水镁石-水体系氢同位素分馏系数的低温实验研究

分别以氯化镁和氮化镁作为起始物料，在低温（25～90℃）溶液中采用化学方法合成水镁石．然后分别测定水镁石和水的氢同位素组成，得到水镁石-水体系的氢同位素分馏方程为：103lna=－4．88×106／T2－22．54。由于矿物-水体系氢同位素分馏的压力效应，本研究通过大气压力下的实验得到的分馏系数值，系统地低于根据高温（510～100℃）高压（100～103MPa）热液交换实验结果向低温方向外推获得的分馏系数值。与已知的其它含羟基矿物-水体系低温氢同位素分馏曲线比较的结果表明，在热力学平衡条件下，水镁石相对于高岭石和伊利石／蒙脱石亏损D，但相对于外铁矿富集D。因此，合羟基矿物存在如下D富集顺序：Al－OH＞Mg－OH＞Fe－OH。

EXPERIMENTAL STUDY ON HYDROGEN ISOTOPE FRACTIONATION BETWEEN BRUCITE AND WATER AT LOW TEMPERATURE

Using MgCl2 and Mg3N2 as reactants to synthesize brucite in aqueous soutions at low temperatures of 25 - 90℃, hydrogen isotope fractionation between brucite and water is experimentally calibrated, which can be presented in the following form of algebraic equation:10ln = - 4. 88 × 106/ T2 - 22. 54 Because of the effect of pressure on hydrogen isotope fractionation between minerals and water, the present calibrations at atmospheric pressure are systematically lower than the fractionations extrapolated from hydrothermal exchange experiments at high temperatures (510 - 100℃ ) and high pressures (100 - 103 MPa). By comParing with known calibrations of hydrogen isotope fractionation between other hydsoxyl-bearing minerals and water at low temperatures, it is obtained that brucite is depleted in D relative to kaolinite and illite/smectite but enriched in D relative to goethite. Thus there is the folfowing Sequence of D-enrichment in the hydroxyl-bearing minerals: Al-OH> Mg-OH>Fe-OH.