Argon and halogens (Cl, Br and I) have been measured, using 40Ar‐39 Ar stepped heating method, in diamonds from Jwaneng, Orapa (both in Botswana) and Zaire. The samples analysed included cubic (coated) stones and polycrystalline diamonds of eclogitic association. Both these types of diamond contain H2O, CO2, carbonate and silicate inclusions. Coated stones have relatively constant 40Ar*/Cl and Br/Cl, show limited variation in I/Cl, and have normal mantle δ13C values (−5 to −7%). This contrasts with polycrystalline diamonds which, although having similar Br/Cl values to coated stones, possess significantly higher and more variable 40Ar*/Cl and I/Cl values coupled with lower δ13C values (≤−20%). The origin of polycrystalline diamonds with high I/Cl‐low δ13C is tentatively considered in terms of the subduction of organic carbon and iodine in pelagic sediment. Coated stones have Br/Cl, I/Cl and δ13C values that are similar to depleted upper mantle (MORB source). Mantle fluid trapped in the coated stones is enriched in halogens 40Ar by about a factor of 5000 relative to present‐day upper mantle values. However, the estimated halogen content of the source from which the fluid derived is 7 ppm Cl, 25 ppb Br and 0.1‐2.5 ppb I. These values are strikingly similar to those estimated previously for the source of MORB and therefore indicates that the halogens, like other volatile elements (e.g., noble gases, C and N), are homogeneously distributed throughout large portions of the upper mantle.