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

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 13 C-depletion and 15 N-enrichment relative to mantle values (δ 13 C =-4.3 to-22.2 ‰ and δ 15 N =-4.9 to +23.2 ‰). In contrast the 3 He/ 4 He 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 R a). We find no evidence for deep mantle He in these diamondites, or indeed in any diamonds from southern Africa. A correlation between 3 He/ 4 He ratios and 3 He concentration suggests that the low 3 He/ 4 He are largely the result of ingrowth of radiogenic 4 He 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 3 He/ 4 He of low δ 13 C diamondites reflects the high 3 He concentration in the mantle fluids relative to the slab-derived fluids. The presence of post-crystallisation 4 He in the fluids means that all 3 He/ 4 He 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 13 C-depleted system.

A genetic metasomatic link between eclogitic and peridotitic diamond inclusions

“…Furthermore, our models satisfy some stable isotope data, such as strong isotopic evidence for the role of both mantle and crustal sources within single samples and single sample populations. For example, mantle-like high 3 He/ 4 He is observed for diamonds with crustal-like 15 N-enrichment and 13 C-depletion (Gautheron et al, 2005;Mikhail et al, 2019b;Timmerman et al, 2019) and the occasional observation for coupled and progressive 13 C-depletion and 18 O-enrichment trends between diamond and inclusion (Schulze et al, 2013).…”