罗布泊自生胶黄铁矿对磁性参数χARM/SIRM指示意义的影响

Influence of Greigite on Interpretation of Magnetic Parameterχ_(ARM)/SIRM in Lop Nur

沉积物中磁性矿物的颗粒大小具有重要环境指示意义,是环境磁学研究的重要内容。磁性参数非磁滞剩磁磁化率/饱和等温剩磁(χ_(ARM)/SIRM)常被用于指示磁性颗粒大小,当沉积物磁性特征为较粗的多畴(MD)颗粒主导时表现为低值,而为较细的单畴(SD)颗粒主导时表现为高值。本研究对罗布泊盐湖LB剖面的沉积物进行了系统环境磁学测试,结果发现剖面中含有5个χ_(ARM)/SIRM值低于上覆层和下伏层的层位,指示其磁性颗粒较粗。但磁滞参数和扫描电镜结果显示这5个层位磁性颗粒明显细于上覆层和下伏层沉积物,显示χ_(ARM)/SIRM在这5个层位的指示意义失真。矿物鉴定结果显示这5个层位的主导磁性矿物为早期成岩成因的SD胶黄铁矿,赋存形态主要为颗粒团聚体、单颗粒或颗粒团聚体附着于硅酸盐矿物表面以及分布于薄片状硅酸盐内,这种赋存形态导致其具有较强的磁相互作用,并限制了χ_(ARM)的增加,最终造成χ_(ARM)/SIRM下降,指示意义失真。本研究显示当沉积物中存在胶黄铁矿等自生矿物时,利用比值参数χ_(ARM)/SIRM判断磁性矿物颗粒特征需谨慎。

The grain size of magnetic minerals has been widely applied in both magnetostratigraphic and paleoenvi⁃ronmental studies.Anhysteretic remanent/saturation isothermal remanent magnetization(χ_(ARM)/SIRM)is commonly used as a grain-size indicator for magnetic minerals,and peaks in the SD range and decreases with increasing grain size.In this study,rock magnetic analyses were conducted on the sediments from the LB profile at Lop Nur,a former saline lake in the Tarim Basin,China.Five layers with relatively low values ofχ_(ARM)/SIRM were identified,indicating that the magnetic minerals were coarser than in other layers.However,the results of magnetic hysteresis and scan⁃ning electron microscopy demonstrate that the magnetic particles in the five layers are significantly finer than those in other layers,suggesting that the indicative significance ofχ_(ARM)/SIRM in those five layers may be incorrect.By combin⁃ing the minerals and rock magnetic analyses it was found that the predominant magnetic mineral in the five layers is authigenic greigite of early diagenetic origin,present in the stable single domain(SD)state,whereas the magnetic properties of the other layers are dominated by detrital multidomain magnetite.SD greigite is mainly present as adja⁃cent particle aggregates or adjacent inclusions embedded in silicate.This morphology of magnetic particles causes sig⁃nificant magnetic interaction,with markedly lowerχ_(ARM)than SIRM,consequently giving lowerχ_(ARM)/SIRM values.The lowχ_(ARM)/SIRM values within the five layers therefore reflects stronger magnetic interaction,not the grain size of the magnetic particles.Our study demonstrates that if the signal indicates the presence of authigenic greigite(e.g.,higher SIRM/χ)in sediments,caution is needed when usingχ_(ARM)/SIRM values to identify the grain size of magnetic particles.