蛇纹石化过程中铁活动性的高温高压实验研究

Experimental investigation of iron mobility during serpentinization

蛇纹石化过程中铁的活动性影响铁氧化物的形成和体系的氧逸度。然而,关于橄榄岩蛇纹石化过程中各矿物（橄榄石、斜方辉石和单斜辉石）蚀变过程中铁的活动性仍没有详细的研究。本文报道了80～200℃、饱和蒸气压下,不同的初始流体、水岩比条件下,橄榄岩蛇纹石化过程中铁的活动性。结果表明,蛇纹石化流体的铁含量较低（0.0～0.7mmol/kg）,没有形成铁氧化物或铁氢氧化物,铁主要富集在蛇纹石和未反应的初始矿物中（例如,橄榄石和辉石）。由橄榄石蚀变形成的蛇纹石和由斜方辉石蚀变生成的蛇纹石化学组成相差较大,前者富铁而贫铝,后者贫铁而富铝。但当初始流体为碱性的0.5mol/L Na Cl（aq）时,两种不同来源的蛇纹石组成相差不大。尤为重要的是,单斜辉石蚀变生成的蛇纹石铁含量较高,8.1%～10.2%Fe O,远高于单斜辉石的铁含量（2.6%Fe O）。以上表明,低温蛇纹石化过程中,铁不能够大规模、长距离的运移,但在微米尺度上是活动的。

The mobility of iron during serpentinization affects the formation of iron oxide and oxygen fugacity of serpentinized peridotite. Until now,there has been no detailed experimental study on the mobility of iron during alteration of olivine,orthopyroxene and clinopyroxene. Here we conducted experiments on hydrous alteration of peridotites at 80 ～ 200℃ and vapor saturated pressures with different starting fluids and water / rock ratios and run durations（ 20 ～ 60 days）. The results show that fluids consist of low iron concentrations（ 0. 0 ～ 0. 7mmol / kg）,and no iron oxide or hydroxide forms. Therefore,iron is mainly incorporated into serpentine and residual silicate minerals（ e. g.,olivine and pyroxene）. Serpentine formed after alteration of olivine has distinct compositions from that after alteration of orthopyroxene. The former is enriched in iron but depleted in aluminum relative to the latter. By contrast,for experiments with alkaline 0. 5mol / L Na Cl（ aq）,the two types of serpentines consist of comparable amount of iron and aluminum.Serpentine formed after alteration of clinopyroxene contains 8. 1% ～ 10. 2% Fe O,which is much higher than primary clinopyroxene（ 2. 6% Fe O）. This suggests that iron release from olivine and orthopyroxene can not be transported a long distance,but iron is still mobile at least in micrometer scales.