Recycled noble gas and nitrogen in the subcontinental lithospheric mantle: Implications from N-He-Ar in fluid inclusions of SE Australian xenoliths

抄録

To elucidate the source of an air-like component in fluid inclusions of xenoliths from the subcontinental mantle, we measured N, He and Ar elemental and isotopic composition in gases released by crushing from spinel-lherzolites of the Newer Volcanics, south-eastern Australia. Gas released from fluid inclusions in olivine separates shows δ¹⁵N ranging from −6.0 ± 1.2‰ to +2.0 ± 1.7‰. The range of measured δ¹⁵N values are in contrast with a remarkably uniform ³He/⁴He ratio of 10.1 ± 0.2 × 10^-6. The lightest δ¹⁵N value of −6.0 ± 1.2‰ is consistent with the measured MORB-like ³He/⁴He ratio of 10.1 ± 0.2 × 10^-6 and suggests that gases in xenoliths of southeast Australia are derived from a well-mixed upper mantle reservoir. The heavier nitrogen isotopic signatures (from ∼0 to +2‰) and elemental ratio of argon to nitrogen could be explained by the addition of 30% to 40% of a recycled sedimentary component. Nitrogen is indeed recycled more efficiently in the mantle than helium, preserving the trace of present or past subduction. The heavy N component has been observed in xenoliths from the eastern side of the Newer Volcanic province. Sedimentary nitrogen may result from subduction along the eastern margin of Australia, during Paleozoic time. The present nitrogen results, together with the relatively low ⁴⁰Ar/³⁶Ar ratios and apparently correlated ³He and ³⁶Ar contents in those xenoliths, suggest the long-term preservation of recycled surface volatiles in the continental lithospheric mantle.