用稀盐酸浸取方法探讨玄武岩风化产物中REE与Al、P、Fe和Mn的依存关系

Relationship between REE and Al,P,Fe and Mn in basaltic weathering products:Evidences from hydrochloric acid leaching

用2mol/LHCl对海南新生代玄武岩风化剖面上的风化产物进行了浸取实验,通过浸取出来的稀土元素(REE)比例及其REE与Al、P、Fe和Mn的关系探讨风化产物中REE的赋存状态。结果显示,大部分REE、Al和Mn(>50%)、相当一部分P(20%～40%)和Fe(～20%)被稀盐酸浸出。这些盐酸提取出来的REE(不包括Ce)根据含量不同与其他元素有不同的对应关系,较高的REE含量(>250μg/g)与Al显示出良好的正相关关系,而与Fe、P和Mn相关性不明显;较低的REE含量(<150μg/g)与Fe和P显示很好的正相关关系,与Mn正相关性不明显,与Al存在负相关关系;Ce与Mn存在较好的正相关关系,与Al、P和Fe没有明显的相关性。结合风化产物全样的相应元素分布特征可以判断,这些风化产物中高含量的REE(不包含Ce)是以比较松散的形式存在,并容易被流体淋滤出去,而正常含量的REE主要赋存于Fe氧化物/氢氧化物和次生磷酸盐矿物中,Ce的赋存则与Mn氧化物/氢氧化物有密切关系。同时也做了原岩的淋洗实验,发现Al、P及REE等元素被大量析出,因此不支持用2mol/L以上的酸对岩石样品进行淋洗前处理。

An experiment of using 2 mol/L HCl to leach the weathering products from a profile developed on Cenozoic basalts in northern Hainan was made. The percentage for rare earth elements （REEs）, Al, P, Fe and Mn to be leached out of the weathering products was estimated, and the relationship between REEs and Al, P, Fe and Mn in the leachate may give hints for the possible hosts for REEs in weathering products. The results indicate that more than 50 percent of REEs, Al and Mn, 20 to 40 percent of P and about 20 percent of Fe in the weathering products have been leached out by 2 mol/L HCl. The relationship between REEs and other elements in the leachate depends on the concentrations of REEs （excluding Ce）. In details, REEs （excluding Ce） with high concentrations （〉 250 μg/g）show significant positive correlation with Al and no correlation with Fe, P and Mn, whereas REEs with low concentrations（〈 150 μg/g）show significant positive correlation with Fe and P, poor positive correlation with Mn and significant negative correlation with Al. Unlike other REEs, Ce show significant positive correlation with Mn, and no correlation with Al, P and Fe in the leachate. These characteristics, together with the re-distributions of these elements in the weathering products （Ma et al., 2007）, suggest that the REEs （excluding Ce） with extreme high concentrations in the weathering products are loosely combined, and are easily by leached away by liquids. However, the REEs （excluding Ce） with normal concentrations seem mainly hosted in Fe oxides/ hydroxides and secondary phosphate minerals. The most important host for Ce is Mn oxides/hydroxides. Additionally, fresh basalt （in powder） was also leached using 2 mol/L HC1 in this paper. The results indicate that many of the Al, P and REEs were also leached out of the fresh basalts. Therefore, using 2 mol/L or more concentrated HCl to pre-clean basaltic rock samples before elemental/isotopic measurement may cause the problem of partial dissolution.