水镁石-水体系氧同位素分馏系数的低温实验研究

EXPERIMENTAL STUDIES OF OXYGEN ISOTOPE FRACTIONATIONS BETWEEN BRUCITE AND WATER AT LOW TEMPERATURES

在15—120℃的低温范围内分别应用氮化镁法、氯化镁法和氧化镁法3种化学合成方法,对水镁石-水体系氧同位素分馏系数进行了实验测定。所有合成样品的晶体结构均由XRD测定,其形貌特征则由SEM确定。应用3种不同合成方法得到了一致的水镁石—水体系氧同位素分馏系数,证明同位素平衡分馏已经达到。在实验温度范围内,水镁石—水体系氧同位素分馏系数主要决定于温度,而溶液的酸碱度、化学组分和陈化时间的影响不明显。由实验数据得到的氧同位素分馏曲线方程为:10~3Inα=1.59×10~6/T^2-14.10(r=0.9921)。结合前人对三水铝石—水体系和针铁矿—水体系氧同位素分馏系数的低温实验测定,可以得到氢氧化物中金属M—OH键的^(18)O富集顺序:Al^(3+)-OH>Fe^(3+)-OH>Mg^(2+)-OH。应用化学合成方法实验测定低温条件下水镁石—水体系氧同位素分馏系数,不仅克服了同位素交换反应实验的一些缺陷(如交换速率缓慢、仪器设备复杂昂贵等),而且可以应用不同的化学合成反应机理来检验同位素平衡是否达到,这为研究低温地球化学过程作用提供了有价值的基本参数。

Oxygen isotope fractionation for the brucite-water system was determined experimentally by chemical synthesis techniques at temperatures of 15 to 120 C. Magnesium oxide, magnesium nitride and magnesium chloride were used as reactants, respectively, to produce brucite in aqueous solutions. All of the synthesis products were identified by X-ray for crystal structure and by SEM for morphology. Isotope equilibrium was achieved between the synthesized brucite and water , yielding the consistent results from the three different methods of synthesis. In the range of experimental temperatures, the oxygen isotope fractionations between brucite and water are temperature dependent and hot relevant to the chemical compositions, aging time and acid concentration of solutions. The oxygen isotope fractionation equation is obtained as follows;103lna=l. 59×106/T2-14. 10 (r=0. 9921)When the results for the brucite-water system are compared with those for systems gibbsite-wa-ter and goethite-water, it suggests the following order of 18O enrichment in the metal-OH bonds (M-OH) of hydroxides; AP3+-OH > Fe3+-OH > Mg2+-OH. The results from the synthesis experiments agree well with the theoretical calculations by increment method , providing experimental evidence for the theoretical prediction. The synthesis methods can overcome the problems due to the extremely low rates of isotope exchange at the low temperatures. According to the differ ent reaction paths in the different synthesis experiments, the mechanism of the low temperature geochemistry can be explained in addition to testing for isotope equilibrium.