A Secretive Mechanical Exchange Between Mantle and Crustal Volatiles Revealed by Helium Isotopes in 13C-Depleted Diamonds

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 C-13-depletion and N-15-enrichment relative to mantle values (delta C-13 = -4.3 to -22.2 parts per thousand and delta N-15 = -4.9 to +23.2 parts per thousand. In contrast the He-3/He-4 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 lie in these (Eamondites, or indeed in any diamonds from southern Africa. A correlation between He-3/He-4 ratios and He-3 concentration suggests that the low He-3/He-4 are largely the result of ingrowth of radiogenic He-4 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 He-3/He-4 of low delta C-13 diamondites reflects the high He-3 concentration in the mantle fluids relative to the slab-derived fluids. The presence of post-crystallisation He-4 in the fluids means that all He-3/He-4 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 C-13-depleted system.