Magnetic Inclusions in Diamonds

Iron sulfide inclusions in diamonds may provide a new source of paleointensity data from key geological intervals. Pyrrhotite and monosulfide solid solutions (Mss between FeS and NiS, and with minor Cu) are the primary mineral inclusions in diamonds [Deines and Harris, 1995]. Through the courtesy of DeBeers we have obtained eleven diamonds from the Orapa kimberlite pipe (intruded at 92 Ma) in Botswana. The diamonds are all octahedra with slightly rounded or rounded edges and have diameters ranging between 2 and 4 mm. Average mass is 0.03734 grams. The diamonds have abundant internal metallic black disc or rosette fracture systems in the centers of which lie the opaque sulfide inclusions which are 20-50 microns in size. The inclusions are strongly magnetic. The magnetic moments range from 0.8 to 13 x 10-5Am2. This corresponds to minimum estimates of the remanent magnetization of the inclusions of 1 to 8 x10-2Am2/kg (based on estimated masses of the inclusions). The stability of the inclusions' magnetizations is remarkable given the large size of the inclusions (larger than the single-domain grain size of pyrrhotite). Progressive alternating field demagnetization of the natural remanent magnetization yields well-defined magnetization components that demagnetize with peak field values of 90 mT or higher. Isothermal remanent magnetizations saturate in fields of 0.2 to 0.8 Tesla. The strong remanent magnetizations and low coercivities are consistent with a sulfide remanence carrier. Ongoing experiments using thermal demagnetization of orthogonal isothermal remanent magnetizations, hysteresis loop measurements and low temperature measurements, will further constrain the role of iron sulfides as the carrier of these magnetizations. These results will determine if sulfide inclusions in diamonds may provide much needed paleointensity data from the Cretaceous and late Proterozoic. Deines, P. and Harris, J. W. (1995). Sulfide inclusion chemistry and carbon isotopes of African diamonds: Geochimica et Cosmochimica Acta, v. 59, p. 3173-3188.