A secretive mechanical exchange between mantle and crustal volatiles revealed by helium isotopes in 13C-depleted diamonds

Mikhail, S.; Crosby, J.C.; Stuart, F.M.; DiNicola, L. and Abernethy, F.A.J (2019). A secretive mechanical exchange between mantle and crustal volatiles revealed by helium isotopes in 13C-depleted diamonds. Goechemical Perspectives Letters, 11 pp. 39–43.

DOI: https://doi.org/10.7185/geochemlet.1923

Abstract

Fluid inclusions trapped in fast-growing diamonds provide a unique opportunity to examine the origin of diamonds, and the conditions under which they formed. Eclogitic to websteritic diamondites from southern Africa show ¹³C-depletion and ¹⁵N-enrichment relative to mantle values (δ¹³C = -4.3 to -22.2 ‰ and δ¹⁵N = -4.9 to +23.2 ‰). In contrast the ³He/⁴He of the trapped fluids have a strong mantle signature, one sample has the highest value so far recorded for African diamonds (8.5 ± 0.4 Ra). We find no evidence for deep mantle He in these diamondites, or indeed in any diamonds from southern Africa. A correlation between ³He/⁴He ratios and ³He concentration suggests that the low ³He/⁴He are largely the result of ingrowth of radiogenic ⁴He in the trapped fluids since diamond formation. The He-C-N isotope systematics can be best described by mixing between fluid released from subducted altered oceanic crust and mantle volatiles. The high ³He/⁴He of low δ13C diamondites reflects the high ³He concentration in the mantle fluids relative to the slab-derived fluids. The presence of post-crystallisation ⁴He in the fluids means that all ³He/⁴He are minima, which in turn implies that the slab-derived carbon has a sedimentary organic origin. In short, although carbon and nitrogen stable isotope data show strong evidence for crustal sources for diamond-formation, helium isotopes reveal an unambiguous mantle component hidden within a strongly ¹³C-depleted system.

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