Helium in the continents and in recycled crust and lithosphere

James M.D. Day & David R. Hilton

Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0244, USA; jmdday@ucsd.edu ; drhilton@ucsd.edu

This webpage is a summary of: Day J.M.D., Barry P.H., Hilton D.R., Pearson D.G., Burgess R., Taylor L.A. (2015) The helium flux from the continents and ubiquity of low-³He/⁴He recycled crust and lithosphere. Geochimica et Cosmochimica Acta, 153, 116-133.

Helium isotope ratios (³He/⁴He) are sensitive tracers of mantle heterogeneity, indicating preservation of relatively undegassed reservoirs in Earth’s mantle since planetary accretion (e.g., Kurz et al., 1982). Two important but poorly characterized reservoirs for He are subducted oceanic crust and lithosphere and cratonic continental lithosphere. These reservoirs may play a direct role in the genesis of some intraplate lavas (e.g., Hofmann & White, 1982; McKenzie & O’Nions, 1983), and in He isotope mantle evolution (Gonnermann & Mukhopadhyay, 2009). It has been proposed that these reservoirs might also host high-³He/⁴He domains (e.g., Anderson, 2000; Albarède, 2008). If correct, they may have a significant impact on the noble gas compositions of volcanic rocks (Moreira & Kurz, 2001; Gautheron & Moreira, 2002; Day et al., 2005).

Helium isotope compositions for both cratonic and non-cratonic peridotite xenoliths can be explained through pervasive open-system behaviour of He in cratonic, mobile belt and tectonically-active regions (Gautheron & Moreira, 2002; Barry et al., 2015; Day et al., 2015). Using He isotope data for cratonic peridotites, and assuming that:

1. significant portions (>50 %) of the Archaean and Proterozoic continental lithospheric mantle (CLM) are stable and unaffected by melt or fluid infiltration on geological timescales (>0.1 Ga), and
2. U and Th contents vary between cratonic lithosphere and non-cratonic lithosphere, a ³He flux of 0.25-2.2 atoms/s/cm² is derived for the CLM (Day et al., 2015).

These estimates differ by a factor of ten from non-cratonic lithospheric mantle (Gautheron & Moreira, 2002) and are closer to the observed ³He flux from the continents (<1 atoms/s/cm²) (Sano, 1986).

Pyroxenite and eclogite materials from the continental regions are widely interpreted to be the metamorphic/metasomatic equivalents of recycled oceanic crustal protoliths, and all are characterized by relatively low-³He/⁴He (0.03-5.6 RA) (Day et al., 2015). Together with oceanic peridotites (Moreira & Kurz, 2001), and the CLM, these materials represent mantle reservoirs with low time-integrated ³He/(U+Th). Pyroxenite and eclogite are also typically characterized by higher Fe/Mg, more radiogenic Os-Pb isotope compositions, and more variable δ¹⁸O values (~3 to 7‰), compared with peridotitic mantle. The low-³He/⁴He values of these reservoirs and their distinctive compositions make them probable end-members to explain the compositions of some low-³He/⁴He OIB, and provide an explanation for the low-³He/⁴He measured in most HIMU lavas. For example, it has been argued that He and O isotope compositions of Canary Island lavas as well as lavas from other HIMU islands are correlated, with this relationship representing mixing between at least four different mantle components (Day & Hilton, 2011):

1. Recycled oceanic lithosphere that has suffered high-temperature hydrothermal alteration to generate low-δ¹⁸O and ³He/⁴He signatures with ~4 to 6 RA.
2. Recycled oceanic crust, with extremely low-³He/⁴He (<0.1 RA), and a range of δ¹⁸O, broadly ranging from mantle-like to high-δ¹⁸O values.
3. A high-³He/⁴He component with ≥11 RA and mantle-likeδ¹⁸O.
4. A depleted mid-ocean-ridge-basalt mantle (DMM) component with mantle δ¹⁸O_olivine (5.2 ± 0.3 ‰) and ³He/⁴He of 8 ± 1 RA.

These end-member compositions also allow assessment of the approximate concentration of He in recycled oceanic crust, making the following assumptions: ³He/⁴He for DMM is 8 ± 1 RA with an He concentration of 1 × 10^-5 cm³STP g^-1 (Moreira & Kurz, 2001), and ³He/⁴He for ancient pyroxenites is equal to 0.05-1.2 with a ~10% contribution of ³He/⁴He from recycled oceanic crust. These calculations yield an estimated He concentration in recycled oceanic crust of ~2 to 10 × 10^-7 cm³STP g^-1. Helium isotope data for pyroxenites and eclogites, therefore, support models for the generation of HIMU ³He/⁴He from the partial melting of recycled oceanic crustal protoliths (e.g., Hanyu & Kaneoka, 1997; Hilton et al., 2000; Moreira & Kurz, 2001; Parai et al., 2009; Day & Hilton, 2011).

A basic tenet of partial melting models is that the concentrations of elements in different protoliths involved in melting are fundamental to generating the resulting composition of magmas. For example, a He-rich source (e.g., DMM, high-³He/⁴He mantle) can ‘overprint’ pre-existing mantle He-isotope heterogeneity, if He contents are sufficiently low in the host protoliths (e.g., recycled oceanic crust and lithosphere). Alternatively, at high temperatures, diffusion of He will lead to isotopic equilibration of He such that only completely isolated, or volumetrically large mantle reservoirs will preserve unique ³He/⁴He signatures. Transformation of recycled oceanic crustal protoliths to pyroxenite and eclogite at high temperatures will therefore inevitably result in them inheriting the ³He/⁴He signature of their ambient surrounding mantle. Thus, at locations where a strong recycled crust or lithospheric component is implicated in OIB mantle sources (HIMU; EM), the helium isotope ratio will be dominantly controlled by He derived from DMM, and possibly from a high-³He/⁴He component. Conversely, where OIB are associated with a high plume flux, or high degrees of partial melting and tholeiitic magmatism (e.g., Hawai’i, Iceland, Reunion, Galapagos, Siberian LIP), there may be greater interaction between DMM and a high-³He/⁴He mantle reservoir as enriched flavours (EM, HIMU) become more diluted as a consequence of higher degrees of partial melting.

CLM and recycled oceanic crust and lithosphere protoliths are not reservoirs for high-³He/⁴He and so alternative, volumetrically significant, He-rich reservoirs, such as less-degassed (lower?) mantle, are required to explain high-³He/⁴He signatures measured in some intraplate lavas.

Figure 1: (a) ³He/⁴He versus ¹⁸O/¹⁶O variations in olivine phenocrysts from OIB and South African melilitites (SA melilitites) with HIMU-type Pb isotope compositions. Samples of OIB are divided into those with HIMU (here defined as ²⁰⁶Pb/²⁰⁴Pb >20), HIMU-type Canary Island, and OIB without HIMU (e.g., EM1, EM2, FOZO) affinity. Note the low-³He/⁴He of most HIMU and HIMU-type OIB and the larger range in δ¹⁸O_olivine for these samples compared with ‘OIB’. (b) Expanded view showing models of interaction between MORB (8 RA, δ¹⁸O_olivine = 5.3‰; models 2 and 3) or high-³He/⁴He peridotite (>15 RA, δ¹⁸O_olivine = 5.3‰; model 1) mantle melts mixing with pyroxenite and eclogite components with low-δ¹⁸O and low-³He/⁴He (<0.1 RA) and assuming 80% degassing of the peridotite mantle source. Uncertainties are generally smaller than symbols. See Day et al. (2015) for further details.

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