Abstract
We report new neon isotope data obtained for well-characterised basaltic glasses from the Manus back-arc basin where helium studies have identified a mantle plume component (mean 3He/4He∼12 RA). 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 21Ne/22Ne ratio for a given 20Ne/22Ne 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 3He/4He ratios having nucleogenic neon isotope systematics, indicating a unique decoupling of He 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 3He/4He 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.
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