Noble gases isotopic compositions and sources of cobalt-rich crusts from west Pacific Ocean seamounts

Noble gases abundances and isotopic compositions of various layers in the three-layered Cobalt-rich ferromanganese crusts and their substrate rocks, which were collected from Magellan seamounts in west Pacific, were analyzed by using a high vacuum gas mass spectrum, and the results show that: 1. Most of the ferromanganese crusts in the west Pacific Ocean were hydrogenous origin and the noble gases in the crusts derived from different sources. The He was sourced predominantly from extraterrestrial grains such as interplanetary dust particles( IDPs), with a little from aeolian continental dust grains in the seawater. The Ar derived mainly from the air dissolved in the ambient sea water, and partly from the aeolian dust grains or formation water in the submarine sediments. While the Xe and Ne were mostly sourced from dissolved air in the sea water, and partly from the interplanetary dust particles (IDPs). (2) In the three-layered crusts, the compact layers possess quite different noble gases isotopic compositions compared with the outer and porous layers, suggesting that marine phosphatization might have greatly modified the noble gases isotopic compositions in the old crusts, resulting in dramatic increase of He-4 concentrations and decrease of(3) He/(4) He. (3) He-3/(4) He ratios in the basaltic substrates of the Cobalt-rich crusts are very low, varying between 0. 0095Ra to 0.074Ra, which are similar to that of the phosphorite substrates (0. 087Ra), but much lower than those of normal submarine basalts, suggesting that the basaltic substrates might have been strongly reacted with radiogenic He-4 and P enriching upwelling marine currents or formation water in the sediments. Large amount of ore-forming elements might have been released during basalt/water interaction and which might favor growth of the Cobalt-rich crusts. The relative low He-3/(4) He ratios in the basaltic substrates may be used as one of the important exploration criterions for the Cobalt-rich ferromanganese crusts.