Mantle helium isotopes

Hanan B. and Graham D. (1996) Lead and helium isotope evidence from oceanic basalts for a common deep source of mantle plumes. Science 111, 991-995. 

Gautheron C. and Moreira M. (2002) Helium isotopic signature of the subcontinental lithospheric mantle. Earth Planet. Sci. Lett. 199, 39-47. 

Measurements of xenon in high He/" He OIB samples have often found atmospheric isotopic ratios (e.g., Allegre et al, 1983) that appear to be due to overwhelming air xenon contamination (Harrison effll., 1999 Patterson efal., 1990) rather than reflecting mantle xenon with an air composition. Although Samoan samples with intermediate (9-20Ra) helium isotope ratios have been found with xenon isotopic ratios distinct from those of the atmosphere (Poreda and Farley, 1992), the xenon in these samples may have been derived largely from the MORB source. Harrison et al. 

Note that storage of helium in the core remains only one component of a noble gas model that can describe the range of noble gas observations. The core has only been evaluated as a possible storage of He. The incorporation in the core of other noble gases, and their relative fractionations, cannot be clearly evaluated without more data. Also, the distribution of radiogenic nuclides such as "" Ar, Xe, and Xe that are produced within the mantle must be explained with a model that fully describes the mantle reservoirs. While these issues may be tractable, a comprehensive model that incorporates a core reservoir remains to be formulated. It should be emphasized that the core does not completely explain the distribution of helium isotopes, since the issue of the " He-heat imbalance is not addressed at all by this model. It appears that even if high He/ He ratios are the signature of involvement of core material in the source of mantle plumes, several mantle reservoirs are still required. 

Dodson A., DePaolo D. J., and Kennedy B. M. (1998) Helium isotopes in lithospheric mantle evidence from Tertiary basalts of the western USA. Geochim. Cosmochim. Acta 62, 3775-3787. 

Hilton D. R., Maepherson C. G., and Elliott T. R. (2000a) Helium isotope ratios in mahe phenocrysts and geothermal fluids from La Palma, the Canary Islands (Spain) implications for HIMU mantle sources. Geochim. Cosmochim. Acta 64, 2119-2132. 

Kurz M. D., Jenkins W. J., and Hart S. R. (1982) Helium isotope systematics of ocean islands and mantle heterogeneity. Nature 297, 43-47. 

Marty B., Pik R., and Gezahegn Y. (1996) Helium isotopic variations in Ethiopian plume lavas—nature of magmatic sources and limit on lower mantle contribution. Earth Planet. Sci. Lett. 144, 223-237. 

Figure 1 Helium isotope data from various mantle-derived volcanics. The upper axis is the He/ He ratio R) normalized to the atmospheric ratio Rj f. As indicated by the data for selected segments of the MORB away from ocean islands falls almost entirely within the range of (7-9)Ra- While there are hotspot basalts that are characterized by high U/Pb ratios and low He/ He ratios (HIMU), many major oceanic hotspots, as well as continental hotspots, have high He/" He ratios (source Porcelli and Ballentine, 2002).

Nitrogen at hotspots. values from hotspots has been found to be largely positive and as high as = +8%o as (Dauphas and Marty, 1999 Marty and Dauphas, 2003). It has been argued that these values are due to the presence of subducted components in mantle plumes (Marty and Dauphas, 2003), and is supported by the lack of correlation with helium isotope compositions. 

Rison W. and Craig H. (1983) Helium isotopes and mantle volatiles in Loihi Seamount and Hawaiian Island basalts and xenoUths. Earth Planet. Set Lett. 66, 407-426. 

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