大洋岛屿玄武岩低温蚀变作用及其对大洋过渡金属循环的贡献

Low-temperature alteration of oceanic island basalts and their contribution to transition metal cycle of the ocean

用化学方法和ICP-MS方法分别对中、西太平洋海山富钴铁锰结壳产出区玄武岩的主元素、微量元素和稀土元素（REE）含量进行了测定,结果表明,研究区玄武岩经受了强烈的洋底低温蚀变作用,主元素成分发生了明显的变化,失去了原岩的特征.样品与新鲜大洋岛屿玄武岩（OIB）极为相似的稀土元素配分模式和微量元素含量特征表明,所研究的岩石属典型的大洋板内玄武岩.受洋底低温蚀变作用的影响,样品的Al2O3,Fe2O3,MnO、K2O,P2O5含量增加,MgO,FeO的含量降低.蚀变作用使大洋岛屿玄武岩中的镁、铁等活动组分大量流失,从而表现出相对富SiO2的特征（标准矿物计算结果中出现石英）.由于蚀变作用,活动组分的流失使样品的REE相对富集,而富REE铁锰氧化物在玄武岩气孔和裂隙中的沉淀不仅使样品的REE含量增大,而且引起轻稀土元素（LREE）与重稀土元素（HREE）分馏,表现为∑c（Ce）/∑c（Yb）值增大.以REE富集机制为基础,对样品中铁锰氧化物的沉淀量和单位质量新鲜玄武岩中活动组分的流失量进行了理论计算,结果表明,因低温蚀变作用所引起的新鲜玄武岩的单位质量亏损为0.150-0.657,而单位质量新鲜玄武岩中铁锰氧化物的沉淀量为0.006-0.042.主元素中以铁、镁的流失亏损最为明显,新鲜玄武岩中铁、镁的流失比例分别为18.28%-70.95%和44.50%-93.94%,超过了岩石总量的流失亏损比例（15.0%-65.7%）,因而样品相对贫铁、镁.其他元素的流失量和流失比例都很好地印证了地球化学研究的结果.样品中铝、钾、磷负的流失量是由于沸石在岩石气孔中的充填和岩石的磷酸盐化.理论计算结果和地球化学研究都表明,大洋岛屿玄武岩的低温蚀变向海水提供了大量金属,这是大洋海水中金属循环的重要环节.

The major elements,rare earth elements（REE）and trace elements of 4 basalt samples from the central and western Pacific ferromanganese crust provinces have been analyzed using chemical methods and ICP-MS,respectively.The results indicate that the samples have been extensively altered and that the contents of their major elements have changed significantly,with the samples losing the characteristics of the original rocks.However,the similarity of REE partition patterns and trace element contents of basalt samples to those of fresh oceanic island basalts（OIB）indicate that the basalt samples originated as oceanic island basalts（OIB）.Because of low-temperature alteration,the contents of Al2O3,Fe2O3,MnO,K2O,and P2O5 increased relative to fresh oceanic island basalts,while MgO and FeO decreased.Active components,such as magnesium and iron,were leached from OIB resulting in the relative enrichment of SiO2 of the samples than fresh OIB（appeare in standard mineral quartz）,although the samples having lower SiO2 contents（40.4%-41.4%）than the lower limit（43%） of fresh basalt.The leaching of active components can cause the relative enrichment of REE of altered basalts,while the precipitation of REE-rich ferromanganese oxides in vesicles and fissures of altered basalts not only causes an increase of REE contents,but also induces fractionation of LREE and HREE,thereby increasing of ∑c（Ce）/∑c（Yb） ratios of the samples.Based on the enrichment mechanism of REE contents of altered basalts,the theoretical quantities of precipitated ferromanganese oxides and the depleted quantities of active components were calculated for per unit mass of fresh basalt.As a result of low-temperatrue alteration,the depleted quantities of active components for per unit mass of fresh basalts vary in the range of 0.150-0.657,and the precipitated quantities of ferromanganese oxides for per unit mass of fresh basalts vary in the range of 0.006-0.042.Of the major elements,the two most depleted are iron,and magnesium,with 18.28%-70.95% of iron and 44.50-93.94% of magnesium in the fresh basalts was leached out.This exceeded the depletion ratios of bulk samples（15.0%-65.7%）,and as a result the samples are depleted in iron and magnesium relative to fresh basalts.The negative depletion quantities of aluminium,potassium,and phosphorus for some samples resulted from the precipitation of zeolite in vesicles of samples and phosphatisation of the basalts.Theoretical calculation and geochemistry results both indicate that low-temperature alteration of basalts can supply abundant among of metals to seawater,and may play an important role in metal cycle of the ocean.