西オーストラリア,ハマースレイ層群のブールギーダ鉄鉱層の全岩化学組成

その起源と低微量元素濃度の示唆

抄録

Major, minor, and rare earth element abundances are reported for the Boolgeeda Iron Formation of the Hamersley Group, Western Australia. The banded iron formations (BIFs) of the present study are divided into five types based on their color; black, gray, yellow, red, and white layers. Black, red, and white layers are exclusively composed of SiO₂ and Fe₂O₃*, while gray and yellow ones contain significant amounts of lithogenous components such as AL₂O₃, TiO₂, MgO, K₂O, and Na₂O. There are excellent positive correlations among these lithogenous major elements and minor elements including Sc, Cr, Ni, Zr, Hf, and Th. These elements were supplied from the exposed continental crust as clastic materials. High Th/Sc and (La/Yb)cn ratios and negative Eu anomalies of the gray and yellow layers indicate a significant contribution of granitic clastics.
Most of red and white layers with a minor terrigenous component generally exhibit no Eu anomaly when normalized to chondrite. This implies that these layers were genetically related to a submarine hydrothermal solution, taking it into consideration that river waters with a significant negative Eu anomaly should have drained considerably into the ocean. A positive corre-lation between Fe and minor elements such as P, V, Ni, Y, U, and REEs is analogous to that of modern hydrothermal sediments at mid-ocean ridge, indicating that iron-rich suspended particulates, precursors of BIFs, scavenged these minor elements as like today. However, their metal/Fe ratios of the BIFs are generally lower than those of modern hydrothermal sediments by one order of magnitude. It is likely that much more abundant iron-rich suspended particulates were formed in the 2.5 Gyr ocean due to much higher Fe contents in the hydrothermal solution than today. Large quantities of iron-rich suspended particulates scavenged competitively the minor elements, resulting in very low contents of them in the BIFs.