Noble Gas Geochemistry Of Ferromanganese Crusts From Pacific Ocean And Their Significations For The Formation Of Crusts

The nuclide contents and isotopic ratios of ferromanganese crusts from the western and central Pacific Ocean vary significantly with regional characteristics. Based on the relationship between the He isotope ratios of samples and that of MORB, the samples can be classified into two types: a low ~3He/~4He type and a high ~3He/~4He type. The sample from the Magellan Seamounts, Marcus-Wake Seamounts, Marshall Island Chain and the Mid-Pacific Seamounts are low ~3He/~4He type crusts, while the samples from the Line Island Chain are high ~3He/~4He type crusts.The noble gases in ferromanganese crusts originate predominantly from the lower mantle of the Earth. The differences in noble gas characteristics between the low ~3He/~4He type and the high ~3He/~4He type crusts indicate the differences of lower mantle end-member of the correspondent seamount chains. Noble gases of the low ~3He/~4He type crusts originate mainly from a HIMU-type lower mantle source contaminated by recycled oceanic crust, whereas noble gases in the high ~3He/~4He type samples originate mainly from an EM-type lower mantle source contaminated by recycled sediments which captured the air component. The difference in the lower mantle end-member is the fundamental reason results in the differences in the noble gas composition between the two type crusts.The platinum group elements (PGE) contents of crust samples vary from one seamount chain to another and there are significant differentiation between different elements of PGE, indicating the diversity of PGE source of crusts and the variation of the enrichment mechanism in seamount chains. The PGE in crusts origin mainly from alteration of mantle-deirived submarine basalts and river and eolian input, while the contributin of extraterrestrial material is limited. This proves the mantle origin hypotheses of noble gases in ferromanganese crusts.The study of low-temperature alteration and the mass balance theoretical calculation of oceanic island basalts (OIB) indicate that low-temperature alteration of OIB can supply abundant amount of metals to seawater and OIB play as an important source of transition metals for the formation of ferromanganese crusts.