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

The major elements, rare earth elements (REE) and trace elements of four basalt samples from 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.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 OIB.Because of low-temperature alteration, the contents of Al₂O₃, Fe₂O₃, MnO, K₂O and P₂O₅ increased, while MgO and FeO decreased.Active components, such as magnesium and iron, were leached from OIB resulting in the relative enrichment of SiO₂.The leaching of active components can cause the relative enrichment of REE, while the precipitation of LREE-rich ferromanganese oxides in vesicles and fissures not only causes an increase of REE contents, but also induces "fractionation" of LREE and HREE.Based on the enrichment mechanism of REE contents, the theoretical quantities of precipitated ferromanganese oxides and the depleted quantities of active components are calculated:the depleted quantities of active components for the unit mass of fresh basalts vary in the range of 0.15~0.657, and the precipitated quantities of ferromanganese oxides for the 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.Theoretical calculation and geochemistry results both indicate that low-temperature alteration of basalts can supply abundant amount of metals to seawater, and may play an important role in ocean metal circulation.