南非巴伯顿绿岩带条带状铁建造岩石磁学及磁性矿物的组成特征

Rock magnetism of the banded iron formation in Barberton greenstone belt,South Africa

条带状铁建造(BIFs)中含有大量的亚铁磁性矿物,其组成及来源是认识BIF成因的重要依据。本文研究了南非巴伯顿绿岩带无花果树群(距今约32亿年)恩圭尼亚组的BIFs样品的磁学和矿物学特征。通过测量富铁层与富硅层的磁滞回线、等温剩磁获得曲线与退磁曲线、矫顽力谱分析、一阶反转曲线(FORC)、低温(20~300K)有场/无场冷却曲线以及k-T曲线、Lowrie三轴热退磁曲线,结合扫描电镜观测,揭示出研究样品中磁性矿物主要为赤铁矿和磁铁矿。基于矫顽力谱分析,富铁层中磁铁矿主要是多畴及假单畴颗粒,相对含量平均为2. 1%;赤铁矿的相对含量平均为97. 9%。富硅层中磁铁矿主要为假单畴及超顺磁性颗粒,相对含量平均为4. 6%;赤铁矿相对含量平均为95. 4%。测试样品具有Morin转变特征,转变温度介于250~260K,说明BIFs中主要为赤铁矿(0. 5~6mm)。富硅层样品出现~107K、~125K两个Verwey转变温度,表明其中可能存在生物成因和非生物成因两种类型磁铁矿。

Ferrimagnetic minerals and their origins in banded iron formations( BIFs) are contentious research topics. Here we present an integrated rock magnetic result of BIFs sample from Barberton greenstone belt( South Africa). Hysteresis loops,acquisition of saturation isothermal remanent magnetism and its back-field demagnetization curves,coercivity spectrum analysis,first-order-reversal curves( FORC), low-temperature( 20 ~ 300 K) magnetic measurements( zero-field cooling/field cooling curves), thermal demagnetization of three-component isothermal remanent magnetization curves and scanning electron microscopy observation were performed. We found that hematite and magnetite are dominated in the studied sample. The magnetite grains in the iron-rich layers are mainly multi-domain and pseudo-single-domain states with average relative content 2. 1% and the average relative content of hematite is97. 9%. In the silica-rich layer,magnetite grains mainly consist of pseudo-single-domain and superparamagnetic states with an average relative content of 4. 6% and the mean of relative content of hematite is 95. 4%. Morin transition ranging in temperature from 250 K to260 K reveal that hematite grains in BIFs is dominated by multi-domain( 0. 5 ~ 6 mm). Interestingly,the silica-rich layer displays the Verwey transition at ~ 107 K and ~ 125 K,implying presence of biogenic and abiogenic magnetite in the sample.