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Non-equilibrium degassing and a primordial source for helium in ocean-island volcanism

Abstract

Radioactive decay of uranium and thorium produces 4He, whereas 3He in the Earth’s mantle is not produced by radioactive decay and was only incorporated during accretion—that is, it is primordial1. 3He/4He ratios in many ocean-island basalts (OIBs) that erupt at hotspot volcanoes, such as Hawaii and Iceland, can be up to sixfold higher than in mid-ocean ridge basalts (MORBs). This is inferred to be the result of outgassing by melt production at mid-ocean ridges in conjunction with radiogenic ingrowth of 4He, which has led to a volatile-depleted upper mantle (MORB source) with low 3He concentrations and low 3He /4He ratios2,3,4,5,6. Consequently, high 3He/4He ratios in OIBs are conventionally viewed as evidence for an undegassed, primitive mantle source, which is sampled by hot, buoyantly upwelling deep-mantle plumes3,6,7. However, this conventional model provides no viable explanation of why helium concentrations and elemental ratios of He/Ne and He/Ar in OIBs are an order of magnitude lower than in MORBs. This has been described as the ‘helium concentration paradox’8 and has contributed to a long-standing controversy about the structure and dynamics of the Earth’s mantle. Here we show that the helium concentration paradox, as well as the full range of noble-gas concentrations observed in MORB and OIB glasses, can self-consistently be explained by disequilibrium open-system degassing of the erupting magma. We show that a higher CO2 content in OIBs than in MORBs leads to more extensive degassing of helium in OIB magmas and that noble gases in OIB lavas can be derived from a largely undegassed primitive mantle source.

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Figure 1: Equilibrium degassing trajectories.
Figure 2: Conceptual framework of our disequilibrium degassing model.
Figure 3: Model results for disequilibrium degassing of MORBs and OIBs.
Figure 4: Predicted CO 2 content of parental magmas.

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Acknowledgements

We thank C. Ballentine for a constructive review; M. Saar and J. Rice for providing computational resources; C. Langmuir for helpful suggestions; and R. O’Connell, S. Parman and S. Jacobsen for discussions. H.M.G. was supported by the Daly Fellowship (Department of Earth and Planetary Sciences, Harvard University).

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Correspondence to Helge M. Gonnermann.

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Gonnermann, H., Mukhopadhyay, S. Non-equilibrium degassing and a primordial source for helium in ocean-island volcanism. Nature 449, 1037–1040 (2007). https://doi.org/10.1038/nature06240

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