A primordial solar-neon enriched component in the source of EM-I-type ocean island basalts from the Pitcairn Seamounts, Polynesia

Abstract

Volcanic glasses dredged from the Pitcairn Seamounts of southeast Polynesia provide the least-altered basaltic material available with compositions corresponding to the EM-I mantle end-member [A. Zindler and S.R. Hart. Chemical geodynamics. Ann. Rev. Earth Planet. Sci., 14, (1986) 493–571]. We report here the observation of apparent correlations between mantle neon and radiogenic isotopes (Sr, Nd and Pb) in these samples. Combined geochemical data from the Pitcairn Seamounts suggest apparent mixing of a MORB (mid-ocean ridge basalt) component and an enriched mantle component of EM-I type. The neon data indicate that the EM-I component is extremely enriched in primordial solar noble gases. Since radiogenic isotope and trace element data strongly suggest the influence of a subducted crustal component in the EM-I source, the neon results require a hypothetical reservoir in the mantle in which primordial and recycled components are mixed. These results provide significant new evidence for the deep origin of some mantle plumes. In contrast to neon, there is no apparent correlation between helium isotope compositions and radiogenic isotopes in the Pitcairn Seamount samples. Positive correlations are observed, however, between ratios of observed 4He to nucleogenic 21Ne ( 21Ne n) against the total helium concentrations and MgO contents, suggesting that preferential loss of mantle helium has occurred, possibly by interaction with seawater during assimilation–fractional crystallization processes. This observation provides a cautionary note for the interpretation of helium isotope results from other oceanic basalt suites.