Systematic behaviour of 3He/4He in Earth’s continental mantle

Abstract

Helium isotopes are unrivalled tracers of the origins of melts in the Earth’s convecting mantle but their role in determining melt contributions from the shallower and rigid lithospheric mantle is more ambiguous. We have acquired new ³He/⁴He data for olivine and pyroxene separates from 47 well-characterised mantle xenoliths from global on- and off-craton settings. When combined with existing data they demonstrate a new systematic relationship between fluid-hosted ³He/⁴He and major and trace element composition of host minerals and whole rock. We show that a significant proportion (>70 %) of mantle peridotites from continental off-craton settings with depleted major element compositions (e.g., olivine Mg# ≥ 89.5) have ³He/⁴He in the range of modern-day mid-ocean ridge basalt (MORB) source mantle (7–9 R_a) and we propose that they represent underplated melt residues, which initially formed in the convecting upper mantle. Furthermore, we observe that off-craton mantle xenoliths with signatures often attributed to enrichment by melts or fluids from ‘ancient’ subducted oceanic lithosphere have lower ³He/⁴He (<7 R_a). Modest correlations between ³He/⁴He and whole rock incompatible trace element signatures commonly used as proxies for metasomatism by small-fraction carbonatite and silicate melts or C-O-H fluids characterise lithospheric mantle with ³He/⁴He ranging from 5 to 8 R_a.

Using a numerical model that integrates temperature-dependent melt extraction from the upper mantle with in-situ radiogenic ingrowth of ⁴He in the continental mantle we show that the initial ³He/⁴He of continental lithosphere mantle has decreased over time. This is consistent with previous observations demonstrating that ancient (2.5–3.5 Ga) cratonic mantle has a depleted mineral chemistry (e.g., olivine Mg# = 91–94) and low ³He/⁴He (0.5–6.7 R_a), while continental off-craton mantle (<2.5 Ga) is more fertile (olivine Mg# = 88–92) and has less radiogenic ³He/⁴He (4–8.8 R_a). This relationship defines a ‘global lithospheric mantle array’ for intraplate peridotites on plots of ³He/⁴He vs olivine Mg#. Peridotites influenced by past and present subduction fluids, including those that contain amphibole, plot off this array. Our findings have broad implications for the ³He/⁴He signatures observed in continental magmas. Many of Earth’s deepest melts, i.e. proto-kimberlites, are characterised by relatively low ³He/⁴He. We attribute this to assimilation and incorporation of low ³He/⁴He cratonic mantle material during ascent of carbonate-rich melts through thick lithosphere, which overprints the original signatures. Moreover, our findings suggests that the lithospheric mantle acts as a long-term reservoir for other fluid-hosted volatiles (e.g., CO₂, CH₄, H₂O), and in some cases able to sequester these over billion-year timescales until physio-chemical perturbation (e.g., during major rifting or heating events).