The enigmatic high 3He/4He mantle: Characteristics and Origins. (Invited)

Noble gas isotopes measured in some oceanic island basalts (OIBs) exhibit ratios that are associated with the solar wind and the atmosphere of Jupiter, suggesting that the lavas tap portions of an ancient reservoir that still resides in the Earth’s mantle [e.g., 1]. High 3He/4He, as seen in the sources of some OIBs, can therefore serve as a powerful indicator for tracing ancient signatures that have survived in the Earth’s interior. However, the storage mechanisms and reasons for long-term survival of the high 3He/4He signature in the Earth’s convecting mantle are poorly understood. One important observation is that high 3He/4He lavas have 143Nd/144Nd ratios that are higher than chondrites, suggesting that they were derived from a mantle reservoir that suffered ancient depletion. The association of primitive, high 3He/4He with depleted, nonprimitive 143Nd/144Nd in OIBs is not straightforward and a number of models have been developed to resolve this apparent complexity [e.g., 2,3,4,5,6]. It is also becoming apparent that the high 3He/4He reservoir is heterogeneous. High 3He/4He (>30 times atmospheric) lavas from Hawaii, Iceland and Galapagos have more depleted 143Nd/144Nd (0.51294-0.51297) than lavas with similarly high 3He/4He from Samoa (0.51283). In fact, the highest 3He/4He sample from each southern hemisphere high 3He/4He hotspot (FOZO-A, austral) exhibits lower 143Nd/144Nd ratios their northern hemisphere (FOZO-B, boreal) counterparts. The mechanism for this separation is unknown, but it is similar in spatial scale to the DUPAL anomaly, a globe-encircling feature of isotopic enrichment observed primarily in southern hemisphere OIBs. With the exception of Baffin Is. picrites [7], high 3He/4He OIBs also exhibit evidence for Ti, Ta, and Nb (TITAN) enrichment relative to low 3He/4He OIBs. This was interpreted as the result of addition of refractory, rutile-bearing eclogite to a peridotitic high 3He/4He reservoir [8]. This hypothesis is supported by the observation of radiogenic 187Os/188Os in some high 3He/4He lavas. The Baffin Is. exception may suggest that its source is free of 4He-producing eclogite, and may explain why Baffin lavas have such high 3He/4He. A recent discovery [9] demonstrated that chondrites have 142Nd/144Nd ~20 ppm higher than all measured modern terrestrial rocks. If BSE (bulk silicate earth) is chondritic, the elevated terrestrial 142Nd/144Nd is simply a feature of the residue of an early terrestrial differentiation of a chondritic earth. If BSE is not chondritic, the 142Nd/144Nd difference between a non-chondritic BSE and chondrites can be used to constrain the implied difference in Sm/Nd between these two reservoirs. A 5% increase in Sm/Nd in BSE is sufficient to explain the difference in 142Nd/144Nd, and BSE would evolve a present-day 143Nd/144Nd of ~0.51297, a value similar to the 143Nd/144Nd measured in Baffin Is. picrites with the most primitive 3He/4He. This presents the intriguing possibility that the most primitive 143Nd/144Nd in the earth resides in the high 3He/4He mantle [10]. References: 1. Kurz et al., EPSL 1982; 2. Hart et al., Science 1992; 3. Class and Goldstein, Nature 2005; 4. Parman et al., Nature 2005; 5. Albarede, Science 2008; 6. Farley et al., EPSL 1992; 7. Starkey et al., EPSL 2009; 8. Jackson et al., G3 2008; 9. Boyet and Carlson, Science 2005; 10. Caro et al., Nature 2008.