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2011
Day, JMD, Hilton DR.  2011.  Origin of (3)He/(4)He ratios in HIMU-type basalts constrained from Canary Island lavas. Earth and Planetary Science Letters. 305:226-234.   10.1016/j.epsl.2011.03.006   AbstractWebsite

New helium isotope and abundance measurements are reported for olivine and clinopyroxene phenocrysts from HIMU-type (high-mu=elevated (238)U/(204)Pb) lavas and xenoliths spanning the stratigraphies of El Hierro and La Palma, Canary Islands. Some pyroxene phenocrysts have suffered post-eruptive modification, either by less than 1% assimilation of crustal-derived He, or by closed-system ageing of He. Olivine phenocrysts record mantle source (3)He/(4)He compositions, with the average (3)He/(4)He for La Palma olivine (7.6 +/- 0.8R(A), where R(A) is the atmospheric (3)He/(4)He ratio of 1.38 x 10(-6)) being within uncertainty of those for El Hierro (7.7 +/- 0.3R(A)), and the canonical mid-ocean ridge basalt range (MORB: 8 +/- 1R(A)). The new helium isotope data for El Hierro and La Palma show no distinct correlations with whole-rock (87)Sr/(86)Sr, (143)Nd/(144)Nd, (187)Os/(188)Os, or Pb isotopes, but (3)He/(4)He ratios for La Palma lavas correlate with (18)O/(16)O measured for the same phenocryst populations. Despite limited (3)He/(4)He variations for El Hierro and La Palma, their He-O isotope systematics are consistent with derivation from mantle sources containing distinct recycled oceanic basaltic crust (El Hierro) and gabbroic lithosphere (La Palma) components that have mixed with depleted mantle, and a high-(3)He/(4)He component (>9.7R(A)) in the case of La Palma. The new data are consistent with models involving generation of compositionally and lithologically (e.g., pyroxenite, eclogite, peridotite) heterogeneous mantle sources containing recycled oceanic crust and lithosphere entrained within upwelling high-(3)He/(4)He mantle that has been severely diluted by interaction with depleted mantle. We propose that the noble gas systematics of HIMU-type lavas and ocean island basalts (OIB) in general, are most simply interpreted as being controlled by the most gas-rich reservoir involved in mixing to generate their mantle sources. In this scenario, HIMU and enriched mantle (EM) sources are dominated by depleted mantle, or high-(3)He/(4)He mantle, because recycled crust and lithosphere have low He concentrations. Consequently, high-(3)He/(4)He OIB would predominantly reflect derivation from a less depleted mantle source with sub-equal to higher He contents than depleted mantle. The available coupled He-O isotope systematics measured for OIB lavas are consistent with this hypothesis. (C) 2011 Elsevier B.V. All rights reserved.

2005
Day, JMD, Hilton DR, Pearson DG, Macpherson CG, Kjarsgaard BA, Janney PE.  2005.  Absence of a high time-integrated He-3/(U+Th) source in the mantle beneath continents. Geology. 33:733-736.   10.1130/g21625.1   AbstractWebsite

Volcanic rocks from ocean island and continental flood basalt provinces can exhibit He-3/He-4 ratios greatly in excess of those of mid-oceanic-ridge basalts (MORB). High He-3/He-4 ratios must indicate derivation from a mantle source with high time-integrated He-3/(U+Th) relative to depleted MORB-source mantle. The location of the high He-3/He-4 mantle reservoir is a poorly resolved but important issue because of the constraints it places upon the structure and convective style of Earth's mantle. It has been proposed that the high He-3/He-4 reservoir resides in the upper mantle, rather than the lower mantle, because Earth should be volatile poor and highly differentiated, with incompatible elements (such as He) concentrated in the upper mantle and crust. This hypothesis can be tested using continental intraplate alkaline volcanics (CIAV) that are generated at or near the boundary between the conducting lithospheric and convecting asthenospheric mantle. Olivine and clinopyroxene phenocrysts from Cretaceous to Miocene CIAV from Canada, South Africa, and Uganda have He-3/He-4 ratios more radiogenic than MORB, strongly arguing against a widespread high He-3/He-4 source in the continental lithosphere or the underlying convecting upper mantle. Combined with a global data set of CIAV and continental lithosphere mantle xenoliths, these results provide no evidence for high He-3/He-4 in any samples known to originate from this environment. Therefore, volcanic rocks with He-3/He-4 greater than MORB He-3/He-4 are likely to sample a mantle source with high time-integrated He-3/(U+Th) that cannot exist within or below the continents. This reservoir is also unlikely to exist within the upper mantle as defined by the He-3/He-4 distribution in MORB.

Macpherson, CG, Hilton DR, Day JMD, Lowry D, Gronvold K.  2005.  High-He-3/He-4, depleted mantle and low-delta O-18, recycled oceanic lithosphere in the source of central Iceland magmatism. Earth and Planetary Science Letters. 233:411-427.   10.1016/j.epsl.2005.02.037   AbstractWebsite

New helium and oxygen isotope data and trace element concentrations are reported for volcanic rocks from central Iceland. Basalts that are depleted in the most incompatible trace elements possess a wide range in He-3/He-4 but most ratios are similar to or higher than those of mid-ocean ridge basalt (MORB:similar to 8R(A)[1] [D.W. Graham, Noble gas geochemistry of mid-ocean ridge and ocean island basalts: characterisation of mantle source reservoirs, in: D.P. Porcelli, C.J. Ballentine, R. Wieler (Eds.), Noble gases in Geochemistry and Cosmochemistry, Rev. Mineral. Geochem., vol. 47, 2002, pp. 247-317]). The low concentrations of helium in these rocks suggest that significant degassing has made them susceptible to contamination by low-He-3/He-4 crust, therefore all measured He-3/He-4 are considered minimum estimates for their sources. Elevated helium isotope ratios in the source of these rocks result from interaction with high-He-3/He-4 mantle. The highest oxygen isotope ratios in the depleted rocks are similar to those in melts from typical depleted upper mantle and the range of delta(18)O values is consistent with variable, limited amounts of contamination by Icelandic crust. Most of the incompatible trace element-enriched rocks possess He-3/He-4 ratios that are similar to or lower than those in MORB. These rocks were erupted close to the postulated centre of the Iceland plume. This observation contradicts models in which high-He-3/He-4 characterises the focus of mantle upwelling. A source with MORB-like He-3/He-4 ratios may also be common to other parts of the North Atlantic Igneous Province. The highest delta(18)O values in the enriched rocks are lower than those in MORB and do not appear to have been affected by interaction with low-delta(18)O Icelandic crust. Recycling of hydrothermally altered oceanic crust that has been subducted into the mantle provides a plausible mechanism for generating an O-18-poor source with the trace element and isotopic characteristics of the enriched lavas. (C) 2005 Elsevier B.V All rights reserved.