化率现象原因初步探讨

A Preliminary Study on the Causes of High Magnetic Susceptibility in Limestones in Kunming and Its Surrounding Areas

摘要: 磁化率一词自第五届土壤学会议提出后便受到广泛关注，以往母岩说中普遍认为高磁化率风化壳应当发育在高磁性玄武岩上，而近年来的研究发现昆明及其周边地区弱磁性石灰岩上却发育目前已知最高磁化率风化壳，通过分别测试该区域玄武岩与石灰岩上覆风化壳Fe元素含量、有机质含量、pH值发现，玄武岩以及石灰岩的Fe含量分别为14%和15%；玄武岩和石灰岩有机质含量分别为1.43%和1.69%；两种岩石的Fe含量与有机质含量总体差异不大，但是玄武岩和石灰岩的pH值分别为5.14和8.16，且基于pH值于实验室模拟合成磁赤铁矿得出pH值为5时产率为13.38%；pH值为8时产率达75.5%，差异显著。由此认为pH是引起昆明及其周边地区玄武岩与石灰岩两种岩石上覆风化壳磁化率差异且引起石灰岩上覆风化壳出现高磁化率现象的重要因素。

Abstract: The term “magnetic susceptibility” has garnered widespread attention since its introduction at the Fifth Pedology Conference. Previous parent rock theories generally assumed that high-magnetic-susceptibility weathering crusts would develop on highly magnetic basalts. However, recent studies have revealed that the highest-known magnetic-susceptibility weathering crusts develop on weakly magnetic limestones in Kunming and its surrounding areas. Testing of the Fe content, organic matter content, and pH of the weathering crusts overlying basalts and limestones in this region revealed that the Fe content of the basalts and limestones was 14% and 15%, respectively; the organic matter content of the basalts and limestones was 1.43% and 1.69%, respectively; and the Fe and organic matter contents of the two rocks were generally similar. However, the pH values of the basalts and limestones were 5.14 and 8.16, respectively. Laboratory simulations of the synthesis of hematite based on pH values revealed a significant yield of 13.38% at pH 5 and 75.5% at pH 8, indicating significant differences. Therefore, we conclude that pH is a key factor influencing the difference in magnetic susceptibility between the weathering crusts overlying basalt and limestone in Kunming and its surrounding areas, and in causing the high magnetic susceptibility of the weathering crust overlying the limestone.