Phenocryst He-Ar isotopic and whole-rock geochemical constraints on the origin of crustal components in the mantle source of Cenozoic continental basalt in eastern China

摘要

Continental basalts commonly exhibit similar geochemical compositions to oceanic island basalts, but their origin is still enigmatic in chemical geodynamics. This involves a resolution to the nature of crustal components in their mantle sources. For this purpose, we have studied the He and Ar isotopic compositions of mineral phenocrysts from Cenozoic continental basalts in eastern China The results are combined with existing data for whole-rock geochemistry, yielding two trends between noble gas isotopic and other geochemical indicators. First is high ~3He/~4He ratios of 6.6 to 7.5 R-A and low ~4He/~(40)Ar' ratios of 0.07 to 139 in association with varying (La/Yb)_N values from 10.4 to 31.8. Second is low~3He/~4He ratios of 0.6 to 2.4 R-A and variable ~4He/~(40)Ar* ratios from 0.68 to 15.47 in association with low (La/Yb)_N values of 113 to 12.0. Phenocryst ~(40)Ar/~(36)Ar ratios vary from 331.0 to 1677.4, in which the low ratios are associated with the low (La/Yb)_N values. In combination with whole-rock Ba/Th and Sr/Y ratios as well as initial Sr and Nd isotopic ratios, we suggest three-component mixing between enriched MORB mantle, altered oceanic basalt and seafloor sediment to generate the mantle sources. The two kinds of crustal components would be incorporated into the mantle sources in the form of felsic melts. One is adakitic melt deriving from the altered oceanic basalt and thus low in Ba/Th ratios but high in Sr/Y, (La/Yb)_N and ε_(Nd)(t) values. The other is sialic melt originating from the seafloor sediment and thus high in Ba/Th ratios but low in Sr/Y, (La/Yb)_N and ε_(Nd)(t) values. The atmospheric Ar and crustal He noble gas components would be carried by the both seawater-hydrothermally altered basalt and seafloor sediment on the oceanic crust These oceanic crust-derived melts would serve as a metasomatic agent to transfer the supracrustal He and Ar isotopic signatures to the mantle sources. The felsic melts would react with the mantle wedge peridotite during slab-mantle interaction in oceanic subduction channel. Therefore, the noble gas isotopic compositions of mantle sources for intraplate basalts are primarily determined by the budget between the mantle and crustal components during the melt-peridotite in oceanic subduction factory.