A secretive mechanical exchange between mantle and crustal volatiles revealed by helium isotopes in 13C-depleted diamonds
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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.
Mikhail , S , Crosby , J , Stuart , F , DiNicola , L & Abernethy , F A J 2019 , ' A secretive mechanical exchange between mantle and crustal volatiles revealed by helium isotopes in 13 C-depleted diamonds ' , Geochemical Perspectives Letters , vol. 11 , pp. 39-43 . https://doi.org/10.7185/geochemlet.1923
Geochemical Perspectives Letters
Copyright © The Authors. Open Access article. Published by the European Association of Geochemistry under Creative Commons License CC BY-NC-ND 4.0
DescriptionSM acknowledges support from the National Environmental Research Council (grant no. NE/PO12167/1).
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