赤铁矿-水体系氧同位素分馏的实验研究  

An Experimental Study of Oxygen Isotope Fractionation in the Hematite-water System

作　　者：徐宝龙[1] 周根陶[1] 郑永飞[1] 

机构地区：[1]中国科学技术大学地球和空间科学学院,合肥230026

出　　处：《地质学报》2003年第1期85-97,共13页Acta Geologica Sinica

基　　金：国家自然科学基金(编号49903001;40033010)

摘　　要：本文分别以3种不同的可溶性三价铁盐作为Fe^(3+)源物质的强迫水解法和以针铁矿和四方纤铁矿为起始物质的溶液转化法,在90～315℃范围内合成赤铁矿,测定了赤铁矿与水之间的氧同位素分馏。矿物合成实验和氧同位素分析结果显示,在90～225℃范围内,实验获得的赤铁矿与水之间氧同位素分馏为亚稳态分馏,并且不同合成实验方法得到不同的分馏关系,前者相对于后者富集(18)O约为2‰,即:10~31nα_(赤铁矿-水)=1.17±0.02×10~6/T^2-9.14±0.20(强迫水解法);10~31nα_(赤铁矿-水)=1.46±0.18×10~6/T^2-14.52±0.03(溶液转化法)。但温度在315℃以下,无论强迫水解法还是溶液转化法,在实验误差范围内实验测定的分馏值几乎不可区分,并且与增量方法的理论预测相近,表明该温度下获得的赤铁矿与水之间氧同位素分馏代表了赤铁矿-水体系氧同位素平衡分馏。此外,两种不同方法获得了不同的分馏关系,显示低温下赤铁矿-水体系氧同位素分馏不仅依赖于赤铁矿形成的温度,而且取决于赤铁矿的成因机制,因此对应于不同形成环境下的动力学亚稳态平衡,这对解释低温环境中赤铁矿的氧同位素数据具有重要意义。Hematite was synthesized by hydrolysis-forced and solution-transformation approaches in a temperature range of 90 to 315 ℃. The three different kinds of soluble iron(Ⅲ) salts and synthesized goethite and aka-ganeite were used as initial Fe3+ sources. The mineral synthesis results and oxygen isotope data show that oxy- gen isotope fractionations between hematite and water derived from the hydrolysis-forced and transformation approaches fit two different steady-state fractionation relationships, respectively, with the former being enriched about 2‰ in 18O relative to the latter:Hydrolysis-forced approach: 103lna = 1.17 ± 0.02 × 106/T2- 9.14 ± 0.20 Solution-transformation approach: 103lna = 1.46±0. 18 × 106/T2-14. 52 ± 0. 03At 315 ℃ , however, the fractionations between hematite and water measured by the two different approaches are not only in agreement with each other within the analytical errors, but also close to the theoretical predictions by the increment method. This suggests that the measured fractionations represent the equilibrium ones for the hematite-water system at this temperature. The different fractionation relationships were obtained from the two different types of synthetic experiments at 90 to 225℃, indicating that the measured fractionations depend not only on the formation temperatures of hematite, but also on the formation mechanism of hematite. This corresponds to the kinetically controlled, steady-state fractionations of oxygen isotopes between hematite and water at the low temperatures. This may be of great importance to geochemical interpretation of oxygen isotope data for hematite formed in low temperature environments.

关 键 词：赤铁矿 氧同位素 分馏系数 亚稳态平衡 针铁矿 古环境 强迫水解法 溶液转化法 

分 类 号：P597[天文地球—地球化学]