Nucleogenic neon in high He-3/He-4 lavas from the Manus back-arc basin: a new perspective on He-Ne decoupling

Shaw, AM, Hilton DR, Macpherson CG, Sinton JM.  2001.  Nucleogenic neon in high He-3/He-4 lavas from the Manus back-arc basin: a new perspective on He-Ne decoupling. Earth and Planetary Science Letters. 194:53-66.

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argon, back-are basins, basalts, Bismarck Sea, gases, helium, isotope systematics, mantle plumes, neon, noble, noble-gas systematics, oceanic-crust, rare-gases, ratios, ridge, stable isotopes


We report new neon isotope data obtained for well-characterised basaltic glasses from the Manus back-are basin where helium studies have identified a mantle plume component (mean He-3/He-4 similar to 12 R-A). In three-isotope neon space. seven of the Manus samples lie along a trajectory between air and an endmember more nucleogenic than MORB i.e., compared to typical MORB, samples have a higher Ne-21/Ne-22 ratio for a given Ne-20/Ne-22 ratio. Thus the slope of the Manus Basin line is less than that of the MORB line [Sarda et al., Earth Planet. Sci. Lett. 91 (1988) 73-88]. This is the first observation of lavas with high He-3/He-4 ratios having nucleogenic neon isotope systematics, indicating a unique decoupling of lie from Ne. We evaluate five possible explanations for the observed trend. We discount: (1) crustal contamination, (2) devolatisation of the subducting Solomon Sea plate and (3) addition of neon from an ancient recycled slab component - based upon mass balance considerations of the availability of nucleogenic Ne. Two possibilities remain - both of which must produce an elevated He/Ne ratio in the Manus Basin source region to account for the nucleogenic neon: (4) a previous degassing event which would leave a Ne-depleted residual reservoir, or (5) a deep mantle source heterogeneity preserving a unique signature inherited from Earth's accretion. We find that isolation times as short as 10 Ma for a previously degassed source are sufficient to grow in the nucleogenic Ne without significantly altering the plume-like He-3/He-4 ratios. Alternatively, solubility-controlled outgassing/ingassing of a magma ocean in contact with a proto-atmosphere may have produced the requisite high He/Ne ratio, although an open-system style of equilibration is necessary. At present, insufficient evidence is available to discriminate between these alternatives. (C) 2001 Elsevier Science B.V. All rights reserved.