长江河流沉积物磁铁矿化学组成及其物源示踪

运用电子探针分析了长江干流和主要支流河漫滩沉积物中磁铁矿的元素组成.磁铁矿中的FeO平均含量稍高于其标准组成,而Fe2O3平均含量则明显低于标准组成;Ti、Al、Cr、V、Mn、Mg、Co和Zn等元素在磁铁矿中含量变化大,不同支流的磁铁矿的元素组成不同,同一取样点不同样品磁铁矿的元素组成变化也较大.金沙江、湘江、汉江及长江干流磁铁矿与钛磁铁矿、钛尖晶石、钒钛磁铁矿和铬铁矿等出溶交生,TiO2、Cr2O3和V2O3等元素含量高且变化大.金沙江磁铁矿富Mg、Al和Cr;大渡河、雅砻江和岷江磁铁矿中微量元素含量大多低于0.5%;涪江、汉江磁铁矿富Ti和V,而湘江磁铁矿富Ti和Al;总体上,长江干流上游磁铁矿富Ti,而下游磁铁矿中Ti、Al、Cr、V、Mg和Mn含量低于0.15%.干流磁铁矿的元素组成变化反映主要支流源岩组成及对干流影响程度的差异.

Magnetite compositions of Changjiang River sediments and their tracing implications

The magnetite compositions of the Changjiang River sediments were analyzed by electron microprobe. The results show that the average contents of FeO and Fe2O3 in magnetite grains are slightly higher and much lower than their theoretical values, respectively, and that the contents of Ti, Al, Cr, V, Mn, Mg, Co and Zn vary obviously. Elemental compositions in magnetites vary considerably among the main tributaries and the mainstreams. The magnetite grains from the Jinshajiang River, Xiangjiang River and Hanjiang River and the mainstream exhibit exsolved intergrowths with occurrences of titanmagnetite, titanospinel, vanadic titanmagnetite and chromite, which results in the variations of elemental contents in magnetites. The magnetites in the Jinshajiang River are rich in Mg, Al, and Cr. All trace elements in the magnetites in the Daduhe River, Yalongjiang River and Minjiang River are almost below 0.5% . Ti and V are concentrated in magnetites from the Fujiang River and Hanjiang River, and Ti and Al are rich in the Xiangjiang River. The magnetite grains from the Changjiang upstream sediments are rich in Ti whereas those from the downstream sediments are poor in most elements, such as Ti, Al, Cr, V, Mg and Mn (mostly lower than 0.15% ). The compositional variations in magnetite indicate the complex controls of source rocks compositions.