Plume–ridge interaction revisited: Evidence for melt mixing from He, Ne and Ar isotope and abundance systematics

Submarine volcanic glass He, Ne and Ar data from the Pacific–Antarctic-Rise at its intersection with the Foundation Seamount Chain have been obtained to further elucidate one of the main outstanding problems in plume–ridge interaction studies: the physical state in which mantle flow and mixing occur in those settings. Mixing of plume mantle and mid-ocean ridge basalt source material may either be in the form of two solids, of solid-melt or of two melt phases only. The positive correlation of 4He/40Ar⁎ with 206Pb/204Pb as well as the negative correlation of 4He/40Ar⁎ with 40Ar⁎ and 4He concentrations, combined with a lack of correlation between 4He/40Ar⁎ and ridge depth or MgO concentration, indicate that the enriched mantle plume material has been extensively degassed prior to mixing with the mid-ocean ridge source material. Degassing of the plume source and mid-ocean ridge source material prior to mixing is also suggested by the He versus Pb isotope systematics. However, the occurrence of degassing processes requires the existence of melts, implying that mixing between the plume and the mid-ocean ridge material occurs in the physical form of melts.