Noble gas geochemistry of the Cameroon line volcanic chain.

Abstract

A noble gas isotopic study of xenoliths, basalts and CO₂ fluids from the Cameroon line volcanic chain has yielded the first determination of the Ne and Ar isotopic composition of a HIMU system. These data provide evidence that models of HIMU genesis involving the recycling of subducted slabs are inadequate to explain the high content of primordial noble gases found in HIMU basalts. However, the isotopic compositions of He, Ne and Ar may be explained by intramantle melting and equilibration (and atmospheric contamination). Cameroon line basalts display a correlation of <super>4</super>He/<super> 3</super>He with <super>206</super>Pb/<super>204</super>Pb and many trace element ratios, indicating coupled geochemical behavior of noble gases and lithophile isotopes. Neodymium-Hf isotopes and trace element compositions argue against a mixing origin for the Pb-He correlation. The Pb-He data are consistent with a recent (&sim;125 Ma) melt fractionation process. The Pb-He correlation requires He to be more compatible than the parent nuclide U (and Th). The <super>20</super>Ne/<super>22</super>Ne ratio of MORB appears to have a maximum upper limit of &sim;12.5, less than the inferred initial <super> 20</super>Ne/<super>22</super>Ne of 13.8. This reduction of <super>20</super>Ne/<super> 22</super>Ne in the MORB-source region may be achieved by air contamination (<super>20</super>Ne/<super>22</super>Ne_AIR = 9.8) and/or enhanced <super> 22</super>Ne production. Post-eruptive air contamination cannot explain the consistent upper limit of <super>20</super>Ne/<super>22</super>Ne observed in MORB. Addition of atmospheric volatiles to MORB magma chambers is insignificant. Subduction of air is also insufficient to perturb the composition of the upper mantle. The reduction of <super>20</super>Ne/<super>22</super>Ne in the MORB-source mantle is most likely caused by an enhancement of the <super>19</super>F(alpha,n)<super> 22</super>Ne reaction, requiring a mum scale association of F, U and Th. This association is likely to be related to enrichment of grain boundary regions possibly accompanied by minor accessory phases rich in trace elements.