Kr and Xe isotopic compositions of Fe-Mn crusts from the western and central Pacific Ocean and implications for their genesis

Abstract

Kr and Xe nuclide abundance and isotopic ratios of the uppermost layer of Fe-Mn Crusts from the western and central Pacific Ocean have been determined. The results indicate that the Kr and Xe isotopic compositions, like that of He, Ne and Ar, can be classified into two types: low 3He/4He type and high 3He/4He type. The low 3He/4He type crusts have low 84Kr and 132Xe abundance, while the high 3He/4He type crusts have high 84Kr and 132Xe abundance. The 82Kr/84Kr ratios of the low 3He/4He type crusts are lower than that of the air, while the 83Kr/84Kr and 86Kr/84Kr ratios are higher than those of the air. The Kr isotopic ratios of the higher 3He/4He type crusts are quite similar to those of the air. The 128Xe/132Xe, 130Xe/132Xe and 131Xe/132Xe ratios of the low 3He/4He type sample are distinctly lower than those of the air, whereas the 129Xe/132Xe, 134Xe/132Xe and 136Xe/132Xe ratios are higher than those of the air. The low 3He/4He type samples have the diagnostic characteristics of the MORB, with excess 129, 131, 132, 134, 136Xe relative to 130Xe compared with the solar wind. The 128Xe/132Xe, 130Xe/132Xe and 131Xe/132Xe ratios of the high 3He/4He type samples are slightly higher than those of the air, and the 129Xe/132Xe, 134Xe/132Xe and 136Xe/132Xe ratios are qiute similar to those of the air. The noble gases in the Fe-Mn crusts are derived from the lower mantle, and they are a mixture of lower mantle primitive component, radiogenic component and subduction recycled component. The helium isotopic ratios of the low mantle reservoir are predominantly controlled by primitive He (3He) and U and Th radiogenic decayed He (4He), but the isotopic ratios of the heavier noble gases, such as Ar, Kr and Xe, are controlled to different extent by recycling of subduction components. The difference of the noble isotopic compositions of the two type crusts is the result of the difference of the noble isotopic composition of the mantle source reservoir underneath the seamounts the crusts occurred, the noble gases of the high 3He/4He type crusts are derived mainly from EM-type lower mantle reservoir, and the noble gases in the low 3He/4He type crusts are derived mainly from HIMU-type lower mantle reservoir.