Kaufman A J, Hayes J M, Klein C
Department of Geological Sciences, Indiana University, Bloomington 47405-5101.
Geochim Cosmochim Acta. 1990;54:3461-73. doi: 10.1016/0016-7037(90)90298-y.
Mineralogic, chemical, and isotopic compositions have been determined for 97 carbonate microbands in five core segments from the early Proterozoic (2.5 Ga) Dales Gorge Member of the Brockman Iron Formation, Hamersley Basin, Western Australia. Samples were obtained both from banded iron-formation (BIF) macrobands 9-12 at Paraburdoo, on the southern margin of the basin, and from BIF macroband 13 at Wittenoom, 130 km NW. At Paraburdoo, oxygen-isotopic compositions of coexisting chert and magnetite microbands were measured and indicated final equilibration temperatures ranging from 60-160 degrees C. This range is consistent with observed mineral assemblages and indicates a considerable temperature gradient across the basin (cf. T approximately 300 degrees C at Wittenoom; BECKER and CLAYTON, 1976). Carbon-isotopic compositions of carbonates are near -7% vs. PDB at Paraburdoo and -10.5% at Wittenoom, but the greater isotopic depletion at Wittenoom appears related to primary or diagenetic processes, not metamorphism. Contents of organic carbon are consistently low. Isotopic depletion is roughly correlated with iron abundance and, together with petrographic observations and chemical balances, is consistent with the model of BIF deposition introduced by BEUKES et al. (1990): primary siderite (delta approximately -5%) precipitated from an anoxic water column depleted in 13C; additional depletion of 13C is associated with coprecipitation of iron oxides and organic carbon. Oxygen-isotopic abundances of microbanded carbonates are similar to those under- and overlying massive marine carbonates, ranging from 17.6 to 21.0% vs. SMOW (-9.6 to -12.9% vs. PDB). Millimeter-scale variations in abundances of 13C and 18O are associated with diagenetic replacement of primary siderite by secondary ankerite and/or magnetite. It is shown that these isotopic variations cannot result from mineral-dependent fractionations, metamorphism, or the influence of large volumes of water in an open system.
已测定了西澳大利亚哈默斯利盆地早元古代（25亿年）布罗克曼铁矿组戴尔斯峡谷段五个岩芯段中97个碳酸盐微带的矿物学、化学和同位素组成。样品取自盆地南缘帕拉布尔杜的条带状含铁建造（BIF）大带9 - 12，以及西北130公里处维特努姆的BIF大带13。在帕拉布尔杜，对共生的燧石和磁铁矿微带的氧同位素组成进行了测量，结果表明最终平衡温度范围为60 - 160摄氏度。该范围与观察到的矿物组合一致，表明整个盆地存在相当大的温度梯度（参考维特努姆的温度约为300摄氏度；贝克尔和克莱顿，1976年）。帕拉布尔杜碳酸盐的碳同位素组成相对于PDB接近 - 7%，维特努姆为 - 10.5%，但维特努姆更大的同位素亏损似乎与原生或成岩过程有关，而非变质作用。有机碳含量一直很低。同位素亏损大致与铁含量相关，并且与岩石学观察和化学平衡一起，与博克斯等人（1990年）提出的BIF沉积模型一致：原生菱铁矿（δ约为 - 5%）从贫13C的缺氧水柱中沉淀；13C的额外亏损与铁氧化物和有机碳的共沉淀有关。微带碳酸盐的氧同位素丰度与下伏和上覆的块状海相碳酸盐相似，相对于SMOW范围为17.6%至21.0%（相对于PDB为 - 9.6%至 - 12.9%）。13C和18O丰度的毫米级变化与次生铁白云石和/或磁铁矿对原生菱铁矿的成岩替代有关。结果表明，这些同位素变化不是由矿物依赖分馏、变质作用或开放系统中大量水的影响导致的。
